Task 2 Working Group Report on 

Supercooled Large Droplet Rulemaking 

Prepared by: 

Ice Protection Harmonization Working Group (IPHWG) 

With Recommendations from: 

Flight Test Harmonization Working Group (FTHWG) 

Powerplant Installation Harmonization Working Group (PPIHWG) 

Engine Harmonization Working Group (EHWG) 

Revision A - December 2005
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 1 

Revision Tracking

Revision A - December 2005

Location of 

change

Description

Table of 

Contents

Updated as required

Appendix K of 

WG report

Page K-55

through K-58

Removed interim placeholder comments and inserted 

language on an example certification using visual cues 

(revisions as marked).

Appendix N of 

WG report

Page N-43

Added section A1.4.1.3 discussing visual detection means and 

recognition times and removed interim placeholder comments

(revisions as marked).
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TABLE OF CONTENTS 

1. BACKGROUND:.............................................................................................................. 11

a. Safety Issue Addressed/Statement of the Problem................................................... 11

b. Current Standards or Means to Address.................................................................... 13

2. DISCUSSION of PROPOSAL ........................................................................................ 19

a. Section-by-Section Description of Proposed Action.................................................. 19

Revised Regulations......................................................................................................... 34

b. Alternatives Considered............................................................................................... 49

c. Harmonization Status................................................................................................... 52

3. COSTS AND OTHER ISSUES THAT MUST BE CONSIDERED.............................. 54

a. Costs Associated with the Proposal............................................................................ 54

b. Other Issues ................................................................................................................. 55

4. ADVISORY MATERIAL................................................................................................... 56
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Table of Appendices 

Appendix A Typical Airworthiness Directive on Severe Icing .......................... A-1 

Appendix B FAA Issue Paper on Roll Control in SLD Conditions ................... B-1 

Appendix C JAA Interim Policy on Severe Icing Conditions............................ C-1 

Appendix D Acceptable compliance methods for Appendix X Conditions 

(Natural Ice Flight Testing Necessity) .................................................... D-1 

 Necessity to Flight Test in Natural Appendix X Conditions.......................... D-2 

 Initial Authorities’ Position............................................................................ D-2 

 Initial Manufacturers’ Position...................................................................... D-5 

 Authorities’ Response................................................................................ D-19 

 Majority Working Group Position ............................................................... D-22 

 Remaining Minority Position ...................................................................... D-24 

 Majority Response to Remaining Minority Position.................................... D-26 

Appendix E Necessity of a Means to Determine Exceedance of Appendix X . E-1 

 Necessity of a Means to Determine Exceedance of Appendix X................. E-2 

 Conclusions................................................................................................. E-3 

Appendix F Rule Applicability based on aircraft design features .....................F-1 

 Position statement supporting exclusion from §25.1420 for Airplanes with 

Certain Design Features .........................................................................F-2 

 Majority Position - Response to Exclusion from §25.1420 for Aircraft with 

Certain Design Features .........................................................................F-9 

Appendix G Certification to a Portion of Appendix X.......................................G-1 

 Minority Position Regarding Certification to a Portion of Appendix X ..........G-2 

 Majority Response.......................................................................................G-3 

 Minority Position Against Certifying to Freezing Drizzle or Freezing Rain 

Using a Subset of the Appendix X Curves .............................................G-5 

 Majority Response.......................................................................................G-6 

Appendix H Development of 14 CFR Part 25 Appendix X & 14 CFR Part 33 

Appendix D............................................................................................. H-1 

 14 CFR Part 25, Appendix X ....................................................................... H-2 

 14 CFR Part 33, Appendix D ....................................................................... H-8 
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Table of Appendices (continued) 

Appendix I Accidents and Incidents Relevant to the Proposed Rule.................I-1 

 Icing Incident Database Review ....................................................................I-2 

 Icing Accident Database Review ...................................................................I-3 

 Minority Position on Appendix I of the IPHWG Working Group Report 

(Accidents and Incidents Relevant to the Proposed Rule) .......................I-5 

 Regional Airline Association Statement of Support of Embraer Minority 

Position ....................................................................................................I-8 

 Majority Response.........................................................................................I-8 

 EHWG/PPIHWG Review of Accidents and Incidents ..................................I-11 

Appendix J Cost Estimates ..............................................................................J-1 

 General Assumptions ...................................................................................J-2 

 Example Costs .............................................................................................J-2 

 Engine Certification Costs ............................................................................J-4 

 Non recurring costs (development, certification)...........................................J-7 

 Recurring costs.............................................................................................J-8 

Appendix K IPHWG Recommended Advisory Materials 

§§25.1419, 25.1420 ............................................................................... K-1 

 1. Background. ......................................................................................... K-10 

 2. Definition of Terms ............................................................................... K-13 

 3. Icing Envelopes .................................................................................... K-17 

 4. Certification Plan .................................................................................. K-23 

 5. Analyses ............................................................................................... K-24 

 6. Simulated Ice Shapes and Roughness ................................................. K-32 

 7. Compliance Tests (Flight/Simulation) ................................................... K-32 

 8. Certification To § 25.1420 ..................................................................... K-41 

 9. Pneumatic Deicer Boots ....................................................................... K-44 

 10. Emergency and Abnormal Operating Conditions ................................ K-45 

 11. Ice and Icing Conditions Detection Systems ....................................... K-45 

 12. Perform Intended Function In Icing ..................................................... K-50 

 13. Ice Inspection Light(s) ........................................................................ K-51 

 14. Caution Information ............................................................................ K-51 

 15. Airplane Flight Manual (AFM) ............................................................. K-52 

 16. Guidance for Amended TCs, And STCs ............................................. K-54 

 Appendix A - Example of Certification to §25.1420(a)(1) 

using Visual Cues ............................................................................... K-55 

 Appendix B - Guidance for Amended TC's and STC's .............................. K-59
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Table of Appendices (continued) 

Appendix L Necessity for an AFM statement concerning cumuliform cloud large 

droplet icing.............................................................................................L-1 

 Minority Position on the Necessity of an AFM Statement Concerning 

Cumuliform Cloud Large Droplet Icing (Exceedance of Appendix X) .....L-2 

 Majority Position on the Necessity of an AFM Statement Concerning 

Cumuliform Cloud Large Droplet Icing (Exceedance of Appendix X) .....L-3 

Appendix M FTHWG Recommendations for 14 CFR 25 Sub-Part B ..............M-1 

 Introduction..................................................................................................M-2 

 The FTHWG interpretation of "operating safely" .........................................M-2 

 Continued operation in the icing conditions of Appendix X .........................M-2 

 Continued operation limited to a portion of Appendix X or to a 

flight phase ............................................................................................M-3 

 "Detect and exit" from the icing conditions of Appendix X into clear air ......M-3 

 "Detect and exit" Appendix X certification integral with Appendix C 

certification ............................................................................................M-4 

 Performance and handling qualities for "detect and exit" ...........................M-4 

 Entry into icing conditions ...........................................................................M-5 

 Summary of proposed Subpart B requirements for "detect and exit" ..........M-5 

 Proposed Amendment to CS 25.21(g) Rule to Accommodate Appendix X M-9 

Appendix N FTHWG RECOMMENDATIONS FOR ADVISORY MATERIAL .. N-1 

 1. Purpose .................................................................................................. N-5 

 2. Related Requirements ............................................................................ N-5 

 3. Reserved ................................................................................................ N-6 

 4. Requirements and Guidance .................................................................. N-6 

 5. Acceptable Means of Compliance - General ........................................ N-14 

 6. Acceptable Means of Compliance - Flight Test Programme ................. N-18 

 A1. Appendix 1 - Airframe Ice Accretion ................................................... N-36 

 A2. Appendix 2 - Artificial Ice Shapes ....................................................... N-49 

 A3. Appendix 3 - Design Features ............................................................ N-51 
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Table of Appendices (continued) 

Appendix O EHWG and PPIHWG SUB-GROUP RECOMENDATIONS FOR 

ADVISORY MATERIALS (Revision to AC 20-147) ................................O-1 

 1. Purpose .................................................................................................O-2 

 2. Applicability. ...........................................................................................O-3 

 3. Related Regulations ..............................................................................O-3 

 4. Related Reading Material (Latest Revisions). ........................................O-3 

 5. Background. ..........................................................................................O-4 

 6. Definitions ..............................................................................................O-5 

 7. Discussion .............................................................................................O-8 

 Section 1. Induction System Icing (§§ 33.68 And 33.89(B)) ....................O-11 

 8. Critical Point Analysis (CPA). .............................................................O-11 

 9. Test Point(S) Selection. ......................................................................O-13 

 10. Test Setup Considerations ................................................................O-18 

 11. Test Results And Compliance Issues ................................................O-19 

 12. Appendix X Water Impingement ........................................................O-29 

 Section 2. Ice Slab Ingestion (§ 33.77) ....................................................O-31 

 13. Intent Of Ice Slab Ingestion Test .......................................................O-31 

 14. Compliance Considerations ...............................................................O-31 

 15. Test Results .......................................................................................O-34 

 16. Communication Of Test Results ........................................................O-35 

 Section 3. Induction System Icing Protection (§§ 23.1093 And 25.1093) O-36 

 17. Airframe Ice Sources Including Inlet ...................................................O-39 

 18. Compliance with Appendix D Of Part 33 ............................................O-40 

 19. Compliance with Appendix X Of Part 25 ............................................O-40 

 20. Natural Icing Flight Tests ...................................................................O-41 

 21. Falling and Blowing Snow ..................................................................O-42 

 22. Test Results .......................................................................................O-42 

 23. Concluding Remarks ..........................................................................O-44 
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Table of Figures 

Figure 1 - 14 CFR 25, Appendix X, Freezing Drizzle, Liquid Water Content.......22 

Figure 2 - 14 CFR 25, Appendix X, Freezing Drizzle, Drop Diameter 

Distribution ...............................................................................................23 

Figure 3 - 14 CFR 25, Appendix X, Freezing Drizzle, Temperature 

and Altitude ..............................................................................................24 

Figure 4 - 14 CFR 25, Appendix X, Freezing Rain, Liquid Water Content ..........25 

Figure 5 - 14 CFR 25, Appendix X, Freezing Rain, Drop Diameter 

Distribution ...............................................................................................26 

Figure 6 - 14 CFR 25, Appendix X, Freezing Rain, Temperature 

and Altitude ..............................................................................................27 

Figure 7 - 14 CFR 25, Appendix X, Horizontal Extent, Freezing Drizzle and 

Freezing Rain...........................................................................................28 

Figure 8 - 14 CFR Part 33, App. D, Figure D-1 Convective Cloud Ice Crystal 

Envelope ..................................................................................................42 

Figure 9 - 14 CFR Part 33, App. D, "Figure D-2" Total Water Content ...............43 

Figure 10 - 14 CFR Part 33, App. D, "Figure D-3" Exposure Length 

Influence on TWC ....................................................................................44 

Figure D-1 - Example Impingement Characteristic ........................................... D-8 

Figure D-2 - LWC as a Function of Drop Size................................................. D-19 

Figure I-1 - Fan Damage Events Tally ..............................................................I-11 

Figure I-2 - Rollback/Flameout/Stall and Core Damage Events Tally ...............I-12 

Figure O-1 - Appendix D Icing Envelopes.......................................................O-27 

Figure O-2 - Appendix D Icing Total Water Content........................................O-28 

Figure O-3 - Droplet Impingement Efficiency for Different Shapes .................O-29 

Figure O-4 - Ballistic Boundary as Function of Target and Droplet Size.........O-30 

Figure O-5 - Normal Component of Kinetic Energy of Ice Slab.......................O-33 

Figure O-6 - Ice Slab Orientation Effects ........................................................O-33 

Figure O-7 - Ice Slab Breakup Experience .....................................................O-34 
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Table of Tables 

Table 1 - "Table 25.1093-1"................................................................................35 

Table 2 - "Table 33.68-1" ....................................................................................38 

Table 3 - "Table 33.68-2" ....................................................................................39 

Table 4 - "Table 33.77 Minimum Ice Slab Requirements Based on Engine Inlet 

Size".........................................................................................................41 

Table 5 - 14 CFR 33, App. D, "Table D-1" Supercooled Liquid Portion of TWC .43 

Table D-1 - Large Manufacturer Development Timeline ................................. D-16 

Table G-1 - Comparison of In-flight versus Ground Observations ....................G-6 

Table I-1 - Applicable Aircraft Icing Incidents......................................................I-3 

Table I-2 - Applicable Accidents Of Aircraft With Pre-Appendix –C Icing 

Certification Criteria..................................................................................I-4 

Table I-3 - Applicable Aircraft Accidents For Aircraft With Appendix C 

Certification Criteria..................................................................................I-5 

Table J-1 - Example Costs.................................................................................J-3 

Table J-2 - Example Costs/Engines...................................................................J-6 

Table J-3 - Operating Costs...............................................................................J-9 

Table K-1 - FZDZ Distributions Represented using 10 Bins ........................... K-20 

Table K-2 - FZRA Distributions Represented using 10 Bins ........................... K-21 

Table M-1 - Subpart B Requirements for Detect and Exit of Appendix X..........M-6 

Table N-1 - Means for Showing Compliance .................................................. N-15 

Table N-2 - Trim Speeds................................................................................. N-21 

Table N-3 - Holding Scenario - Manoeuvres................................................... N-32 

Table N-4 - Approach/Landing Scenario - Manoeuvres.................................. N-33 

Table O-A - Inlet Lip and Runback Ice............................................................O-37 
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Abbreviations & Acronyms 

Item Description 

μm Micron (one millionth of a meter) 

AMS American Meteorological Society 

APM Aerodynamic performance monitor 

Appendix C 14 CFR 25, Appendix C 

Appendix X Proposed 14 CFR 25 new appendix with "X" as a placeholder until specific 

Appendix is assigned. 

CAD Computer Aided Design 

CANICE Canadian Ice Accretion Code 

CAST Commercial Aviation Safety Team 

CFD Computational Fluid Dynamics 

CIP Current Icing Potential 

CPA Critical Point Analysis 

Dmax Maximum diameter dropsize within a distribution 

EHWG Engine Harmonization Working Group 

FMC Flight Management Computer 

FSAT Flight Standards Information Bulletin for Air Transportation 

FTHWG Flight Test Harmonization Working Group 

FZDZ Freezing Drizzle 

FZDZ/G Freezing Drizzle with an MVD greater than 40μm 

FZDZ/L Freezing Drizzle with an MVD less than 40μm 

FZRA Freezing Rain 

FZRA/G Freezing Rain with an MVD greater than 40μm 

FZRA/L Freezing Rain with an MVD less than 40μm 

IPHWG Ice Protection Harmonization Working Group 

IPS Ice Protection System 

JSIT Joint Safety Implementation Team 

k/c Indicator of relative roughness, k=roughness height, c=chord dimension 

LEWICE Ice Accretion simulation code produced by NASA Glenn, Icing Branch 

LOC Loss of control 

LWC Liquid Water Content 

MED Mean Effective Drop Diameter 

MMD Median mass dimension 

MMEL Master Minimum Equipment List 

MSC Meteorological Service of Canada 

MSL Mean sea level 

MTOW Maximum Take-off Weight 

MVD Median Volume Diameter 

NCAR National Center for Atmospheric Research 

PIREPS Pilot Weather Reports 

PMS Particle Measurement System 

PPIHWG Powerplant Installation Harmonization Working Group 

SLD Supercooled large drops 

STC Supplemental type certificates 

SWC Stall Warning Computer 

TWC Total water content, liquid water content plus ice water content 

VFR Visual Flight Rules 

WAT Weight, Altitude Temperature limits 

ZL Freezing Drizzle 

ZR Freezing Rain 
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ATTENTION: This Working Group Report was written with the assumption that 

three icing rules, which have been proposed by ARAC to the FAA, have become 

final rules by the time that this report is developed into an NPRM. If those three 

rules are not final at that time, the NPRM will need to be revised to reflect the 

status of the proposed draft rules. The three rules are: §§25.21(g); 25.1419(e), 

(f), (g), and (h); and 121.321. 

This Working Group Report contains references to the Joint Aviation Authorities 

(JAA). The report was coordinated with IPHWG members who participated as 

representatives of the JAA. Coordination with EASA has not been accomplished. 
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1. BACKGROUND: 

This section “tells the story.” 

• It should include all the information necessary to provide context for the 

planned action. Only include information that is helpful in understanding the 

proposal -- no extraneous information (e.g., no “day-by-day” description of 

Working Group’s activities). 

• It should provide an answer for all of the following questions: 

A. SAFETY ISSUE ADDRESSED/STATEMENT OF THE PROBLEM

(1) What prompted this rulemaking activity (e.g., accident, accident 

investigation, NTSB recommendation, new technology, service history, 

etc.)? What focused our attention on the issue? 

On October 31, 1994, an accident involving an Aerospatiale Model ATR72 series 

airplane occurred in which icing conditions, believed to include freezing drizzle 

drops, were reported in the area. The FAA, Aerospatiale, the French Direction 

Général de l’Aviation Civile, Bureau Enquete Accident, National Aeronautics and 

Space Administration, National Transportation Safety Board, and others have 

conducted an extensive investigation of this accident. This investigation led to 

the conclusion that freezing drizzle conditions created a ridge of ice on the wing’s 

upper surface aft of the deicing boots and forward of the ailerons. It was further 

concluded that the ridge of ice resulted in uncommanded roll of the airplane. The 

atmospheric conditions (freezing drizzle) that may have contributed to the 

accident are outside of the icing envelope specified in Appendix C of part 25 of 

the Federal Aviation Regulations (14 CFR part 25) for certification of the airplane. 

Freezing rain is an atmospheric condition that also is outside of the icing 

envelope. The FAA has not required that airplanes be shown to be capable of 

operating safely in freezing drizzle or freezing rain icing conditions. 

NTSB Safety Recommendations

The NTSB issued various safety recommendations to the FAA following the 

Model ATR72 accident. One of the recommendations, A-96-56, states in part 

that: 

If safe operations in certain icing conditions cannot be 

demonstrated by the manufacturer, operational limitations 

should be imposed to prohibit flight in such conditions and 

flightcrews should be provided with the means to positively 

determine when they are in icing conditions that exceed the 

limits for aircraft certification 
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Another recommendation, A-96-54, states: 

Revise the icing criteria published in 14 Code of Federal 

Regulations (CFR), parts 23 and 25, in light of both recent 

research into aircraft ice accretion under varying conditions of 

liquid water content, drop size distribution, and temperature, and 

recent developments in both the design and use of aircraft. Also, 

expand the appendix C icing certification envelope to include 

freezing drizzle/freezing rain and mixed water/ice crystal 

conditions, as necessary. 

In response to the NTSB safety recommendations, the FAA tasked the Aviation 

Rulemaking Advisory Committee (ARAC), by notice published in the Federal 

Register on December 8, 1997 (62 FR 64621), to do the following: 

. . . consider the need for a regulation that requires installation of 

ice detectors, aerodynamic performance monitors, or another 

acceptable means to warn flightcrews of ice accumulation on 

critical surfaces requiring crew action (regardless of whether the 

icing conditions are inside or outside of appendix C of 14 CFR 

part 25). 

and to: 

. . . define an icing environment that includes supercooled large 

drops (SLD), and devise requirements to assess the ability of 

aircraft to safely operate either for the time to exit or to operate 

without restriction in SLD aloft, in SLD at or near the surface . . . . 

(2) What is the underlying safety issue to be addressed in this proposal? 

The Part 25 and Part 33 airworthiness rules identify icing conditions (14 CFR 25, 

Appendix C) upon which approval of airplane operations in icing conditions is 

based. Appendix C conditions do not include supercooled large drops conditions 

(which include freezing drizzle and freezing rain), nor does it cover mixed phase 

and ice crystal icing conditions. The operating rules do not prohibit operations in 

supercooled large drop conditions or mixed phase and ice crystal icing 

conditions. The accident and incident history indicates that flightcrews of certain 

types of aircraft have had power losses and in some cases lost control of their 

aircraft in such conditions. 

(3) What is the underlying safety rationale for the requirement? 

Many airplanes have operated without incident in these conditions. However, the 

performance and handling safety margins are not known for these operations. A 
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required safety margin should be established to help ensure the safety of 

operations in these conditions. 

(4) Why should the requirement exist? 

The proposed rule would expand the certification icing environment to include 

certain freezing drizzle and freezing rain conditions in which an airplane must be 

capable of safely operating either for the time to exit or without restriction. The 

proposed rule would require specified safety margins when operating in these 

icing conditions. Additionally the proposed rule would expand the engine and 

installation certification icing environment to include certain mixed phase and ice 

crystal icing conditions. 

(5) Concerns with the proposed rule. 

NASA is developing codes and test methods to provide airplane manufactures 

with a means of compliance with the proposed rule. However, the distribution of 

the NASA products is limited to US companies. This will result in non-US 

companies being limited in the methods that can be used to comply with the 

proposed rule. Having all possible means of compliance available to all airplane 

companies will improve aircraft safety not only in the US but worldwide. 

Therefore, the IPHWG recommends that NASA take the necessary steps to 

ensure their aircraft icing products are available to all airplane companies 

worldwide. 

B. CURRENT STANDARDS OR MEANS TO ADDRESS

(1) If regulations currently exist: 

(a) What are the current regulations relative to this subject? (Include both the 

14 CFR’s and JAR’s.) 

None of the current FAA 14 CFR 25, 14 CFR 33 rules addresses safe operations 

in freezing drizzle and freezing rain or mixed phase and ice crystal icing 

conditions JAR E and JAR 25 JAR (EASA) 25 guidance materials do define a 

mixed phase/ice crystal environment that has had application to pitot probes and 

engine inlets (ex. AMC 25.1419). 

Certification Regulations 

The current certification icing regulations that are applicable to transport category 

airplanes for flight in icing conditions are contained in Title 14, Code of Federal 

Regulations (14 CFR) part 25 (§§ 25.21(g),25.773, 25.929, 25.1093, 25.1323, 
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25.1325, 25.1419) and JAR 25.773, 25.929, 25.1093, 25.1323, 25.1325, 

25.1419. Both 14 CFR 25.1419 and JAR 25.1419 require that an airplane must 

be able to safely operate in the continuous maximum and intermittent maximum 

icing conditions of 14 CFR part 25, Appendix C and JAR 25, Appendix C. Also, 

icing certification requirements exist for engines in Part 33, sections 33.68 for 

induction system icing and section 33.77 for ice slab ingestion. Some of the 

other regulations that address flight in icing conditions require the affected 

equipment operate to various extents of the icing conditions defined in Appendix 

C while others do not specify the icing conditions that must be considered. 

Appendix C characterizes continuous maximum and intermittent maximum icing 

conditions within stratiform and cumuliform clouds. Appendix C defines icing 

cloud characteristics in terms of mean effective drop diameters, liquid water 

content, temperature, horizontal and vertical extent, and altitude. Freezing 

drizzle and freezing rain precipitation are not included as these environments 

typically contain mean effective diameters that are larger than the cloud mean 

effective drop diameters defined in Appendix C. Consequently, these icing 

conditions containing freezing drizzle and freezing rain are not considered during 

the certification of the airplane’s ice protection system, and exposure to these 

conditions could result in hazardous ice accumulations because the larger 

diameters typically impinge farther aft on airfoil surfaces. Also, mixed phase and 

ice crystal icing conditions are not currently considered during the certification of 

the engine, and exposure to these conditions could result in hazardous ice 

accumulations within the engine that could result in engine damage and power 

loss. 

Operating Regulations

There are relevant regulations that apply to airplane operations in icing 

conditions, which are found in 14 CFR part 91 (“General Operating and Flight 

Rules”), 14 CFR part 121 (“Operating Requirements: Domestic, Flag, and 

Supplemental Operations”), and 14 CFR part 135 (“Operating Requirements: 

Commuter and On Demand Operations and Rules Governing Persons on Board 

Such Aircraft”). 

Specifically, § 91.527 (“Operating in icing conditions”) and § 135.227 (“Icing 

conditions: Operating limitations”) address limitations in icing conditions for 

aircraft operated under these rules. 

Specific requirements regarding exiting hazardous icing conditions are found in 

§ 121.629(a) (“Operation in icing conditions”) which states: 

No person may dispatch or release an aircraft, continue to operate 

an aircraft en route, or land an aircraft when in the opinion of the 

pilot in command or aircraft dispatcher (domestic and flag 

operations only), icing conditions are expected or met that might 

adversely affect the safety of the flight. 
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Also, § 121.341 (“Equipment for operations in icing conditions”) 

requires the installation of certain types of ice protection 

equipment and wing illumination equipment. 

14 CFR 121.321 requires airplanes with less than 60,000 lbs 

maximum takeoff weight and equipped with reversible flight 

controls for the pitch and/or roll axis to exit icing conditions when 

the airplane is operated in supercooled large drop conditions 

conducive to ice accumulation aft of the airframe’s protected 

areas. The regulations require either a substantiated visual cue or 

a caution level alert to the flightcrew when the airplane is operated 

in supercooled large drop conditions conducive to ice 

accumulation aft of the airframe’s protected areas. 

(b) How have the regulations been applied? (What are the current means of 

compliance?) If there are differences between the 14 CFR and JAR, what 

are they and how has each been applied? (Include a discussion of any 

advisory material that currently exists.) 

Advisory material such as JAA interim policy and FAA issue papers are 

discussed in the section 1.b.(2)(a) of this report. 

(c) What has occurred since those regulations were adopted that has caused 

us to conclude that additional or revised regulations are necessary? Why 

are those regulations now inadequate? 

Investigation of the 1994 accident described above resulted in the NTSB, JAA, 

FAA, and others concluding that the existing icing conditions used for certification 

are not adequate. The FAA issued § 121.321 to improve the safety of certain 

existing airplanes when operated in supercooled large drop conditions not 

considered by the certification regulations. This proposed rule addresses the 

safe operation in supercooled large drop conditions of all future part 25 airplanes. 

Section 1.b.(2)(b) of this report contains a full discussion on the inadequacies of 

the existing rules. Additionally, there have been documented cases of over 100 

ice crystal and mixed phase engine events, with six occurrences of multi-engine 

flameouts, during the period of 1988 through 2003. During this same period 

there were 54 aircraft level events of SLD icing engine damage where 56% 

occurred on multiple engines on an aircraft and two events resulted in an air-turnback. 
Transport Airplane Directorate 

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 (2.) If no regulations currently exist: 

(a) What means, if any, have been used in the past to ensure that this safety 

issue is addressed? Has the FAA relied on issue papers? Special 

Conditions? Policy statements? Certification action items? Has the JAA 

relied on Certification Review Items? Interim Policy? If so, reproduce the 

applicable text from these items that is relative to this issue 

As a result of activities following the above-mentioned ATR accident in 1994, the 

FAA issued a series of Airworthiness Directives (ADs) to minimize the potential 

hazards associated with operating certain airplanes in severe icing conditions. 

[Amendment 39-9698, AD 96-09-22 (61 FR 20674, May 7, 1996) is typical of 

these ADs.] The ADs require certain airplanes to exit icing when the conditions 

exceed the capabilities of the ice protection equipment. 

Additionally, JAA interim policy INT/POL/25/11 “Severe Icing Conditions” and 

FAA generic issue paper “Roll Control in Supercooled Large Droplet Conditions” 

have been applied for new certifications on aircraft equipped with unpowered roll 

axis controls and pneumatic de-icing boots. The interim policy and issue paper 

do not address operation of other types of aircraft in freezing drizzle and freezing 

rain conditions. The interim policy and issue paper are intended to provide some 

protection against loss of control by providing for means of detection and exiting 

from freezing drizzle and freezing rain conditions. However, they are not 

intended to certify an airplane for unrestricted flight in supercooled large drops or 

any other conditions which are outside of the Appendix C icing envelope. 

See Appendices A, B, and C of this report for copies of a typical AD, the FAA 

generic issue paper, and the JAA interim policy. 

The FAA has been working with engine manufacturers to identify means to 

understand ice crystal icing conditions effects on engine operation and develop 

mitigation design techniques and validated analytical and test techniques for 

these conditions. Although JAR-E 780(d) provides consideration for ice crystal 

icing conditions, in practice, engine manufacturers have not provided objective 

pre-certification evidence of substantiated operation in these conditions. 

(b) Why are those means inadequate? Why is rulemaking considered 

necessary (i.e., do we need a general standard instead of addressing the 

issue on a case-by-case basis?) 

It is important that all airplanes operate safely in freezing rain and freezing drizzle 

conditions as well as mixed phase and ice crystal icing conditions. However, the 

JAA interim policy and FAA Airworthiness Directives and issue paper are only 

applicable to airplanes with unpowered roll controls and pneumatic deicing boots. 

The § 121.321 rule only addresses airplanes operated under Part 121 that have 
Transport Airplane Directorate 

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12/19/2005 Page 17 

a MTOW less than 60,000 lbs and are equipped with unpowered roll or pitch 

controls. 

The scope of these actions are also limited because they do not address the 

underlying safety concern of the unknown performance and handling safety 

margins for airplanes operating in freezing drizzle or freezing rain or mixed phase 

and ice crystal conditions. Requirements resulting from these actions are not 

comparable to those established by the addition of part 25 §25.21(g) that defined 

safe performance and handling qualities for flight in part 25 Appendix C icing 

conditions. 

The ADs are intended to minimize the potential hazards associated with 

exposure of the identified airplane types to severe icing conditions by having the 

flightcrews exit icing conditions when certain visual cues are observed. 

The FAA issue paper and the JAA interim policy were intended to address the 

safety concern of possible roll upset. The large drop icing environment and the 

dynamics of ice accretion in such conditions were not well understood at the time 

the policies were issued. Therefore the tests only address possible roll upset 

due to a ridge of ice aft of the protected area and forward of the ailerons. 

Extensive research and analyses have been performed to characterize freezing 

drizzle and freezing rain icing conditions and the resultant definition of the 

supercooled large drop icing environment differs from the interim environment 

contained in the issue paper and interim policy. It is recognized that airplanes 

could develop ice shapes other than those addressed by the issue paper and 

interim policy which could result in unsafe operations. Also, other considerations 

in addition to the roll control anomaly identified by the 1994 accident, such as 

engine operability and other airplane flying qualities, should be addressed when 

evaluating airplane airworthiness during flight in icing. 

The § 121.321 rule only partially addresses the concern of airplanes operating in 

icing conditions that exceed Appendix C conditions. The rule is not performance 

based but instead identifies specific conditions (icing conditions conducive to ice 

accretions aft of the protected area) from which the airplane must exit. Like the 

AD’s, the rule minimizes the potential hazards associated with exposure of the 

identified airplanes to severe icing conditions. 

No aircraft, including those affected by these ADs, policies, or operations rule, 

are evaluated for potential performance and handling problems when exposed to 

freezing drizzle and freezing rain. 

New and modified airplanes approved for flight in icing conditions, except those 

limited by the Issue Paper and JAA interim policy, are permitted to operate in 

freezing drizzle and freezing rain icing conditions, based on recommendations of 

the airplane manufacturers and decisions of the airplane operators. Operators of 

future airplanes limited by the Issue Paper and JAA interim policy, will suffer a 

commercial disadvantage relative to operators of airplanes not subjected to these 
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requirements when these icing conditions exist. There are no airworthiness 

requirements, similar to those established by the addition of part 25 §25.21(g), 

for safe flight in freezing drizzle and freezing rain that may be used by affected 

airplane manufacturers or operators to demonstrate that their airplanes are 

unfairly limited from such operations, and thereby be relieved of the commercial 

disadvantage. Conversely, there are no requirements to determine if those 

airplanes not affected by the Issue Paper and JAA Interim policy should so be 

limited. 

The proposed standards for SLD and mixed phase icing conditions effects on 

engines were developed when industry, EASA, Transport Canada, and the FAA 

combined together as an engine and engine installation subgroup under the Ice 

Protection Harmonization Working Group. A thorough review of service 

experience in conjunction with meteorological data has allowed a joint approach 

to developing standards for these icing conditions. This group has developed 

propulsion rules and guidance material that will be phased in as the industry 

advances in its understanding of these weather conditions. For Propulsion 

systems, SLD is addressed in proposed rule changes for 14 CFR Part 33, 

sections 33.68 and 33.77, and 14 CFR Part 25, section 25.1093. Mixed phase 

and ice crystal conditions are addressed in proposed rule changes for 14 CFR 

Part 33, section 33.68, and 14 CFR Part 25, section 25.1093. Guidance 

material has been developed and is proposed in a revision to AC 20-147. The 

JAA has previously had ice crystals (glaciated ice) addressed under JAR-E 

780(d), but with no specific definition of the environmental threat or related ACJ 

guidance material defining acceptable means of compliance. 
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2. DISCUSSION OF PROPOSAL 

This section explains: 

• what the proposal would require, 

• what effect we intend the requirement to have, and 

• how the proposal addresses the problems identified in Background. 

• Discuss each requirement separately. Where two or more requirements are 

very closely related, discuss them together. 

• This section also should discuss alternatives considered and why each was 

rejected. 

A. SECTION-BY-SECTION DESCRIPTION OF PROPOSED ACTION

(1) What is the proposed action? Is the proposed action to introduce a new 

regulation, revise the existing regulation, or to take some other action? 

Propose a new part 25.1420 regulation, a new part 25 Appendix X, and a new 

Part 33 Appendix D. In addition, revise §§25.773, 25.903, 25.929, 25.1093, 

25.1323, 25.1325, 33.68, 33.77 and 121.321. See Appendix M of this report for 

details on the proposed revisions to Part 25 sub-part B. 

Note that paragraph (b) of the proposed part 25.1420 requires “one, or more as 

found necessary, of the following methods must be used:” The words “as found 

necessary” will be applied in the same way as they are applied in §25.1419(b). 

During the certification process the applicant will demonstrate compliance with 

the rule using a combination of analysis and test(s). The applicant’s means of 

compliance will consist of analysis and the amount and types of testing they find 

are necessary to demonstrate compliance with the regulation. The applicant will 

choose to use one or more of the tests identified in paragraphs (b)(1) through 

(b)(5). Although the applicant may choose their means of compliance, it is 

ultimately the FAA (EASA) which must make a finding that the applicant has 

performed sufficient test(s) and analysis to substantiate compliance with the 

regulation. Similarly, the words “as required” which appear in (b)(3) and (b)(5) 

will result in the applicant choosing the means of compliance that is necessary to 

support the analysis but the authorities will make a finding whether the means of 

compliance is acceptable. 

Additionally note that following the proposed 14 CFR § 25.21(g) icing certification 

performance and handling qualities requirements in 14 CFR part 25 Appendix C 

icing conditions, similar requirements are proposed for the icing conditions of 

Appendix X, including defining the ice accretions needed for showing compliance 

with the proposed revised 14 CFR § 25.21(g) requirements for Appendix X. The 

proposed ice accretions are defined in part II of the proposed Appendix X. 
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(2) If regulatory action is proposed, what is the text of the proposed regulation? 

Proposals 1 through 25 - 14 CFR 25 Sub-Part B:

See Appendix M of this report for Proposals 1 through 25 relative to 14 CFR 25, 

subpart B 

Proposal 26 - New Part 25.1420 Regulation:

§25.1420 Supercooled large drop icing conditions 

(a) If certification for flight in icing conditions is desired, in addition to the 

requirements of §25.1419 the airplane must be capable of (a)(1), (a)(2) or (a)(3). 

(1) Operating safely after encountering Appendix X conditions: 

(i) There must be a means provided to detect that the airplane is operating in 

Appendix X; and 

(ii) Following detection, the airplane must be capable of operating safely while 

exiting all icing conditions. 

(2) Operating safely in a portion of Appendix X as selected by the applicant. 

(i) There must be a means provided to detect that the airplane is operating in 

conditions that exceed the selected portion of Appendix X; and 

(ii) Following the exceedance of the selected portion of Appendix X, the airplane 

must be capable of operating safely while exiting all icing conditions. 

(3) Operating safely in the icing conditions of Appendix X. 

(b) To establish that the airplane can operate safely as required in paragraph (a), 

an analysis must be performed to establish that the ice protection for the various 

components of the airplane is adequate, taking into account the various airplane 

operational configurations; and one, or more as found necessary, of the following 

methods must be used: 

(1) Laboratory dry air or simulated icing tests, or a combination of both, of the 

components or models of the components. 

(2) Laboratory dry air or simulated icing tests, or a combination of both, of models 

of the airplane. 

(3) Flight tests of the airplane or its components in simulated icing conditions, 

measured as required to support the analysis. 

(4) Flight tests of the airplane with simulated ice shapes. 

(5) Flight tests of the airplane in natural icing conditions, measured as required to 

support the analysis. 
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(c) For an airplane certificated in accordance with (a)(2) or (a)(3) the 

requirements of 25.1419(e), (f), (g), and (h) must be met for the selected portion 

or all of Appendix X as applicable. 

Proposal 27 - New Part 25 Appendix X

Part 25, Appendix X 

Appendix X consists of two parts. Part I defines Appendix X as supercooled 

large drop (SLD) icing conditions in which the drop median volume diameter 

(MVD) is less than or greater than 40 μm, the maximum mean effective drop 

diameter (MED) of Appendix C continuous maximum (stratiform clouds) icing 

conditions. For Appendix X, supercooled large drop icing conditions consist of 

freezing drizzle and freezing rain and can consist of precipitation in and/or below 

stratiform clouds. Part II defines ice shapes used to show compliance with 14 

CFR § 25.21(g) requirements for continuous flight or for flight in a portion of 

Appendix X. 

PART I – METEOROLOGY 

Appendix X icing conditions are defined by the parameters of altitude, vertical 

and horizontal extent, temperature, liquid water content, and water mass 

distribution as a function of drop diameter distribution. 

a. Freezing Drizzle (Conditions with spectra maximum drop diameters from 

100μm to 500 μm) 

1. Pressure altitude range: 0 to 22,000 feet MSL 

2. Maximum vertical extent: 12,000 feet 

3. Horizontal extent: standard distance of 17.4 nautical miles 
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4. Total liquid water content (cloud and precipitation): 

Note: LWC based on horizontal extent standard distance of 17.4 nm. 

Figure 1 - 14 CFR 25, Appendix X, Freezing Drizzle, Liquid Water Content 
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5. Drop diameter distribution: 

Figure 2 - 14 CFR 25, Appendix X, Freezing Drizzle, Drop Diameter Distribution 
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6. Altitude and temperature envelope: 

Figure 3 - 14 CFR 25, Appendix X, Freezing Drizzle, Temperature and Altitude 
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b. Freezing Rain (Conditions with spectra maximum drop diameters greater than 

500 μm) 

1. Pressure altitude range: 0 to 12,000 ft MSL 

2. Maximum vertical extent: 7,000 ft 

3. Horizontal extent: standard distance of 17.4 nautical miles 

4. Total liquid water content (cloud and precipitation) 

Note: LWC based on horizontal extent standard distance of 17.4 nm. 

Figure 4 - 14 CFR 25, Appendix X, Freezing Rain, Liquid Water Content 
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5. Drop Diameter Distribution 

Figure 5 - 14 CFR 25, Appendix X, Freezing Rain, Drop Diameter Distribution 
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6. Altitude and temperature envelope: 

Figure 6 - 14 CFR 25, Appendix X, Freezing Rain, Temperature and Altitude 
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c. Horizontal extent 

The liquid water content for freezing drizzle and freezing rain conditions for 

horizontal extents other than the standard 17.4nm can be determined by the 

value of the liquid water content determined from Figure 1 or Figure 4, multiplied 

by the factor provided in Figure 7. 

Figure 7 - 14 CFR 25, Appendix X, Horizontal Extent, Freezing Drizzle and 

Freezing Rain 
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PART II – AIRFRAME ICE ACCRETIONS FOR SHOWING COMPLIANCE WITH 

SUBPART B 

a. General 

14 CFR §25.21(g) states that if certification for flight in icing conditions is desired, 

the requirements of subpart B must be met (except as specified otherwise) with 

the ice accretions of part II of Appendix C and part II(b) of this appendix. The 

most critical ice accretion in terms of handling characteristics and performance 

for each flight phase must be determined, taking into consideration the 

atmospheric icing conditions of part I of this appendix, and the flight conditions 

required by §25.21(g) (for example, configuration, speed, angle of attack, and 

altitude). 

b. Ice accretions 

(1) Certification for Appendix X icing conditions defined in Part I of this appendix, 

or a portion thereof, as required by §§ 25.1420(a)(2) and 25.1420(a)(3). 

(a) Takeoff Ice 

Takeoff ice is most critical ice accretion on unprotected surfaces, and any ice 

accretion on the protected surfaces appropriate to normal ice protection system 

operation, occurring between liftoff and 400 feet above the takeoff surface, 

assuming accretion starts at liftoff. 

(b) Final takeoff ice 

Final takeoff ice is the most critical ice accretion on unprotected surfaces, and 

any ice accretion on the protected surfaces appropriate to normal ice protection 

system operation, between 400 feet and 1,500 feet above the takeoff surface, 

assuming ice accretion starts at liftoff. 

(c) En route Ice 

En route Ice is the most critical ice accretion on the unprotected surfaces, and 

any ice accretion on the protected surfaces appropriate to normal ice protection 

system operation, during the en route flight phase. 

(d) Holding Ice 

Holding ice is the most critical ice accretion on the unprotected surfaces, and any 

ice accretion on the protected surfaces appropriate to normal ice protection 

system operation, resulting from 45 minutes of flight within the 17.4 nautical miles 

cloud horizontal extent standard distance during the holding phase of flight. 
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(e) Approach ice 

Approach ice is the more critical ice accretion of the following: 

(i) Ice accumulated during descent from the maximum vertical extent of 

the icing environment described in part I of this appendix to 2,000 feet 

above the landing surface in the cruise configuration, and transition to 

the approach configuration and maneuvering for 15 minutes at 2,000 

feet above the landing surface. 

(ii) Holding ice as defined by Part II b(1)(d). 

(f) Landing Ice 

Landing ice is the more critical ice accretion of the following: 

(i) The ice accretion defined by Part II b(1)(e)(i)) plus ice accumulated 

during a descent from 2,000 feet above the landing surface to a height 

of 200 feet above the landing surface with a transition to the landing 

configuration, a go-around maneuver, beginning with the minimum 

climb requirements of 14 CFR § 25.119, from a height of 200 feet 

above the landing surface to 2,000 feet above the landing surface, 

maneuvering for 15 minutes at 2,000 feet above the landing surface in 

the approach configuration, and a descent to the landing surface 

(touchdown) in the landing configuration. 

(ii) Holding ice as defined by Part II b(1)(d). 

(g) Ice accretion for the takeoff phase 

For both unprotected and protected surfaces, the ice accretion may be 

determined by calculation, assuming the icing conditions described in part I of 

this appendix, and that: 

(i) Airfoils, control surfaces, and, if applicable, propellers are free from 

frost, snow, or ice at the start of takeoff, 

(ii) The ice accretion begins at liftoff, 

(iii) The critical ratio of thrust/power-to-weight, 

(iv) Failure of the critical engine occurs at VEF, and 

(v) Crew activation of the ice protection system is in accordance with a 

normal operating procedure provided in the Airplane Flight Manual in 

accordance with § 25.1581(a)(1), except that after commencement of 

the takeoff roll no crew action to activate the ice protection system 

should be assumed to occur until the airplane is 400 feet above the 

takeoff surface. 

(h) Ice accretion before normal system operation 

Ice accretion prior to normal system operation (pre-activation ice) is the ice 

accretion formed on the unprotected and normally protected surfaces before the 
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activation and effective operation of any ice protection system. The ice accretion 

includes that accumulated during the detection of the icing condition, during 

activation of the ice protection systems, and during the period required for the ice 

protection systems to become fully effective in performing their intended 

functions. 

(i) Failure ice 

Failure ice is the critical holding ice on unprotected surfaces, as defined by 

paragraph part II b(1)(d) of this appendix, plus the critical ice accretion on 

protected surfaces accumulated during 22.5 minutes of exposure to the critical 

icing conditions of part I of this appendix (total accumulation limited to 45 

minutes). If failure of the ice protection system is unannounced or if Airplane 

Flight Manual procedures do not call for exiting icing conditions following an ice 

protection system failure, the failure ice is the same as the ice accretion normally 

considered for unprotected surfaces. For airplanes certificated in accordance 

with 25.1420(a)(1) failure ice need not be considered. For airplanes certificated 

in accordance with 25.1420(a)(2) failure ice need only be considered for the 

selected portion of appendix X. 

(2) Certification for detecting and exiting the icing conditions defined in part I of 

this appendix, as required by §§ 25.1420(a)(1) and 25.1420(a)(2). 

For determining detect-and-exit ice accretions, pre-existing ice accretions may 

exist from operations in approved icing conditions prior to encountering the icing 

conditions requiring an exit. For aircraft certified under 25.1420(a)(2), these preexisting ice accretions should be based on the approved portion of Appendix X 

as determined in paragraph part II b(1) of this appendix. For aircraft certified 

under 25.1420(a)(1), the pre-existing ice accretions should be based on 

Appendix C. 

(a) En route detect-and-exit Ice 

En route detect-and-exit Ice is en route ice, as defined by paragraph part II 

b(1)(c) of this appendix (25.1420(a)(2) certifications) or by paragraph part II (a)(3) 

of 14 CFR part 25 Appendix C (25.1420(a)(1) certifications), as applicable, plus 

pre-detection ice, as described by paragraph part II b(2)(e) of this appendix, and 

the ice accumulated during the transit of one standard cloud horizontal extent 

(17.4 nautical miles) of the icing environment described in part I of this appendix 

and one standard cloud horizontal extent (17.4 nautical miles) of the continuous 

maximum icing environment described in Appendix C of 14 CFR part 25. 

(b) Holding detect-and-exit Ice 

Holding detect-and-exit ice is holding ice, as defined by paragraph part II b(1)(d) 

of this appendix (25.1420(a)(2) certifications) or by paragraph part II (a)(4) of 14 

CFR part 25 Appendix C (25.1420(a)(1) certifications), as applicable, plus pre-
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detection ice, as described by paragraph part II b(2)(e) of this appendix, and the 

ice accumulated during the transit of one standard cloud horizontal extent (17.4 

nautical miles) of the icing environment described in part I of this appendix and 

one standard cloud horizontal extent (17.4 nautical miles) of the continuous 

maximum icing environment described in part I of 14 CFR part 25 Appendix C. 

The total exposure to the icing conditions need not exceed 45 minutes. 

(c) Approach detect-and-exit ice 

Approach detect-and-exit ice accretion is the more critical of the following: 

(i) Ice accumulated during descent from the maximum vertical extent of 

the icing environment described in part I of this appendix to 2,000 feet 

above the landing surface in the cruise configuration (25.1420(a)(2) 

certifications) or descent from the maximum vertical extent of the 

maximum continuous icing environment described in part I of 14 CFR 

part 25 Appendix C (25.1420(a)(1) certifications), as applicable, 

transition to the approach configuration, plus pre-detection ice, as 

described by paragraph part II b(2)(e) of this appendix, and the ice 

accumulated during the transit at 2,000 feet above the landing surface 

of one standard cloud horizontal extent (17.4 nautical miles) of the 

icing environment described in part I of this appendix and one standard 

cloud horizontal extent (17.4 nautical miles) of the continuous 

maximum icing environment described in part I of 14 CFR part 25 

Appendix C. 

(ii) Holding detect-and-exit ice as defined by Part II b(2)(b). 

(d) Landing detect-and-exit ice 

Landing detect-and-exit ice accretion is the more critical of the (i) or (iii): 

(i) Approach ice, as defined by paragraph part II b(1)(f)(i) of this appendix 

plus a descent from 2,000 feet above the landing surface to the a 

height of 200 feet above the landing surface with a transition to the 

landing configuration (25.1420(a)(2) certifications) or approach and 

landing ice in 14 CFR part 25 Appendix C icing conditions as described 

by ii of this section (25.1420(a)(1) certifications), as applicable, plus 

pre-detection ice, as described by paragraph part II b(2)(e) of this 

appendix, and the ice accumulated during an exit maneuver, beginning 

with the minimum climb requirements of 14 CFR part 25 Section 

25.119, from a height of 200 feet above the landing surface through 

one standard cloud horizontal extent (17.4 nautical miles) of the icing 

environment described in part I of this appendix and one standard 

cloud horizontal extent (17.4 nautical miles) of the continuous 

maximum icing environment described in part I of 14 CFR part 25 

Appendix C. 
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(ii) Approach & Landing ice in the maximum continuous icing conditions 

described in part I of 14 CFR part 25 Appendix C is the ice 

accumulated during a descent from the maximum vertical extent of 

Appendix C part I maximum continuous icing conditions to 2,000 feet 

above the landing surface in the cruise configuration, and transition to 

approach configuration and maneuvering for 15 minutes at 2,000 feet 

above the landing surface, a descent from 2,000 feet above the 

landing surface to the a height of 200 feet above the landing surface 

with a transition to the landing configuration. 

(iii) Holding detect-and-exit ice as defined by Part II b(2)(b). 

(e) Ice accretion before detection of Appendix X icing conditions (pre-detection 

ice) 

Ice accretion before detection of Appendix X conditions which require exiting per 

25.1420(a)(1) and (a)(2), is the ice accretion formed on the unprotected and 

normally protected surfaces or on pre-existing ice. The pre-detection ice must 

consider the ice accumulated during the period required to detect the icing 

conditions (considering the provided means of detection) followed by two minutes 

of ice accumulation to represent the period for the flight crew to reference and 

implement prescribed procedures to exit the icing conditions including 

coordination with air traffic control. 

(3) Use of conservative ice accretions 

To reduce the number of ice accretions needed to show compliance with 14 CFR 

§25.21(g), the most conservative ice accretion may be used. 

------------------------END OF APPENDIX X------------------------- 
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REVISED REGULATIONS

(Note: new words are in bold) 

Proposal 28 - Reserved

Proposal 29 - Sec. 25.773 – Pilot compartment view

Sec. 25.773(b)(1)(ii): The icing conditions specified in Appendices C and 

X § 25.1419 if certification with ice protection provisions for flight in icing 

conditions is requested desired.

Proposal 30 - Section 25.903 - Engines

Add section (a)(3) 

 (3) Each turbine engine must comply with one of the following:

 (i) Section 33.68 of this chapter in effect on [insert effective date of 

final rule], or as subsequently amended; or 

 (ii) Comply with § 33.68 of this chapter in effect on February 23, 

1984, or as subsequently amended before [insert effective date of final 

rule], unless that engine’s ice accumulation service history has resulted in 

an unsafe condition; or 

 (iii) Comply with § 33.68 of this chapter in effect on October 1, 1974, 

or as subsequently amended prior to February 23, 1984, unless that 

engine’s ice accumulation service history has resulted in an unsafe 

condition; or 

 (iv) Be shown to have an ice accumulation service history in similar 

installation locations which has not resulted in any unsafe conditions. 

Proposal 31 - Sec. 25.929 – Propeller deicing 

Sec. 25.929(a): If certification for flight in icing is desired. For 

airplanes intended for use where icing may be expected, there must be a 

means to prevent or remove hazardous ice accumulations that could 

form in the icing conditions specified in Appendices C and X of this 

part, on propellers or on accessories where ice accumulation would 

jeopardize engine performance. 
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Proposal 32 - Sec. 25.1093 – Induction system icing protection 

Revise section (b) Turbine Engines. 

Each engine, with all icing protection systems operating, must: 

1. Operate throughout its flight power range, in continuous maximum and 

intermittent maximum icing conditions as defined in Appendix C, Appendix 

X of Part 25 of this chapter, and Appendix D of Part 33 of this chapter, in 

falling and blowing snow within the limitations established for the airplane 

for such operation, including the minimum descent idling speeds in icing, 

without the accumulation of ice on the engine, inlet system components or 

airframe components that: 

I. Adversely affects installed engine operation or that causes a 

permanent loss of power or thrust ; or unacceptable increase in 

operating temperature; or cause an airframe/engine incompatibility; 

or 

II. Results in unacceptable temporary power loss or engine damage; or 

III. Causes a stall, surge, or flameout or loss of engine controllability (for 

example, rollback). 

2) Idle for a minimum of 30-minutes on the ground in the following icing 

conditions shown in Table 25.1093-1 [sic: ref. Table 1 of this report], unless 

replaced by similar test conditions that are more critical. These conditions 

must be demonstrated with the available air bleed for icing protection at its 

critical condition, without adverse effect, followed by an acceleration to 

takeoff power or thrust. During the idle operation the engine may be run up 

periodically to a moderate power or thrust setting in a manner acceptable 

to the Administrator. The applicant must document any demonstrated run 

ups and minimum ambient temperature capability during the conduct of 

icing testing in the limitations section of the Aircraft Flight Manual (AFM). 

Condition Total Air 

Temperature 

Water 

Concentration 

(minimum) 

Mean Effective 

Particle 

Diameter 

Demonstration 

1. Rime ice 

condition 

0 to 15 °F 

(-18 to -9 °C) 

Liquid - 0.3 

gm/m3 

 15-25 microns By test, analysis or 

combination of the 

two. 

2. Glaze ice 

condition 

 20 to 30 °F 

(-7 to -1 °C) 

Liquid - 0.3 

gm/m3 

 15-25 microns By test, analysis or 

combination of the 

two. 

3. Large 

droplet 

condition 

15 to 30 °F 

(-9 to -1 °C) 

Liquid - 0.3 

gm/m3 

100 microns 

(minimum) 

By test, analysis or 

combination of the 

two. 

Table 1 - "Table 25.1093-1" 
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Proposal 33 - Sec. 25.1323 – Airspeed indicating system

Sec. 25.1323(e): Each system must have a heated pitot tube or an 

equivalent means of preventing malfunction due to icing conditions 

specified in appendices C and X of this part. 

Proposal 34 - Sec. 25.1325 – Static pressure system

Sec. 25.1325(b): . . . and that the correlation between air pressure in the 

static pressure system and true ambient atmospheric static pressure is not 

changed when the airplane is exposed to the continuous and intermittent 

maximum icing conditions defined in appendix C and the icing 

conditions defined in appendix X of this part. 

Proposal 35 - Section 33.68 - Induction System Icing

Each engine, with all icing protection systems operating, must: 

A. Operate throughout its flight power range, including the minimum 

descent idling speeds, in icing conditions as defined in Appendix C, 

Appendix X of Part 25 of this chapter, and Appendix D of Part 33 of this 

chapter without the accumulation of ice on the engine components that: 

1) Adversely affects engine operation or that causes an 

unacceptable permanent loss of power or thrust or unacceptable 

increase in engine operating temperature; or 

2) Results in unacceptable temporary power loss or engine damage; 

or 

3) Causes a stall, surge, or flameout or loss of engine controllability 

(for example, rollback). The applicant must account for in-flight 

ram effects (for example; scoop factor amplification, water 

temperature, air density) in any critical point analysis or test 

demonstration of these flight conditions. 

B. Operate throughout its flight power range (including minimum descent 

idling speeds in icing) in continuous maximum and intermittent 

maximum icing conditions as defined in Appendix C, and Appendix X of 

Part 25 of this chapter. In addition,: 

1) It must be shown through Critical Point Analysis (CPA) that the 

complete ice envelope has been analyzed, and that the most critical 

points must be demonstrated by engine test, analysis or a 

combination of the two to operate acceptably. Extended flight in 

critical flight conditions such as hold, descent, approach, climb, and 

cruise, must be addressed, for these ice conditions. 

2) It must be demonstrated by engine test, analysis or a combination 

of the two that the engine can run for a duration that achieves the 

following compliance requirements: 

(a) At engine powers that can sustain level flight: 
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A duration that achieves repetitive, stabilized operation in part 25, 

Appendix C and in large droplet icing conditions of part 25, 

Appendix X. 

(b) At engine power below that which can sustain level flight: 

i. Demonstration in altitude flight simulation test facility: 

A duration of 10 minutes consistent with a simulated flight 

descent of 10000 ft (3 km) operation in Continuous Maximum 

icing conditions, plus 40 percent liquid water content margin, at 

the critical level of airspeed and air temperature, or 

ii. Demonstration in ground test facility: 

A duration of 3 cycles of alternating icing exposure 

corresponding to the LWC levels and standard cloud lengths in 

Intermittent Maximum and Continuous Maximum icing 

conditions, at the critical level of air temperature. 
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C. In addition to complying with paragraph B. of this section, the following 

conditions shown in Table 33.68-1 [sic: ref. Table 2 of this report], unless 

replaced by similar CPA test conditions that are more critical or produce an 

equivalent level of safety, must be demonstrated by engine test: 

Condition Total Air 

Temperature 

LWC 

(minimum) 

Median 

Volume 

Droplet 

Diameter 

(+/- 3 

microns) 

Duration 

1. Glaze ice 

conditions 

21 to 25 °F 

(-6 to -4 °C) 

2 gm/m3 25 microns (a) 10-minutes for power 

below sustainable 

level flight (idle 

descent). 

(b) Must show repetitive, 

stabilized operation 

for higher powers 

(50%, 75%, 100%MC). 

2. Rime ice 

conditions 

-10 to 0 °F 

(-23 to –18 °C) 

1 gm/m3 15 microns (a) 10-minutes for power 

below sustainable 

level flight (idle 

descent). 

(b) Must show repetitive, 

stabilized operation 

for higher powers 

(50%, 75%, 100%MC). 

3. Glaze ice 

holding 

conditions 

(Turboprop 

and turbofan, 

only) 

Turbofan, 

only: 

10 to 18 °F 

(-12 to –8 °C) 

--------------------

- 

Turboprop, 

only: 

2 to 10 °F 

(-17 to –12 °C) 

Alternating 

cycle: 

0.3 gm/m3 (6 

minute) 

1.7 gm/m3 (1 

minute) 

20 microns Must show repetitive, 

stabilized operation (or 

45 minutes max) 

4. Rime ice 

holding 

conditions 

(Turboprop 

and turbofan, 

only) 

Turbofan, 

only: 

-10 to 0 °F 

(-23 to –18 °C) 

--------------------

- 

Turboprop, 

only: 

2 to 10 °F 

(-17 to –12 °C) 

0.25 gm/m3 20 microns Must show repetitive, 

stabilized operation (or 

45 minutes max). 

Table 2 - "Table 33.68-1" 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 39 

D. Idle for a minimum of 30-minutes on the ground in the following icing 

conditions shown in Table 33.68-2 [sic: ref. Table 3 of this report], with the 

available air bleed for icing protection at its critical condition, without 

adverse effect, followed by an acceleration to takeoff power or thrust. 

During the idle operation the engine may be run up periodically to a 

moderate power or thrust setting in a manner acceptable to the 

Administrator. The applicant must document any demonstrated run ups 

and minimum ambient temperature capability during the conduct of icing 

testing in the engine operating manual as mandatory in icing conditions. 

The applicant must demonstrate, with consideration of expected airport 

elevations, the following: 

Condition Total Air 

Temperature 

LWC 

(minimum) 

Mean Effective 

Particle 

Diameter 

Demonstration 

1. Rime ice 

condition 

0 to 15 °F 

(-18 to -9 °C) 

Liquid - 0.3 

gm/m3 

 15-25 microns By engine test 

2. Glaze ice 

condition 

 20 to 30 °F 

(-7 to -1 °C) 

Liquid - 0.3 

gm/m3 

 15-25 microns By engine test 

3. Snow ice 

condition 

26 to 32 °F 

(-3 to 0 °C) 

Ice - 0.9 

gm/m3 100 microns 

(minimum) 

By test, analysis or 

combination of the 

two. 

4. Large 

droplet glaze 

ice condition 

15 to 30 °F 

(-9 to -1 °C) 

Liquid - 0.3 

gm/m3 

100 microns 

(minimum) 

3000 microns 

(maximum) 

By test, analysis or 

combination of the 

two. 

Table 3 - "Table 33.68-2" 

E. The applicant must demonstrate by test, analysis or combination of the 

two acceptable operation in ice crystals and mixed phase icing conditions 

throughout the Part 33, Appendix D icing envelope throughout its flight 

power range, including minimum descent idling speeds. 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 40 

Proposal 36 - Section 33.77 - Foreign object ingestion ice

 (a) Compliance with the requirements of this paragraph shall be 

demonstrated by engine ice ingestion test or by validated analysis 

showing equivalence of other means for demonstrating soft body 

damage tolerance. 

(b) [Reserved] 

(c) Ingestion of ice under the conditions of this section may not -- 

 (1) Cause an immediate or ultimate unacceptable sustained power or 

thrust loss; or 

 (2) Require the engine to be shutdown 

 (d) For an engine that incorporates a protection device, compliance with this 

section need not be demonstrated with respect to ice formed forward of the 

protection device if it is shown that-- 

 (1) Such ice is of a size that will not pass through the protective device; 

 (2) The protective device will withstand the impact of the ice and 

 (3) The ice stopped by the protective device will not obstruct the flow of 

induction air into the engine with a resultant sustained reduction in power or 

thrust greater than those values required by paragraph (c) of this section. 

(e) Compliance with paragraph (c) of this section may be shown by engine test 

under the following ingestion conditions: 

 (1) The minimum ice quantity will be established by the engine 

size as defined in Table 33.77 [sic: ref. Table 4 of this report] – dimensions 

should be linearly interpolated based on actual hilite area 

 (2) The ingestion velocity will simulate ice being sucked into the engine from 

the inlet 

 (3) Engine operation will be at the maximum cruise power or thrust unless 

lower power is more critical 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 41 

Inlet Hilite area 

 (sq inch) 

Thickness 

(inch) 

Width 

(inch) 

Length 

(inch) 

0 0.25 0 3.6 

80 0.25 6 3.6 

300 0.25 12 3.6 

700 0.25 12 4.8 

2800 0.35 12 8.5 

5000 0.43 12 11.0 

7000 0.50 12 12.7 

7900 0.50 12 13.4 

9500 0.50 12 14.6 

11300 0.50 12 15.9 

13300 0.50 12 17.1 

16500 0.5 12 18.9 

20000 0.5 12 20.0 

Table 4 - "Table 33.77 Minimum Ice Slab Requirements Based on Engine Inlet 

Size" 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 42 

Proposal 37 - 14 CFR Part 33 Appendix D – Mixed Phase and Ice Crystal 

Icing Envelope (Deep Convective Clouds) 

Ice crystal conditions associated with convective storm cloud formations exist 

within the 14 CFR Part 25 Appendix C Intermittent Maximum Icing envelope 

(including the extension to -40 deg C) and the Mil Standard 210 Hot Day 

envelope. This ice crystal icing envelope is depicted in Figure D-1. [sic: refer to 

Figure 8 of this report]. 

FAR 33 Appendix D Icing Envelope Limits

-70

-60

-50

-40

-30

-20

-10

0

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000

Altitude - ft

Ambient Temperature - deg C

Appendix D

Figure 8 - 14 CFR Part 33, App. D, Figure D-1 Convective Cloud Ice Crystal 

Envelope 

Within the envelope, total water content (TWC) in gms/m3 have been assessed 

based upon the adiabatic lapse defined by the convective rise of 90% relative 

humidity air from sea level to higher altitudes and scaled by a factor of 0.65 to a 

standard cloud length of 17.4 nautical miles. TWC is displayed for this distance 

over a range of ambient temperature within the boundaries of the ice crystal 

envelope in Figure D-2 [sic: refer to Figure 9 of this report]. 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 43 

TWC Levels: Standard Exposure Length of 17.4 Nautical Miles

(Scaled from Adiabatic Lapse from Sea Level @ 90% Relative Humidity) 

Legend : Ambient Temperature

0

1

2

3

4

5

6

0 5 10 15 20 25 30 35 40 45 50

Altitude - K ft

TWC - gms/m3

- 60 C - 50 C - 40 C - 30 C - 20 C

- 10 C 0 C Upper App D B'ndy Lower App D B'ndy

Figure 9 - 14 CFR Part 33, App. D, "Figure D-2" Total Water Content 

Ice crystal size median mass dimension (MMD) range is 50 - 200 microns 

(equivalent spherical size) based upon measurements near convective storm 

cores. 

The TWC can be treated as completely glaciated except as noted in the Table D1 [sic: refer to Table 5 of this report]. 

Temperature 

Range – deg C 

Horizontal Cloud 

Length 

LWC – gm/m3 

0 to -20 </= 50 miles </=1.0 

0 to -20 Indefinite </=0.5 

< -20 0 

Table 5 - 14 CFR 33, App. D, "Table D-1" Supercooled Liquid Portion of TWC 

The TWC levels displayed in Figure D-2 [sic: refer to Figure 9 of this report] 

represent TWC values for a standard exposure distance (horizontal cloud length) 

of 17.4 nautical miles that must be adjusted with length of icing exposure. The 

assessment from data measurements in References 1 supports the reduction 

factor with exposure length shown in Figure D-3 [sic: refer to Figure 10 of this 

report]. 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 44 

Altitude Ice Crystal Conditions 

Total Water Content Distance Scale Factor

0.6

0.7

0.8

0.9

1

1.1

1.2

1 10 100 1000

Horizontal Extent - Nautical Miles

Total Water Content (TWC) Factor

Figure 10 - 14 CFR Part 33, App. D, "Figure D-3" Exposure Length Influence on 

TWC 

Proposal 38 - Sec. 121.321 – Operations in Icing Conditions

Sec 121.321: After [a date 24 months after the effective date of the final rule], no 

person may operate an airplane with a maximum certified takeoff weight less 

than 60,000 pounds in conditions conducive to airframe icing unless it complies 

with this section or 25.1419 Amendment 25-xx, and 25.1420, 25.773, 25.903, 

25.929, 25.1093, 25.1323, and 25.1325 Amendment 25-xx. 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 45 

(3) If this text changes current regulations, what change does it make? For each 

change: 

What is the reason for the change? 

What is the effect of the change? 

The proposed 25.1420 text changes the icing environment used to evaluate safe 

operation of the airplane in icing conditions. The reason for the change is that 

the accident history of some aircraft has shown that the current icing environment 

requirements are inadequate. The effect of the change is to require: evaluation 

of safe operation in the revised icing environment; and, if necessary, a means to 

differentiate environmental icing conditions and development of exit procedures. 

The proposed 25.1420 extends the requirements of 25.1419(e), (f), (g), and (h), 

regarding the activation and operation of airframe ice protection systems, to the 

certificated portion of Appendix X icing conditions. These requirements are not 

applicable to non-certificated Appendix X icing conditions because 25.1420(a) 

requires a method to identify these conditions and safely exiting the conditions. 

Current knowledge suggests that large-droplet dynamic phenomena such as 

breakup and splashing can reduce the amount of ice that forms. These 

mechanisms can contribute to mass leaving the airfoil surface after impact and 

smaller droplets (either from breakup or splash-off) not impinging. This can lead 

to substantial amounts of water in SLD conditions not staying on the airfoil. 

As has been widely documented, ice ridges may form due to the impingement 

and freezing of large drops aft of protected areas. However, there is 

experimental evidence indicating that, in some conditions, in particular when the 

total air temperature is near the freezing point or at low freezing fractions, aft ice 

accretions may result from runback ice or ice migration (ice-on-surface 

movement), as well as from direct drop impingement. Ice migration has been 

observed in videos of icing tunnel experiments at total air temperature near the 

freezing point and is characterized by ice-buildup that initially grows at or near 

the leading edge, then slides aft due to aerodynamic forces overcoming ice-toairfoil adhesion forces. The videos indicate that the ice migration can contribute 

to the potential for development of ice ridges behind protected areas. Drop 

impingement onto such formations would result in their enlargement. 

If certification for flight in icing is desired, part 25 requires that the airplane must 

be capable of safe operation in icing conditions. The airplane and its 

components are considered during flight in icing certification programs. There 

are several rules in part 25 (§§25.773, 25.929, 25.1093, 25.1323, and 25.1325) 

which reference icing requirements for specific components. Since these rules 

reference Part 25, Appendix C icing conditions or generically mention icing 

conditions, the proposed expansion of icing conditions to be considered for safe 

operation of the airplane necessitates revisions to these rules. The effect of the 

changes is to require that certain components function properly when exposed to 

Part 25, Appendices C and X icing conditions. 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 46 

The proposed part 25 Appendix X provides definitions of supercooled large drop 

icing conditions (freezing drizzle and freezing rain) addressed by the proposed 

§25.1420 and the proposed revisions of §§25.773, 25.929, 25.1093, 25.1323, 

and 25.1325. The supercooled large drop icing conditions described in the 

proposed part 25 Appendix X are the icing meteorological conditions in which the 

airplane must either safely operate unrestricted or safely exit following detection. 

Appendix X includes supercooled drops larger than those considered by current 

icing regulations. These larger drops will impinge and freeze farther aft on 

airplane surfaces and may affect the airplane’s performance, flying qualities, 

engine and systems operation. The Appendix X icing conditions may affect 

design considerations of airplane ice protection provisions. 

Sections 25.929 and 25.1323 generically reference icing instead of specifically 

mentioning Appendix C. Historically the icing conditions of Appendix C have 

been considered as applicable to these rules. For clarity, the rules are revised to 

specifically reference Appendices C and X. 

Section 25.1419 requires that the airplane be able to safely operate in all of the 

conditions specified in Appendix C. Section 25.1420 allows substantiation that 

the airplane is able to safely operate in all of Appendix X, a portion of the 

Appendix X, or detect SLD and exit icing conditions. Airplanes certificated to a 

portion of Appendix X must have a method to detect that the airplane is operating 

in conditions which exceed the selected portion of Appendix X. Airplanes 

certificated to a portion of Appendix X or certificated to detect and exit Appendix 

X conditions may encounter a range of Appendix X conditions while exiting icing. 

It is not practical to certificate probes, components, or engines to varying portions 

of Appendix X. Therefore the icing regulations for probes, components, and 

engines (§§25.773, 25.929, 25.1093, 25.1323, and 25.1325) will be modified to 

reference Appendix X rather than the icing conditions specified in §25.1420. The 

wording is chosen to preclude the interpretation that the components may be 

certificated for only the certificated portions of Appendix X conditions. 

Section 25.773(b) is only applicable if there are ice protection provisions. Future 

airplanes may be capable of safely operating in icing conditions without ice 

protection provisions. Therefore, the rule is also being revised to apply to those 

airplanes certificated for flight in icing. 

Section 25.929 is also being modified to clarify the meaning of the words, “for 

airplanes intended for use where icing may be expected.” The intent is for the 

rule to be applicable to airplanes certificated for flight in icing. The regulation is 

being revised to clarify this intent. 

Section 121.321 is being modified to remove the requirement to comply with the 

rule providing the airplane complies with certain part 25 regulations. 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 47 

Section 121.321 addresses two issues: when to activate the ice protection 

system and when to exit icing conditions. 

Section 25.1419 Amendment 25-xx addresses when the ice protection system 

must be activated. An airplane that is certificated to this amendment level would 

comply with this aspect of §121.321. Section 121.321 partially addresses the 

concern of airplanes operating in icing conditions that exceed Appendix C 

conditions. The part 121 rule is not performance based but instead identifies 

specific conditions (icing conditions conducive to ice accretions aft of the 

protected areas) from which the airplane must exit. The rule does not allow the 

option of substantiating that the airplane can safely operate in icing conditions 

conducive to ice accretions aft of the protected areas. 

The proposed §25.1420 is a performance-based rule that would allow for 

substantiation that the airplane can be safely operated in Appendix X icing 

conditions and that the flight crew need not exit icing conditions when Appendix 

X conditions are encountered. 

Section 25.1420(a)(1) requires a method to detect that the airplane is operating 

in Appendix X; following detection, the airplane must be capable of operating 

safely while exiting all icing conditions. 

When complying with §25.1420(a)(2), certification may be requested for portions 

of Appendix X, such as for freezing drizzle only or for specific phases of flight. 

Following detection, the airplane must be capable of operating safely while 

exiting all icing conditions. Certification for a portion of Appendix X 

(§25.1420(a)(2)) or detect and exit from Appendix X (§25.1420(a)(1)) requires 

that substantiated methods be provided to alert flight crews when those portions 

of Appendix X are exceeded (§25.1420(a)(2)) or Appendix X conditions are 

encountered (§25.1420(a)(1)). 

Certification to Appendix X (§25.1420(a)(3)) requires that the airplane operate 

safely throughout Appendix X. 

An airplane that is certified to proposed §25.1420 would not need to comply with 

Part 121.321(c), which requires certain airplanes to exit icing conditions 

conducive to ice accumulations aft of the protected areas. 

The proposed 33.68, 33.77 and 25.1093 text changes the icing environment 

used to evaluate safe operation of the installed engine in icing conditions. The 

reason for the change is that the incident history of some aircraft has shown that 

the current icing environment requirements are inadequate. The effect of the 

change is to require an evaluation of safe operation in the revised icing 

environment. Engine and engine installation certification to Appendix X of part 

25 and Appendix D of part 33 requires that the plane operate safely throughout 

these icing envelopes. Acceptable compliance methods are proposed within a 

revised AC 20-147. The proposed text change to 25.903 is consistent with the 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 48 

current § 25.903, and allows flexibility for installation of pre § 33.68 certification 

basis engines into new aircraft applications at the FAA’s discretion. 

(4) If not answered already, how will the proposed action address (i.e., correct, 

eliminate) the underlying safety issue (identified previously)? 

This is answered in section 1.a(4) of this report in response to the question, “Why 

should the requirement exist?” 

(5) Why is the proposed action superior to the current regulations? 

The proposed action is superior to the current regulations because it requires 

airplanes to be able to safely operate in supercooled large drop icing conditions, 

and for engines, in ice crystal and mixed phase conditions. These conditions 

have resulted in flightcrews losing control of their aircraft and in some cases in 

engine power loss, and are not addressed by the current regulations. The JAA 

interim policy, FAA issue paper, and FAA Airworthiness Directives, and part 121 

operations rule address a portion of the safety issue. However, as noted in 

section 1.b.(2)(b), there are limitations associated with the JAA and FAA actions. 

Similar to rulemaking that added §25.21(g) to define airplane performance and 

handling qualities that ensure safe flight in part 25 Appendix C icing conditions, 

appropriate part 25 Subpart B requirements have been defined to ensure safe 

flight in the freezing drizzle and freezing rain icing conditions defined by 

Appendix X. These requirements apply to all part 25 airplanes and permit 

determination of airplanes that must be limited, in part or completely, from 

operations in supercooled large drop icing conditions. Subsequently, part 25 

Subpart B requirements that ensure safe flight in icing are comparable for both 

part 25 Appendices C and X. 

Considerations have been given to engine and engine installation, pilot 

compartment view, propeller, and air data instrumentation ice protection 

requirements to ensure safe flight in freezing drizzle and freezing rain as defined 

by Appendix X. These requirements have been revised to address inadvertent 

exposure to supercooled large drop icing conditions, even if the airplane is 

certified for operation in a limited portion of Appendix X or certified to operate 

safely while exiting Appendix X. 

Preliminary definitions of the supercooled large drop icing environment provided 

by the Issue Papers and the JAA Interim Policy on Severe Icing Conditions are 

replaced by a new part 25 Appendix X that resulted from extensive research and 

analyses performed by the Meteorological Service of Canada, the National 

Aeronautics and Space Administration, the FAA, and others. Appendix H of this 

report summarizes the development of the new part 25 Appendix X, and a more 

detailed description is provided in DOT/FAA/AR-04/7. 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 49 

B. ALTERNATIVES CONSIDERED

(1) What actions did the working group consider other than the action proposed? 

Explain alternative ideas and dissenting opinions. 

Alternate Means of Addressing Tasking

Research is ongoing to address avoidance operations in supercooled large 

drops. The use of terminal area radar and sensors for detection and 

characterization of icing conditions and holding areas is being developed.

These have limited capability to determine supercooled large drops in between 

terminals or holding areas and could not provide avoidance information to flight 

crews for these areas. In addition, in development are airborne radars and 

sensors that would allow identification of supercooled large drop conditions in 

sufficient time for avoidance. Although active, these developments are not 

mature enough to provide sufficient aircraft protection over the entire flight 

operations area. 

Additionally, icing diagnostic and predictive weather tools have been developed 

that can provide information on icing and SLD potential, with acceptance of the 

icing diagnostic tool by the Aviation Weather Center. Internet access to these 

experimental tools can be used to provide flight planning information guidance for 

avoidance of SLD conditions. However, aircraft safety margins for inadvertent 

flight in such conditions are not addressed. 

• Aerodynamic Performance Monitor as an Alternative to § 25.1420 and 

Part 25 Appendix X

The working group considered the use of aerodynamic performance monitors 

(APMs) to detect sources of boundary layer instabilities similar to those caused 

by ice accretions. The logic is that flightcrews would be alerted to take 

precautionary actions once the wing or a control surface’s boundary layer 

conditions had reached a safety threshold, regardless of what caused the 

boundary layer disturbance. Several attempts have been made to investigate the 

feasibility of APMs on civil aircraft. Currently no APMs are used on commercial 

aircraft and there is little operational experience to establish confidence in the 

system’s capabilities. The working group concluded that APM’s are not 

sufficiently mature to support a rule based on its technology. However, the rule 

is written broadly enough to allow the use of an APM as a means of compliance 

should the APM’s reach the maturity required to obtain approval for installation 

on a Part 25 airplane. 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 50 

Alternative Concept Relative to Caution Information

The working group evaluated whether the proposed §25.1420 should contain a 

requirement similar to §25.1419(c). Paragraph 25.1419(c) requires that caution 

information, such as an amber caution light or equivalent, must be provided to 

alert the flightcrew when the anti-ice or de-ice system is not functioning normally. 

(Amdt. 25-72, July 20, 1990). In the preamble to Amendment 25-72 it is stated: 

One commenter suggests that the requirement of proposed 

§25.1419(b)(3) for flightcrew caution indication is unnecessary as 

system failure indication requirements are adequately covered by 

§25.1309(c). The FAA concurs that such indication would be 

required by current §25.1309(c) in the absence of a specific rule, 

such as proposed §25.1419(b)(3). The general nature of 

§25.1309(c), however, introduces a degree of uncertainty as to its 

applicability to specific airplane systems. It is, therefore, 

considered appropriate to retain the specific requirement of 

proposed §25.1419(b)(3). 

It should be noted that the proposed §25.1419(b)(3) became §25.1419(c) in the 

final rule. The working group found that in the years since amendment 25-72 

was adopted, §25.1309(c) has been interpreted as applying to any airplane 

system that can experience unsafe system operating conditions. The uncertainty 

that existed in 1990 no longer exists. Paragraph 25.1309(c) does apply to ice 

protection systems. Therefore, the proposal for §25.1420 does not contain the 

requirement for caution information as contained in §25.1419(c). 

Necessity to Flight Test in Natural Appendix X Conditions

Another area of extended debate within the IPHWG regards the topic of whether 

flight testing in natural icing conditions should be required. The discussions 

centered on the difficulty of performing certification testing within Appendix X and 

the ability to use the simulation methods as a means of demonstrating the effects 

of Appendix X conditions. For a more detailed discussion, see Appendix D of 

this report. 

Necessity of a Means to Determine Exceedance of Appendix X

The tasking states: "In addition, consider the need for a regulation that requires 

installation of a means to discriminate between conditions within and outside the 

certification envelope." This topic was debated within the working group and a 

consensus was achieved that a means for discrimination of conditions outside 

the certification envelopes is not necessary. See Appendix E for a detailed 

statement of the final consensus position. 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 51 

Exclusion of Aircraft with Certain Design Features

Airbus, Boeing, and Embraer, with a supporting statement from Cessna, have put 

forth a Minority Position advocating exclusion from §25.1420 for aircraft with 

specific design features. The rationale for the position of these manufacturers is 

that large transport aircraft still in production have experienced no accidents and 

few incidents as a result of flying in SLD conditions. Therefore, the imposition of 

new, burdensome and costly certification requirements for flight in SLD 

conditions for these types of aircraft is unwarranted as no safety improvement 

can be expected. These manufacturers have proposed that aircraft with the 

following three design features be excluded from compliance with §25.1420: (1) 

gross weight in excess of 60,000 lbs (27,000 kg); (2) irreversible powered flight 

controls; and (3) wing leading-edge high-lift devices. It is the intent of these 

manufacturers that all three of these design features would be required to qualify 

for exclusion. The details of this position are included in Appendix F of this 

report. 

Certification to a Portion of Appendix X

Two minority positions against the option for an applicant to certify to a portion of 

Appendix X (§25.1420(a)(2)) were submitted, and consensus of these positions 

was not achieved. Both minority positions address aspects of the appropriate 

use of Appendix X for certification. See Appendix G of this report for the position 

statements. 

Potential Benefits of §25.1420

Embraer, with the support of the Regional Airline Association, holds a minority 

view of the applicability of some of the accidents identified for potential use in the 

economic analysis. See Appendix I of this report for the position statements. 

Necessity for an AFM Statement Concerning Cumuliform Cloud Large Droplet 

Icing

Proposed 14 CFR 25 Appendix X was derived from flight data obtained in stratus 

clouds. As such, information on large droplet icing that may be present in 

cumuliform clouds is not defined. See Appendix L of this report for the minority 

and majority position statements on this issue and Appendix H of this report for 

more information on the development of the proposed 14 CFR 25 Appendix X. 

(2) Why was each action rejected (e.g., cost/benefit? unacceptable decrease in 

the level of safety? lack of consensus? etc.)? Include the pros and cons 

associated with each alternative. 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 52 

Necessity to Flight Test in Natural Appendix X Conditions

See Appendix D of this report. 

Necessity of a Means to Determine Exceedance of Appendix X 

See Appendix E of this report. 

Exclusion of Aircraft with Certain Design Features

The position advocating exclusion from §25.1420 for aircraft with specific design 

features was rejected since the basis of excluding aircraft with the specified 

design features may not be applicable for future airplane designs. The design 

features of future and innovative airplanes are unknown. Therefore, the 

effectiveness of the proposed design features to ensure safe operation in icing 

conditions, including freezing drizzle and freezing rain, is unknown. Safe 

in-service experience for current airplanes with the proposed design features will 

not ensure safe flight for future new and innovative airplane with similar design 

features. For this reason, airplane airworthiness requirements typically do not 

include exclusions unless those exclusions are addressed by other more 

appropriate regulations or if the requirement is not applicable to a specific type of 

engine or airplane. (Examples of appropriate exclusions include turbojet versus 

reciprocating engines, and land based versus water based airplanes.) See 

Appendix F of this report for the full response. 

Certification to a Portion of Appendix X

See Appendix G of this report. 

Potential Benefits of §25.1420

See Appendix I of this report. 

Necessity for an AFM Statement Concerning Cumuliform Cloud Large Droplet 

Icing

See Appendix L of this report. 

C. HARMONIZATION STATUS

(1) Is the proposed action the same for the FAA and the JAA? 

Yes for large drop aspects. Mixed phased icing relative to §§ 25.1323 & 25.1325 

will be addressed by IPHWG Task 5. 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 53 

For 14 CFR 33 engine icing requirements, the rules are somewhat different. The 

objective is to maintain equivalency, just as has been the case historically. 

 (2) If the proposed action differs for the JAA, explain the proposed JAA action. 

No difference is anticipated, but the JAA, now EASA is currently making their 

final determination of their proposed actions. 

(3) If the proposed action differs for the JAA, explain why there is a difference 

between FAA and JAA proposed action (e.g., administrative differences in 

applicability between authorities). 

No difference is anticipated with respect to large drops. For 14 CFR 33 engine 

rules, the JAA/EASA is expected to maintain equivalency to FAA rules, and not 

direct similarity. This equivalency allows for all manufacturers to continue their 

equivalent methods of compliance demonstrations. 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 54 

3. COSTS AND OTHER ISSUES THAT MUST BE CONSIDERED 

The Working Group should answer these questions to the greatest extent 

possible. What information is supplied can be used in the economic evaluation 

that the FAA must accomplish for each regulation. The more quality information 

that is supplied, the quicker the evaluation can be completed. 

A. COSTS ASSOCIATED WITH THE PROPOSAL

(1) Who would be affected by the proposed change? How? (Identify the parties 

that would be materially affected by the rule change – airplane manufacturers, 

airplane operators, etc.) 

Airplane manufacturers of new Part 25 airplane type certificate programs, engine 

manufacturers of new Part 33 engine type certificate programs and applicants for 

type certificate amendments, supplemental type certificates (STCs), or amended 

STCs that involve significant changes to the ice protection system, airframe, or 

engines would be required to provide additional substantiation. 

(2) What is the cost impact of complying with the proposed regulation? Provide 

any information that will assist in estimating the costs (either positive or negative) 

of the proposed rule. 

(For example: 

What are the differences (in general terms) between current practice and the 

actions required by the new rule? 

• If new tests or designs are required, how much time and costs would be 

associated with them? 

• If new equipment is required, what can be reported relative to purchase, 

installation, and maintenance costs? 

• In contrast, if the proposed rule relieves industry of testing or other costs, 

please provide any known estimate of costs. 

• What more-- or what less -- will affected parties have to do if this rule is 

issued? 

NOTE: “Cost” does not have to be stated in terms of dollars; it can be 

stated in terms of work-hours, downtime, etc. Include as much detail as 

possible.)

See Appendix I for the accidents and incidents relevant to this proposed rule. 

See Appendix J for an estimate of the costs. 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 55 

B. OTHER ISSUES

(1) Will small businesses be affected? (In general terms, “small businesses” 

are those employing 1,500 people or less. This question relates to the 

Regulatory Flexibility Act of 1980 and the Small Business Regulatory 

Enforcement Fairness Act of 1996.] 

APO to determine. 

(2) Will the proposed rule require affected parties to do any new or additional 

recordkeeping? If so, explain. [This question relates to the Paperwork 

Reduction Act of 1995.] 

No. 

(3) Will the proposed rule create any unnecessary obstacles to the foreign 

commerce of the United States -- i.e., create barriers to international trade? 

[This question relates to the Trade Agreement Act of 1979.] 

No. 

(4) Will the proposed rule result in spending by State, local, or tribal 

governments, or by the private sector, that will be $100 million or more in 

one year? [This question relates to the Unfunded Mandates Reform Act of 

1995.] 

APO to determine. 
Transport Airplane Directorate 

IPHWG Task 2 Report 

12/19/2005 Page 56 

4. ADVISORY MATERIAL 

a. Is existing FAA or JAA advisory material adequate? Is the existing FAA 

and JAA advisory material harmonized? 

No. 

b. If not, what advisory material should be adopted? Should the existing 

material be revised, or should new material be provided? 

New advisory material is proposed as part of this report. 

c. Insert the text of the proposed advisory material here (or attach), or 

summarize the information it will contain, and indicate what form it will be 

in (e.g., Advisory Circular, Advisory Circular – Joint, policy statement, 

FAA Order, etc.) 

A new combined Advisory Circular to address §25.1419 and §25.1420 

icing requirements has been drafted and will be submitted as a separate 

document. It is intended to replace the current AC 25.1419. The AC 

contains information necessary to clarify the icing guidance for 

certification to Appendix C and the new Appendix X. The current release 

of AC 25.1419 contains materials that provide direction relative to 

compliance to subpart B requirements for icing. Upon recommendation 

of the FTHWG, the subpart B aspects were removed from the IPHWGdrafted Advisory Circular. 

The subpart B aspects for compliance to both §25.1419 as well as 

§25.1420 are to be addressed in the materials prepared by the FTHWG 

in association with AC 25.21-1. The AC prepared by the IPHWG focuses 

on the icing environment and systems compliance aspects. 

The engine and engine installation aspects are to be addressed in a 

revision to AC 20-147. 
IPHWG Task 2 WG Report - Appendix A 

Typical Airworthiness Directive 

12/19/2005 Page A-1 

APPENDIX A - TYPICAL AIRWORTHINESS DIRECTIVE ON 

SEVERE ICING 
IPHWG Task 2 WG Report - Appendix A 

Typical Airworthiness Directive 

12/19/2005 Page A-2 

PART 39 - AIRWORTHINESS DIRECTIVES 

 1. The authority citation for part 39 continues to read as follows: 

Authority: 49 USC 106(g), 40113, 44701. 

§ 39.13 - [Amended] 

 2. Section 39.13 is amended by adding the following new airworthiness 

directive: 

96-09-26 FOKKER: Amendment 39-9602. Docket 96-NM-21-AD. 

 Applicability: All Model F27 Mark 100, 200, 300, 400, 500, 600, and 700 

series airplanes and Model F27 Mark 050 series airplanes, certificated in any 

category. 

 NOTE 1: This AD applies to each airplane identified in the preceding 

applicability provision, regardless of whether it has been modified, altered, or 

repaired in the area subject to the requirements of this AD. For airplanes that 

have been modified, altered, or repaired so that the performance of the 

requirements of this AD is affected, the owner/operator must request approval for 

an alternative method of compliance in accordance with paragraph (b) of this AD. 

The request should include an assessment of the effect of the modification, 

alteration, or repair on the unsafe condition addressed by this AD; and, if the 

unsafe condition has not been eliminated, the request should include specific 

proposed actions to address it. 

 Compliance: Required as indicated, unless accomplished previously. 

 To minimize the potential hazards associated with operating the airplane 

in severe icing conditions by providing more clearly defined procedures and 

limitations associated with such conditions, accomplish the following: 

 (a) Within 30 days after the effective date of this AD, accomplish the 

requirements of paragraphs (a)(1) and (a)(2) of this AD. 

 NOTE 2: Operators must initiate action to notify and ensure that flight 

crewmembers are apprised of this change. 

 (1) Revise the FAA-approved Airplane Flight Manual (AFM) by 

incorporating the following into the Limitations Section of the AFM. This may be 

accomplished by inserting a copy of this AD in the AFM. 

"WARNING

Severe icing may result from environmental conditions outside of 

those for which the airplane is certificated. Flight in freezing rain, 

freezing drizzle, or mixed icing conditions (supercooled liquid water 

and ice crystals) may result in ice build-up on protected surfaces 

exceeding the capability of the ice protection system, or may result 

in ice forming aft of the protected surfaces. This ice may not be 

shed using the ice protection systems, and may seriously degrade 

the performance and controllability of the airplane. 

• During flight, severe icing conditions that exceed those for which 

the airplane is certificated shall be determined by the following 

visual cues. If one or more of these visual cues exists, immediately 
IPHWG Task 2 WG Report - Appendix A 

Typical Airworthiness Directive 

12/19/2005 Page A-3 

request priority handling from Air Traffic Control to facilitate a route 

or an altitude change to exit the icing conditions. 

- Unusually extensive ice accreted on the airframe in areas not 

normally observed to collect ice. 

- Accumulation of ice on the lower surface of the wing aft of the 

protected area. 

- Accumulation of ice on the propeller spinner farther aft than 

normally observed. 

• Since the autopilot may mask tactile cues that indicate adverse 

changes in handling characteristics, use of the autopilot is 

prohibited when any of the visual cues specified above exist, or 

when unusual lateral trim requirements or autopilot trim warnings 

are encountered while the airplane is in icing conditions. 

• All icing detection lights must be operative prior to flight into icing 

conditions at night. [NOTE: This supersedes any relief provided by 

the Master Minimum Equipment List (MMEL).]" 

 (2) Revise the FAA-approved AFM by incorporating the following into the 

Procedures Section of the AFM. This may be accomplished by inserting a copy 

of this AD in the AFM. 

"THE FOLLOWING WEATHER CONDITIONS 

MAY BE CONDUCIVE TO SEVERE IN-FLIGHT ICING:

• Visible rain at temperatures below 0 degrees Celsius ambient air 

temperature. 

• Droplets that splash or splatter on impact at temperatures below 

0 degrees Celsius ambient air temperature. 

PROCEDURES FOR EXITING 

THE SEVERE ICING ENVIRONMENT:

These procedures are applicable to all flight phases from takeoff to 

landing. Monitor the ambient air temperature. While severe icing 

may form at temperatures as cold as -18 degrees Celsius, 

increased vigilance is warranted at temperatures around freezing 

with visible moisture present. If the visual cues specified in the 

Limitations Section of the AFM for identifying severe icing 

conditions are observed, accomplish the following: 
IPHWG Task 2 WG Report - Appendix A 

Typical Airworthiness Directive 

12/19/2005 Page A-4 

• Immediately request priority handling from Air Traffic Control to 

facilitate a route or an altitude change to exit the severe icing 

conditions in order to avoid extended exposure to flight conditions 

more severe than those for which the airplane has been 

certificated. 

• Avoid abrupt and excessive maneuvering that may exacerbate 

control difficulties. 

• Do not engage the autopilot. 

• If the autopilot is engaged, hold the control wheel firmly and 

disengage the autopilot. 

• If an unusual roll response or uncommanded roll control 

movement is observed, reduce the angle-of-attack. 

• Do not extend flaps during extended operation in icing conditions. 

Operation with flaps extended can result in a reduced wing 

angle-of-attack, with the possibility of ice forming on the upper 

surface further aft on the wing than normal, possibly aft of the 

protected area. 

• If the flaps are extended, do not retract them until the airframe is 

clear of ice. 

• Report these weather conditions to Air Traffic Control." 

 (b) An alternative method of compliance or adjustment of the compliance 

time that provides an acceptable level of safety may be used if approved by the 

Manager, Standardization Branch, ANM-113, FAA, Transport Airplane 

Directorate. Operators shall submit their requests through an appropriate FAA 

Principal Operations Inspector, who may add comments and then send it to the 

Manager, Standardization Branch, ANM-113. 

 NOTE 3: Information concerning the existence of approved alternative 

methods of compliance with this AD, if any, may be obtained from the 

Standardization Branch, ANM-113. 

(c) Special flight permits may be issued in accordance with sections 

21.197 and 21.199 of the Federal Aviation Regulations (14 CFR 21.197 and 

21.199) to operate the airplane to a location where the requirements of this AD 

can be accomplished. 
IPHWG Task 2 WG Report - Appendix B 

FAA Issue Paper on SLD 

12/19/2005 Page B-1 

APPENDIX B - FAA ISSUE PAPER ON ROLL CONTROL IN SLD 

CONDITIONS 
IPHWG Task 2 WG Report - Appendix B 

FAA Issue Paper on SLD 

12/19/2005 Page B-2 

ISSUE PAPER

 

PROJECT: GENERIC AIRPLANE COMPANY ITEM: S-2 

 Model XYZ STAGE: 2 

 

REG. REF.: §§ 21.16, 21.21(b)(2), 25.1419 

 

DATE: June 25, 1998 

 

NATIONAL 

POLICY REF.: ANM-100 Memorandum dated July 

23, 1997 (Signed: Ron Wojnar) 

ISSUE STATUS: OPEN 

 

SUBJECT: Roll Control in Supercooled Large 

Droplet Conditions 

BRANCH ACTION: ANM-111, 

112 

 

Based on GIP # S-x13 COMPLIANCE 

TARGET: Pre-TC 

 

Known Unsafe Condition 

STATEMENT OF ISSUE:

The requirements for the certification of an airplane with ice protection provisions 

are defined in o 25.1419. The current regulation requires that an airplane with ice 

protection provisions must be able to safely operate in the conditions defined in 

the Federal Aviation Regulations, part 25, appendix C. The regulation is not 

adequate to address freezing drizzle and freezing rain conditions (hereafter called 

supercooled large droplets or SLD) that are outside of the appendix C envelope. 

BACKGROUND:

On October 31, 1994, an Aerospatiale ATR-72-212 was involved in an accident in 

which severe icing conditions were reported in the area. During extensive testing 

the accident profile was replicated by ice shapes developed from testing in an 

icing cloud having droplets in the size range of freezing drizzle at a temperature 

near freezing. This condition created a ridge of ice aft of the deicing boots and 

forward of the ailerons, which resulted in uncommanded motion of the ailerons and 

rapid roll of the aircraft. 

The National Transportation Safety Board recommended that the FAA develop a 

test procedure to identify unsafe aileron hinge moment characteristics. The 

procedure described herein is the procedure used during the FAA's program to 

screen airplanes for susceptibility to aileron control anomalies. The airplanes that 

were evaluated in the program are used in regularly scheduled passenger service 

equipped with non-powered controls and pneumatic deicing boots. 
IPHWG Task 2 WG Report - Appendix B 

FAA Issue Paper on SLD 

12/19/2005 Page B-3 

FAA POSITION: 

The FAA has identified the susceptibility to loss of control following exposure to 

supercooled large droplets as an unsafe condition that may exist on other 

airplanes. The FAA is particularly concerned with airplanes with non-powered 

flight controls, since non-powered flight controls do not have the physical 

advantage of hydraulic or electrical power to assist the pilot in overcoming the 

large control forces that may exist from differential pressure resulting from flow 

separation over the roll control surfaces.

The FAA proposes to test airplanes with non-powered roll controls and pneumatic 

deicing boots for susceptibility to roll control anomalies in certain supercooled 

large droplet conditions. This test is not intended to certify an airplane for flight in 

supercooled large droplets or any other conditions which are outside of the 

appendix C icing envelope. 

The following test is proposed: 

a. Tests and analyses must show that the airplane characteristics meet the 

criteria specified in paragraph b. following a 20 minute icing encounter: 

 1. with supercooled droplets having maximum diameters of approximately 

400 μm (microns), 

 2. an LWC (liquid water content) of approximately 0.6 grams per cubic meter1, 

 3. a median volumetric diameter of approximately 170 microns2, 

 4. temperatures near freezing such that runback conditions exist at the 

stagnation line3, and 

 5. at holding speeds and approved holding configurations. 

b. When manually flying the airplane: 

 1. The pilot roll force to counter any uncommanded roll control surface 

deflection may not exceed 50 pounds with two hands available for control, 

and 

 2. the airplane must not exhibit a hazardous degradation of flying qualities. 

Rapid control force onset, unsteady and oscillatory forces must be 

considered carefully as these dynamic conditions may be hazardous even 

though the peak force may be less than the static limit. 

c. The tests and analyses in paragraph a. must consider the effects of asymmetric 

shedding of the ice. 

 

1 For this condition, the LWC strongly affects the rate at which the ice feature develops. A higher LWC results in more 

rapid formation of the ice feature while a lower LWC results in a slower formation of the ice feature. The LWC should 

be adequate to produce an ice feature during the exposure interval that will start to self shed and then reform. 

2 The cloud physics instrumentation, calibration, and data processing methodologies must be acceptable to the FAA. 

3 For this test, temperature is a critical factor. Not only is the temperature critical to the development of the ice shape and

dimension, static air temperature excursions above freezing, although short in duration, can reverse the ice accretion 

process. 
IPHWG Task 2 WG Report - Appendix B 

FAA Issue Paper on SLD 

12/19/2005 Page B-4 

 

d. There must be a means for the flightcrew to determine when the airplane has 

entered into a supercooled large droplet environment, to enable the crew to take 

appropriate action. 

e. There must be appropriate crew information provided in the airplane flight 

manual that describes the limitations and procedures to be observed while exiting 

the supercooled large droplet environment. The FAA finds the limitations and 

procedures contained in AD 96-09-25 are an acceptable means of compliance 

with this paragraph. These limitations and procedures include but are not limited 

to: 

 1. visual cues that the airplane is in severe icing conditions, 

 2. prohibition on the use of the autopilot when the visual cues are 

observed, 

 3. all icing detection lights operative prior to flight into icing conditions at 

night, 

 4. immediate exiting of the severe icing conditions, and 

 5. if the flaps are extended, do not retract them until the airframe is clear of 

ice. 

 

Note: For paragraph e.5., the retraction of the flaps is contingent upon the aircraft 

having a means to determine if the airframe is clear of ice. 

f. One means of compliance with paragraphs a., b., and c. is to perform a high 

speed taxi test to evaluate control wheel force characteristics that may result from 

flow separation over the roll control surfaces induced by an artificial ice shape as 

described below. The testing should include the following: 

 1. Installation of a one-inch high quarter-round molding, flat side forward, 

located on the upper surface of the wing, at the chord position aft of the 

active portion of the boots and forward of the non-powered roll control 

surfaces (i.e. ailerons and/or inflight spoilers) that produces the most 

adverse lateral wheel force. 

 2. Locate this shape in front of the roll control surfaces on one wing only. 

As a minimum the shape must cover the entire span of the roll control 

surface. 

 3. Perform high speed taxi tests with the flaps retracted and at various 

angles of attack. The maximum angle of attack should be obtained at the 

highest takeoff weight such that the airplane does not become airborne. 

 4. Measure the forces required to maintain wings level. 

 5. Extrapolate the maximum forces obtained from the high speed taxi tests 

to the maximum speeds expected while in holding conditions. In most 

cases the maximum forces will occur at the maximum angle of attack 

achieved during the high speed taxi tests. 

 6. The extrapolated forces may not exceed 50 pounds with two hands 

available for control. 
IPHWG Task 2 WG Report - Appendix B 

FAA Issue Paper on SLD 

12/19/2005 Page B-5 

 7. Airplanes equipped with non-powered inflight spoilers may require tunnel 

or flight testing to evaluate the effect on airplane control and handling 

characteristics. 
IPHWG Task 2 WG Report - Appendix C 

JAA Interim Policy on Severe Icing Conditions 

12/19/2005 Page C-1 

APPENDIX C - JAA INTERIM POLICY ON SEVERE ICING 

CONDITIONS 
IPHWG Task 2 WG Report - Appendix C 

JAA Interim Policy on Severe Icing Conditions 

12/19/2005 Page C-2 

Policy Paper Number : INT/POL/25/11 Issue 1 Date: 01.10.98 

Subject : Severe Icing Conditions 

Regulation affected : JAR 25.1419 

Problem: 

A fatal accident and some incidents related to loss of aeroplane control have 

raised the concern that aeroplane operating in certain meteorological conditions 

can accrete ice, not only aft of the protected areas but also on the underwing 

airfoil surface. This may result particularly in lateral control difficulties due to 

disturbed flow over the control surfaces and (or) drag penalties. This JAA Interim 

Policy focuses on the hazard of sudden handling qualities degradation resulting 

from Ice accretion downstream the protected area on the overwing. 

The meteorological conditions involved so called large supercooled droplets, are 

outside the conditions defined in Regulation part 25, Appendix C. 

An inadvertent encounter of such conditions may result in an unsafe condition 

that must be addressed. 

This issue is particularly of interest for aircraft equipped with pneumatic de-icing 

boots and lateral axis non powered flight control system. 

JAA Policy: 

As long as, Severe Icing Conditions, so called large supercooled droplets are not 

defined in the required atmosphere of JAR-25 Appendix C used for certification, 

means must be provided to the crew to detect these conditions and procedures 

to exit them are to be established. 

The JAA consider that the following issues must be addressed: 

1.) Means for the flight crew to determine when the aeroplane has inadvertently 

entered Severe Icing Conditions. 

2.) Assessment of the design to ensure that the aeroplane can safely exit such 

environment (susceptibility to loss of control leading to an unsafe condition). 

3.) Crew information that describes the limitations to be observed while exiting 

such environment. 

4.) Means for the crew to determine when the hazard no longer exists. 
IPHWG Task 2 WG Report - Appendix C 

JAA Interim Policy on Severe Icing Conditions 

12/19/2005 Page C-3 

5.) Appendices: 

Appendix A: provides guidance for Freezing drizzle (ZL) and freezing rain 

(ZR) included in Severe Icing Conditions addressed by this CRI. 

Appendix B: provides guidance material on means to demonstrate 

acceptable handling qualities in case of inadvertent encounter of Severe 

Icing Conditions. 

APPENDIX A

[sic: Refers to Appendix A of INT/POL/25/11] 

For guidance in the definition and representative characteristic values of freezing 

drizzle and freezing rain the under mentioned document may be used. 

"REPRESENTATIVE VALUES OF ICING RELATED VARIABLES ALOFT IN 

FREEZING DRIZZLE AND FREEZING RAIN". 

Richard K. JECK 

March 1996 

DOT/FAA/AR-TN 95/119 

FAA 

Technical Centre 

Atlantic City International Airport, NJ 08405 

The hereafter set of values are extracted from the above mentioned document 

but for accurate details about how these values have been originated and could 

be used, refer to the FAA document. 

Due to the few amount of measurement, the meteorological statistics/data are 

preliminary but focused on the under mentioned characteristics. 

Freezing drizzle (ZL):

Dropsize diameter range 50 to 500 micron meters, 

Ambient temperature between 0 and -11oC, 

Liquid water content 0.1 (0.3 g/m3 maximum), 

Altitude range up to 12 500/17 000 ft. 

Freezing rain (ZR):

Dropsize diameter range 500 up to 2.000/4000 micron metres, 

Ambient temperature between 0 and -11oC, 

Liquid water content 0.1 up to 0.3 g/m3 for 1000 micron-meters RMVD 

(Raindrop Median Volume Diameter), 

Altitude range up to 7.000 ft 

Horizontal extend about 100 Miles, 

Depth of ZR layer about 6 to 8000 ft. 
IPHWG Task 2 WG Report - Appendix C 

JAA Interim Policy on Severe Icing Conditions 

12/19/2005 Page C-4 

APPENDIX B

[sic: Refers to Appendix B of INT/POL/25/11] 

Guidance Material for 

AIRPLANE CERTIFICATION IN CASE OF INADVERTENT SEVERE ICING 

ENCOUNTER OF CONDITIONS OUTSIDE THE "APPENDIX - C" 

ATMOSPHERE: 

Large Supercooled Droplets (Freezing drizzle and freezing rain) 

The guidance material outlines an acceptable means of compliance with 

the above mentioned requirements. 

To establish that the aeroplane can safely exit inadvertent Severe Icing, the 

applicant must: 

- Determine if conditions (in terms of droplet size diameter, liquid water content, 

temperature) outside the Appendix C can lead to an unsafe situation and 

- Demonstrate by an agreed selection of means of compliance (analysis, test in 

simulated icing conditions, test in dry air with artificial ice shapes, test in 

measured natural icing conditions,...) that the aeroplane can safely exit such 

non-authorised conditions. 

For that purpose, the most critical Ice accretions in terms of shape, thickness 

and location should be established for each flight phase to determine the 

combinations which have the most adverse effect on handling qualities to be 

flight tested. 

The applicant should apply the method consisting in: 

- Ice shape determination either by test in simulated Icing Condition (tanker test 

or wind tunnel or analysis) or using the arbitrary defined Ice shape "Quarter 

Round/One Inch/high/flat side forward." 

- Wind tunnel test in dry air with the above agreed artificial Ice shape and 

location to determine the most critical flight conditions/flight characteristics 

effects. 

- In flight evaluation with the agreed Artificial Ice Shape using the following 

criteria: 
IPHWG Task 2 WG Report - Appendix C 

JAA Interim Policy on Severe Icing Conditions 

12/19/2005 Page C-5 

While flying the airplane to assess handling qualities, at speeds up to the 

maximum anticipated for holding or for flap configurations (VFE), the criteria 

about flight characteristics specified below are applicable. Manoeuvres should 

include constant airspeed level banked turns up to 40o either direction, 30o to 

30o rolls in both directions using up to full lateral control wheel throw, and 

wing level deceleration down to the stall warning. 

-1 When manually flying the aeroplane: 

There shall be no hazardous degradation of the handling qualities of the 

airplane. The pilot roll forced needed to maintain lateral control during the 

aforementioned manoeuvres may not exceed 50 lbs. (22,7 daN) with two 

hands available for control. 

-2 When Auto pilot is engaged: 

Any uncommanded control surface movement occurring during the 

manoeuvres specified above with the Autopilot connected during entry into 

the manoeuvre, then disconnected after establishing the maximum 

Autopilot coupled bank angle for normal operation or 40o, whichever is 

less; or disconnected with the maximum roll rate is established when 

changing the direction of bank angle with the Autopilot heading control (in 

both directions); or disconnected at stall warning during a deceleration 

with Autopilot in attitude hold and heading hold mode, must not result in 

the following: 

- bank angle of more 60o, 

- a load on any part of the structure greater than its limit or beyond 2g, 

- a normal acceleration less 0g, 

- a roll force greater than 50 lbs (22,7 daN) during the recovery action, 

- an excessive altitude loss, 

- hazardous degradation of the handling qualities of the airplane, 

- engagement or disengagement of a mode leading to hazardous 

consequences. 

Recovery action should be initiated: 

- until three seconds after recognition point for straight flight 

- until one second after the recognition point for manoeuvring flight 

(turns included). 

Influence of power setting should be evaluated. 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-1 

APPENDIX D - ACCEPTABLE COMPLIANCE METHODS 

FOR APPENDIX X CONDITIONS (NATURAL ICE FLIGHT 

TESTING NECESSITY) 

 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-2 

NECESSITY TO FLIGHT TEST IN NATURAL APPENDIX X CONDITIONS

The requirement to flight test the airplane in Appendix X conditions was a 

major issue when drafting the proposed §25.1420. There is a requirement 

to flight test in natural Appendix C conditions to show compliance to 

§25.1419. The only specified method of compliance that is required in the 

proposed §25.1420 is analysis. The analysis must be supplemented by at 

least one type of test. The proposed rule lists the tests, and natural icing 

Appendix X flight tests is one of them. The group recognizes that flight 

testing in natural Appendix X conditions is not as practical as in Appendix 

C conditions, and substantiated tools could be used to show compliance 

to §25.1420 for most airplane configurations. After considerable 

discussion, concerns for requiring flight testing in Appendix X natural icing 

conditions were addressed by modifications to the proposed rule and AC. 

The working group concluded that the tools must be substantiated and 

that there may be some configurations or design features that would 

warrant natural Appendix X flight tests. Guidance on these critical issues 

are contained in the draft AC. This appendix provides a history of the 

issue including the background, key points from both sides of the issue, a 

Minority Position, and the Majority Response. 

INITIAL AUTHORITIES’ POSITION

The initial position of the FAA, JAA, and Transport Canada was that flight 

testing in measured natural Appendix X conditions was required to show 

compliance to §25.1420. 

The rationale was as follows: 

Although there is some variability in the various manufacturers’ methods 

for certification for flight in icing conditions, the fundamentals are similar. 

Each manufacturer typically uses a variety of analysis and test methods to 

optimize the airplane and ice protection systems design. For compliance 

with 14 CFR/JAR 25.1419, the following means are used in addition to 

analysis and flight in measured natural icing conditions: 

• Wind Tunnel / Icing Tunnel Tests 

• Dry Air and Simulated Ice Shape Flight Tests 

• Artificial Icing Flight Tests 

• Ancestor Airplane Comparison (Similarity Analysis) 

NOTE: The FTHWG proposed AC 25.21-1 identifies similar means of 

showing compliance to the 25.21(g) requirement. 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-3 

These means of compliance have limitations with regard to Appendix X 

icing conditions. For example: 

• Engineering tools, such as software for CFD, computer aided 

design analysis (CAD), aerodynamics, and drop and ice accretion 

effects (LEWICE, CANICE, CFD, etc.), may be used in conjunction 

with testing of scale, or full-size, models of the airplane or its 

components in wind tunnels and icing tunnels to confirm the 

analysis results. However, the current software codes and icing 

tunnels typically address Appendix C simulations for drop 

impingement and ice accretion effects. The applicability of the 

engineering tools to Appendix X icing conditions has not been 

verified. 

• There are variations in the Appendix C ice shapes produced in 

different icing tunnel facilities that cannot be explained. These 

differences are expected to continue in Appendix X icing conditions. 

• Airborne icing tankers can only produce an icing plume of a 

consistent drop distribution and liquid water content over a limited 

area. The test area of the plume may not adequately expose the 

areas of the airplane that are of interest. There are also concerns 

of low humidity effects, ambient temperature, uniformity of the 

plume relative to liquid water content, and drop spectra. 

• Ancestor airplane data has been utilized to support new models 

where sufficient similarity exists between derivative models. 

However, the use of ancestor airplane data can only be applied for 

certification to Appendix X after at least one airplane model has 

been certificated to Appendix X. Otherwise, the data that would be 

used to support a similarity analysis would not exist. 

As required by 14 CFR 25.1419, flight tests in measured natural icing 

conditions, along with at least one other test method, are required to verify 

the ice protection analysis, to check for icing anomalies, and to 

demonstrate that the ice protection system and its components are 

effective. It is important to note that these tests are conducted with the 

airplane and its ice protection system (IPS) operating as a complete 

system, not as a series of discrete components. The natural icing flight 

tests demonstrate that the components function properly when installed 

and interact together as a system to perform their intended function in 

normal operating modes and in failure cases. Confirmation of the AFM 

Normal and Abnormal procedures is also carried out, including 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-4 

demonstrating adequate performance and handling of the airplane in 

measured natural icing conditions. Some recent examples of problems 

found during measured natural icing flight tests that were not accounted 

for by other tests and analyses include: 

• Asymmetric ice accretions on the wing and horizontal tail. 

• Ice accretions that affected air data probes. 

• A tendency for ice build up on the elevator horn after ten minutes 

in a hold. The ice prevented elevator movement and caused 

autopilot disconnect and a 10o pitch up in a turn (the airplane had 

reversible flight controls). 

• Water reaching the brush block area of the propeller deicing system 

that caused brush block/slip ring hydroplaning and loss of power to 

propeller deicing boots. 

• Unexpected ice accretions. 

These Appendix C “Lessons Learned” can apply to Appendix X conditions. 

Some additional concerns for Appendix X that can only be evaluated by 

flight testing in measured natural icing conditions include: 

• Ice accretions aft of the protected surfaces. 

• Experience has shown that SLD ice accretions on the far aft 

portions of the radome can occur. These ice accretions could 

affect the accuracy of the angle of attack vanes used by stall 

warning and stick pusher systems. Flight deck airspeed indications 

may also be affected. 

 

• Qualitative performance checks of propeller airplanes are done in 

natural Appendix C icing conditions because intercycle propeller ice 

shapes are not tested and propeller codes do not account for 

runback ice. The need to evaluate intercycle propeller ice shapes 

and runback ice in Appendix X conditions will exist and natural icing 

flight tests will likely continue to be the only means of examination. 

• Nacelle cooling inlets may be blocked or partially blocked and this 

may affect thrust or power (maximum engine power or thrust 

cannot be obtained due to over-temperature). 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-5 

• Antennae that do not build up significant ice in Appendix C can do 

so in Appendix X. Communication and navigation systems would 

have to be checked with the ice accretions on the antennae. 

Similarly, systems such as the weather radar, fuel venting, bleed air 

cooling, and APU performance may be affected in Appendix X icing 

conditions. 

• Unexpected ice accretions on aircraft surfaces and an ice accretion 

on surfaces not predicted by icing simulation methods e.g. ice on 

fuselage, nacelle, pylons, etc. 

Airplane icing certification is recognized as a complex task. Even though 

the aviation industry has been using 14 CFR 25.1419 and Appendix C 

standards for decades and the tools used for certification have matured, 

flight tests in measured natural icing conditions are still considered a 

valuable engineering tool and continue to identify areas of design or 

operating procedures where improvements can be made. The authorities 

see no reason why Appendix X should be treated differently than 

Appendix C. 

INITIAL MANUFACTURERS’ POSITION

The airplane manufacturers examined the practicality of certification 

testing in natural SLD conditions. They concluded that there are multiple 

issues that contribute to the difficulty and hence the practicality of 

certification testing in natural SLD conditions and that natural icing flight 

tests in Appendix X conditions should not be required. 

Encounter Probability

One issue that has been brought up repeatedly in IPHWG deliberations is 

the probability of encountering the SLD conditions. The Meteorological 

Service of Canada (MSC) research data indicates that SLD are a relatively 

frequent event and can be found readily. However, this indication is 

biased by the definition of SLD that has been used to reach this 

conclusion. At the May, 1996, FAA International Conference of Aircraft 

Inflight Icing, a working group deliberated on the current knowledge of the 

SLD environment. Co-Chairs for this working group were George Isaac 

(MSC) and Richard Jeck (FAA Technical Center). In the published 

proceedings of this event, one of the consensus items was that the 

definition of SLD was “any droplets larger than 50 microns diameter.” 
IPHWG Task 2 WG Report - Appendix D 

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12/19/2005 Page D-6 

As has been previously brought before the working group, this definition 

does not consider the drop distribution effects of icing clouds. In a 

previously presented technical paper,4 it was proposed that the definition 

of SLD as drops greater than 50μm should stand. However, it was also 

recommended that agreement should be reached on the definition of “SLD 

conditions”. This new term would address distribution effects and align 

more closely with today’s Appendix C. This agreement has not been 

reached and is a contributing factor in the discussion on the practicality of 

flight-testing in SLD conditions. 

The concept of the drop distribution is inherent in the definition of the 

current certification icing envelopes with the term “mean effective 

diameter” (MED). This term5 is associated with the rotating multi-cylinder 

method for sampling cloud properties and requires the assumption of a 

specific drop distribution. FAA research has shown that the historic use of 

MED is in general agreement with the term Median Volume Diameter 

(MVD) as used today. It follows that the concept of the drop distributions 

has long been associated with the current certification icing envelopes.6 

The most commonly used distributions are the Langmuir & Blodgett 

distributions “A” through ”E” (“A” is monotonic and “E” has the widest 

range of drop sizes). 

To illustrate the point relative to the probability of encountering SLD, 

consider a 20μm MED condition assuming the Langmuir & Blodgett “E” 

distribution. Per the definition, 5 percent of the liquid water content (LWC) 

is contained in drops of ~54μm (2.71*20=54.2μm). Given that the 

distribution contains drops in excess of 50μm, it would be classified as 

SLD per the 1996 definition. However, this 20μm distribution is well within 

current certification envelopes and is a probable icing condition. 

However, it is unlikely that an airworthiness authority would accept a 20μm 

MVD encounter as a certification test representative of flight in SLD 

conditions. As an example, consider a wing system designed with the aft 

protection limits set at the 40μm or 50μm impingement limit 

(mono-distributed as discussed in current AC20-73 and ACJ25.1419). 

Using the Langmuir “E” distribution, only 5 percent of the water even has 

the potential for accreting behind the protected area due to the drop size 

exceeding the 50μm protection limit. The remaining 95 percent of the 

water (smaller drops) will not have sufficient inertia to impinge behind the 

 

4 Anil D. Shah, Michael W. Patnoe and Ervin L. Berg: Engineering Analysis of the Atmospheric 

Icing Environment Including Large Droplet Conditions, SAE 2000-01-2115 

5 Richard Jeck: Terminology for Droplet Size Measurements, (Icing Information Note -August 

1994), 

6 Such as: FAA ADS-4, DOT/FAA/CT-88/8-1, FAA AC20-73, FAA AC25.1419, and JAA 

ACJ25.1419. 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-7 

protected areas directly. An additional reduction factor is the effect of the 

distribution of the local collection efficiency. The local collection efficiency 

approaches zero at the aft edges of the protected area as shown in Figure 

D-1. Using the example, the local collection efficiency near the protection 

limits is less than 1 percent. Assuming the flight conditions noted in the 

figure, the theoretical maximum accumulation potential is approximately 

0.012 inches film thickness over a 10-minute exposure.7 This analysis has 

some simplifying assumptions8 but provides an order of magnitude 

estimate to help illustrate the point. 

The purported purpose of natural ice flight testing evaluations is as 

follows: 

The primary purposes for flight testing an airplane equipped 

for flight in icing conditions is to evaluate such degradation, 

and to determine the flying qualities remain adequate and 

that performance levels are acceptable for this flight 

environment9 

and 

to consolidate the results obtained from flight testing with 

artificial shapes.10 

In addition, this evaluation of degradation effects is listed among 

“significant” items in Transport Canada AMA 525/5 (September, 1996). 

However, given the slight amount of ice accretion behind the protected 

areas in the example, it is unlikely than any meaningful evaluation of 

degradation effects could be performed. 

This effect indicates that the real acceptance threshold for a SLD 

certification test would require significantly larger drop sizes and/or 

significantly higher water contents in the large drops. In fact, the real 

acceptance threshold will likely be closer to the 50μm MVD, 135μm 

maximum drop size distribution proposed in the previously referenced 

technical paper.4

 The result of this line of reasoning is that the reduced 

 

7 Using maximum accumulation potential methods such as Eq. 2-29 and Eq. 2-30 of 

DOT/FAA/CT-88/8-1, "Aircraft Icing Handbook". 

8 Does not account for any runback, assumes ice growth normal to surface, assumes clean 

efficiency characteristics, conversely does not account for splash effects which would reduce 

accumulation potential. 

9 FAA/AC 25.1419-1, Certification of Transport Category Airplanes for Flight in Icing 

Conditions, 8/18/99. 

10 JAA/NPA 25F-219, Flight in Icing Conditions Acceptable Handling Characteristics and 

Performance Effects, May 1996. 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-8 

acceptance envelope for SLD certification validation testing (relative to the 

1996, 50μm definition) would greatly decrease the probability of 

encountering such conditions. 

Figure D-1 - Example Impingement Characteristic 

Previous Testing Experience

It has been reported11 that previous attempts to perform development 

and/or certification testing in SLD conditions were of limited success. The 

testing involved installation of an ice detector positioned to alert to the 

presence of ice behind the protected areas. The devices were installed on 

two flight test airplanes and seven in-service airplanes in normal revenue 

service over the winter of 1997-1998. During the evaluation only two 

unusual icing encounters (characterized as “severe large droplet icing 

encounters”) were experienced. It was later reported12 that three 

successive winter flight test campaigns were flown on company-owned 

aircraft, as well as evaluations of in-service experience, in the US and 

Europe for this same purpose. The flight tests were unsuccessful for a 

variety of reasons including, “[t]he rarity of severe SLD icing encounters.” 

 

11 FAA letter dated 12/14/98 response to NTSB recommendation A-96-59 

12 FAA letter dated 3/28/00 response to NTSB recommendation A-96-59 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

-0.25 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 0.25

Surface Distance (s/c)

Local Collection Efficiency (Beta)

Example: NACA 23012, 6ft chord, 200 KTAS, 

10,000ft, 2° AOA, -10°C, LWC 0.4g/m3

20μm, Langmuir "E"

40μm & 50μm Mono-distributed

If the limits of protection are designed 

considering the 40μm or 50μm 

impingement limits, the 20μm "SLD" 

case will not result in significant 

impingement aft of the protected area.
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-9 

In addition, there is other evidence that the successful pursuit of SLD 

conditions is improbable. It was reported to the IPHWG that the NASA 

SLD data collection for the 1998-1999 flight season was not productive 

due to the inability and difficulty of locating and sampling significant SLD 

conditions. This was approximately a six-week program dedicated to the 

pursuit of SLD conditions for data collection. No data was collected due to 

either extinction of the conditions before arrival, lack of significant SLD 

conditions, or data issues due to instrumentation problems. This was a 

research program where the aircraft was put on station awaiting weather 

patterns conducive to SLD conditions. This program had access to 

significant meteorological resources, such as frequent forecasts and 

in-flight updates through a satellite telephone. A typical certification 

program would likely not have this level of meteorological support. This 

would decrease the probability of success. 

As a practical matter, aircraft certification programs would incur a large 

expense to station an aircraft for an extended period to await SLD weather 

patterns to move into the test region. This expense would not only include 

aircraft flight and crew costs but would also include costs associated with 

accompanying flight test engineer(s), maintenance personnel, inspection 

personnel (both company and FAA delegates) to release the aircraft for 

flight, and likely a photographer to document the process. These 

expenses are aside from the logistics of hotels, meals, and car rentals that 

would be required for an extended test period. For certification testing, 

icing conditions are commonly “chased” based on available meteorological 

data (including NCAR products and pilot reports). The ability to forecast 

SLD conditions is much less mature than currently available icing 

products. Pilot Reports (PIREPS) will be of limited use since there is no 

requirement to distinguish SLD conditions from conventional icing 

encounters. In addition, the success of “chasing” a particular icing 

condition is heavily influenced by aircraft range, speed, and service 

ceiling. Due to the transient nature of icing, aircraft with limited range, 

speed, or service ceiling have a decreased probability of reaching the 

reported icing conditions before the conditions dissipate. 

These factors provide actual flight test evidence of the difficulty in finding 

SLD conditions that would provide a significant validation of SLD 

conditions. 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-10 

Variability of Conditions

Representative distributions have been presented to the IPHWG that 

demonstrate the wide range of drop sizes and distributions present in the 

SLD environment. This range of distributions can provide a substantial 

amount of water in small drops and a relatively few large drops. 

Conversely, the distribution can provide a relatively large population of 

large drops with virtually no small cloud-sized drops. 

Current certification encounters have no firm criterion for acceptability of 

the extent and intensity. 

• AC20-73 states: 

To establish the airplane's tolerance to the continuous 

accumulation of ice on unprotected surfaces, flight tests 

should explore stratiform icing clouds (Continuous Maximum 

Fig. 1) for a period of time representative of today's air traffic 

"holding" conditions . . . . It is recommended that the tests 

include a continuous exposure for at least 45 minutes. 

• AC25-7A states: 

The amount of ice on the airplane should be representative 

of what would be accumulated in a 45 minute hold in icing 

conditions prior to approach and landing (typically, one to 

two inches on the unprotected wing leading edge surfaces 

has been found acceptable for this testing). 

• JAA advisory materials NPA 25F-219 state: “with natural glaze ice . 

. . various quantities of ice on unprotected surfaces (between 0 and 

3 inches).” 

Although not a specific criteria, the above guidance typically drives the 

testing towards the warm, small-drop corner of the Appendix C (Figure 1) 

liquid water content envelope with no horizontal extent adjustment. The 

primary variable sought is water content. Temperatures are a function of 

water content and altitudes are taken as available considering sufficient 

ground clearance for safety. 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-11 

Given the potential variability of SLD ice shapes due to distribution effects, 

the presence of large drops alone may not provide adequate confidence to 

certification engineers that the conditions have been validated. This 

forces the potential candidate for SLD certification to pursue not only 

water content but also a specific distribution, again considering sufficient 

altitude for safe evaluation. Given that the SLD conditions tend to be lowaltitude events, pursuing water content, altitude, and distribution effects 

reduces the probability of finding adequate SLD conditions beyond the 

limits of practicality. Accounting for this variation in distribution effect is 

best suited to analytical methods. 

Instrumentation Requirements

Research flights have carried extensive instrumentation for particle 

classification/sizing. As an example, the Canadian MSC Convair carries 

the following Particle Measurement Systems (PMS) sizing equipment: 

• PMS FSSP 100 3-45 μm Drop concentration/size 

• PMS FSSP 100 5-95 μm Drop concentration/size 

• PMS 2D-C Mono 25-800 μm Hydrometeor conc./size 

• PMS 2D-C Grey 25-1600 μm Hydrometeor conc./size 

• PMS 2D-P Mono 200-6400 μm Hydrometeor conc./size 

This equipment is carried in addition to any for water content and 

temperature measurement requirements. The instrumentation probes 

require expert attention to maintain calibration and ensure functionality. 

The difficulty in maintaining this instrumentation was a contributing factor 

in the previously reported unsuccessful NASA research flight testing to 

obtain SLD data. Maintaining this quantity of equipment, both from 

reliability and calibration standpoints, would be a significant burden for an 

applicant and would require highly specialized skills. It has been reported 

to the IPHWG that post-processing of the data from these instruments is 

critical to accurately quantify the conditions and ensure that icecontaminated conditions are understood. The data processing from SLD 

encounters is very time consuming and has taken months within the 

research environment to complete sufficient post-flight processing and 

analysis required to determine the full extent of the conditions. The 

aircraft certification environment is a fast-paced, costly environment where 

it would be extremely difficult to justify such program delays. Maintaining 

this quantity of equipment, both from a reliability and a calibration 

standpoint, would be a significant burden for an applicant. 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-12 

Test aircraft typically require modification to provide the structural support 

required for particle measurement systems for Appendix C certifications. 

Suspending pylon mounted, multiple 35- to 50-pound devices on an 

airframe is not a trivial task, particularly on small transport category 

aircraft. This effort would require significant design and analysis support. 

The modification includes extensive analysis to determine local drop 

trajectory effects and additional engineering to perform aero-elastic 

stability analysis. Requiring the multiple ranges of PMS would require 

multiple hard mounting points. This extensive aircraft modification effort 

would be a significant expense. 

This quantity of instrumentation represents state of the art as applied to 

quantifying large drop conditions. However, imposing this level of 

instrumentation requirement for an SLD certification effort is extreme and 

contributes to the impracticality of testing in natural SLD conditions. 

Risk Mitigation

Most manufactures have stringent safety process in place to reduce the 

risk of experimental flight testing. While many airplanes are derivative in 

nature and have extensive service history, this cannot be assumed while 

proposing new certification rulemaking. With the low-altitude nature of 

SLD icing, increased risk would be present due to the lack of altitude for 

recovery in the event of an upset. Taking an unproven aircraft design into 

an SLD condition and performing any type of maneuver testing would 

represent excessive risk for flight test personnel. Most manufacturers will 

require some method of risk reduction. Wind tunnel testing is commonly 

used, as is dry air flight testing with simulated ice shapes. Dry air flight 

testing with “built-up” shapes allows for testing with varying degrees of 

contamination to assess and reduce risk before flights with the full-span, 

representative ice shapes. 

These simulated ice shapes will likely be the result of analysis and icing 

tunnel tests. This analysis can consider a wide variety of conditions to 

determine critical ice shapes. The simulated ice shapes can be flown with 

sufficient altitude and in VFR conditions that allow safe recovery in the 

event of an upset. If this risk mitigation process is based on sufficiently 

validated simulation tools, the risk is mitigated not only for experimental 

flight test but also for in-service encounters with such conditions. This 

removes the necessity for testing in natural SLD conditions. 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-13 

Simulation

The best alternative to flight tests in natural icing testing is through the 

increased use of simulation. This simulation would take the form of 

analysis or physical testing such as icing wind tunnels, icing tankers, dryair wind tunnels, and dry-air flight testing with simulated ice shapes. Each 

form of simulation has particular strengths that can be used to resolve 

particular areas of concern relative to SLD environments. The primary 

areas of interest particular to SLD conditions include: 

 

• Impingement aft of protected areas (potential hinge moment and 

stall handling effects) 

• Ice shapes on unprotected areas (CLmax, CM, and CD) 

• Verification of exit cues (visual or device based) 

• Performance effects (drag from extended accretion areas)13

Each of these areas of concern can be addressed through simulation 

methods. The following discussion provides examples of the simulation 

techniques and the relationships to the areas of interest. 

Analytical Simulation

Analytical simulation methods include impingement and accretion models 

based on computational fluid dynamics. There are existing Appendix C 

tools which are in the process of being modified to account for large drop 

effects. Part of the development process includes validation of the large 

drop physics. These methods have the potential of simulating 

impingement and accretion aft of the protected areas, as well as resulting 

shapes on unprotected areas. Analytical simulation appears to provide 

the best method of accounting for the variability in drop distributions that 

are present in nature. Once the SLD ice accretions are defined, 

aerodynamic results could be obtained through wind tunnel or dry-air flight 

testing. 

In addition to determination of resultant SLD ice shapes, analytical 

simulation provides the capability to examine impingement relative to 

visual icing cues and analyze the location of detection devices for any 

detrimental local flow effects. This will become more accurate when the 

 

13 The issue of performance (power margins relative to drag rise) in SLD conditions is a lesser 

concern for many transport category aircraft. Large-scale aircraft are less susceptible to all icing 

conditions due to the reduced collection efficiency of the airfoils. High-speed aircraft (large or 

small scale) typically have significant power margins when operating at low altitude holding 

speeds where the large droplets would likely be encountered. 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-14 

SLD physics are incorporated (e.g., splashing). For aircraft where 

performance effects could be a concern,13 analytical techniques have the 

potential to predict SLD accretion areas over the total airframe. For 

aircraft that have potentially limited performance, these SLD accretion 

areas could potentially be used in estimating overall aircraft drag 

decrements. The accretion areas could be used with other proven 

methods (such as wind tunnels or dry-air flight testing) to determine 

aircraft performance effects. 

The ability to analytically simulate complex physical conditions continues 

to grow at a rapid rate due to the rapid growth in computer hardware 

capabilities. This trend will likely continue and provide increasingly 

realistic simulation of icing effects. 

Icing Tunnel Tests

Icing tunnels are likely to be the primary method of predicting ice 

accretions aft of protected areas, as well as ice shapes on unprotected 

areas. This method has the ability to combine the effects of thermal 

systems, such as runback, with potential accretion due to SLD. The icing 

tunnels will likely be limited in cloud size and in the ability to simulate 

distribution effects. However, the distribution effects could conceivably be 

addressed through superposition of large and small drop effects 

(time/LWC scaled) or for accretions behind protected areas through LWC 

scaling (time) of water catch for the area of interest. The defined SLD ice 

accretions could then be use to determine aerodynamic results through 

wind tunnel or dry-air flight testing. 

Icing tunnels could conceivably be used to examine impingement limits 

relative to fuselages or windscreens with scale models. While scaling 

SLD icing shapes is still an emerging area, scaling of drop inertia effects 

and subsequent impingement limits is in a more mature state. This 

technique could be used to examine visual icing cues, validate location of 

detection devices, and determine total SLD accretion areas that could be 

used in drag estimates. 

Icing Tanker Testing

The use of an icing tanker would provide high confidence in local icing 

effects. Tankers typically have limited cloud sizes, but they can produce 

clouds of approximately four to eight feet. A plume of this size is sufficient 

to examine local icing effects relative to large drop impingement. This 

technique could potentially be used in a manner similar to icing tunnel 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-15 

testing with respect to ice shape development on unprotected areas and 

on areas behind protection zones. The plume could be applied to sections 

of the airframe (four to eight feet at a time) to examine any potential hinge 

moment or CLmax effects. Alternately, the testing could be used to define 

simulated ice shapes to evaluate aerodynamic effects. This method also 

has the advantage of being able to combine the effects of thermal systems 

(such as runback) with direct accretion to simulate the resulting ice 

accumulations. Similar to previous discussion, this method could be used 

to estimate SLD accretion areas that could be used in drag estimates (if 

required).13

Wind Tunnel and Dry-Air Flight Testing

The use of wind tunnel and dry-air flight testing has long been used as a 

cornerstone of ice protection system development and certification. The 

primary differences between Appendix C and SLD conditions are in the 

formation of the ice shapes. Once the shapes are defined, the testing 

methods for aerodynamic degradation would be the same. These are 

mature techniques with no new development required. 

Current Appendix C icing regulations such as 14 CFR §25.1419 and JAR 

25.1419 require the use of multiple methods of compliance. Simulation of 

SLD conditions will likely be similar as it may take a combination of a 

couple of methods to adequately represent the conditions and determine 

the degradation effects. 

The realities of a new aircraft development and certification program are 

such that early evaluation of icing effects via simulation is not only 

desirable but essential. While program practices vary among 

manufacturers, the following illustrates a general timeline for a large 

aircraft manufacturer prior to first flight of a new aircraft model. 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-16 

Activity Time Frame 

PRIOR TO FIRST FLIGHT 

Preliminary estimate of ice 

shapes for dry-air wind tunnel 

testing (is/can be a factor in 

establishing final configuration) 

5-10 yrs (all new) 

3-5 yrs (derivative) 

Preliminary IPS design 5-10 yrs (all new) 

3-5 yrs (derivative) 

Final estimate of ice shapes & 

dry-air wind tunnel testing 

2-3 yrs 

Final IPS design 2-3 yrs 

Preliminary icing data for 

onboard computers (AFM, FMC, 

SWC, etc.) and simulator 

database 

0-2 yrs 

Table D-1 - Large Manufacturer Development Timeline 

Between the time of first flight of the aircraft until the time that it is certified, 

data for the systems, onboard computers, and simulator are finalized. 

Flight-testing for icing certification (either in natural icing or with 

manufactured ice shapes) has primarily become a “declare and verify” 

effort. It is expected that there will be no surprises during flight test which 

will require substantial re-analyses of the pre-flight data. Should a 

manufacturer be required to wait until AFTER the aircraft is flying to 

assess the icing effects, clearly there would be a substantial risk of the 

unknown placed upon the manufacturer. This is particularly true in the 

case of SLD, where today there is relatively little upon which to base 

pre-flight analyses. Waiting until the aircraft flies to investigate ice 

accretion is highly impractical and unrealistic. It is essential that tools and 

methods be available to estimate SLD ice accretion and assess its effects 

prior to first flight. 

Relation to current §25.1419

Much of the previous working group discussion has centered on the 

current requirements of §25.1419 relative to testing in natural icing 

conditions. This requirement was imposed by Amendment 25-23 in 1970. 

In the preamble materials, the stated reason for this change is: “The 

current state of the art and current practice indicate that the flight tests in 

measure natural atmospheric icing conditions specified in paragraph (c)(4) 

are necessary in all ice protection system certification programs . . . .” 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-17 

This precedent indicates that the decision to require natural icing tests 

should be based on the confidence level of the simulation techniques that 

could alternatively be used. The simulation tools and capabilities that 

NASA, et al., are developing for SLD are planned to be validated tools. In 

fact, many of the large drop effects (such as splashing) are being 

developed through physical testing which also serves as validation points 

for that particular aspect of the large drop physics. Another factor to be 

considered is that the proposed simulation tools are not completely new 

techniques; they are a continued progression of current Appendix C 

simulation techniques. 

In addition, there is a precedent to not require testing in specific conditions 

throughout the Appendix C envelopes. In AC 25.1419-1 it states: 

Past experience indicates that flight testing in natural Intermittent 

Maximum icing conditions may be accompanied by severe 

turbulence and possible hail encounters that could extensively 

damage the test airplane. When design analyses show that the 

critical ice protection design points (i.e., heat loads, critical ice 

shapes, accumulation, and accumulation rates, etc.) are adequate 

under these conditions, and sufficient ground or flight test data exist 

to verify the analysis, the flight test in intermittent maximum 

conditions may not be necessary. 

Given this precedent, it follows that natural SLD conditions could be 

treated in a manner analogous to the Appendix C Intermittent Maximum 

conditions without requiring flight tests in such conditions. 

Conclusion

There are multiple factors that contribute to the impracticality of testing in 

natural SLD conditions. As presented, the primary factor is the difficulty in 

locating such conditions on demand in the time constraints of a 

certification program. Other factors include: 

• Perceptions on the availability of SLD have been biased by the use 

of differing definitions to discriminate such conditions. 

• Previous flight test experiences in SLD conditions illustrate the 

difficulty in finding SLD conditions that would be adequate to 

validate the aerodynamic degradation predictions. 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-18 

• Requiring natural SLD flight-testing with current state-of-the-art 

particle measurement technology would be excessively 

burdensome for applicants. 

• The risk of flight in SLD (either without restriction or continuously) 

can be effectively mitigated with simulated shapes through wind 

tunnel or dry-air flight testing. 

• A precedent has been established to not require testing within 

potentially hazardous icing conditions by the allowance of validation 

methods other than natural icing flight tests for Appendix C 

Intermittent Maximum conditions. 

In summary, a requirement for flight testing in natural SLD conditions 

would be impractical and an excessive burden on airframe manufacturers 

with no significant increase in the level of safety. The value added by 

such an exercise would not approach the cost of the testing. There are 

other methods of mitigating the risk such that natural ice flight testing is 

not required. The alternative methods of validating SLD conditions can be 

designed to be conservative and reliable through steady-state, controlled 

testing. This has the added benefit of providing a more rigorous validation 

than a random encounter in natural SLD conditions. Therefore, it is 

recommended that certification flight tests in natural SLD conditions not be 

required in the proposed IPHWG Task 2 rulemaking. 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-19 

AUTHORITIES’ RESPONSE

The authorities’ response to the issues raised by the manufacturers was 

as follows: 

Probability of Encountering Appendix X Conditions

The IPHWG has moved away from the 1996 definition of SLD conditions. 

As described in the September 2003 version of the draft AC 

23.1419/1420, distributions with maximum drop sizes (Dmax) less than 

100 μm are now classified as Appendix C. For a given temperature, the 

FZDZ/L water content at 110 μm is approximately four times the water 

content of an Appendix C, Continuous Maximum, 40 MVD Langmuir E 

distribution, see Figure D-2. With SLD conditions defined with a Dmax of 

greater than or equal to 100 μm, SLD conditions were encountered 4 

percent of the time during SLD research flights conducted from 1995 to 

2000. Encountering SLD icing conditions 4 percent of the time is less 

probable than encountering icing 24 percent of the time. However, 4 

percent of the time is not improbable. Applicants may have to relocate to 

geographical areas where the probability of encountering SLD icing 

conditions is higher, as they similarly do to find Appendix C icing 

conditions. 

LWC Mass

23 deg F

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 20 40 60 80 100 120

Droplet Size, μm 

LWC, g/m3 

Appendix C CM 40 MVD

Appendix C CM 20 MVD

FZDZ/L

Figure D-2 - LWC as a Function of Drop Size 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-20 

The authorities concede that finding SLD conditions that will be 

meaningful may take a longer time than Appendix C conditions. 

However, the authorities have comments on several of the statements: 

“A typical certification program would likely not have this level of 

meteorological support.” This statement was made in reference to a 

research program that, “had access to significant meteorological 

resources, such as frequent forecasts and in-flight updates through a 

satellite telephone.” 

It is recognized that manufacturers may not have in-house weather 

support, but they typically contract a company to provide this service in 

real time to the test team. A manufacturer has already utilized such a 

contractor and found icing conditions every time that they looked for it. 

“The ability to forecast SLD conditions is much less mature than currently 

available icing products. Pilot Reports (PIREPS) will be of limited use, 

since there is no requirement to distinguish SLD conditions from 

conventional icing encounters.” Freezing rain can be predicted and tools, 

such as the Current Icing Potential (CIP), are available on the internet to 

assist in finding SLD conditions. This tool represents a big improvement 

over what was available only a few years ago. It is undergoing refinement 

to make it less conservative, which will be a benefit to flight testers. It is 

hoped that by the time §25.1420 becomes effective, the SLD forecast 

capabilities of CIP will be just as mature as current icing products. In 2004, 

the PIREP format will include an “airplane effects” code that will include 

“severe.” Although “severe” does not differentiate between SLD and 

conventional icing encounters, it does represent conditions that exceed 

the capabilities of the airplane’s ice protection system, which could be an 

indication of SLD conditions. 

An acceptable Appendix X natural icing flight test program has not been 

defined. However, it may not be too different from current Appendix C 

natural icing flight testing. Specific Appendix C LWC/drop 

size/temperature points cannot be targeted due to the improbability of 

finding specific conditions. AC 25.1419-1 states: “At least one exposure 

to icing conditions within the part 25, Appendix C, continuous maximum 

icing envelope should be obtained.” Many times now it is difficult to judge 

the acceptability of an Appendix C encounter. For this reason the draft AC 

20-73A will have appendices that provide tools for such determination. 

The authorities concede that Appendix X may be different since specific 

drop spectra are defined. 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-21 

Instrumentation Requirements

The authorities agree that the instrumentation required to measure 

Appendix X conditions is more extensive than that currently required for 

Appendix C. Efforts are underway to develop Appendix X instrumentation 

that is appropriate for certification. Commercial resources are currently 

available to measure Appendix X icing conditions. Equipment for 

measuring Appendix X icing conditions may also be rented. Some 

manufacturers may contract this service for Appendix C certifications. 

Analysis is required to mount Appendix C equipment on test airplanes. 

This would not be different for Appendix X equipment. For small 

airplanes, instrumentation can be mounted on a larger chase airplane. 

Risk Mitigation

Although freezing rain may be concentrated at lower altitudes, it is 

anticipated that flight testing in Appendix X would not be different than 

Appendix C. Maneuvers such as stall approaches would be accomplished 

in VFR conditions and at a safe altitude after desired ice accretions are 

obtained. One of the objectives of natural icing flight test would be to 

evaluate the whole system (for example, propeller) as well as to look for 

unexpected places where ice can collect. In this regard, flight testing with 

simulated ice shapes mitigates the risk of natural icing flight tests but is 

not a replacement for natural icing flight tests. 

Simulation Tools

The authorities agree that simulation tools need to be developed and 

validated in order to be a means of compliance. The simulation tools will 

be required to mitigate risk to the manufacturer regardless of whether 

natural icing flight tests in Appendix X are required. 

Relation to §25.1419

The authorities agree that the decision to require natural icing tests should 

be based on the confidence level of the simulation techniques. It should 

be pointed out that the authorities believe that Appendix C natural icing 

flight tests are required regardless of whether natural Appendix X flight 

testing is required. To reiterate the comment under “Simulation Tools,” 

validation of the simulation tools is critical if they are to be used in lieu of 

natural Appendix X icing flight test. For an airplane certificated to the 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-22 

entire Appendix X icing envelope, the authorities do not agree that flight 

testing in Appendix X is analogous to flight testing in intermittent maximum 

icing conditions. Flight testing in intermittent maximum icing conditions is 

not done because there is sufficient data to verify the intermittent 

maximum icing conditions analysis. The conditions are avoided due to 

non-icing hazards such as lightning and turbulence. Some manufactures 

do test in intermittent maximum icing conditions. Natural icing tests in 

Appendix X icing conditions are analogous to testing in Appendix C 

maximum continuous icing conditions since they are both related to 

operations in stratiform clouds. 

Authorities Conclusions:

The authorities fully appreciate the difficulties that the manufacturers face 

with respect to flight testing in natural icing in Appendix X conditions. This 

includes the aspects of probability of encounter, variability of conditions, 

instrumentation requirements, and risk mitigation. The authorities have 

taken into account these factors in determining the need for natural icing 

flight test in Appendix X but have to point out that the safety objectives 

must be achieved for flight in Appendix X, and flight testing in natural icing 

can only be not required if adequate alternative means are validated and 

employed satisfactorily. 

MAJORITY WORKING GROUP POSITION 

NASA has obtained natural ice shapes in SLD icing conditions. This 

information will contribute to a validation database for assessing SLD icing 

simulation methods. NASA will be using these inflight ice shapes for 

validating their SLD icing simulation methods. This would be useful 

during a certification program. However, it is not required prior to the rule, 

and a means of compliance using icing tankers and tunnels may be used 

as a verification method. 

The rule language "as found necessary" was not intended to force 

applicants to perform testing in natural icing conditions. Acknowledging 

the difficulties in flight testing in natural Appendix X conditions, the 

majority of the IPHWG agree that flight testing in natural Appendix X 

conditions would not be required as in §25.1419 for Appendix C icing 

conditions. Flight testing in natural Appendix X icing conditions was then 

considered as one of the available methods to support the required 

analysis. 

Flight testing in natural Appendix X conditions should not be necessary if: 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-23 

 1. The design analyses show that the critical ice protection 

design points (i.e., heat loads, critical ice shapes for performance and 

handling qualities, accumulation, and accumulation rates, etc.) are 

adequate under the conditions of Appendix X and various airplane 

operational configurations; and 

 2. The analyses performed for item 1. are accomplished with 

methods that have been validated for Appendix X icing conditions (see 

paragraphs (c) and (d)); and 

3. Adequate analyses and/or tests are accomplished if there is 

a need to evaluate more than one airplane component simultaneously. As 

examples, the evaluation of airplane performance with propeller and 

airframe ice accretion, asymmetric ice accretions due to propeller wash, 

engine performance with inlet (including cooling) ice accretions, or stall 

warning and characteristics with ice accretion that affects air data used by 

stall protection systems; and 

4. If the airplane is a derivative and is being certificated for 

flight throughout Appendix X icing conditions, the service record of 

ancestor airplanes does not include a pattern of accidents or incidents due 

to in-flight encounters with Appendix X conditions; and either 

 5. Flight testing and/or dry air wind tunnel testing with 

simulated Appendix X ice shapes shows performance and handling 

qualities characteristics that meets the requirements of § 25.21(g); or 

 6. The aircraft approval being sought is limited to exiting from 

Appendix X conditions § 25.1420(a)(1). 

(c) At least two different methods of predicting Appendix X ice 

accretions should be shown to provide similar results (ice accretion 

thickness, location). One method should be either an icing wind tunnel or 

icing tanker test. 

(d) Similarity analysis may be used if the methods used on similar 

designs were validated as in (c). 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-24 

MINORITY POSITION OF AIRBUS, BOEING, EMBRAER AND SAAB

Airbus and Boeing remain the minority IPHWG members on this issue. In 

their view, the Majority Working Group Position presented above is 

somewhat misleading regarding the AC language: “Flight testing in natural 

Appendix X conditions should not be necessary if . . . .” Even Airbus and 

Boeing agree that if the numerous criteria are satisfied, then natural icing 

flight testing should not be necessary. However, that is not the point. The 

primary reasons for sustained disagreement with the majority are that the 

criteria remain overly burdensome in terms of cost and resources to be 

expended by the manufacturers, and they believe that a flight test program 

in natural Appendix X conditions is more likely to result in an extensive 

research project than an airplane certification. 

Airbus and Boeing maintain that at the time of release to the user 

community, any one of the engineering tools currently being developed for 

determining Appendix X ice shapes should be validated/substantiated to 

such a degree that it is acceptable to the authorities and thus not require 

the use of two methods. While it is likely that for some period of time, 

applicants may find it prudent to use more than one tool, eventually 

enough expertise will be developed that applicants may have sufficient 

confidence in a single method, yet the rule will still require two. Boeing 

does not believe that the natural SLD ice shapes obtained by NASA will 

be of much value for them since the research airplane utilized by NASA is 

not at all similar to Boeing’s large jets. This opinion is supported by the 

current situation with NASA’s Appendix C ice-shape generation CFD 

codes, e.g., Lewice 2.0: it has not been validated for large, high-speed 

airplanes. The experimental data range used for validation by NASA is 

limited to a wing chord length of 78 inches but the wing chord of the 737-

300, for example, is approximately 135 inches; the highest Reynolds 

Number is 13 million, whereas the in-flight Reynolds Number for Boeing 

airplanes ranges from approximately 20 to 40 million; the wings of NASA’s 

Twin Otter aircraft are not swept, whereas those of all large modern 

aircraft are swept. Given these exceedances of the validation data, 

whether LEWICE 2.0 generates ice shapes that correlate with those 

accreted in natural Appendix C conditions is unknown, and thus Boeing 

does not utilize the LEWICE 2.0 code for certification purposes. There is 

no reason to believe that the situation will be improved for Appendix X. 

Validation of NASA’S LEWICE 2.0 code utilized an extensive icing tunnel 

ice-shape database including modern airfoils; however, no such extensive 

database exists or is planned for NASA’s Appendix X code validation. 

Unfortunately, based upon historical experience with Appendix C 

certifications, Airbus and Boeing believe that it will be many years and 

numerous certification attempts before the authorities will accept even a 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-25 

combination of two methods of determining Appendix X ice shapes without 

natural icing flight test validation performed with the airplane model for 

which certification is sought. 

The potential for natural icing flight testing in Appendix X conditions to be 

“found necessary” at the discretion of the authorities is an unacceptable 

business situation. The Airbus and Boeing view is that the manufacturers 

should have the liberty and responsibility of determining which means of 

compliance are appropriate for their situation. They must not be burdened 

with the uncertainty that at the final hour of a certification program, the 

authorities will declare that natural icing flight testing in Appendix X 

conditions is necessary and therefore required. The rationale presented in 

the “Initial Manufacturers’ Position” remains valid. Airbus and Boeing 

stand by that position. 

Notwithstanding the majority’s contention that the phrase “as found 

necessary” is not intended to force the applicant to perform natural icing 

flight testing in Appendix X conditions, the fact is that the language 

remains in the proposed rule. If the majority sincerely accepts that natural 

icing flight testing in Appendix X conditions is merely one of the 

acceptable additional means of compliance and is not intended to be 

required, then they should be amenable to deleting the phrase “as found 

necessary” rather than stating in guidance material that they don’t actually 

intend for it to be necessary. Another good-faith alternative would be to 

incorporate into the rule itself language similar to the “should not be 

necessary if . . . “ provision from the Majority Working Group Position and 

proposed AC (of course, changing “should not be” to “is not” and removing 

the stipulation for the use of at least two methods of predicting ice 

shapes). 

Failure to achieve full group consensus on this issue stems from a 

fundamental difference of opinion that has not changed since the 

beginning of this effort: the authorities (or the majority) believe that the 

proposed new rule for Appendix X conditions is necessary to improve 

safety for all Part 25 airplanes; Airbus and Boeing do not believe that 

safety will be improved for large jet transports. Therefore, although some 

additional burden may be acceptable when the rule is first implemented, 

care should be taken to eliminate the potential for unnecessary long-term 

undue burden.
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-26 

MAJORITY RESPONSE TO MINORITY POSITION (ALPA, CAA/UK, CESSNA,

FAA/FAA TECH CENTER, METEOROLOGICAL SERVICES CANADA, NASA,

TRANSPORT CANADA/TRANSPORT DEVELOPMENT CENTER)

Criteria Overly Burdensome

The minority state, “the criteria remain overly burdensome in terms of 

costs and resources . . . .” 

The working group has prepared cost and benefit information to be 

considered in the economic evaluation. If the benefits outweigh the costs, 

the proposed rule will not be considered overly burdensome. 

Extensive Research Project

The minority state, “that a flight test program in natural Appendix X 

conditions is more likely to result in an extensive research project than an 

airplane certification.” 

The Majority Working Group Position acknowledges the difficulties in flight 

testing in natural Appendix X conditions. Since natural icing flight test are 

controversial, the majority of the working group developed and agreed 

upon the conditions when the natural Appendix X conditions would not be 

necessary. 

Ice Shape Determination

1. The minority believes that it will be many years and certification 

attempts before the authorities will accept a combination of two methods 

of determining ice shapes without natural icing flight tests. 

The majority does not concur. The Majority Working Group Position of 

when flight testing in natural Appendix X conditions should not be 

necessary has been included in the proposed AC. The authorities concur 

with the proposed AC, which provides options other than icing flight tests 

to verify Appendix X ice shapes. The development of the methods for 

determining the ice shapes is expected to be incremental relative to 

existing Appendix C capabilities. This should be able to be accomplished 

in a shorter time frame than that required for the development of Appendix 

C methods. 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-27 

2. The minority states that, “any one of the engineering tools currently 

being developed for determining Appendix X ice shapes should be 

validated/substantiated to such a degree that it is acceptable to the 

authorities and thus not require the use of two methods.” 

The majority acknowledges that government entities are actively pursuing 

the development of engineering tools for use in determining Appendix X 

ice shapes. However, it is ultimately the applicant’s responsibility to 

substantiate that the engineering tools employed during certifications are 

valid. The manufacturers cannot and should not expect that the 

government will deliver a package that does not require additional 

verification effects by the manufacturer. 

3. The minority states: “While it is likely that for some period of time, 

applicants may find it prudent to use more than one tool, eventually 

enough expertise will be developed that applicants may have sufficient 

confidence in a single method yet the rule will still require two.” 

The majority interprets this to mean that the minority would like the rule 

changed to only require one tool. The majority does not concur. The rule 

requires two methods: analysis and, as found necessary, one or more 

types of tests. The requirement for more than one method is similar to the 

existing icing rule § 25.1419. The AC 25.1419-1A and the proposed AC 

for this rule provide guidance on the use of similarity analyses that allows 

an applicant to take advantage of past certifications by applying the 

previous substantiation to another program. The use of similarity analyses 

has been successfully used on many programs. There have been cases 

where a manufacturer has substantiated compliance with § 25.1419 based 

on a similarity analysis and no additional testing was required for the new 

program. The majority believes that the use of similarity analyses for 

compliance with § 25.1420 will result in similar reductions in testing as 

experience with Appendix X certification programs increases. There is no 

need to revise the rule as suggested by the minority. 

“As Found Necessary”

1. The minority position states: ‘The potential for natural icing flight testing 

in Appendix X conditions to be “found necessary” at the discretion of the 

authorities is an unacceptable business situation.’ 

This appears to imply that the authorities will arbitrarily decide that natural 

icing flight tests in Appendix X conditions is necessary. The majority 

believe that adequate explanations have been provided in the Working 

Group Report and the advisory material so the rule may be applied without 
IPHWG Task 2 WG Report - Appendix D 

Acceptable Compliance Methods For Appendix X Conditions 

(Natural Ice Flight Testing Necessity)

12/19/2005 Page D-28 

ambiguity regarding the meaning of the words, “as found necessary.” The 

Working Group Report, which provides information to be included in the 

preamble to the proposed rule, contains an explanation of the words “as 

found necessary.” The report states, in part: 

The words “as found necessary” will be applied in the same 

way as they are applied in § 25.1419(b). During the 

certification process, the applicant will demonstrate 

compliance with the rule using a combination of analysis and 

test(s). The applicant’s means of compliance will consist of 

analysis and the amount and types of testing they find are 

necessary to demonstrate compliance with the regulation. 

The applicant will choose to use one or more of the tests 

identified in paragraphs (b)(1) through (b)(5). Although the 

applicant may choose their means of compliance, it is 

ultimately the FAA (EASA) which must make a finding that 

the applicant has performed sufficient test(s) and analysis to 

substantiate compliance with the regulation. 

As mentioned above, the working group has developed AC material that 

identifies when flight testing in Appendix X conditions should not be 

necessary. The majority of the working group, which includes the 

authorities, concurs with the proposed AC. 

2. The minority has interpreted the AC language to mean that with regard 

to flight testing in natural icing conditions, the majority “don’t actually 

intend for it to be necessary.” 

The majority finds this to be an incorrect interpretation of the AC language. 

The advisory material simply explains when fight testing in Appendix X 

should not be necessary. 

3. The minority proposes that the language in the AC be included in the 

rule. 

The majority does not concur. Including the language from the AC into the 

rule would unnecessarily increase the complexity of the rule. 

Safety Not Improved for Large Jet Transports

The minority position brings up the issue of the applicability of the rule. 

This issue is addressed in Appendix F of this Working Group Report. 
IPHWG Task 2 WG Report - Appendix E 

 Necessity to Require a Means to Determine Exceedance of Appendix X 

12/19/2005 Page E-1 

APPENDIX E - NECESSITY OF A MEANS TO DETERMINE 

EXCEEDANCE OF APPENDIX X 
IPHWG Task 2 WG Report - Appendix E 

 Necessity to Require a Means to Determine Exceedance of Appendix X 

12/19/2005 Page E-2 

NECESSITY OF A MEANS TO DETERMINE EXCEEDANCE OF APPENDIX X

The tasking statement requires: "In addition, consider the need for a regulation 

that requires installation of a means to discriminate between conditions within 

and outside the certification envelope.” For aircraft certified for detect and exit 

under §25.1420(a)(1) and (a)(2), a method to discriminate between conditions 

within and outside of the certification envelope is required by the rule language 

and clarified in the advisory material. This discussion addresses aircraft certified 

under §25.1420(a)(3) and the necessity for a means to discriminate conditions 

outside of Appendix X. 

Appendix X characterizes the SLD environment in terms of an engineering 

design standard. The database used for developing Appendix X consists of 

measurements made in wintertime stratiform clouds. No measurements were 

made in cumulus clouds. Convective flows within cumulus clouds are known to 

produce higher liquid water (LWC) contents than stratiform clouds. Appendix H 

of this report provides more detail on why it was reasonable to develop Appendix 

X using wintertime stratiform cloud data only. 

The IPHWG proposed Advisory Circular AC 121-XX contains the following 

statements: “The Appendix C conditions were designed to include 99% of icing 

conditions. Evaluation of icing data has indicated that the probability of 

encountering icing outside of Appendix C droplet conditions is on the order of 

10–2”. 

The IPHWG has proposed to use the 99th percentile of liquid water content 

(LWC) for Appendix X. Therefore, for Appendix X encounters the probability of 

LWC values greater than Appendix X is on the order of 10–2. 

There are other mitigating considerations that are conservative aspects of an ice 

protection system certification program. For example, the probability of being in 

icing is assumed to be one for certification purposes. Ice shapes described in 

Part II of Appendix X for use in demonstrating compliance with the airplane 

handling and performance requirements of 14 CFR § 25.21(g)(2) & (g)(4) are 

defined to provide a conservative assessment of the icing effects to ensure an 

acceptable level of safety. Holding operations are assumed to last for extended 

durations, typically at the most critical water catch point. Current and future 

advisory material considers the hold to occur within a 17.4 nm cloud extent which 

produces a high LWC. The critical water catch point is typically a very specific 

set of conditions and conservative assumptions such as aircraft weight, holding 

airspeeds, and ambient temperatures. The probability of an aircraft 

simultaneously achieving or exceeding these design assumptions is not 

quantifiable but is conservative. Many certification programs have further 

conservative factors such as assuming minimum system tolerances such as 

minimum pressure regulators or temperature controller set points. 
IPHWG Task 2 WG Report - Appendix E 

 Necessity to Require a Means to Determine Exceedance of Appendix X 

12/19/2005 Page E-3 

CONCLUSIONS

No definitive quantitative analysis can be performed to show that the hazard of 

flight in conditions beyond certification limits meets commonly accepted 

probabilities for catastrophic events. However, given the conservative factors 

described above, the IPHWG believes that the probability of exceeding the limits for 

aircraft certification is sufficiently low to prevent a catastrophic event, and a means 

to alert flight crews when 14 CFR part 25 Appendix X icing conditions are exceeded 

is not needed. 
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-1 

APPENDIX F - RULE APPLICABILITY BASED ON AIRCRAFT 

DESIGN FEATURES 
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-2 

POSITION STATEMENT SUPPORTING EXCLUSION FROM §25.1420 FOR AIRPLANES WITH 

CERTAIN DESIGN FEATURES

(Airbus, Boeing, Embraer, Cessna14)

Overview

The current airworthiness requirements of 14 CFR/JAR Part 25 for flight-in-icing 

certification have proven to be sufficient to provide the desired level of safety for 

certain transport-category airplanes. The position of Airbus, Boeing, Embraer, 

and Cessna14 (“the major airplane manufacturers”) is strongly supported by the 

safety record of the class of airplanes proposed for exclusion, which today 

routinely fly in icing conditions both within and outside of Appendix C. 

Compliance with the proposed new certification requirements of §25.1420 to 

address icing environments including supercooled large droplet conditions 

(“SLD”) would be extremely costly, and no safety improvement can be expected. 

Consequently, the major airplane manufacturers contend that there is no 

justification for making either the certification requirements or the certification 

process more burdensome for these types of airplanes. 

Introduction

The Ice Protection Harmonization Working Group (“IPHWG”) assembled accident 

and incident databases of events in SLD conditions that are considered relevant 

to the proposed §25.1420 (“relevant” meaning that had it been in place at the 

time, the proposed regulation could have prevented the event). Review of the 

databases revealed that airplanes of certain types or with certain design features 

have experienced no accidents and very few incidents due to in-flight icing, either 

inside or outside of Appendix C environments. It is therefore reasonable to 

conclude that no increase in safety with regard to in-flight icing need be achieved 

for these types of airplanes. The major airplane manufacturers thus propose the 

concept of “exclusionary design features,” with the intent that future airplane 

designs which incorporate these features be exempt from demonstrating 

compliance with proposed §25.1420. 

The major airplane manufacturers propose three primary exclusionary design 

features, discussed in greater detail below: size, irreversible powered flight 

controls, and wing leading-edge high-lift devices. The intention of the major 

airplane manufacturers is that in order to qualify for exemption from proposed 

§25.1420, an airplane should have all three of these main features. However, 

there are other design features that warrant consideration for exemption as well. 

 

14 See appended Cessna position statement. 
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-3 

Requirements for Flight-in-Icing Certification

The icing certification rules were established in the 1940’s after World War II. The 

certification process was subsequently defined by gradual development of the 

advisory material. The 14 CFR/JAR requirement is that an airplane be able to 

operate safely when flying in icing conditions. Compliance is shown by 

evaluating the ice protection system (“IPS”) design and the airplane’s 

performance when subjected to an environment defined by design icing 

envelopes known as 14 CFR/JAR Part 25, Appendix C. Advisory material 

provides guidance on the conservative critical ice shapes to be considered when 

showing compliance. Ice accretion associated with a 45-minute holding scenario 

has often been found to result in the most critical ice shapes per Appendix C, and 

historically this ice measured a nominal 3 inches at the upper horn and has come 

to be known as “3-inch ice.” Both 3-inch simulated ice shapes and natural 

accretion of as much as 3 inches on the wing leading edge are commonly used 

for flight testing to show compliance. 

It is often quite difficult to accrete 3 inches of ice in natural Appendix C 

conditions. Natural icing conditions generally do not match the Appendix C 

conditions which generated the critical ice shapes. Appendix C conditions are a 

statistical definition where each point is defined by 3 icing parameters for a given 

size cloud: mean effective diameter (“MED,” droplet size), air temperature, and 

liquid water content (“LWC”). Icing clouds seldom provide an environment 

adequate for achieving a 3-inch accumulation during a certification flight test, and 

as a result, the airplane is often flown for several hours (rather than 45 minutes) 

and the authorities accept a lesser accumulation. In-flight assessment of the 

actual shape and thickness is very difficult and results are approximate. These 

difficulties are compounded by another factor relative to natural ice flight testing 

for certification compliance: measurement of CLmax with a naturally iced wing. 

Generally, natural buffet or buffet generated by the accreted ice causes the ice to 

break off or shed in parts, resulting in inconsistent measurements. For these 

reasons, more and more reliance has come to be placed upon dry-air wind tunnel 

and flight tests with simulated ice shapes. By manufacturing ice shapes out of a 

light-weight material (e.g., wood, plastic, or foam) and bonding them onto 

airplane leading edges, it is possible to make a consistent assessment of 

airplane behavior with ice accretion; flight tests can take place in a quiet 

atmosphere. Preceding dry-air tests, it remains necessary to define the critical 

ice shapes. Ground simulation techniques and the emergence of computational 

fluid dynamic (“CFD”) computer codes assist in determination of the critical ice 

shapes. This has assisted the transition away from reliance upon natural icing 

flight testing as the primary means of certification compliance. 

An airplane certified for flight in icing today typically has no environmental 

limitations on its in-flight operation in icing conditions. Since Appendix C does 

not describe all icing conditions encountered by airplanes in flight, and given 
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-4 

recent meteorological data as to the probability of encountering various 

conditions, the conclusion can be made that many airplanes safely fly today, and 

have for decades, in SLD conditions described by the proposed Appendix X. As 

they are flying safely, it must be the case that certification compliance to 

Appendix C conditions is conservative enough that the conditions of Appendix X 

do not pose a greater or additional threat. Airplanes also take off today in icing 

conditions outside of Appendix C, i.e., freezing drizzle and light freezing rain, with 

no limitation on vertical or horizontal extent of the conditions after takeoff. No 

accidents nor incidents have occurred due to takeoff and subsequent flight in 

these conditions. Therefore, for airplanes possessing the exclusionary design 

features, certification for flight in icing using Appendix C conditions for design of 

the IPS and determination of the critical ice shapes should be considered an 

equivalent method of compliance providing global protection (or “equivalent 

protection”) for all icing encounters. 

At an FAA symposium in 1969 regarding review of the Appendix C icing criteria, 

William Lewis stated: 

Since these criteria have stood the test of use, and since the total 

of experience with existing aircraft is more comprehensive than any 

data-collecting program, it is suggested that the future changes in 

criteria be based primarily on operating experience rather than on 

meteorological data. 

This opinion is very much in unison with the assertion of the major airplane 

manufacturers – that the current large transport category airplanes, a fleet of 

nearly 20,000, already demonstrate safe operation in icing. 

Exclusionary Design Features

The major airplane manufacturers propose that airplanes possessing a 

combination of all three specific design features be exempt from demonstrating 

compliance with the proposed §25.1420. The three exclusionary design features 

are: 

1. Size. Size has a direct bearing on an airplane’s susceptibility to the 

adverse effects of ice accretion. From Appendix 3 of the draft AC 

25.21(g): 

The size of an airplane determines the sensitivity of its 

flight characteristics to ice thickness and roughness. The 

relative effect of a given ice height (or ice roughness 

height) decreases as airplane size increases. 
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-5 

Notwithstanding the difficulty of defining the limit between “larger” and 

“smaller” airplanes, review of the icing safety record reveals the facts: 

• Zero in-flight accidents with larger airplanes15

• Numerous fatalities from accidents of smaller airplanes. 

The advantage of size with regard to in-flight icing considerations is well 

known based upon decades of testing and experience and does not have 

to be re-proven. 

Based upon review of the accident/incident database, it is proposed that 

size be an exclusionary design feature for airplanes with gross weight in 

excess of 60,000 pounds (27,000 kilograms). There is no compelling 

rationale for the choice of 60,000 pounds other than it being the threshold 

used for the IPHWG’s proposed operating rule §121.321. Wing chord or 

other such parameters would no doubt be better technical criteria but, as 

was decided for §121.321, a simple weight threshold is easier to 

implement. 

2. Irreversible Powered Flight Controls. The use of irreversible powered 

flight controls is another factor that reduces an airplane’s susceptibility to 

SLD conditions. The concern that SLD accretions can produce hinge 

moment or other anomalous control force/trim effects is not applicable to 

such systems. Since the loads generated at the control surfaces are 

reacted against the actuator and its mounting, changes in these loads 

cannot be transmitted through the cockpit controls. The use of irreversible 

powered flight controls is implicitly related to reduced airplane 

susceptibility. The choice of irreversible flight controls is related to the 

aerodynamic loads on the control surfaces. The high loads that require 

powered flight controls are typical of high speed or large-scale airplanes. 

The high-speed characteristic requires significant power that results in 

significant power margins at the low speeds typical of low altitude 

operations within the icing environment. The scale issue is discussed in 

item 1 above. 

3. Wing Leading-Edge High-Lift Devices. For wings without ice 

contamination, wing leading-edge high-lift devices (e.g., slats, Krueger 

flaps) provide a considerable increase in the maximum lift coefficient, 

CLmax, compared to fixed leading edges. When contaminated with ice, 

 

15 Regarding the 28 March 1989 McDonnell Douglas DC-8-73F accident, this airplane type would not 

qualify for the proposed exemption since it does not have the proposed required design features (no wing 

leading-edge devices nor irreversible powered flight controls). 
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-6 

wings equipped with these devices have smaller relative CLmax losses 

due to ice accretion than are suffered by wings with fixed leading edges. 

This reduced sensitivity to the adverse effects of ice accretion is 

applicable to the critical takeoff, initial climb, approach, and landing 

phases of flight. 

Significant facts associated with the proposed exclusionary design features 

should be pointed out. 

• Manufacturers of airplanes with the proposed exclusionary design 

features tend to apply an IPS to fewer and fewer lifting surfaces. As 

airplanes have grown in size, it has been found to be unnecessary to 

protect the entire wing leading edge and the leading edges of the 

horizontal and vertical tails. For example, the B777 IPS protects only 

three out of seven wing leading-edge slats, less than half of the span, 

and does not protect the tails at all. As a precautionary measure for 

flight testing, the A380 prototype design incorporates protection for 

only one wing slat out of eight; the production airplane is intended to 

have no IPS for the wings and tails. 

• The safety record of today’s commercial jet airplane types that would 

qualify for the proposed exclusion shows not one single accident due 

to SLD icing conditions. As of 31 December 2001, there were 

approximately 16,144 airplanes, 7 manufacturers, and 33 types of 

these airplanes in service. The cumulative departures were 396 

million, registering approximately 645 million flight hours. These 

metrics are impressive, with not one accident due to SLD icing 

conditions. 

There are other design features which may have a positive effect on mitigating 

performance degradation due to ice accumulation. Such features should be 

considered as potentially providing a level of safety equivalent to the three 

primary exclusionary design features. 

a. Configuration or Airfoil Design. The configuration of an airplane’s critical 

surface may be such that degradation due to icing effects falls within the 

operational design envelope. For example, some tails are designed to 

provide full functionality with critical ice accretion. Some airfoil designs 

are less susceptible to icing effects than others. The scale of a 

leading-edge radius has a dramatic effect on collection efficiency and the 

resulting ice accretion, as shown by the comparison among the thickness 

of ice collected during the same test on the following Airbus airplane parts: 

Beluga wing tip, 3 inches; wing root, 1.5 inches; upper front fuselage, 

zero. This result is not unique and is indisputable. It may be explained by 

the experimental fact that water droplets are susceptible to bursting when 
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-7 

subjected to the shear forces that appear in front of any leading edge, due 

to the bending of flow lines. Another finding is the spectrum of droplets 

after the burst: larger droplets split into thousands of very small ones 

which are easily carried away by the airflow and never hit the airframe. 

This type of “protection by design” would likely have to be evaluated on a 

case-by-case basis. 

b. Stall Protection Devices. An airplane equipped with a stall protection 

device such as a “slat gapper” (a device that automatically extends 

leading-edge slats from a sealed to a slotted position, typically when the 

airplane reaches a specific angle of attack) is likely able to generate a 

greater CLmax with ice accretion than the uncontaminated wing with 

sealed slats. The extension and gapping of the slats increases the 

maximum lift capability of the wing and may provide effective protection 

against degradation in stall performance or characteristics. 

c. Thermal Ice Protection System. The accident/incident database revealed 

that most airplane types that have had accidents in SLD conditions did not 

have a thermal IPS or, at least, not one that was prudently activated. The 

greatest advantage of a thermal IPS is achieved when it is designed as an 

evaporative system and operated as an anti-icing system to prevent ice 

accretion. 

Conclusion

Airplanes possessing the proposed exclusionary design features enjoy an 

exemplary safety record with regard to in-flight icing. Tribute for this must be 

given to the current certification standards and methods, which are successful for 

these types of airplanes. This success demonstrates the validity of the concept 

of “equivalent protection” afforded by using the current Appendix C as the 

environmental icing standard for means of compliance. Given actual flight 

activity, it is clear that current icing certification standards adequately protect 

many airplanes against any type of icing encounter, including SLD conditions as 

described by proposed §25.1420 Appendix X. 

The major airplane manufacturers contend that the current icing certification 

process is not only adequate for these types of airplanes, it is already 

overprotective; each new version of more stringent certification requirements 

penalizes new airplanes relative to previous ones, which have proven to be safe 

as certified to the previous requirements. The major airplane manufacturers 

therefore consider it unnecessary and unacceptable to impose the requirements 

of §25.1420 upon these types of airplanes. Such a change to icing certification 

cannot be justified by any safety benefit but would be enormously costly. With 

the growing challenges of economics within the aviation industry, it is more 

important than ever to invest for safety improvements in the most productive 
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-8 

manner. Further, the IPHWG’s rulemaking activities with regard to SLD, as 

assigned by TAEIG in Task 2, are not consistent with the recommendations of 

the Commercial Aviation Safety Team, which has determined that the operational 

data support application of an expanded icing envelope only to airplanes without 

thermal anti-icing systems.16 Burdening all airplanes with certification efforts that 

result in no significant increase in safety does not benefit the flying public and is 

contrary to the purposes of ARAC rulemaking. 

Cessna Position Statement

As a manufacturer of small-scale Transport and Normal Category airplanes, a 

scale-based discriminator will not include most Cessna products. However, it is 

recognized that there is technical justification for the reduced susceptibility of 

certain airplanes based on scale and other design features. Cessna is 

supportive of the recognition that airplanes with certain design features are less 

susceptible and will have a reduced safety benefit as a result of large drop 

rulemaking. It is also recognized that improved operating procedures and 

training is a major factor in increasing safety in icing conditions, whether in 

Appendix C or large droplet conditions and independent of certification or specific 

operating rules (Part 121, 135 or 91). 

 

16 The following excerpts are from the Commercial Aviation Safety Team - Loss of Control Joint Safety 

Implementation Team’s Implementation Plan for Criteria for Flight in Icing Conditions for New Airplane 

Designs (emphasis added): 

Safety Enhancement: New designs for airplanes not equipped with evaporative systems 

accommodate flight in an expanded icing envelope and additional de-ice/anti-ice system 

malfunctions. 

Output 1: Regulations and guidance materials are in place that adopt the principles embodied in 

the final reports of the ARAC Ice Protection Harmonization Working Group and the ARAC Flight 

Test Harmonization Working Group to establish new icing certification criteria, for airplanes 

not equipped with evaporative systems, that include performance and handling qualities 

requirements for the following: Residual ice; Intercycle ice; Delayed anti-icing/de-icing system 

activation; De-icing/anti-icing system malfunction. 

Actions: 

1. The ARAC Ice Protection Harmonization Working Group publishes expanded 

icing envelope. 

2. The ARAC Flight Test Harmonization Working Group publishes recommendations 

that address airplane performance and handling characteristics in icing conditions. 
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-9 

MAJORITY POSITION - RESPONSE TO EXCLUSION FROM §25.1420 FOR AIRCRAFT WITH 

CERTAIN DESIGN FEATURES

(ALPA, CAA/UK, FAA/FAA Tech Center, Meterological Services of Canada, 

NASA, SAAB, Transport Canada/Transport Development Center)

Background

The existence of SLD icing conditions, consisting of freezing drizzle and freezing 

rain, is unquestioned. Transport airplanes, regardless of airplane size, weight, 

high-lift configuration design, or flight control system design, routinely operate or 

may be inadvertently exposed to these icing conditions. Yet, during the approval 

of the airplane for flight in icing conditions, transport airplanes are not required to 

show compliance with minimum airworthiness requirements that would ensure 

safe operation in these SLD icing conditions. The safety margins when operating 

in these conditions are unknown. 

While some transport airplane manufacturers remain silent on the operation of 

their aircraft in SLD icing conditions, others recommend that their airplanes not 

be operated in SLD icing conditions. For example, Gulfstream states the 

following in the operations manual for their Gl-GV airplanes: "Under no 

circumstances will flights be planned through forecast or known severe icing 

conditions. . . . Flight in freezing rain or freezing drizzle should be avoided." 

Assessments by the FAA, TC, ALPA, and the National Safety Transportation 

Board (NTSB) of transport airplane operational experiences in SLD indicate that 

the 14 CFR part 25 Appendix C icing conditions standards are no longer 

sufficient and that the icing conditions standards of 14 CFR part 25 should be 

expanded to include SLD icing conditions and mixed-phase icing conditions, if 

mixed-phase icing conditions are found to be more hazardous than the liquid 

SLD icing conditions. These assessments and decisions are congruent with a 

statement made relative to 14 CFR part 25, Appendix C, at a 1969 FAA 

symposium by William Lewis, a National Advisory Committee for Aeronautics’ 

researcher, who contributed to the information used to establish Appendix C: 

Since these criteria have stood the test of use, and since the total 

of experience with existing aircraft is more comprehensive than any 

data-collecting program, it is suggested that the future changes in 

criteria be based primarily on operating experience rather than on 

meteorological data. 

Action by the FAA to expand the 14 CFR part 25 icing condition standards to 

include SLD is “based primarily on operating experience.” 
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-10 

Beyond the assessment by the NTSB and the FAA of the safety history of 

transport airplane operations in SLD icing conditions, , the transport airplane 

operational experience in SLD icing conditions is further mired in related 

confused industry practices and unclear information provided to pilots relative to 

deciding when operating their airplane in SLD icing conditions is safe. Annually, 

the FAA issues a Flight Standards Information Bulletin for Air Transportation 

(FSAT) that provides de/anti-icing fluid holdover protection guidelines in freezing 

drizzle and light freezing rain for ground operations. Even though the de/antiicing fluid holdover time guidelines explicitly state that the fluids do not provide 

protection during flight, some flight crews interpret the inclusion of light freezing 

rain fluid protection guidelines in the FSAT as an FAA approval for flight 

operations in SLD icing conditions. Action by the FAA to expand the 14 CFR 

part 25 icing condition standards to include SLD will address operational issues 

associated with flightcrews lacking clear guidelines for when operation in freezing 

rain would be safe or unsafe. 

Issue No. 1- Exclude airplanes that have the following: 

1. Weight greater than 60,000 pounds, 

2. Equipped with irreversible flight controls, and 

3. Wing leading edge high-lift devices. 

Response No. 1

The response to the minority proposal to exclude airplanes with specific 

characteristics or design features is addressed in two parts: first, a general 

response addressing whether it is appropriate to limit the applicability of a 

certification requirement; secondly, a response to some of the specific design 

features that are proposed to be used to limit the applicability of the proposed 

rule. 

General

Part 25 prescribes airworthiness standards for the issue of type certificates, and 

changes to those certificates, for transport category airplanes. In general, 

specific airworthiness standards are defined with the intent of ensuring safety of 

flight. These standards state airworthiness objectives or absolute requirements, 

and have been formatted to be generally applicable to future airplane designs. 

Typically, a limit is included in the airworthiness requirements when the 

requirement applies to a specific configuration class, such as: reciprocating or 

turbojet engines; propeller or turbojet powered airplanes; land or sea airplane; 

nose or tail wheel landing gear; high-lift devices retracted or extended. 
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-11 

Limiting the applicability of an airworthiness requirement is only appropriate 

when there is high confidence that the boundary of the applicability can be 

appropriately chosen. To evaluate the confidence in the proposed boundary one 

must consider if there are unknown factors involved. For example, the safety 

record of large aircraft may be due in part to the prevalence of safety equipment 

that are not typically installed in smaller airplanes, a situation that may change 

due to improved technology and changes in the economy that could drive the 

removal of the equipment on future airplanes. Another example would be the 

unknown effect of future new, modified, or innovative airplane designs and 

modifications on past operational experience. These two examples illustrate that 

we cannot predict with confidence that the past service experience of airplanes 

with specific design features will be applicable to future designs. 

Historically the FAA has not limited the applicability of icing requirements 

contained in part 25. Recent Aviation Rulemaking Advisory Committee (ARAC) 

activities have followed this philosophy. The Flight Test Harmonization Working 

Group (FTHWG), consisting of industry and government specialists of Europe, 

North America, and South America, was tasked on June 10, 1994 (59 FR 

30082), to recommend to the ARAC new or revised requirements and 

compliance methods related to airplane performance and handling 

characteristics in part 25, Appendix C, icing conditions. ARAC has 

recommended to the FAA proposed rulemaking drafted by the FTHWG, 

commonly referred to as §25.21(g), et al. These proposed airworthiness 

requirements are applicable to all transport category airplanes, regardless of 

size, weight, high-lift configuration design, or flight control system design. 

The Ice Protection Harmonization Working Group (IPHWG) has requested 

support from the FTHWG relative to devising requirements to assess the ability 

of airplanes to safely exit or operate without restriction in Appendix X SLD icing 

conditions. The IPHWG has proposed that the FTHWG generally extend the 

proposed requirements of §25.21(g), et al., to Appendix X SLD icing conditions. 

Operations in Appendix X SLD icing conditions are expected to result in airplane 

performance degradations and adverse changes in airplane stability and control 

different than, and potentially greater than, those resulting from operations in part 

25, Appendix C. Limiting the applicability may result in degraded airplane 

performance and handling qualities relative to the airworthiness standards that 

have been relaxed in the proposed in §25.21(g), et al. Therefore, limiting the 

applicability of the proposed §25.1420 would result in the unacceptable situation 

of requiring higher levels of safety for certain airplanes and lower levels of safety 

for other airplanes with regard to performance, stability, and control in icing 

conditions. 
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-12 

Weight

Airplane takeoff weight is suggested as a design feature that reflects the alleviating 

effect of airplane size on the adverse aerodynamic effects of ice accretion. 

However, the ratio of wing and control surface sizes to airplane weight varies 

between airplane designs. Also, the chord of wings and control surfaces vary with 

the aspect ratio of these surfaces. Therefore, airplane takeoff weight is not a 

consistent indicator of lifting and control surface size or chord. 

The proposed 14 CFR §25.21(g), et al., recognizes the beneficial effects of airplane 

size, relative to the adverse effects of ice accretion, by the establishment of airplane 

performance thresholds at which airplane performance degradations resulting from 

ice accretion must be included in the airplane performance requirements of subpart 

B of 14 CFR part 25. The proposed 14 CFR §25.21(g), et al., establishes thresholds 

for when ice-related degraded airplane performance must be addressed. This 

benefits larger airplanes since the adverse aerodynamic effects of ice accretions 

diminish as the airplane size increases. Airplane size is also addressed by the 

diminished effects of ice accretion on large airplanes relative to the ice contaminated 

stability and control requirements proposed by §25.21(g), et al. Actual 

substantiation of the effects of airplane size on the aerodynamic effects of ice 

accretion, as is required by the proposed §25.21(g), et al., is considered more 

encompassing and appropriate than inferring benefits based on takeoff weight. The 

proposed 25.21(g), et al., currently requires analysis based on 14 CFR Appendix C 

icing envelopes. The larger drops in SLD conditions will yield impingements, and 

ice accretions, further aft than Appendix C conditions, along with potentially different 

accretions in the leading edge area. These SLD shapes and locations have not 

been quantified in terms of performance degradation and handling qualities, and 

they need to have the same evaluation expected of Appendix C ice shapes to 

assure safe operations. 

Irreversible Flight Controls

Irreversible flight controls will provide protection against uncommanded motion of a 

flight control surface due to a ridge of ice aft of the protected area and in front of the 

flight control surface. However, this is not the only concern in SLD icing conditions. 

An irreversible control surface may not be deflected by the SLD accumulation but 

the aerodynamic efficiency of the control is likely to be degraded by the presence of 

SLD icing in front of the control surface. Moreover, the thinner outboard wing 

sections are in general more susceptible to collecting ice than the thicker inboard 

wing sections, and the chord of the airfoil decreases outboard, which makes the 

outboard sections more sensitive for ice accretion. Thus, especially SLD ice 

accretion might change the stall characteristics of the wing in the direction of wingtip 

stall. 
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-13 

Wing leading edge high-lift devices 

The aerodynamic benefits resulting from wing leading edge high-lift devices are 

well known. The leading edge high-lift devices afford wing boundary layer 

control that delays wing stall to higher angles of attack. Airplane performance 

benefits can be realized since many performance margins defined by subpart B 

of 14 CFR part 25 are predicated on stall speed. However, any aerodynamic 

benefits attributed to the devices in icing conditions would not be present when 

the devices are stowed for the cruise configuration. 

Arguments have been made that airplanes with leading edge high-lift devices are 

less sensitive to leading edge surface roughness. However, maintenance and 

repair requirements of 14 CFR part 41 and the requirements of 14 CFR 

§121.629, and similar other operating rules, ensure that airplane surface finishes 

are maintained to manufacturers’ standards and ensure that transport airplane 

surfaces are free of adhering ice prior to takeoff. Also, means used for 

demonstrating compliance with 14 CFR part 25, subpart B, provide for using the 

most adverse of allowable tolerances, such as equipment tolerances and surface 

finishes, for those demonstrations. The issues of surface smoothness and the 

adverse aerodynamic effects of non-smooth surfaces are addressed by 

compliance with existing regulations and conformance with existing certification 

practices. 

The benefits of various leading edge high-lift device designs relative to leading 

edge surface roughness may vary. A significant portion of these benefits are 

due to boundary layer control resulting from a well-designed slot between the 

leading edge high-lift device and the basic wing leading edge. Some leading 

edge slats are not slotted for takeoff but instead are extended and deflected to 

increase wing area and leading edge camber for improved high-lift performance. 

Airplanes with these features may exhibit the same sensitivity to leading edge 

surface roughness as “hard wing” airplanes. The takeoff position of slats may be 

automated, with the slat extended and sealed at takeoff operational attitudes and 

further extended and deflected, opening a slot at the slat trailing-edge, as stall 

angles of attack are approached. Such slat designs may provide tolerance to 

wing leading surface roughness. The level of surface roughness tolerance 

provided by such designs may vary. 

High confidence in limiting a rule based on the presence of wing leading edge 

high-lift devices cannot be achieved because of the variations in these devices. 

Wing leading edge high-lift device designs include: slats that may be slotted or 

sealed to the basic wing leading edge, over or under deflected, deflection and 

slotting of the slat that may be automated as a function of stall warning or 

airplane angle of attack; Krueger flaps that may be slotted or sealed to the wing 

leading edge, flexed to optimum curvature or conformed to the wing’s leading 

edge lower surface; vortilons or some other vortex creating devices; or some 

other innovative device. In addition the spanwise extent of ice protection for 
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-14 

transport airplanes with leading edge high-lift devices varies from 100 percent for 

some early turbo-jet airplane slats to the span of two slats for later airplane 

designs and to none for Krueger flaps. The variations in the designs lead to 

varying degrees of aerodynamic benefit. Without defining these specific 

performance benefits associated with the above designs, it is indeterminate as to 

the potential safety margins for SLD conditions. 

A comprehensive review of the effects of ice accretion on airplane aerodynamics 

published in 2001 by Frank T. Lynch and Abdollah Khodadoust17 states that there is 

very little applicable data available addressing the possible range of maximum lift 

penalties which could be caused by “large” in-flight ice accretions on multi-element 

high-lift wings. A well know correlation of the effect of wing surface roughness on 

maximum lift coefficient by Ralph E. Brumby,18 indicates that for roughness at the 

leading edge, the maximum lift benefits afforded by slats, relative to “hard wing” 

designs, no longer exists for k/c greater than approximately 0.0015. SLD ice is often 

rougher than Appendix C ice. On a large transport aircraft this kind of rough SLD 

ice may result in adverse aerodynamic effects. 

Use of wing leading edge high-lift devices to limit the applicability of the draft 14 

CFR §25.1420 is considered inappropriate since: the adverse aerodynamic 

effects of icing must be addressed for all phases of flight and airplane 

configurations; the variability of wing leading edge high-lift device and airplane 

ice protection designs (and associated benefits that may be attributed to 

utilization of the devices); and, the lack of applicable data available that address 

the aerodynamic benefits resulting from the devices during flight in icing. 

Issue No. 2 – Exclude airplanes with other design features (configuration of 

airfoil design, stall protection devices, thermal ice protection system) 

Response No. 2 

Airplane design features may exist that mitigate the adverse effects of ice 

accretion on airplane performance and stability and control, such as leading 

edge sweep, airfoil camber and thickness distribution, and stall margins. 

However, the mitigation may vary, depending on the attributes of the design 

feature. Since the value of mitigation varies with specific airplane design 

features, consideration of these design features should be recognized when 

decisions are made relative to the extent of needed ice protection and when 

compliance with airworthiness standards is demonstrated.

 

17 Lynch, Frank T. and Khodadoust, Abdollah, “Effects of ice accretion on aircraft aerodynamics,” Progress 

in Aerospace Sciences, Vol. 37, p. 730, 2001.

18 Reinmann, John J., “Icing: Accretion, Detection, Protection, ”AGARD Lecture Series 197, “Flight in an 

Adverse Environment,” AGARD-LS-197, November 1994, p. 4-14.
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-15 

Below are comments on some of the specific design features the minority has 

proposed to be used to limit the applicability of the proposed rule. 

Configuration or Airfoil Design

The minority states that larger drops split into thousands of small ones, which 

never hit the airframe. Although understood for other applications (e.g., ink jet 

printing and fuel atomization), the bursting of large water drops because of shear 

forces as the drops approach surfaces and the bouncing and splashing of large 

drops is an emerging area of icing physics research. The science of large 

droplet shear and break-up dynamics is not well understood, and there is no 

current research that defends the notion of droplets bursting into thousands of 

very small ones for the speed, leading edge radius, and droplet sizes involved. 

The claim cannot be substantiated at this time. It would not be appropriate to 

limit the applicability of a rule without a better understanding of large drop icing 

physics and any potential impingement effects on airfoil surfaces. 

Thermal Ice Protection System

The design of thermal ice protection systems varies with respect to coverage and 

capability. Running wet systems are becoming more prevalent as heat sources 

become scarce with the use of high by-pass ratio engines, resulting in runback 

ice aft of the protected surfaces. Runback ice can exacerbate ice accretion 

resulting from SLD icing conditions. Also, thermal ice protection system 

coverage is becoming less extensive as the size of airplanes increases and as 

the heat sources become scarce with the use of high by-pass ratio engines, 

resulting in more unprotected surfaces that would be exposed to SLD icing 

conditions. As such, the benefits of thermal ice protection systems may vary and 

offer little confidence as a basis for limiting the applicability of the proposed 14 

CFR § 25.1420 

Issue No. 3 – Extremely costly and there are no benefits for the airplanes that 

are proposed to be excluded. 

Response No. 3 

A cost/benefit analysis has not been accomplished. Therefore the majority does 

not concur that it is possible to conclude that the costs would outweigh the 

benefits for the airplanes that are proposed to be excluded from the Task 2 SLD 

rule. 
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-16 

Identification of benefits would need to consider: 

1. That the past service history may not predict future service experience 

with respect to safe operations in icing because there is a trend toward 

minimizing the areas protected from ice accretions and increasing the 

areas which run wet rather than minimizing the protected surface wetness 

by evaporation of the impinging cloud drops. 

2. Zero accidents or incidents do not mean that sufficient safety margins are 

always maintained. 

The majority acknowledges that the manufacturer would incur costs to comply 

with the proposed rule. 

Issue No. 4 – An aircraft certificated for flight in icing typically has no 

environmental limitations on its in-flight operation in icing conditions. Certain 

airplanes already safely fly in icing conditions of proposed Appendix X. Since 

they are safely flying the certification compliance to Appendix C conditions is 

conservative enough that the conditions of Appendix X do not pose a greater or 

additional threat. 

Response No. 4 

The majority concur that certain large transport aircraft presently operate without 

environmental limitations on their in-flight icing operation. These aircraft have 

been designed and certified according to Appendix C. Thus, compliance with 

Appendix C may provide some measure of conservatism. However, it is not 

known if this conservatism results in a safety margin that is appropriate for 

operation in all icing conditions of Appendix X. The general trend for new 

aircraft designs is to further improve airplane efficiency by reducing the protected 

areas, reducing the amount of heat, changing the mode of the thermal IPS from 

evaporative anti-icing to running-wet de-icing systems etc. Therefore, the 

majority does not concur that current practices ensure future airplanes will have 

an acceptable safety record in Appendix X icing conditions. 

The safety margin for Appendix X conditions afforded by compliance with 

Appendix C would be dependent upon the specific airplane and the means of 

compliance. Therefore, without determining the effect of Appendix X on the 

performance and handling for each airplane, the majority does not concur that it 

is possible to state that compliance with Appendix C is sufficient to ensure that 

Appendix X does not pose a greater or additional threat to safe operations. 
IPHWG Task 2 WG Report - Appendix F 

Rule Applicability Based on Aircraft Design Features 

12/19/2005 Page F-17 

Issue No. 5 – Task 2 is not consistent with recommendations of the Commercial 

Aviation Safety Team (CAST), which determined that the operational data 

support application of an expanded icing envelope to only airplanes without 

thermal anti-icing systems.

Response No. 5 

The majority concur that there is an inconsistency. This issue references the 

CAST Loss of Control (LOC) Joint Safety Implementation Team (JSIT) 

implementation plan for safety enhancement No. 39. The implementation plan is 

based upon recommendations from the CAST LOC Joint Safety Analysis Team 

(JSAT). The LOC JSAT only looked at three icing accidents, all of which 

involved airplanes equipped with pneumatic deicing boots. With such a limited 

evaluation, no data was examined to justify a decision regarding the adequacies 

or inadequacies of a thermal anti-icing system or aircraft size. Under issue No. 

2, the majority explains why this proposed rule should not be limited to airplanes 

without thermal ice protection systems. 

Issue No. 6 – Cessna Position Statement 

a. There is technical justification for the reduced susceptibility of 

certain aircraft based on scale and other design features. 

b. Supportive of recognition that aircraft with certain design 

features are less susceptible and will have a reduced safety 

benefit as a result of supercooled large drop rulemaking. 

c. Improved operating procedures and training is a major factor in 

increasing safety in icing conditions. 

Response Nos. 6.a and 6.b 

The Cessna position statement does not propose the exclusion of certain aircraft 

from the proposed SLD rule. As stated in response No. 2, the majority concurs 

that airplane scale and other design features help reduce the effect of icing. 

However, these design features must be evaluated on a case-by-case basis. 

The majority concurs that airplanes with certain design features may have 

reduced susceptibility. 

Response No. 6.c 

The Cessna position statement does not propose specific operating procedures 

or training that should be implemented. However, the majority does concur with 

the general statement that operating procedures and training are important 

factors that improve the safety in icing conditions. 
IPHWG Task 2 WG Report - Appendix G 

Certification to a Portion of Appendix X 

12/19/2005 Page G-1 

APPENDIX G - CERTIFICATION TO A PORTION OF APPENDIX X 
IPHWG Task 2 WG Report - Appendix G 

Certification to a Portion of Appendix X 

12/19/2005 Page G-2 

MINORITY POSITION REGARDING CERTIFICATION TO A PORTION OF APPENDIX X

(ALPA, Metrological Services of Canada) 

The Appendix X rule and AC provide for “certification with a range of techniques 

for example, limiting the range of liquid water contents, or potentially water 

contents in a specific drop size.” The AC further requires “the ice shapes 

developed for the portion of the envelope should account for the icing conditions 

in terms of drop distribution and water content . . . .” There is no guidance for 

either an applicant or an approving authority for how to apply the four spectra 

and their individual LWC’s in the Appendix X rule to “account for the icing 

conditions” in any proposed limited environment. 

Using Appendix X, ALPA does not believe it is possible to know how any 

proposed limited condition relates to actual SLD icing environments. There are 

no “envelopes” in Appendix X to determine what spectrum or LWC should be 

used to represent the chosen “portion of the envelope” for a partial spectrum or 

limited LWC certification. Appendix X contains four “average spectra” which 

were not designed or demonstrated to represent actual SLD environmental data 

for other drop sizes or other LWC’s than those contained in Appendix X. 

Perhaps AC material could be developed to show a validated method to derive 

spectra and LWC for use to certify to a portion of Appendix X. Use of the SLD 

database would be necessary to validate the method as representing actual SLD 

conditions, over any proposed range of drop sizes and LWC’s, to the same 

degree as does Appendix X. Such a process would be necessary to insure valid 

ice shapes and provide consistent safety levels in various certification programs. 

Certification to a portion of Appendix X based on limited LWC’s or drop sizes 

should be removed from the advisory material, since Appendix X provides no 

way to relate such a proposal to actual SLD icing environments. Accurate 

representation of the environment is essential to provide accurate airplane ice 

shapes for determining the performance and handling quality impact of SLD. 
IPHWG Task 2 WG Report - Appendix G 

Certification to a Portion of Appendix X 

12/19/2005 Page G-3 

MAJORITY RESPONSE

(Airbus, Boeing, CAA/UK, Cessna, Embraer, FAA/FAA Tech Center, NASA, 

SAAB, Transport Canada/TDC) 

The majority of the IPHWG recommend that certification to a portion of Appendix 

X be retained. Specific AC language has been added in an attempt to reach a 

consensus position but the Working Group has been unable to resolve the 

dissenting opinion. In particular, the AC acknowledges that initial certifications to 

a portion of Appendix X will likely include all of freezing drizzle or all of freezing 

rain, or will be restricted by phase of flight. The AC material also discusses some 

potential concepts that could be used to accomplish certification to a portion of 

Appendix X but, as the minority position suggests, does not provide specific 

examples. However, the AC does provide guidance that for limited certifications, 

ice shapes will be developed for the portion of the envelope for which the aircraft 

is approved, as well as “detect and exit” ice shapes, and that the ice shapes 

should account for the icing conditions in terms of drop distribution and water 

content. Additionally, the shapes will need to be related to the proposed method 

of identifying the icing conditions for the portion that requires exiting. Language 

was added to the AC to clarify that certifications to a portion of Appendix X 

should consider the methods used for developing Appendix X, with a reference to 

a detailed report to be released by the FAA Technical Center. Further, proposed 

advisory material discusses that the AFM Limitations section should contain 

appropriate limits that consider the certification assumptions with respect to 

approved portions of Appendix X. As such, if an aircraft is only approved for a 

portion of the freezing drizzle environment and an operator is unable to 

distinguish that portion prior to dispatch (for example, the exiting icing cue is an 

inertia based, in-flight cue only), the aircraft may have to be limited from 

dispatching into any freezing drizzle. These changes to the proposed advisory 

materials, in addition to the rule language, constitute a performance-based 

approach to defining how to certify to a portion of Appendix X. 

Eliminating the alternative for certification to a portion of Appendix X creates a 

more prescriptive rule that has the potential for disproportionately restricting the 

flight operations of future aircraft. Future aircraft designs could potentially 

accommodate portions of Appendix X; however, it is not possible to design all 

transport category aircraft to operate unrestricted in the full SLD environment. 

Removing certification to a portion removes any benefit for manufacturers to 

certify for a part of the Appendix X envelopes if the aircraft can safely operate 

therein. The ability to certify to a portion of the environment allows 

manufacturers to certify to as large an icing envelope as safely possible. This 

provides a balance of economic pressures (aircraft operators desire to dispatch 

with a minimum of restrictions), yet provides a consistent level of safety (with 

established margins) via the type certification process. 

In addition, the ability to certify to a portion of Appendix X provides incentive for 

technology development relative to both detection of conditions requiring exit and 
IPHWG Task 2 WG Report - Appendix G 

Certification to a Portion of Appendix X 

12/19/2005 Page G-4 

the design of protection systems. The desire expressed in the Minority Position 

to provide a validated method for certification to a portion of Appendix X in the 

AC materials is not consistent with allowing potential for future development of 

techniques and compliance methodologies. As stated previously, some example 

concepts are provided in the AC but a fully defined method will likely not be 

available until an applicant chooses to certify with this option. As with many new 

types of certification efforts, extensive negotiations between the applicant and the 

authorities will likely be necessary, and engineering judgment will be applied 

based on the performance-based guidance in the rule. 

Conclusion 

The rule language was specifically drafted to be performance-based and not 

prescriptive. The majority of the IPHWG recommend that allowance of 

certification to a portion of Appendix X is needed to accomplish the overall goal 

of improving safety in icing conditions while balancing the practical aspect of 

allowing aircraft to dispatch in as wide an operating envelope as safely possible. 

The majority of the IPHWG accept that the performance-based rule language 

and the added draft advisory material provide sufficient guidance to future 

applicants and regulatory personnel regarding certification to a portion of 

Appendix X and are therefore sufficient to address the concerns raised by the 

Minority Position. 
IPHWG Task 2 WG Report - Appendix G 

Certification to a Portion of Appendix X 

12/19/2005 Page G-5 

MINORITY POSITION AGAINST CERTIFYING TO FREEZING DRIZZLE OR FREEZING RAIN 

USING A SUBSET OF THE APPENDIX X CURVES

(Meteorological Service of Canada, ALPA) 

The four spectra with associated LWC limits were developed to describe the 

entire Appendix X environment as it could be determined by all of the 

measurements available, rather than producing a continuous envelope that 

covered all SLD conditions. A manageable number (four) of curves were 

selected to allow tests using codes, icing tanker, and wind tunnel simulations. 

During development of these spectra, partial certification to a portion of Appendix 

X was not considered. Later it was proposed that a manufacturer might want to 

certify to freezing drizzle (ZL) or freezing rain (ZR) by using one or two of the four 

Appendix X curves. However, the curves were not designed or selected for this 

purpose. As an example, all of the four spectra contain some cloud droplets. 

They do not necessarily represent ZL or ZR extreme drop spectra that might be 

observed below cloud at the surface. In addition, airport observers do not 

identify freezing drizzle or freezing precipitation using the precise criteria used to 

develop these curves, so this could pose a problem. For example, freezing 

drizzle is defined by the surface observer as containing drops between 200 and 

500 microns, and the rainfall rate should not be greater than 1 mm/hr. However, 

it is very difficult for a surface observer to identify freezing drizzle visually and 

distinguish it from freezing rain. 

Table G-1 below shows the in-flight aircraft data used to develop Appendix X in 

terms of 50, 75, 95, 99 and 99.9 percent values of precipitation rate. It also 

shows the freezing drizzle and freezing rain precipitation rates, measured every 

minute using a heated gauge with the occurrence of freezing drizzle or rain as 

being reported by the ground observer. The surface data were obtained from an 

analysis performed by Transport Canada for several stations in Quebec, and 

contain 3,692 minutes of freezing drizzle and 10,833 minutes of freezing rain. 

Obviously, the gauge data shows higher precipitation rates for the extreme cases 

of freezing drizzle than the in-flight data. The values are also outside of the 

definition of the extreme value of 1 mm/hr to be used by the surface observer as 

the definition of freezing drizzle. This is not very surprising. The surface 

observer needs to quickly estimate the drizzle/rain distinction using visual cues 

which are difficult to use, and precipitation rates are naturally highly variable, 

especially on a time scale of minutes. The extreme drizzle rates as measured by 

the gauge were probably incorrectly classified by the observer who classified 

them as freezing drizzle when they should have been classified as freezing rain. 

Based on the data in Table 2, in excess of 25 percent of the surface observations 

of freezing drizzle would exceed the maximum criteria of Appendix X for freezing 

drizzle because the observer had misidentified freezing rain as freezing drizzle. 

The logical use of a partial certification to Appendix X for freezing drizzle or rain 

would be to allow the aircraft to take off or land in such conditions using the 
IPHWG Task 2 WG Report - Appendix G 

Certification to a Portion of Appendix X 

12/19/2005 Page G-6 

surface reports of precipitation type made by an observer. Given that Appendix 

X was not designed for manufacturers to be able to certify to only freezing drizzle 

or freezing rain, the surface observer definitions of freezing drizzle and rain are 

different than those used for Appendix X, and the fact that extreme values of 

surface observations of freezing drizzle fall well within the freezing rain envelope, 

it would not be prudent to allow certification to freezing drizzle or freezing rain 

alone using Appendix X. 

 50% 75% 95% 99.0% 99.9% 

Method Type Precipitation Rate mm/hr 

In-Flight ZL 0.03 0.07 0.26 0.60 1.13 

Gauge ZL 0.26 1.0 2.0 3.2 4.7 

 

In-Flight ZR 0.50 1.1 2.1 3.2 4.5 

Gauge ZR 0.67 1.7 3.3 4.3 6.0 

Table G-1 - Comparison of In-flight versus Ground Observations 

Note: Shows the 50, 75, 95, 99, 99.9% precipitation rate values of freezing 

drizzle and freezing rain as determined from the in-flight data of Appendix X and 

observations at the ground using heated gauges and the reports of precipitation 

type by the surface observer. 

MAJORITY RESPONSE

(Airbus, Boeing, CAA/UK, Cessna, Embraer, FAA/FAA Tech Center, NASA, 

SAAB, Transport Canada/TDC) 

The majority of the IPHWG support the concept of certification to a portion of 

Appendix X. Discussion of the motivation for retaining the ability to certify to a 

portion is contained in the above IPHWG majority position relative to the ALPA 

position statement against certification to a portion. This position statement will 

focus on the concerns raised by the Minority Position relative to certifying to a 

portion of Appendix X and to the difficulty in differentiating between freezing 

drizzle and freezing rain at the terminal areas. 

Certification to a portion of Appendix X

The minority position states that during the development of Appendix X, 

certification to a portion was not considered. The majority of the IPHWG do not 

agree with this statement, as use of a portion was inherent in the concept from 

the early stages. A review of the concept proposal presented to and approved by 

TAEIG in February of 2002 contains early draft rule language and states: 

"Operating safely in Appendix (X) to an extent as determined by the applicant." 

As such, certification to a portion has been intended since concept approval. 

During much of the development of the draft SLD rule language and supporting 

documentation, some of the IPHWG focused on development of Appendix X as 

part of the Meteorology sub-working group and may have not been fully aware of 
IPHWG Task 2 WG Report - Appendix G 

Certification to a Portion of Appendix X 

12/19/2005 Page G-7 

the draft rule intent of allowing approval of ice protection provisions for a portion 

of Appendix X. 

The majority of the IPHWG acknowledges that freezing precipitation is a 

continuum of many drop sizes and liquid water contents. To characterize the 

environment, measurements of the freezing precipitation were stratified into four 

categories, based on the maximum drop size contained in the spectra of each 

measurement. Measurements containing maximum drop sizes of less than 500 

μm, but greater than the maximum drop sized commonly considered as a cloud 

drop size (100 μm), were defined as freezing drizzle. The maximum drop size of 

100 μm is consistent with the use of the Langmuir D distribution for the Appendix 

C stratiform cloud maximum average drop size. Measurements containing 

maximum drop sizes greater than 500 μm were defined as freezing rain. These 

definitions of freezing drizzle and freezing rain were selected to be consistent 

with accepted meteorological definitions of these recognized types of 

precipitation. The two categories were further divided to sub-categories having 

drop mean volume diameters greater than or less than 40 μm. The Appendix C 

stratiform cloud maximum mean effective drop size was used as a boundary for 

maximum average cloud drop size. The descriptors used for characterizing 

Appendix X therefore stratify the continuum of freezing precipitation into 

commonly accepted regimes of freezing drizzle and freezing rain. The database 

used for characterizing the freezing precipitation environment can be thoughtfully 

and carefully stratified using other definitions. Freezing drizzle and freezing rain 

are each characterized by two curves. By definition, each set of two curves 

completely describes the two different precipitation types, and each set may be 

used independently of the other set. 

Differentiating between freezing drizzle and freezing rain at the terminal areas

The Minority Position warns that ground-based diagnosis of freezing precipitation 

may be inaccurate for takeoff and landing of airplanes in a portion of Appendix X 

and is not compatible with measurements of freezing precipitation contained in 

the Appendix X data base. The proposed 14 CFR § 25.1420 addresses safe inflight airplane operations in Appendix X and does not address terminal area 

freezing precipitation diagnosis and reporting. Ground-based observations of 

freezing precipitation were not included in the Appendix X database. Information 

from ground-based observations were only used to justify the coldest 

temperatures recorded for freezing precipitation, and ground-based 

measurements of precipitation rates were used to ensure that the measured inflight precipitation rates were compatible. This comparison of the precipitation 

intensities provided confidence that airplane deicing and anti-icing practices did 

not permit dispatch of airplanes into freezing precipitation aloft rates greater than 

those addressed by Appendix X. Note that ground-based observations of 

precipitation rates (discussed in the minority position), especially at moderate 

and high intensities, may be influenced by other effects such as downdrafts, 
IPHWG Task 2 WG Report - Appendix G 

Certification to a Portion of Appendix X 

12/19/2005 Page G-8 

sample volumes, and mixed-phase precipitation. Therefore, the majority of the 

IPHWG has concerns about the significance of the ground and in flight 

comparison presented in the minority opinion. 

The need for accurate diagnosis of freezing precipitation in the terminal area 

currently exists for dispatching airplanes for takeoff in freezing precipitation. 

Reported freezing precipitation is used to restrict takeoff in freezing precipitation 

that exceed the capability of anti-icing fluids to protect the airplane from ice 

accumulation during ground operations and the takeoff run. Also, the reported 

freezing precipitation diagnosis is used to estimate the holdover protection time 

provided by deicing and anti-icing fluids. Service history shows that the means 

and procedures used for reporting the category (drizzle or rain) and intensity of 

freezing precipitation have been adequate for safe airplane takeoff operations 

following de-icing and anti-icing. 

The minority position states that the fall rate for freezing drizzle "should" not be 

greater than 1 mm/hr. However, the definition in the American Meteorological 

Society's "Glossary of Meteorology" states that the fall rate is "usually" not 

greater than 1 mm/hr. The minority position also states that "it is very difficult for 

a surface observer to identify freezing drizzle visually and distinguish it from 

freezing rain." However, an expert consulted by the majority states that impact 

behavior is generally accepted as a reliable indicator in distinguishing between 

drizzle drops and raindrops. 

The minority position relies heavily on a single study analyzing data from "several 

stations in Quebec." No other studies are cited, and no evidence is provided to 

show that a single limited study in one geographical area should be taken as 

representative. The study reports some drizzle rates substantially higher than 1 

mm/hr, which as noted is a value usually not exceeded, but not a strict upper 

bound, in the AMS definition. The minority then states the high rates are 

"probably" due to rain being "incorrectly classified" as drizzle. The use of the 

word "probably" acknowledges that other factors may be considered in 

interpreting the results. One such factor is wind. An expert consulted by the 

majority stated that wind can have a much more dramatic effect on fall rates for 

drizzle than for rain, due to the much smaller mass of drizzle drops. Thus, two 

drizzle events characterized by the same liquid water content might have 

substantially different fall rates. 

The majority acknowledges that misdiagnosis can occur in discriminating 

between freezing drizzle and rain, and supports improvements in practices and 

technology to improve reporting accuracy. However, it believes that the 

conservatism inherent in the certification practices will result in safe operations. 

The certification process for Appendix X has many cumulative conservative 

assumptions that support the overall level of safe operations. For example, in 

addition to the use of the 99 percent liquid water content (LWC) levels, the 
IPHWG Task 2 WG Report - Appendix G 

Certification to a Portion of Appendix X 

12/19/2005 Page G-9 

vertical extents are defined as 12,000 ft. The vertical extent is defined by the 

combined icing environment of the cloud and the precipitation levels below the 

cloud. This results in the use of 99 percent LWC over the full 12,000 ft. Previous 

IPHWG deliberations determined that the mean vertical extents are closer to 

3,500 ft and 2,900 ft for freezing drizzle and freezing rain, respectively. As such, 

the majority of actual encounters in the terminal area will not approach the 

12,000 ft limit defined in Appendix X, nor will they approach the 99 percent LWC 

values. Additional conservatism is applied in that every takeoff is planned based 

on engine-out performance; this has the effect of decreasing the aircraft’s vertical 

climb rates and increases the duration of the exposure to the Appendix X 

environment. These combined procedures for assessing the environmental and 

operational processes for certification provide a conservative evaluation. 

Aircraft certificated for a portion of Appendix X will have a means to determine 

exceedance conditions. The result of this situation would be that exceedance 

cues would be present and would trigger an exit from the restricted portion of the 

SLD environment. The results of an aircraft taking off into a freezing rain 

encounter that was misidentified as freezing drizzle would not be significantly 

different than those associated with an inadvertent encounter with freezing rain 

aloft. This type of operation would not result in an unsafe condition as the aircraft 

will have been evaluated with accretions simulating a detect and exit scenario 

from all icing conditions. The end result will likely cause an increase in the 

number of diversions, or other operational considerations, but will not result in an 

unsafe condition. Data has been provided regarding the economic impact of 

diversions for use in the rulemaking process (see “Recurring Costs” section of 

Appendix J of this Working Group Report). 

The majority of the IPHWG believes that current observing and reporting 

definitions, when properly applied, are sufficient to accurately distinguish 

between freezing drizzle and freezing rain, (the difference in impact behavior of 

drizzle drops and raindrops is itself a reliable indicator). Furthermore, the majority 

of the IPHWG also believe that the conservatism built into the certification 

process is sufficient to offset any safety concerns at this time. 
IPHWG Task 2 WG Report - Appendix H 

Development of Part 25 Appendix X & Part 33 Appendix D 

12/19/2005 Page H-1 

APPENDIX H - DEVELOPMENT OF 14 CFR PART 25 APPENDIX X 

& 14 CFR PART 33 APPENDIX D 
IPHWG Task 2 WG Report - Appendix H 

Development of Part 25 Appendix X & Part 33 Appendix D 

12/19/2005 Page H-2 

This appendix provides discussion of the development of the 14 CFR Part 25 

Appendix X and 14 CFR Part 33 Appendix D. Detailed information concerning 

the development of these appendices will be published as FAA Technical Center 

reports. 

14 CFR PART 25, APPENDIX X

Appendix X was derived from a database containing in-situ measurements of 

supercooled large drop (SLD) icing conditions by suitably instrumented research 

aircraft. Data from many research campaigns, in various geographic locations 

and over several years, were used. All data was screened for quality and 

combined to form a database maintained at the FAA’s Wm. J. Hughes Technical 

Center. This database was used to determine the spatial aspects of SLD icing 

conditions, such as altitude and temperature ranges. Concerns about significant 

amounts ice crystals in traditionally measured liquid water content (LWC) data, 

and the desire to more precisely define SLD drop spectra and LWC values, 

resulted in a subset of more precise, multiple sensor data from recent 

Meteorological Service of Canada (MSC) and the National Aeronautics and 

Space Administration (NASA) research flights being used for these purposes. 

This insured both the widest applicability and the greatest confidence in the 

critical details for the Appendix X environment. See following references for 

more information. 

GENERAL

The relationship between Appendix C and Appendix X icing conditions is limited. 

The data measured to produce Appendix X was taken in wintertime stratus cloud 

formations. As such, the Appendix X conditions are most closely associated with 

the stratus conditions used to define the existing Appendix C, Continuous 

Maximum conditions. Nearly all of the data contained in the IPHWG SLD icing 

conditions database are related to wintertime, stratiform cloud conditions. It is 

known, however, that supercooled large drop icing conditions are generally 

plentiful in warm season, vigorously growing convective clouds (Cumulus 

Congestus, Towering Cumulus, Cumulonimbus) above the freezing level. 

Changnon, et al. (1991), report that for large, growing convective clouds in 

Illinois: “Typical in cloud results at –10 °C reveal multiple updrafts that tend to be 

filled with large amounts of supercooled drizzle and raindrops.” This means that 

airplanes penetrating the cores of these clouds in the 0° C to –20° C temperature 

range can expect to encounter intense bursts of SLD icing. The horizontal extent 

of individual cores is on the order of Appendix C Intermittent Maximum icing 

conditions and, as such, the characterization of these cloud penetrations would 

be an extension of this Appendix C icing condition. 
IPHWG Task 2 WG Report - Appendix H 

Development of Part 25 Appendix X & Part 33 Appendix D 

12/19/2005 Page H-3 

Use of only stratiform cloud data for the airframe aspects is considered 

acceptable since stratiform clouds predominate during prolonged freezing 

precipitation conditions, and these conditions are seen as the primary source of 

reported SLD-related accidents and incidents. In addition, some practical factors 

that mitigate the need for a characterization of summer SLD icing conditions are: 

• The relative ease of visually recognizing or using on-board weather radar 

to avoid strong convective cloud activity 

• Operating procedures that limit exposure to such conditions, as well as the 

relative brevity of inadvertent exposures (at least in Cumulus Congestus 

and Towering Cumulus clouds). 

The in situ data obtained for determining Appendix X were divided as follows: 

• Measured icing conditions with an MVD less than 40 μm and a maximum 

drop size of less than 100 μm were excluded from the Appendix X 

definition (categorized as Appendix C icing conditions). 

• Measured icing conditions with an MVD less than 40 μm and a maximum 

drop size between 100 μm and 500 μm were classified as freezing drizzle, 

MVD < 40 μm. 

• Measured icing conditions with an MVD greater than 40 μm and a 

maximum drop size between 100 μm and 500 μm were classified as 

freezing drizzle, MVD > 40 μm. 

• Measured icing conditions with an MVD less than 40 μm and a maximum 

drop size greater than 500 μm were classified as freezing rain, MVD < 40 

μm. 

• Measured icing conditions with an MVD greater than 40 μm and a 

maximum drop size greater than 500 μm were classified as freezing rain, 

MVC > 40 μm. 

SLD icing conditions are defined by Appendix X, including freezing drizzle and 

freezing rain. The two types of freezing precipitation are further divided into 

conditions in which the drop median volume diameters are either less than or 

greater than the 40 μm, the largest mean effective diameter of Appendix C 

continuous maximum icing conditions. This format follows recommendations by 

A. D. Shah, M. W. Patnoe, and E. L. Berg, SAE 2000-01-2115, "Engineering 

Analysis of the Atmospheric Icing Environment Including Large Droplet Icing 

Conditions." Dividing the freezing drizzle and freezing rain into these two 

categories approximates freezing precipitation icing conditions where most of the 

supercooled water mass is either in or outside of an Appendix C stratiform cloud. 
IPHWG Task 2 WG Report - Appendix H 

Development of Part 25 Appendix X & Part 33 Appendix D 

12/19/2005 Page H-4 

Appendix X consists of measured data that was divided into drop distributions 

within these four icing conditions. These conditions were averaged to produce 

representative distributions for each condition. 

The water content versus drop size relationships defined in 14 CFR 25, Appendix 

C, Figures 1 and 4, are defined in terms of mean effective drop diameter. By 

definition, this acknowledges the distributed nature of drop sizes around a mean 

or median value. However, Part 25 does not require specific distributions that 

must be considered for Appendix C icing conditions. As referenced in 

DOT/FAA/CT-88/8-1 “Aircraft Icing Handbook” and AC 20-73A, commonly 

assumed single-mode distributions such as Langmuir & Blodgett have been used 

to represent the range of drop sizes and associated water contents in a specific 

Appendix C icing condition. 

The distributions assumed for Appendix C icing conditions are inappropriate for 

freezing precipitation icing conditions since SLD icing conditions typically consist 

of significant portions of small and large drops and a bi-modal distribution of drop 

sizes. Use of the Langmuir & Blodgett type of distributions for determining the 

drop size intervals do not adequately define the bi-modal nature of freezing 

precipitation. Consequently, the distributions of drop sizes are defined as part of 

Appendix X. The need to include the distributions comes from the larger amount 

of mass in the larger drop diameters of Appendix X. The water mass of the 

larger drops will impinge further aft on surface leading edges than that of 

Appendix C. Ice accretion resulting from this water catch must be considered 

relative to its effects on safe flight of the airplane. 

The variation among freezing precipitation drop diameter distributions is very 

large. Subdividing freezing drizzle and freezing rain allowed selection of the four 

average drop diameter distributions that are representative of the total range of 

drop distributions. Appendix X drop diameter distributions are shown as 

cumulative mass distributions. 

Also, the maximum values of freezing drizzle and freezing rain liquid water 

content decrease with decreasing temperature as defined in 14 CFR 25, 

Appendix X, figures 1 and 4, respectively. Following the same standard as 14 

CFR 25, Appendix C, the values of total liquid water are selected at the standard 

icing condition horizontal extent of 17.4 nautical miles (20 statute miles). 

The maximum vertical extent of 12,000 feet for freezing drizzle includes the cloud 

layer and any drizzle precipitation below the cloud. The maximum altitude of 

freezing drizzle is the same as that of an Appendix C continuous maximum icing 

cloud, 22,000 feet (Appendix X, figure 3). Freezing drizzle has been reported at 

temperatures as cold as -13° F (-25°C). 
IPHWG Task 2 WG Report - Appendix H 

Development of Part 25 Appendix X & Part 33 Appendix D 

12/19/2005 Page H-5 

The maximum vertical extent of freezing rain is 7,000 feet. The maximum 

altitude of freezing rain is 12,000 ft. Freezing rain has been reported at 

temperatures as cold as 8.6° F (-13°C). 

The liquid water contents of continuous freezing drizzle and freezing rain 

conditions decrease with increasing horizontal extents. Horizontal extents of 

exposures in FZDZ and FZRA can be anywhere from a brief encounter to an 

extended encounter. The use of the Appendix X f-factor is the same as for the 

Appendix C f-factor, which is discussed in Appendix N of AC 20-73A. 

TOTAL WATER CONTENT VERSUS LIQUID WATER CONTENT

For approximately 40 to 50 percent of the in-situ measurements in icing 

conditions between -5° C and –30° C, some ice crystals exist in the cloud and the 

cloud may be considered as mixed phase. The fraction of all liquid cloud in-situ 

measurements increases as the temperature approaches 0° C, and the fraction 

of glaciated or all-ice clouds increases as the temperature approaches –30° C. 

However, the portion of mixed-phase cloud measurements remains near 40 to 50 

percent. These mixed-phase clouds tend to cluster near liquid fractions 1 or 0. 

The total water content (TWC) consists of both the liquid water content (LWC) 

and ice water content portions of the cloud. However, removing the glaciated 

clouds, and just considering the all-liquid and all-mixed-phase clouds, the 

probability distributions of both TWC and LWC as a function of temperature are 

very similar. The 90 to 95 percent values are almost the same. Since Appendix 

X includes extreme water content values, it does not appear that there would be 

a significant difference between using TWC or LWC. 

GEOGRAPHICAL RESTRICTIONS

The current archive used to produce representative spectra and extremes for 

LWC for Appendix X consists of data collected at a few locations in North 

America, such as the Great Lakes area, Canadian east coast, and the Canadian 

Arctic. The larger database maintained by the FAA was used to produce the 

extremes for altitude limits, and temperature ranges for Appendix X include 

measurements from more locations (e.g., Colorado, Alaska, California, and 

Kansas), with a limited amount of data outside of North America. Observed data 

provided by NCAR (National Center for Atmospheric Research) was also 

considered in the development of the temperature ranges used in Appendix X. 

There are known differences between geographical areas. For example, 

maritime areas have larger drops and lower drop concentrations. The frequency 

of occurrence of SLD conditions also varies by location according to recent 

climatology studies, and this has been confirmed by aircraft measurement 

campaigns. There is evidence that a high frequency of occurrence of SLD 

occurs in the southern tip of South America and in Europe. However, there are 

very few in-situ measurements from these areas. The micro-physics of large 
IPHWG Task 2 WG Report - Appendix H 

Development of Part 25 Appendix X & Part 33 Appendix D 

12/19/2005 Page H-6 

droplet formation are the same and it is not expected that the drop diameter 

distributions would be greatly affected. Therefore, the existing data is considered 

sufficient for rulemaking. 

PROBABILITY OF OCCURRENCE

The MSC in-situ icing database shows that the SLD icing conditions with MVD 

greater than 40 μm represent an average of 6 percent of the in-flight time. In 

maritime areas, this percentage rises to 20 percent of in-flight time. These 

percentages cannot be used to represent the probability of occurrence of SLD 

icing conditions because these flights were directed into areas with icing that 

were expected to contain SLD. Consequently, the true probability of occurrence 

of SLD icing conditions would be lower. For example, the frequency of 

occurrence of freezing precipitation at the ground represents approximately 1 

percent of the time averaged over the winter season for most of Canada, with 

greater values occurring in the Great Lakes area (2 percent) and in 

Newfoundland (5 percent). 

Climatology studies performed by the National Center for Atmospheric Research 

(NCAR) for North America show that the annual average frequency of 

occurrence of SLD ranges from 0.4 to 5 percent of the time in a spatial column 

over a measuring station. This study indicates significant variations in probability 

of SLD icing conditions with geographic location and season (e.g., in northern 

Canada or Alaska, SLD icing conditions are most prevalent during the summer 

season). In the December to March time period, in the Great Lakes area where 

most of the Canadian data were obtained, the NCAR frequency of occurrence 

was approximately 3 percent in a spatial column over each surface measuring 

site. For sites on the western coast of Alaska for the same December to March 

time frame, the percentage rises to 6 to 8 percent. 

Determining the actual occurrence of SLD icing conditions in the atmosphere is 

difficult using the data sets available. It is recognized that there are large 

geographical differences and changes with season. However, to a first 

approximation, the probability of occurrence of SLD icing conditions for any 

particular location in North America, representing the altitude ranges between 0 

and 15,000 feet which airplanes normally encounter upon takeoff and landing, is 

typically 1 to 5 percent over a winter season for a large part of the continent. 

Currently, airport observers cannot accurately identify the differences between 

freezing drizzle and freezing rain. The precipitation conditions at the ground are 

primarily out of cloud. Appendix X is an engineering design standard that 

considers freezing drizzle and freezing rain aloft, and the four spectra selected 

contain cloud droplets. This can lead to differences between dispatch standards 

and conditions tested during the certification process. However, the working 

group considers this difference to be acceptable for an engineering design 
IPHWG Task 2 WG Report - Appendix H 

Development of Part 25 Appendix X & Part 33 Appendix D 

12/19/2005 Page H-7 

standard due to the limited exposure during transition between ground freezing 

precipitation environment and aloft. 

APPENDIX X REFERENCES

1. Ashenden, R. and Marwitz, J. D. (1996); "Supercooled Large Droplet 

Distribution in the Natural Environment and Comparison to Artificial Drizzle from 

the Air Force Water Spray Tanker", Proc. of the FAA International Conf. On 

Aircraft Inflight Icing, DOT/FAA/AR-96/81, II, p. 33-43, dated August 1996. 

2. Cober, S. G. et al (1995); " Aircraft Icing Measurements in East Coast Winter 

Storms," Journal of Applied Meteorology, Vol. 34, pages 88-100, dated January 

1995. 

3. Cober, S.G., G.A. Isaac, A.V. Korolev, and J.W. Strapp, 2001: Assessing 

cloud-phase conditions. J. Appl. Meteor., 40, 1967-1983. 

4. Cober, S. G. et al (2001); "Characterizations of Aircraft Icing Environments that 

Include Supercooled Large Drops," Journal of Applied Meteorology, Vol. 40, 

pages 1984-2002, dated November 2001. 

5. Cober, S. G. et al.(2003), "Defining Characteristic Cloud Drop Spectra From 

In-Situ Measurements," AIAA-2003-0561, 41st Aerospace Sciences Meeting & 

Exhibit, 6-10 January 2003, Reno, NV 

6. Cober, S.G., T.P. Ratvasky, and G.A. Isaac, 2002: Assessment of aircraft icing 

conditions observed during AIRS. AIAA 40th Aerospace Sci. Meeting and Exhibit, 

Reno Nevada, 14-17 January 2002, AIAA 2002-0674. 

7. Cober, S.G., and G.A. Isaac, 2002: Aircraft environments observed in mixedphase clouds. AIAA 40th Aerospace Sci. Meeting and Exhibit, Reno Nevada, 14-

17 January 2002, AIAA 2002-0675. 

8. Isaac, G. A. et al (1999); “Canadian Freezing Drizzle Experiment”, AIAA1999—0492, dated January 11-14, 1999. 

9. Isaac, G.A., S.G. Cober, J.W. Strapp, A.V. Korolev, A. Tremblay, and D.L. 

Marcotte, 2001: Recent Canadian research on aircraft in-flight icing. Canadian 

Aeronautics and Space Journal, 47, 213-221. 

10. Isaac, G.A., S.G. Cober, J.W. Strapp, D. Hudak, T.P. Ratvasky, D.L. 

Marcotte, and F. Fabry 2001: Preliminary results from the Alliance Icing 

Research Study (AIRS). AIAA 39th Aerospace Sci. Meeting and Exhibit, Reno 

Nevada, 8-11 January 2001, AIAA 2001-0393. 
IPHWG Task 2 WG Report - Appendix H 

Development of Part 25 Appendix X & Part 33 Appendix D 

12/19/2005 Page H-8 

11. Jeck, R. K. (1996); “Representative Values of Icing-Related Variables Aloft in 

Freezing Rain and Freezing Drizzle" Technical note DOT/FAA/AR-TN95/119, 44 

pp., dated March 1996. 

12. Jeck, R. K. editor; "Data and Analyses Contributing to the Development of 

Engineering Specifications for Freezing Rain and Freezing Drizzle Conditions for 

Aviation Purposes," FAA Technical Report DOT/FAA/AR-04/7,To be released. 

13. Shah, A. D. et al (1998); "Droplet Size Distribution and Ice Shapes", AIAA 

98- 0487, Reno, NV. dated January 12-15,1998. 

14. Shah, A. D. et al (2000); "Engineering Analysis of the Atmospheric Icing 

Environment Including Large Droplet Icing Conditions", SAE 2000-01-2115, 

dated April 2000. 

14 CFR PART 33, APPENDIX D

A data base of commercial service icing events (see Appendix I of this report) in 

the mixed phase / glaciated environment established that the current FAR Part 

25 Appendix C icing envelope with supercooled liquid water droplets does not 

cover the range of altitude and ambient temperature established by these events. 

The event conditions are predominately outside FAR Part 25 Appendix C, but 

appear to be bounded by Mil Std 210A Hot Day temperature limits. Hence, the 

range of the new FAR Part 33 Appendix D was increased beyond FAR Part 25 

Appendix C envelope to the Mil Std 210A temperature limits. A common 

characteristic in most of these events was the proximity to convective / tropical 

storm cloud formations, and thus Appendix D is being proposed primarily as a 

deep convective cloud extension to the current 14 CFR 25, Appendix C 

intermittent maximum conditions. 

To develop Appendix D, a literature search was perfomed which revealed limited 

data for deep convective atmospheres. For example, a series of airborne 

measurements of cloud total water content (TWC) were conducted between 

December 1956 and March 1958 in three tropical locations, resulting in an 

extensive set of 101 cloud measurement flights with 44.5 hours of in cloud data 

(McNaughtan data). Most of the more recent data on the properties of mixed 

phase clouds has been collected by the atmospheric research community, but 

these studies were not focused on measuring maximum total water content areas 

of deep convective storms as in the case of the McNaughtan data. 

The accuracy of the TWC measurements in all of the above studies is 

questionable and the traceability and accuracy of the data methods is limited. 
IPHWG Task 2 WG Report - Appendix H 

Development of Part 25 Appendix X & Part 33 Appendix D 

12/19/2005 Page H-9 

As such, an engineering design standard is being proposed that combines facets 

of the theoretical and experimental data. A theoretical adiabatic lifting of air was 

used to establish an upper bound on water contents and scaled to the 99% of the 

McNaughtan data. The events were examined relative to temperature and 

altitude and were consistent with the Mil-Std hot day boundary. As such, the MilStd was used as limiting temperature/altitude condition. 

The general slope of the TWC reduction with distance scale was derived from the 

McNaughtan data set. A standard distance scale of 17.4 nmi was chosen as a 

normalization point. This distance scale also roughly represents a typical onset 

time for an engine event after the recognition of a total air temperature (TAT) 

anomaly, an indicator of the presence of high TWC. The short distance limit of 

4.6 nm is the lowest that could be deduced from the McNaughtan data set. It 

was the opinion of the EHWG that this was adequate given the information in the 

event database and current technical knowledge of onset time for the engine 

problem. 

Given the limited availability of data, the above engineering design standard 

approach is recommended to reduce engine events due to the ice crystal 

environment. EHWG has recommended that a new effort be conducted to collect 

a data base of tropical convergence zone cloud measurements using modern 

instrumentation with accurate TWC measurement capability. 

14 CFR PART 33, APPENDIX D REFERENCES

1. THE ANALYSIS OF MEASUREMENTS OF FREE ICE AND ICE/WATER 

CONCENTRATIONS IN THE ATMOSPHERE OF THE EQUATORIAL 

ZONE, Ian I. McNaughton, B.Sc., Dip. R.T.C., ROYAL AIRCRAFT 

ESTABLISHMENT (Farnborough) TECHNICAL NOTE No: MECH. ENG. 

283, 1959 

2. SNOW AND ICE PARTICLE SIZES AND MASS CONCENTRATIONS AT 

ALTITUDES UP TO 9 KM ( 30,000 FT), R. K. Jeck, DOT/FAA/AR-97/66, 

August, 1998. 

3. CLOUD MICROPHYSICAL MEASUREMENTS IN THUNDERSTORM 

OUTFLOW REGIONS DURING ALLIED/BAE 1997 FLIGHT TRIALS, 

Strapp, J.W., P. Chow, M. Maltby, A.D. Bezer, A. Korolev, I. Stomberg, 

and J. Hallett, 37th AIAA Aerospace Sciences Meeting and Exhibit, Jan. 

11-14, 1999, Reno, NV. AIAA 99-0498. 
IPHWG Task 2 WG Report - Appendix I 

Accidents and Incidents Relevant to the Proposed Rule 

12/19/2005 Page I-1 

APPENDIX I - ACCIDENTS AND INCIDENTS RELEVANT TO THE 

PROPOSED RULE 
IPHWG Task 2 WG Report - Appendix I 

Accidents and Incidents Relevant to the Proposed Rule 

12/19/2005 Page I-2 

The IPHWG examined incidents and accidents to identify those which may be 

considered in the benefit analysis of the regulatory evaluation. It should be noted 

that some of these accidents and incidents have been identified as being 

relevant to the regulatory evaluations of other ARAC recommendations, such as 

the Part 121 Operations in Icing, Part 25 Activation of Ice Protection, and Part 25 

Performance and Handling in Icing Conditions. It will be necessary for the FAA 

economist to ensure that the benefits associated with an accident or incident are 

employed in accordance with acceptable practices. 

Incidents and accidents of Part 23 airplanes that have 14 CFR §25.1419 in their 

certification basis have been included in the events considered as relevant. It is 

the IPHWG’s understanding that the FAA economist will use events of these 

airplanes in the regulatory evaluation. However, not all of the working group 

concurs with this policy. Some contend that these events are not relevant 

because it is not believed that future Part 23 airplanes will have a Part 25 icing 

certification basis. 

ICING INCIDENT DATABASE REVIEW

The icing incident database for Task 2 included 81 events. These 81 events 

were obtained following a review of the NASA Aircraft Safety Reporting System 

(ASRS) database for the time frame 1988 to 2002, five reports of incidents 

extracted from the NTSB 3/31/94 Simmons 4184 accident report, one report of 

an incident that was extracted from the NTSB 1/9/97 Comair 3272 accident 

report, and incidents from foreign authorities. 

Descriptors such as severe ice, freezing rain, freezing drizzle, side window icing, 

heavy icing, large droplets, SLD, and, for mixed ice conditions, descriptors of 

sleet, snow, snow grains, and ice were employed in the selection of these 

events. 

In general, the review encompassed both 14 CFR part 23 and 14 CFR part 25 

aircraft. However, the preponderance of events pertained to small aircraft (14 

CFR part 23). Since the result of Task 2 is to be a Part 25 rule, the group chose 

to eliminate all part 23 aircraft events as not being applicable to Task 2 (except 

for those with a 14 CFR §25.1419 certification basis). Several other events were 

eliminated due to insufficient information. With these eliminations, the number of 

applicable events was reduced from 81 to 28. This includes 6 events associated 

with turbojet aircraft, 18 events associated with turbo-propeller aircraft, and one 

event in which the type of power plant could not be determined. Of these, there 

were 4 references to mixed conditions (2 each for turbo-propeller aircraft and 

turbojet aircraft), and six events which occurred outside the U.S. 

 
IPHWG Task 2 WG Report - Appendix I 

Accidents and Incidents Relevant to the Proposed Rule 

12/19/2005 Page I-3 

A review of incidents related to 14 CFR Part 23 aircraft with a 14 CFR § 

25.1419 certification basis was conducted. This review yielded 3 incidents 

involving Part 23 turbo-propeller aircraft and they are included in Table I-1 

below. Thus, the final list of 28 incidents potentially applicable to Task 2 is: 

Aircraft Type Number of incidents 

Total U.S. Other

Turbo-propeller 18 14 4 

Turbo-jet 6 4 2 

Un-determined 1 1 0 

Part 23 aircraft/w Part 

25.1419 Cert. basis 

3 3 0 

Table I-1 - Applicable Aircraft Icing Incidents 

ICING ACCIDENT DATABASE REVIEW

The IPHWG developed an icing accident database in support of Task 2 that 

included 182 events. These 182 events were obtained after a review of several 

databases, including the National Transportation Safety Board, Flight Safety 

Foundation (FSF), Transport Canada, Eurice, and other sources. The accident 

database covers the time frame from 1940 to 2002. 

Descriptors such as severe ice, freezing rain, freezing drizzle, side window icing, 

heavy icing, large droplets, SLD, and for mixed-ice conditions, descriptors of 

sleet, snow, snow grains, and ice were employed in the selection of events for 

the database. There were limited references to “SLD” since most of these 

accidents occurred prior to the 10/31/94 Roselawn accident and SLD is a term 

that was coined after that accident. In general, the review encompassed many 

aircraft of different categories (including two helicopters). However, the 

preponderance of events pertained to small aircraft (14 CFR part 23). 

Since the result of Task 2 is to be a Part 25 rule, the group chose to eliminate all 

helicopter and part 23 aircraft accidents as not being applicable to Task 2 

(except for the part 23 aircraft with a 14 CFR §25.1419 certification basis). 

Several other events were eliminated due to duplication (4), improper ground 

deicing (4), and insufficient information (2). With these eliminations, the number 

of applicable events was reduced from 182 to 42. A further division of these 

accidents was made for old pre-Appendix C icing certified aircraft and 

post-Appendix C icing certified aircraft. There were 34 events falling into the old 

pre-Appendix C category; 18 were deemed to be potentially applicable for Task 

2. The Bristol Britannia 253 accident on 2/16/80 was included even though this 

aircraft had not been properly deiced prior to takeoff. For this particular event the 

NTSB listed the probable cause of this accident as resulting from an 

accumulation of ice and snow on the airframe before takeoff and a further 
IPHWG Task 2 WG Report - Appendix I 

Accidents and Incidents Relevant to the Proposed Rule 

12/19/2005 Page I-4 

accumulation of ice when the aircraft was flown into moderate to severe icing 

conditions following takeoff (see Table I-2 below). 

DATE AC TYPE LOCATION FATAL INJURY 

12/21/47 DC-3A N. Platte, NE 0 3 Minor 

10/4/57 DC-3 Alberta, Canada 0 2 Serious 

1/29/63 VISC-810 Kansas City, MO 8 8 Fatal 

12/2/78 DC-3 Des Moines, IA 2 2 Fatal 

1/16/81 DC-6A Gambell, AK 0 3 Minor 

12/4/40 DC-3A Chicago, IL 10 3 Serious 

10/30/41 DC-3A Mooreshead,MN 14 1 Serious 

4/11/42 DC-3A LaGuardia, NY 0 2 Serious 

10/9/49 C-46E Cheyenne, WY 3 3 Fatal 

1/7/53 C-46F Fishaven, ID 40 40 Fatal 

3/20/53 DC-4 Alvarado, CA 35 35 Fatal 

1/20/54 DC-3A Kansas City, MO 3 3 Fatal 

2/26/54 CV-240 Wight, WY 9 9 Fatal 

4/6/58 VISC –700 Freelance, MI 47 47 Fatal 

3/10/64 DC-4 Boston, MA 3 3 Fatal 

3/12/64 DC-3 Miles City, MT 5 5 Fatal 

12/27/78 DC-3 Des Moines, IA 0 2 Serious 

2/16/80 BRIT-253 Billerica, MA 7 1 Serious 

Table I-2 - Applicable Accidents Of Aircraft With Pre-Appendix –C 

Icing Certification Criteria 

Of the eight accidents involving aircraft certificated to Appendix C criteria, six, 

including two outside of the U.S., were deemed as being potentially applicable for 

Task 2 (see Table I-3 below). 
IPHWG Task 2 WG Report - Appendix I 

Accidents and Incidents Relevant to the Proposed Rule 

12/19/2005 Page I-5 

DATE AIRCRAFT 

TYPE 

LOCATION FATAL INJURY HULL 

LOSS 

10/31/94 ATR-72 Roselawn, 

IN 

68 0 Yes 

3/28/89 DC-8-73 Edmonton, 

Canada 

0 0 No 

4/29/93 EMB-120 Pine Bluff, 

AR 

0 13 

minor 

Yes 

1/9/97 EMB-120 Monroe, MI 29 0 Yes 

3/19/01 EMB-120 Orlando, FL 0 0 Severe 

Damage 

12/21/02 ATR-72 Pengu 

Island, 

Taiwan 

2 0 Yes 

Table I-3 - Applicable Aircraft Accidents For Aircraft With 

Appendix C Certification Criteria 

MINORITY POSITION ON APPENDIX I OF THE IPHWG WORKING GROUP REPORT 

(ACCIDENTS AND INCIDENTS RELEVANT TO THE PROPOSED RULE)

(Embraer, with support of the Regional Airline Association) 

The airworthiness rules identify icing conditions (14 CFR 25, Appendix C) upon 

which approval of airplane operations in icing conditions is based. Appendix C 

conditions do not include supercooled large drops conditions (includes freezing 

drizzle and freezing rain). The operating rules do not prohibit operations in 

supercooled large drop conditions. The accident history indicates that flight 

crews of certain types of aircraft have lost control of their aircraft in such 

conditions. 

The proposed 25.1420 text changes the icing environment used to evaluate safe 

operation of the airplane in icing conditions. The IPHWG Working Group report 

identifies as “the reason for the change the accident history of some aircraft has 

shown that the current icing environment requirements are inadequate”. Embraer 

agrees with this statement; however, the purpose of this document is to clarify 

that the events related to the EMB-120 aircraft, based on the acceptance criteria 

used on the accidents screening, should not be considered in the IPHWG benefit 

analysis. It must be clear that the reasons for the requirement improvement are 

not associated with the EMB-120 aircraft model. 

The IPHWG examined incidents and accidents to identify those which may be 

considered in the benefit analysis of the regulatory evaluation. Descriptors such 

as severe ice, freezing rain, freezing drizzle, side window icing, heavy icing, large 

droplets, SLD, and for mixed ice conditions descriptors of sleet, snow, snow 

grains and ice were employed in the selection of the events of the database. 
IPHWG Task 2 WG Report - Appendix I 

Accidents and Incidents Relevant to the Proposed Rule 

12/19/2005 Page I-6 

The current IPHWG proposal for “Accidents and Incidents Relevant to the 

Proposed Rule” includes three events with the EMB-120 aircraft (164, 171 and 

179), which are presented in Table I-3 above: 

Some reasons were also taken into account to consider the elimination of an 

event from the benefit analysis. They are: 

- Inoperative IPS 

- IPS not activated 

- Improper ground deicing 

- Improper pilot actions 

All three events with the EMB-120 aircraft considered by the IPHWG as 

applicable to this benefit analysis should be removed from the Working Group 

report, as either the IPS was not activated, there was improper pilot action, or 

both. This is very clear in the NTSB report associated to each event. Below is a 

summary of each of the events that support our request: 

• Pine Bluff

In the Pine Bluff case, as stated in the NTSB report, the probable cause of the 

accident was: “... [T]he probable causes of this accident were the captain’s 

failure to maintain professional cockpit discipline, his consequent inattention to 

flight instruments and ice accretion, and his selection of an improper autoflight 

vertical mode” . 

The report also states that there was no evidence that the crew had turned the 

IPS ON at any time prior to the loss of control. This is based on the performance 

analysis performed by Embraer, during the accident investigation, that 

demonstrated that the loss of performance size wise and time wise was not 

compatible with all data available for the EMB-120 with the de-ice system 

activated. 

• Monroe

In the Monroe accident, according to the performance analysis performed by 

Embraer and fully accepted by the NTSB, it was demonstrated that the aircraft 

boots were most probably not activated prior to the loss of control. FDR data 

shows that the crew let the airspeed decrease to around 138 KIAS that is much 

lower than the minimum of 160 KIAS, recommended in the AFM for icing 

conditions. The NTSB probable causes for this accident are all related with 

dissemination of information between the aircraft manufacturer, certification 

authorities and aircraft operators and are not related to the specific icing 

conditions. 

In this accident also, the performance analysis performed by Embraer, during the 

accident investigation, demonstrated that the loss of performance size wise and 

time wise was not compatible with all data available for the EMB-120 with the deice system activated. 
IPHWG Task 2 WG Report - Appendix I 

Accidents and Incidents Relevant to the Proposed Rule 

12/19/2005 Page I-7 

• Orlando 

In respect to the Comair 5054 (Palm Beach) accident, the NTSB report 

concluded that the probable cause of the accident was: “The failure of the flight 

crew to maintain airspeed during an encounter with severe icing conditions, 

which resulted in an inadvertent stall, loss of control, and structural damage to 

the airplane”. Although there are indications that the crew had activated the deicing boots, the “light” mode was probably selected, was not compatible with the 

severity of the icing encounter. 

A series of improper pilot actions were verified during this accident: 

o Pilots ignored evidences of severe icing and did not attempt to leave 

the icing cloud; 

o Auto pilot was not disengaged as specified in the AFM; 

o Pilots did not pay attention to aircraft speed and let it reach values 

much lower than the 160 KIAS threshold specified in the AFM for 

operation in icing conditions 

o According to Embraer analysis of the accident that shows that there 

was a linear increase in performance degradation for approximately 3 

minutes, it is apparent that the de-icing system was activated in “light” 

mode (three minutes cycle), conflicting with instructions presented in 

the AFM to select “heavy” (one minute cycle) mode in severe icing 

conditions. 

In none of the incidents/accidents above it was demonstrated that the icing 

conditions prior to the upsets were outside the FAA Appendix C envelope. The 

possibility of SLD was discussed and not discarded, but, according to the 

performance analyses, all events were possible with a severe ice encounter 

inside the Appendix C conditions. 

In addition to that, a series of improvements mandated by FAA/CTA have already 

been incorporated to the plane, as follows: 

o Modification in the Limitations Section of the AFM, describing the 

visual cues of severe ice, instructing to immediate leave this ice 

condition and prohibiting the use of the auto-pilot; 

o Removal of the de-icing system “light” mode (now only the “heavy” 

mode is available); 

o Installation of an Ice Detection System; 

o Installation of a Low Speed Alarm, which activates whenever ice 

detection system detects ice or ice protection system is activated and 

the airspeed with flaps up decreases below the recommended 

minimum of 160 KIAS. 

These improvements by themselves, if available before events involving the 

listed EMB-120 happened, would have very likely prevented the events involving 

the EMB-120 aircraft from occurring. 
IPHWG Task 2 WG Report - Appendix I 

Accidents and Incidents Relevant to the Proposed Rule 

12/19/2005 Page I-8 

Because the contribution of SLD (if any) to the accidents listed can never be 

established with absolute certainty, Embraer believes that the only way FAA can 

construct a defendable benefit estimate is to factor the total benefit from 

accidents prevented by the percentage contributed by each corrective action that 

has been or will be implemented. This would allocate the proper credit to each 

action. Since the FDR data from each accident clearly shows that each airplane 

was operated for a significant period of time at airspeeds below the 160 KIAS 

threshold of the low speed alarm, it is clear that the vast amount of benefit would 

be accrued solely through that system. This would leave only accidents in SLD 

conditions so severe that the 160 KIAS threshold would be insufficient to prevent 

stall and loss of control as an accident that would be credibly prevented by the 

proposed regulation. There is nothing indicating that the events involving EMB120 would fall in this category, what reinforces Embraer position that the EMB120 events shall be removed from the list of events relevant to the proposed SLD 

rule. 

As a conclusion, Embraer would like to request the IPHWG to remove the EMB120 events from the Working Group Report. 

REGIONAL AIRLINE ASSOCIATION STATEMENT OF SUPPORT OF EMBRAER MINORITY 

POSITION 

RAA concurs with the Embraer position that with all three of the Emb-120 

accidents/incidents, there is nothing to indicate that the icing conditions were 

outside the current Appendix C envelope and that the Emb-120 events should 

not be cited as justification for the proposed new rule. 

MAJORITY RESPONSE

(ALPA, Boeing, CAA/UK, FAA/FAA Tech Center, MSC, NASA, SAAB, Transport 

Canada/TDC) 

Considering the descriptions of the accidents in the NTSB reports, the three 

EMB-120 events included in Table 5 were more closely reviewed. 

Determining the actual icing conditions after an accident is very difficult. In all 3 

events, the NTSB reports did not quantify the icing environments as inside or 

outside 14 CFR Appendix C envelopes, however, the icing event was believed to 

have affected the performance and handling of the airplane and the effective 

operation of the ice protection system. In all three events, NTSB meteorological 

data showed that conditions in the accident area were conducive to SLD 

conditions. The screening criteria used was “potential SLD conditions”, since the 

actual icing conditions for each event were not measured. 
IPHWG Task 2 WG Report - Appendix I 

Accidents and Incidents Relevant to the Proposed Rule 

12/19/2005 Page I-9 

Although there was no evidence in the Pine Bluff accident that the flight crew had 

activated the deicing boots, there was no evidence that they failed to activate the 

deicing boots. In the Orlando, FL event, the flightcrew did activate the “light” 

mode of the deicing boots, but the AFM did not specify the mode to be used for 

severe icing, and in fact stated that ice forming aft of the deicing boots may not 

be shed using the ice protection systems. The “light” mode was prohibited in the 

EMB-120 several months later as a result of the Orlando, FL accident. Although 

not activating de-ice boots in a high LWC Appendix C could cause the 

performance losses noted in the events, it is also possible that icing conditions 

outside of Appendix C could cause similar performance losses. It is also 

possible that operating boots designed to Appendix C conditions may not 

eliminate ice accretions aft of the boots in SLD conditions. 

The majority does not concur with the supporting statement that there was an 

"absolute" IPHWG criterion for exclusion from the list of accidents for benefit 

analysis. Failure to activate the IPS was identified as a "basis for possible 

exclusion" rather an absolute criterion. In cases where it was unclear whether 

the IPS was activated, the event was not excluded. 

Failure to adhere to the AFM operating procedures such as maintaining a 

minimum airspeed in icing conditions, or failure to be made aware of the latest 

recommended operating procedures were identified as contributing factors 

affecting the crew’s performance and ability to safely operate the airplane in 

these icing conditions. The above improvements (such as improvements to the 

ice detection systems and AFM procedures) are considered appropriate 

responses by applicants to Appendix C as well as Appendix X icing conditions. 

Failure to monitor airspeed in icing conditions is a systemic factor in icing related 

accidents of airplanes without an auto-throttle, not just in the EMB-120. The 

accident and history database show that a redesign for the SLD environment 

may include a stall warning system modification. 

The series of incorporated improvements listed by Embraer for the EMB-120 

were not a requirement for icing certification when the EMB-120 was certificated, 

and is not a requirement for Appendix C icing certification. This is direct 

indication that improvements to the certification requirements are necessary for 

safe flight in the icing conditions. 

It is recognized that rulemaking is in progress that will require adequate stall 

warning in Appendix C icing conditions. However, if one works through the 

numbers available in the NTSB reports, one will find that a stall warning reset to 

provide a 7% margin, as required when the EMB-120 was certificated, with the 

critical Appendix C ice shapes flight tested, would not have been sufficient. The 

current part 25 stall warning margin requirement is even lower, 5%. In addition, 

the Low Speed Alarm referenced by Embraer was designed to the accident FDR 

data, not part 25 regulations. One would find the Low Speed Alarm of 160 knots 
IPHWG Task 2 WG Report - Appendix I 

Accidents and Incidents Relevant to the Proposed Rule 

12/19/2005 Page I-10 

activates 20 knots higher than a stall warning designed to CFR Section 25.207. 

This analysis supports the need for improvements to the icing certification 

requirements. 

The performance analysis that Embraer conducted for the NTSB after the 

Orlando, FL accident, and included in the public docket for that accident, was 

reviewed by the group. The analysis showed that the lift loss and drag increase 

measured in EMB-120 flight testing with critical Appendix C ice shapes could not 

duplicate the performance losses experienced in the three EMB-120 accident 

events included in Table 5, and an EMB-120 incident included in Table 3, derived 

from the flight data recorders. For example, Figure 1 of the NTSB performance 

group report for the Orlando, FL accident, dated March 13, 2002, showed the 

following lift losses at the current stall warning shaker angle of attack: 

Flight test with critical Appendix C intercycle ice: 20% 

Pine Bluff accident: 28% 

Monroe accident: 30% 

Westair incident: 37% 

Orlando accident: 41% 

Based upon the above, improvements to the design of the airplane to meet the 

proposed SLD icing environment rule could be considered as appropriate, thus, 

the three EMB-120 events were retained in the database to support this Task 2 

proposed SLD icing environment rule. 
IPHWG Task 2 WG Report - Appendix I 

Accidents and Incidents Relevant to the Proposed Rule 

12/19/2005 Page I-11 

EHWG/PPIHWG REVIEW OF ACCIDENTS AND INCIDENTS

An assessment of worldwide commercial engine service history was conducted 

for small and large engines covering the period from 1989 through 2003. No 

accidents were attributed to the effects of SLD or Mixed Phase/Glaciated 

conditions on engine operation. 

The following chart (Figure I-1) displays incidents (defined as specific weather 

events which affected multiple aircraft) in SLD conditions that resulted in 

reportable fan damage. The 15 incidents listed resulted in damage to a total of 

59 engines. None of the events resulted in an actual threat to continued safety of 

flight. 

Note: 

H1 - Engines with SLTO thrust < 20,000 lbs 

H2 - Engines with SLTO thrust > 20,000 lbs and < 40,000 lbs 

H3 - Engines with SLTO thrust > 40,000 lbs 

Fan Damage Events Tally

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

H2 Ground (42

Engines)

H2 Hold (5

Engines)

H2 Descent (2

Engines)

H3 Ground (4

Engines)

H3 Hold (2

Engines)

H3 Descent (2 Tail

Engines, 1 Wing

Engine)

H3 Cruise (1

Engine)

Number of Incidents 

Figure I-1 - Fan Damage Events Tally 
IPHWG Task 2 WG Report - Appendix I 

Accidents and Incidents Relevant to the Proposed Rule 

12/19/2005 Page I-12 

The following chart (Figure I-2) displays incidents in Mixed Phase/Glaciated 

conditions that resulted in reportable rollbacks, flameouts, stalls or engine core 

damage. The 70 incidents listed affected a total of 97 engines. 

Rollback/Flameout/Stall and Core Damage Events Tally

0

2

4

6

8

10

12

14

16

18

20

H1 Cruise (27

Engines)

H2

Climb/Cruise

(10 enginies)

H2 Ground (7

Engines)

H2 Descent

(14 Engines)

H3

Climb/Cruise

(8 Engines)

H3 Ground (2

Engines)

H3 Descent

(24 Engines)

Low BPR

Climb/Cruise

(3 Engines)

Prop Descent

(2 Engines)

Number of Incidents 

Figure I-2 Rollback/Flameout/Stall and Core Damage Events Tally 
IPHWG Task 2 WG Report - Appendix J 

Cost Estimates 

12/19/2005 Page J-1 

APPENDIX J - COST ESTIMATES 
IPHWG Task 2 WG Report - Appendix J 

Cost Estimates 

12/19/2005 Page J-2 

GENERAL ASSUMPTIONS

It is admitted among the group that the results of the economic analysis will 

contain some fairly arbitrary numbers, as experience of existing airplanes cannot 

be used as a reliable indicator. 

Data could be split in three main categories: 

-Large transport aircraft operating in full Appendix X ((§25.1420(a)(3)) 

-Regional aircrafts operating in full or a portion of Appendix X 

((§25.1420(a)(3) or (a)(2)) 

-Commuters / business aircrafts certified with the “detect and exit” option 

((§25.1420(a)(1)) 

There is some indication that the development and certification costs will be 

roughly the same for all certification options in Appendix X (full, portion of, detect 

and exit), but the additional operational costs (training, dispatch, diversions, etc.) 

will be strongly dependant upon the type of certification. 

There were no efforts made to identify the costs to government entities 

associated with the development of the simulation tools (e.g., icing codes, test 

methods, and facilities development). 

The potential overlapping with certification in Appendix C was discussed, without 

clear conclusions. The costs were provided assuming no benefit from an 

Appendix C certification applied to an Appendix X certification. There may be 

some reduction in the cost when considering a combined Appendix C and 

Appendix X certification program, but this is an unknown factor. 

EXAMPLE COSTS

Table J-1 provides example costs that were provided to the IPHWG as 

representative of expected costs to show compliance to the proposed rule. 
IPHWG Task 2 WG Report - Appendix J 

Cost Estimates 

12/19/2005 Page J-3 

 Large 

Transport Y 

Large 

Transport Z 

Regional 

Transport 

Aircraft3 

SLD Ice Detection System 

Design, Qualification & 

Certification 

$556,800 $600,000 $280,000 

Aerodynamic Wind Tunnel 

Tests 

$1,219,200 $800,000 $ 1,202,000 

Icing Tunnel Test $2,020,800 $12,300,0002 $456,000 

Analysis (icing codes, CFD, ...) $230,400 n/a $100,000 

Simulated Ice Shapes - Flight 

Test Campaign 

$1,420,800 $1,900,000 $682,000 

Icing Tanker - Flight Test 

Campaign 

$2,434,800 n/a $1,530,000 

Natural Ice - Flight Test 

Campaign 

$4,252,800 $4,200,000 $2,555,000 

SLD Ice Detect Recurring $10,0005 $10,0005 $10,0005 

Total $12,135,6001 $20,300,0001 $6,805,0001 

Table J-1 - Example Costs 

Note 1: Compliance will likely require a combination of the approaches, but 

individual manufactures will have to determine which exact combination will be 

required to meet the certification goals of program risk reduction, development 

and validation of ice shapes, and validation of the methods. It is not possible to 

predict which combinations will be required. In light of this, a conservative 

economic analysis would consider the total costs. For derivative aircraft 

programs that have an ancestor aircraft that has already been certified to 

Appendix X, it is expected that the program costs could be reduced by 50 

percent. For operation in a portion and “detect and exit” aircraft, it should be 

assumed that 50 percent will use an ice detector to exit and 50 percent will use 

visual cues. It is estimated that 50 percent of airplane certification programs will 

require natural icing flight testing. 

Note 2: Quote assumes considerably more tunnel testing. 

Note 3: These quotes are representative of commuters / business aircraft, as well 

as regional aircraft. 

Note 4: Quotes were normalized to the US dollar assuming an exchange rate of 

1.2 US dollar per Euro. Costs are provided in year 2003 US dollars. 

Note 5: The Ice Detector could be a detector for activation of the ice protection 

system and/or for knowing when to exit icing conditions. The large transport 

manufacturers, whose airplanes will likely be certified in accordance with § 

25.1420(a)(3), anticipate installation of an ice detector which will activate the IPS, 
IPHWG Task 2 WG Report - Appendix J 

Cost Estimates 

12/19/2005 Page J-4 

although the rule allows activation of the IPS based on visible moisture and 

temperature. There are fuel savings benefits associated with the installation of 

ice detectors which activate the IPS; conversely, there are fuel-burn penalties 

associated with activation of the IPS based upon temperature and visible 

moisture; neither have been accounted for in this Working Group Report. 

Note 6: Certifications will be approximately equally divided between unrestricted 

operation, operation in a portion of X, and detect and exit. 

Note 7: For unrestricted aircraft, the majority position is that a system to detect 

exceedance of Appendix X conditions is not required. The minority position may 

require some percentage of the aircraft to be equipped with such a system. 

ENGINE CERTIFICATION COSTS

• 14 CFR Part 33.68 – Three additional icing test conditions required, cold 

point plus ground operation in SLD and snow, with allowance for 

certification in the latter two by analysis and/or in the case of snow by 

equivalent liquid water). Five additional test conditions (climb, cruise, idle 

descent, holding, approach) in Mixed Phase/Glaciated icing currently 

addressed in AC20-147. 

• 14 CFR 25.1093- Requirement to demonstrate capability by test or 

analysis including a 30 minute ground SLD exposure. Expect this will drive 

one or more experimental test points to validate analysis. 

New Equipment Purchases 

• Currently, there are limited test facilities for testing with ice crystals. 

Estimates are being prepared for enhancing ground test facilities for this 

purpose. An alternate proposal utilizes current test facilities by prescribing 

an equivalent liquid water test, but requires validation. A technology 

program to validate that approach is now under consideration. If snow 

and Mixed Phase/Glaciated tests must use artificial ice crystals, an 

assessment is required for effects of ice crystal velocity. Existing means 

for generating artificial ice crystals from spray nozzles provide velocities 

lower than flight speed. If facilities for producing the ice crystals with 

adequate velocity to simulate flight speeds are required, they may need to 

include ice processing and high air pressure equipment. 

Industry Relief 

• 14 CFR Part 33.77 – Rule changes allow alternate means for certification 

of soft body impact tolerance associated with ice slab through use of 

validated analysis and other applicable testing. This will minimize engine 

ice slab testing for manufacturers having or developing validated analysis 

procedures. It is anticipated that due to the costs and complexity of 
IPHWG Task 2 WG Report - Appendix J 

Cost Estimates 

12/19/2005 Page J-5 

developing such analyses only the three large engine manufacturers will 

be able to make use of this option, while small engine manufacturers will 

continue to run the ice slab test. 

• 14 CFR Part 33.68 – Manufacturers will not be required to test the legacy 

AC20-147 “Table Points” unless they are more severe than conditions 

defined by the Critical Point Analysis of icing conditions.

• 14 CFR Part 33.77 and Part 33.68 – Rule changes allow for changes in 

engine power resulting from ice slab and natural icing tests within the 

nominal ability to measure this parameter (1.5%). 

• Alternate method of compliance by similarity to engines proven safe to 

operate in mixed phase or glaciated icing conditions: Although it has been 

established that mixed phase or glaciated icing conditions are hazardous 

to turbine engine operation, severe incidents involving this type of 

meteorological condition are not common. Many currently certified engine 

designs have been proven by their field service experience to be safe to 

operate in these conditions. New engine design, similar to those proven 

engines, is allowed to show compliance by similarity analysis. 

Additional Requirements 

• Validated analysis of engine icing in glaciated environment is required for 

assessment of this icing threat. 

• Manufacturers will have to demonstrate to FAA that they have thoroughly 

addressed Critical Point Analysis requirements for natural icing in 14 CFR 

Part 25 Appendix C, the new 14 CFR Part 25 Appendix X and the new 14 

CFR Part 33 Appendix D. 

• The addition of Appendix X to 14 CFR 25.1093 will require additional 

analysis of the engine inlet capability in descent and holding conditions 

• The addition of Appendix X Ground Test point will require additional 

analysis of inlet ice accretion on the ground for comparison with 14 CFR 

33.77 slab. 

• Analysis for 14 CFR 33.77 to show compliance using equivalent soft-body 

testing may require additional component tests to validate analyses. 
IPHWG Task 2 WG Report - Appendix J 

Cost Estimates 

12/19/2005 Page J-6 

 Large 

Engine A 

Large 

Engine B 

Large 

Engine C 

Small 

Engine

Engine Analysis (Mixed Phase 

Glaciated) 

TBD* $10,000 $50,000a TBD* 

Ice crystal facility $1,000,000b 

Engine Analysis SLD TBD* $10,000* $20,000c TBD* 

Engine Icing Tests TBD** TBD** $600,000d $500,000 

Saved Ice Slab Testing -TBD*** -150,000*** -$50,000*** N/A 

Soft Body Tolerance Analysis TBD $182,000 $50,000 N/A 

Table J-2 - Example Costs/Engines 

a Mixed phase compliance by analysis is only short term. Considerable research 

& testing must be done to meet the long-term goals stated in the proposed rule. 

This work may cost $250,000 or more. 

b Modifying manufacturer facility to provide an ice crystal cloud estimated to cost 

approx. $1 million using a known method, however this may not meet 

requirements and result in more expensive facilities. 

c

 If additional test facilities to provide SLD required then investment costs of 

facility changes estimated at $100,000, additional per program costs ~$50,000 

d

 Additional testing in Appendix C atmosphere and new 14 CFR33 Appendix D 

will result in extra test points, if manufacturers facility cannot be used and altitude 

facility required costs will be considerably higher 

* Rule allowance for ice crystal compliance by similarity reduces cost significantly 

and promotes designing proven ice crystal-tolerant features into new engines. 

Comparative analysis done by one engine manufacturer cost $10,000. 

** If new engine size is compatible with limited existing test facilities capable of 

producing ice crystals, or equivalent liquid water can be substituted for ice crystal 

tests, preliminary estimates from one small engine manufacturer are for 

$400,000 -$500,000 additional icing test costs for new engine certification 

program. Estimate from one testing facility with capacity up to 25,000 lb thrust 

turbofan engine are for $200,000 additional icing test costs for new engine 

certification program. 

 ***Cost reduction for eliminated ice slab test partially offset by additional soft 

body tolerance analysis 

NOTE: Additional SLD costs for airplane certification associated with 25.1093 

rulemaking have not been included in IPHWG cost numbers. Cost information is 

being assessed and will be provided to the economist as available. 
IPHWG Task 2 WG Report - Appendix J 

Cost Estimates 

12/19/2005 Page J-7 

NON RECURRING COSTS (DEVELOPMENT, CERTIFICATION)

Flight tests in natural conditions were identified as being a major contributor to 

the overall cost. There is no clear visibility on the amount of flight hours that 

could be needed for certification (low probability for SLD encounters as 

compared to Appendix C conditions; no requirements defined in terms of MVD, 

LWC, or exposure times). A large transport aircraft manufacturer evaluated the 

additional test time at about 100 flight test hours. It is recognized that the 

accretions obtained during the test campaign may not be sufficient to allow the 

validation of the icing codes used to generate the critical ice shapes for a full 

Appendix X certification. The analysis should therefore provide an envelope, 

with a lower limit corresponding to the minimum to comply and an upper limit 

which includes extensive flight-testing in natural conditions. 

Some other factors need to be considered, such as the rank of application to the 

new rule (costs associated with the first certifications will likely be significantly 

higher than subsequent applications) or certification of derivatives (no general 

rule can be proposed on the nature of the modifications being applied). 

For each airplane category assumed (large transport, regional transport, 

commuter/business), the non-recurring costs can be summarized with the 

following break down: 

1. Minimum Cost to Comply (Lower Limit): 

 -Ice detector qualification / certification 

 -Analysis (icing codes, CFD) 

 -Aerodynamic wind tunnel tests 

 -Icing tunnel tests or artificial icing behind a tanker 

 -Flight test with simulated ice shapes 

 -Documentation (Certification reports, AFM, FCOM,..) 

2. Upper Limit: 

 - All of the items in No. 1 above, plus 

 - Flight tests in natural SLD conditions 

Note: The possible overlapping with certification in Appendix C will obviously 

depend upon the performance penalties which could be required for operation in 

Appendix X. Costs provided by the manufacturers are based on a scenario in 

which a specific set of minimum speeds /performance and associated handling 

qualities tests need to be established for Appendix X operation in addition to 

Appendix C conditions. 
IPHWG Task 2 WG Report - Appendix J 

Cost Estimates 

12/19/2005 Page J-8 

RECURRING COSTS

On the manufacturers’ side, introduction of the new rule may lead to major 

modifications in the ice protection systems (e.g., increased size of protected 

surfaces requiring additional power; addition of new detection or warning means). 

An evaluation of these recurring costs is not available. 

On the operators’ side for the non-recurring costs, a breakdown equally divided 

into three categories (large, regional, business/commuter) can be envisaged. 

This is based on the assumption that large transport airplanes will probably apply 

for certification in all of Appendix X, regional airplanes for full or a portion of 

Appendix X, and business/commuters for the detect and exit option. The 

economic analysis should take into account the following consequences: 

- Impact on dispatch (performance limited take-offs and payload decrease) 

- Diversion by exiting (increased flown distance) 

- Diversion to alternate 

- Cancelled flight 

- Pilot training 

Costs associated with the training of Air Traffic Control and weather system 

personnel also need to be considered. 

One large transport manufacturer using available figures (probability to encounter 

Appendix X conditions, number of airplanes certified to the new rule, estimated 

proportions in each category) provided an example calculation of the operational 

costs. The impact on dispatch numbers are based on aircraft certified without 

restriction or certified for a portion of Appendix X (i.e., WAT limited takeoffs). The 

diversion and cancelled-flight numbers are based on aircraft certified to detect 

and exit all or a portion of Appendix X. These estimates are provided in Table J3. 
IPHWG Task 2 WG Report - Appendix J 

Cost Estimates 

12/19/2005 Page J-9 

Table J-3 - Operating Costs 

Category Costs for year one (2007 assumed) 

Impact on dispatch (payload) $113,380 

Exit by Diversion $169,493 

Diversion to alternate airport $16,376,100 

Cancelled flights $5,567,874 

Pilot training not available2

Weather/ATC training not available2

Notes: 

(1) Assumptions of 1,900 flights per year and 169 new aircraft in 2007. 

(2) Pilot training and weather/ATC training costs should be calculable by the FAA 

economist assuming, for example, one hour of training per person per year.
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-1 

APPENDIX K - IPHWG RECOMMENDED ADVISORY MATERIALS 

§§25.1419, 25.1420 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-2 

ATTENTION: This Advisory Circular is based on the existing AC 25.1419-1 

"Certification of Transport Category Airplanes for Flight in Icing Conditions". This 

draft AC is intended to address existing §25.1419 requirements as well as 

§25.1420. As part of the revision, Sub-part B related requirements were moved 

to the FTHWG proposed advisory materials AC 25.21(g). 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-3 

Subject: COMPLIANCE OF 

TRANSPORT CATEGORY 

AIRPLANES FOR FLIGHT IN 

ICING CONDITIONS

Date: Draft 

5/10/04

Initiated By: 

ANM-110 

AC No: 25-XX 

Change: 

WORKING DRAFT -- NOT FOR PUBLIC RELEASE. 

1. PURPOSE.

 a. This Advisory Circular (AC) describes an acceptable means for showing 

compliance with the requirements of § 25.1419, “Ice protection” and § 25.1420, 

“Supercooled Large Drop Icing Conditions,” of Title 14, Code of Federal 

Regulations (14 CFR) part 25, commonly referred to as part 25 of the Federal 

Aviation Regulations (FAR). Part 25 contains the applicable certification 

requirements for transport category aircraft. The means of compliance described 

in this document are intended to provide guidance to supplement the engineering 

judgment that must form the basis of any compliance findings relative to the 

requirements of §§ 25.1419 and 25.1420. 

 b. The guidance provided in this document is directed to airplane 

manufacturers, modifiers, foreign regulatory authorities, and Federal Aviation 

Administration airplane type certification engineers and their designees. 

 c. Like all advisory circular material, this AC is not in itself mandatory, and 

does not constitute a regulation. It is issued to describe an acceptable means, 

but not the only means, for demonstrating compliance with the requirements for 

transport category airplanes. Terms such as “shall” and “must” are used only in 

the sense of ensuring applicability of this particular method of compliance when 

the acceptable method of compliance described in this document is used. While 

these guidelines are not mandatory, they are derived from extensive Federal 

Aviation Administration and industry experience in determining compliance with 

the pertinent regulations. 

 d. This advisory circular does not change, create any additional, authorize 

changes in, or permit deviations from regulatory requirements. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-4 

2. APPLICABILITY.

The guidance provided in this AC applies to certification of part 25 transport 

category airplanes for flight in icing conditions. If certification for flight in icing 

conditions is desired, the airplane must be able to safely operate throughout the 

icing envelope of 14 CFR 25, Appendix C. The airplane must also be able to 

safely operate or exit the icing conditions defined by 14 CFR 25, Appendix X. 

The certification options available are: 1) to safely operate throughout Appendix 

X, 2) To operate safely throughout a portion of Appendix X, 3) Limited to safely 

exiting Appendix X conditions. Sections 25.1419 and 25.1420 set forth the 

specific airframe requirements for demonstrating compliance with the icing 

conditions defined in Appendices C and X. Additionally, for other parts of the 

airplane (i.e., engine, engine inlet, propeller) there are more specific icing 

requirements and associated guidance defined in the referenced regulations and 

advisory circulars listed in paragraph 3 below. Some of the icing-related 

regulations must be complied with, even if the airplane is not certificated for flight 

in icing (e.g., §§ 25.629(d)(3), 25.903, 25.975, 25.1093, 25.1323(e), and 

25.1325(b)). This AC contains information on flight tests for ice protection 

certification; additional information on flight tests for showing compliance with the 

airplane performance and handling qualities requirements for icing certification 

may be found in Advisory Circular 25.21-1. The guidance provided in this AC is 

applicable to new Type Certificates (TC's), Supplemental Type Certificates 

(STC's), and amendments to existing TC's for airplanes certified under Part 4b of 

the Civil Aviation Regulations (CAR) and Part 25, for which approval under the 

provisions of § 25.1419 is desired. 

3. RELATED DOCUMENTS.

 a. Regulations contained in Title 14, Code of Federal Regulations (CFR).

Section Title 

§ 25.21(g) Proof of compliance 

§ 25.105(a)(2) Takeoff 

§ 25.107(h) Takeoff speeds 

§ 25.111(c)(5) Takeoff path 

§ 25.119(b) Landing climb: All-engines-operating 

§ 25.121(b)(2)(ii), 

(c)(2)(ii) & (d)(2)(ii) 

Climb: One-engine-inoperative 

§ 25.123(b)(2) En route flight paths 

§ 25.125(a)(2), & 

(b)(2)(ii) 

Landing 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-5 

§ 25.143(c),(i), & (j) General 

§ 25.207(b), (e), (f), 

(h), & (i) 

Stall warning 

§ 25.237(a)(3)(ii) Wind velocities 

§ 25.253(b) High-speed characteristics 

§ 25.629 Aeroelastic stability requirements 

§ 25.773(b)(1) Pilot compartment view 

§ 25.929 Propeller deicing 

§ 25.975(a)(1) Fuel tank vents 

§ 25.1093 Induction system icing protection 

§ 25.1323 Airspeed indicating system 

§ 25.1325 Static pressure system 

§ 25.1301 Equipment - Function and installation 

§ 25.1309 Equipment, systems, and installations 

§ 25.1316(b) System lightning protection 

§ 25.1321 Instruments Installation - Arrangement and visibility 

§ 25.1322 Warning, caution, and advisory lights 

§ 25.1403 Wing icing detection lights 

§ 25.1419 Ice protection 

§ 25.1420 Supercooled large drop icing conditions 

§ 25.1585 Operating procedures. 

Appendix C to part 

25 

Appendix X to part 

25 

 b. Advisory Circulars (AC). The AC's listed below may be obtained from 

the U.S. Department of Transportation, Subsequent Distribution Office, SVC121.23, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, MD 

20785: 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-6 

Number Title and Date 

AC 20-73A Aircraft Ice Protection, dated XX, 2004. 

AC 20-117A Hazards Following Ground Deicing and Ground 

Operations in Conditions Conducive to Aircraft Icing, 

dated December 17, 1982. 

AC 20-115B Radio Technical Commission for Aeronautics, Inc. 

(RTCA) Document RTCA/DO-178B, dated January 11, 

1993. 

AC 20-147 Turbojet, Turboprop, and Turbofan Engine Induction 

System Icing 

AC 21-16D RTCA Document DO-160C, dated July 21, 1998. 

AC 25-11 Transport Category Airplane Electronic Display 

Systems, dated July 16, 1987 

AC 25.21-1 Performance and Handling Characteristics in the Icing 

Conditions Specified in Part 25, Appendix C, dated XX, 

2004. 

AC 25-22 Certification of Transport Airplane Mechanical Systems 

AC 25.629-1 Flutter Substantiation of Transport Category Airplanes 

AC 25.1309-1A System Design Analysis, dated June 21, 1988. 

 c. Other FAA Documents: 

Number Title 

DOT/FAA/CT-88/8-

1 

“Aircraft Icing Handbook,” issued March 1991, 

updated September 1993 

DOT/FAA/AR-04/7 "TBD", dated TBD (Insert Met group paper that is 

to be written on the derivation of Appendix X) 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-7 

 d. Industry Documents. The following RTCA documents can be obtained 

from Radio Technical Commission for Aeronautics (RTCA), Inc., 1140 

Connecticut Ave., NW, Suite 1020, Washington, DC 20036. The following SAE 

Documents can be ordered from the Society of Automotive Engineers, Inc., 400 

Commonwealth Drive, Warrendale, PA 15096. The SAE documents to date 

have not been updated to reflect supercooled large droplet analyses and testing 

but provide good reference material. 

Number Title 

RTCA/DO178B Software Considerations in Airborne Systems and 

Equipment Certification 

RTCA/DO160D Environmental Conditions and Test Procedures for 

Airborne Equipment 

RTCA/DO-254 Design Assurance Guidance for Airborne Electronic 

Hardware 

SAE/ARP5624 Aircraft Inflight Icing Terminology, to be released 

SAE/ARP5903

 

Droplet Impingement and Ice Accretion Computer 

Codes, dated, October 2003 

SAE/ARP5904 Airborne Icing Tankers, dated 10/2002. 

SAE/ARP5905 Calibration and Acceptance of Icing Wind Tunnels, 

dated September 2003. 

SAE 

ARP1168/4 

Ice, Rain, Fog and Frost Protection, dated 7/30/1990 

SAE AS5498 

(EUROCAE 

ED-103) 

Minimum Operational Performance Specification For 

Inflight Icing Detection Systems 

Signed by: 

Manager, Transport Airplane Directorate 

Aircraft Certification Service 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-8 

Contents

1. Background 

 a. Civil Air Regulations (CAR) 

 b. Part 25 

2. Definition of Terms 

3. Icing Envelopes 

 a. Background 

 b. Use of Icing Envelopes 

 c. Effects of Appendix X Icing Conditions 

 d. Primary areas of Interest for Appendix X 

 (1) Impingement aft of Protected Areas of Lifting Surfaces 

 (2) Ice Shapes on Unprotected areas of Lifting Surfaces 

 (3) Ice Shapes on Unprotected Airframe Regions 

 (4) Drag and Power Effects 

 (5) Engine considerations 

4. Certification Plan 

5. Analyses 

 a. Analysis of Areas and Components to be Protected 

 b. Flutter Analysis 

 c. Power Sources 

 d. Failure Analyses 

 (1) Failure Analysis for § 25.1419 (Appendix C Conditions) 

 (2) Failure Analysis for § 25.1420 (Appendix X Conditions) 

 (a) Hazard Classification 

 (b) Probability of Encountering Appendix X 

 (c) Numerical Safety Analysis 

 (d) Assessment of Visual Cues 

 e. Similarity Analyses 

 f. Impingement Limit Analyses 

 g. Ice Shedding Analyses 

 h. Propeller Deicing Analysis 

i. Pitot Probe Ice Protection 

j. Stall Warning System Ice Protection 

k. Runback Ice 

6. Simulated Ice Shapes and Roughness 

7. Compliance Tests (Flight/Simulation) 

 a. Dry Air Ground Tests 

 b. Dry Air Flight Tests with Ice Protection Equipment Operating 

 (1) Thermal Anti-ice Leading Edge Systems 

 (2) Pneumatic Leading Edge Boots 

 (3) Electrically Heated Propeller Boots 

 (4) Windshield Anti-ice 

 (5) Pitot-Static and Static Pressure Sources 

 (6) Bleed Air Systems 

 c. Dry Air Flight Tests With Predicted Simulated Shapes and Roughness 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-9 

d. Icing Flight Tests 

(1) Instrumentation 

 (2) Artificial Icing 

 (3) Appendix C Natural Icing Flight Testing 

 (4) Appendix X Natural Icing Flight Testing 

 e. Fluid Anti-Icing Deicing Systems 

 f. Icing Wind Tunnel Tests 

 g. Dry-air wind tunnel tests 

8. Certification to § 25.1420 

 a. Appendix X Boundary 

 b. Means for Detecting Exceedance Icing Conditions 

9. Pneumatic Deicing Boots 

10. Emergency and Abnormal Operating Conditions 

11. Ice Detection Systems 

 a. Primary and Advisory Ice Detection 

 b. Aerodynamic Performance Monitor 

 c. Compliance with § 25.1419(e)(1) and (e)(2) 

 d. Compliance with § 25.1419(e)(3) 

 e. Compliance with § 25.1419(f) 

 f. Compliance with § 25.1419(g) 

 g. Compliance with § 25.1420(a)(1) or (a)(2) 

12. Perform Intended Function in Icing 

 a. Engines and Equipment 

 b. Engine Alternate Induction Air Sources 

 c. Fuel System Venting 

 d. Landing Gear 

 e. Stall Warning 

 f. Ice Detection Cues 

 g. Primary and Secondary Flight control Surfaces 

 h. Ram Air Turbine 

 i. Pilot Compartment View 

13. Ice Inspection Light(s) 

14. Caution Information 

15. Airplane Flight Manual (AFM) 

 a. Operating Limitations Section 

 b. Operating Procedures Section 

16. Guidance for TCs, Amended TCs, and STCs 

Appendices

Appendix A Example of Certification of § 25.1420(a)(1) Using Visual Cues 

Appendix B Guidance For Amended TC's, AND STC's 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-10 

1. BACKGROUND. 

 a. Civil Air Regulations (CAR). 

 (1) Prior to 1953, airplanes were certificated under Part 04 of the CAR. 

Section 04.5814 required that if deicer boots were installed, then positive means 

must be provided for the deflation of all wing boots. There were no other 

references to an airplane ice protection system (IPS) in Part 04. 

 (2) Part 4b of the CAR was codified on December 31, 1953. The 

requirement for positive means of deflating deicer boots was incorporated, 

without change, in § 4b.640. Section 4b.640 stated that, “When an ice protection 

system is installed, it shall be of an approved type. If pneumatic boots are used, 

at least two independent sources of power and a positive means for the deflation 

of the boots shall be provided.” Section 4b.351(b)(ii) provided requirements for 

pilot compartment vision in icing conditions. Section 4b.406 provided propellerdeicing requirements. Section 4b.461 provided induction system deicing and 

anti-icing requirements. Section 4b.612(a)(5) required that the airspeed 

indicating system be provided with a heated pitot tube or equivalent means of 

preventing malfunctioning due to icing. 

 (3) Amendment 4b-2, effective August 25, 1955, introduced icing 

envelopes similar to the current Appendix C to part 25. The graphs added by 

Amendment 4b-2 (4b-24a, 4b-24b, 4b-24c, 4b-25a, 4b-25b, and 4b-25c) were 

identical in substance and format to the current Appendix C envelopes with a few 

exceptions. These envelopes described the liquid water content, the mean 

effective diameter of droplets, the temperature, and horizontal and vertical extent 

of the supercooled icing cloud environment. In the figures introduced by 

Amendment 4b-2, the units of the distances shown on the graphs were 

expressed in statute miles instead of nautical miles. The Liquid Water Content 

(LWC), however, is identical between the Amendment 4b-2 figures and the 

current Appendix C envelopes, if the correction for the difference in value 

between nautical and statute miles is made. There are two significant 

differences between the Amendment 4b-2 envelopes and the current Appendix 

C. The minimum mean effective diameter in the intermittent maximum conditions 

was 20 μm versus the current 15 μm, and the minimum cloud horizontal extent in 

the Intermittent Maximum Atmospheric Icing Conditions Variation of LWC Factor 

with Cloud Horizontal Extent chart was 1.5 statute miles versus the current 2.6 

nautical miles (3.0 statute miles). 

 (4) Amendment 4b-6, effective August 12, 1957, revised the icing 

envelopes to the current requirements and revised § 4b.461, “Induction system 

de-icing and anti-icing provisions.” The preamble to the amendment states: 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-11 

There are included herein changes which extend the currently 

effective provisions governing intermittent maximum icing 

conditions so as to cover conditions which might be critical insofar 

as the turbine engine induction system is concerned. In this regard, 

the data are being extended in accordance with NACA Technical 

Note 2738 and involve a revision of Figure 4b-25a to cover drop 

diameters as small as 15 μm and a revision of Figure 4b-25c to 

cover distances down to 3.0 mile. The icing conditions prescribed 

in the currently effective regulations are applicable in the main to 

the airframe. The changes being made in section 4b.461 require 

the turbine powerplant to be subjected to the same icing conditions 

and require that the induction system be protected to prevent 

serious engine power loss. A similar requirement is incorporated 

with respect to certification of turbine engines by an amendment to 

Part 13 which is being made concurrently with this amendment. 

 b. Part 25. 

 (1) Part 4b of the CAR was codified into part 25, effective 

February 1, 1965. After recodification, with minor editorial changes the content 

of § 4b.640, Ice protection, became § 25.1419; § 4b.406, Propeller deicing 

provisions, became § 25.929; § 4b.612(a)(5), Airspeed indicating systems, 

became § 25.1323(e); § 4b.351(b)(1)(ii), Pilot compartment view, became 

§ 25.773(b)(1)(ii). The § 4b.351 reference to the most severe icing conditions for 

which approval of the airplane is desired was changed in part 25 to reference the 

icing conditions specified in § 25.1419. Section 4b.461, Induction system deicing 

and anti-icing provisions, became § 25.1093. 

 (2) Amendment 25-5, effective July 29, 1965, revised § 25.1325(b) to 

require that the correlation between air pressure in the static pressure system 

and true ambient atmospheric static pressure is not changed when the airplane is 

exposed to the continuous and intermittent maximum icing conditions defined in 

Appendix C. 

 (3) Amendment 25-11, effective June 4, 1967, revised § 25.1585 to 

require that the Airplane Flight Manual include information on the use of ice 

protection equipment. 

 (4) Amendment 25-23, effective May 8, 1970, revised § 25.1419 to 

require that the effectiveness of the IPS and its components be shown by flight 

tests of the airplane or its components in measured natural atmospheric icing 

conditions. Previous to this amendment, flight tests in natural icing conditions 

were considered as one means of compliance but were not mandatory. 

Amendment 25-23 also revised § 25.1309 to include additional requirements for 

certificating equipment, systems, and installations. The regulation was revised to 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-12 

require a comprehensive systematic failure analysis, supported by appropriate 

test, to ensure that the safety objectives of the probability of occurrence decrease 

as the hazard of a failure increases. 

 (5) Amendment 25-38, effective February 1, 1977, added § 25.1403 to 

require a means for illuminating or otherwise determining the formation of ice on 

the parts of the wings that are critical from the standpoint of ice accumulation. 

The requirement is not applicable if the operating limitations prohibit operations 

at night in icing conditions. 

 (6) Amendment 25-43, effective April 12, 1978, added § 25.1326 to 

require the installation of a pitot heat indication system on airplanes equipped 

with flight instrument pitot heating systems. 

 (7) Amendment 25-46, effective December 1, 1978, added § 25.1416 to 

require specific standards for pneumatic deicer boots. 

 (8) Amendment 25-72, effective July 20, 1990, transferred § 25.1416 to 

§ 25.1419 for clarification and editorial improvement. In addition, the contents of 

§ 25.1416(c), which required a means to indicate to the flightcrew that the 

pneumatic deicing boot system is receiving adequate pressure and is functioning 

normally, were revised to allow the use of the “dark cockpit” concept (i.e., a 

warning when failure occurs rather than continual pilot monitoring of a healthy 

system). 

 (9) Amendment 25-XX, effective xx xx, 200X, §§ 25.21, 25.103, 25.105, 

25.107, 25.111, 25.119, 25.121, 25.123, 25.125, 25.143, 25.207, 25.237, and 

25.253 were revised to provide performance and handling requirements for safe 

operation in Appendix C icing conditions. Section 25.1419 was revised to require 

compliance with § 25.1419 if certification for flight in icing is desired. Prior that 

revision, compliance with § 25.1419 was only required if the airplane had ice 

protection provisions. Editorial revisions were made to §§ 25.773 and 25.941 to 

retain consistency with the changed § 25.1419 and to revise references to 

changed paragraphs, respectively. 

 (10) Amendment 25-XX, effective xx xx §25.1419(e), (f), (g), and (h) were 

added to provide requirements for determination of when to operate ice 

protection systems to ensure safe operation in Appendix C icing conditions. 

 (11) Amendment 25-XX, effective xx xx, 200X, introduced Appendix X 

and § 25.1420, and revised §§ 25.773, 25.929, 25.1093, 25.1323, and 25.1325. 

The amendment added supercooled large droplet icing conditions that must be 

considered when an airplane is certificated for flight in icing. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-13 

2. DEFINITION OF TERMS. For the purposes of this AC, the following 

definitions should be used. 

NOTE: These definitions of terms are intended for use only with 

respect to § 25.1419 & § 25.1420 

a. ADVISORY ICE DETECTION SYSTEM: An advisory system annunciates 

the presence of icing conditions or ice accretion. The flightcrew is 

responsible for monitoring the icing conditions or ice accretion as defined in 

the AFM, typically using total air temperature and visible moisture criteria, 

visible ice accretion, or specific airframe ice accretion thickness, and 

activation by the flightcrew of the anti-icing or de-icing system(s) is necessary. 

The advisory system provides information to advise the flightcrew of the 

presence of ice accretion or icing conditions, but it can only be used in 

conjunction with other means to determine the need for, or timing of, 

activating the anti-icing or de-icing system. 

b. AIRFRAME ICING: Ice accretions on portions of the airplane, with the 

exception of the propulsion system including the air induction system, on 

which supercooled liquid droplets may impinge. 

c. ANTI-ICING: The prevention of ice formation or accumulation on a 

protected surface, either: 

• by evaporating the impinging water or 

• by allowing it to run back and off the surface or freeze on non-critical 

areas. 

d. APPENDIX C ICING CONDITIONS: The environmental conditions 

defined in Appendix C of 14 CFR Part 25 that are to be used as the 

engineering standard for icing certification.

e. APPENDIX X ICING CONDITIONS: The environmental conditions 

defined in Appendix X of 14 CFR Part 25, characterizing SLD conditions, 

which are to be used as the engineering standard for icing certification. 

f. ARTIFICIAL ICE: Real ice, but formed by artificial means, such as a spray 

rig in a tunnel or by a tanker. 

g. AUTOMATIC CYCLING MODE: A mode of operation of the airframe deicing system that provides repetitive cycles of the system without the need for 

the pilot to select each cycle. This is generally done with a timer, and there 

may be more than one timing mode. 

h. DEICING: Removal or the process of removal of an ice accretion after it 

has formed on a surface. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-14 

i. DRIZZLE DROP: A drop of water of diameter 100 μm to 500 μm (0.1 – 0.5 

mm). 

j. FREEZING DRIZZLE (FZDZ): Supercooled drizzle drops that remain in 

liquid form, and freeze upon contact with objects colder than 0°C. 

k. FREEZING PRECIPITATION: Any form of supercooled liquid precipitation 

that freezes upon impact with the ground or exposed objects, that is, freezing 

rain or freezing drizzle. 

l. FREEZING RAIN (FZRA): Supercooled rain drops that remain in liquid 

form, and freeze upon contact with objects colder than 0°C. 

m. IPS: Ice Protection System 

n. ICING CONDITIONS: The presence of atmospheric moisture and 

temperature conducive to airplane icing. 

o. ICING CONDITIONS DETECTOR: A device that detects the presence of 

atmospheric moisture and temperature conducive to airplane icing.

p. IRREVERSIBLE FLIGHT CONTROLS: All of the force required to move 

the pitch, roll, or yaw control surfaces is provided by hydraulic or electric 

actuators, the motion of which is controlled by signals from the flight deck 

controls. Loads generated at the control surfaces themselves are reacted 

against the actuator and its mounting and cannot be transmitted directly back 

to the flight deck controls.

q. LIQUID WATER CONTENT (LWC): The total mass of water contained in 

liquid drops within a unit volume or mass of air, usually given in units of grams 

of water per cubic meter (g/m3). 

r. MEAN EFFECTIVE DIAMETER (MED): The calculated drop diameter that 

divides the total liquid water content present in the drop size distribution in 

half, i.e., half the water volume will be in larger drops and half the volume in 

smaller drops. The value is calculated, based on an assumed Langmuir drop 

size distribution, which is how it differs from median volume diameter. 

s. MEDIAN-VOLUME DIAMETER (MVD): The drop diameter that divides the 

total liquid water content present in the drop distribution in half, i.e., half the 

water volume will be in larger drops and half the volume in smaller drops. The 

value is obtained by actual drop size measurements. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-15 

t. MIGRATING ICE: Ice resulting from the down-stream migration and 

buildup of ice from wet protected surfaces to areas that cool sufficiently for 

the migrating ice to adhere to the surface. 

u. MONITORED SURFACE: The surface of concern regarding the ice 

hazard. (e.g., the leading edge of a wing). Ice accretion on the monitored 

surface may be measured directly or correlated to ice accretion on a 

reference surface. 

v. PRIMARY ICE DETECTION SYSTEM: An ice detection system which is 

the only means used to determine when the IPS must be activated. The 

system annunciates the presence of ice accretion or icing conditions and may 

also provide information to other aircraft systems. A primary automatic

system automatically activates the anti-icing or de-icing systems. With a 

primary manual system, the flightcrew activates the IPS upon indication from 

the system. 

w. RAIN DROP: A drop of water of diameter greater than 500 μm (0.5 mm). 

 

x. REFERENCE SURFACE: The observed (directly or indirectly) surface 

used as a reference for the presence of ice on the monitored surface. The 

presence of ice on the reference surface must occur prior to – or 

coincidentally with – the presence of ice on the monitored surface. Examples 

of reference surfaces include windshield wiper blades or bolts, windshield 

posts, ice evidence probes, propeller spinner, and the surface of ice 

detectors. The reference surface may also be the monitored surface. 

y. REVERSIBLE FLIGHT CONTROLS: The flight deck controls are 

connected to the pitch, roll, or yaw control surfaces by direct mechanical 

linkages, cables, or push-pull rods such that pilot effort produces motion or 

force about the hinge line. Conversely, force or motion originating at the 

control surface (through aerodynamic loads, static imbalance, or trim tab 

inputs, for example) is transmitted back to flight deck controls. 

 (1) Aerodynamically boosted flight controls: Reversible flight 

control systems that employ a movable tab on the trailing edge of the main 

control surface linked to the pilot’s controls or to the structure in such a way 

as to produce aerodynamic forces that move, or help to move, the surface. 

Among the various forms are flying tabs, geared or servo tabs, and spring 

tabs. 

 (2) Power-assisted flight controls: Reversible flight control 

systems in which some means is provided, usually a hydraulic actuator, to 

apply force to a control surface in addition to that supplied by the pilot to 

enable large surface deflections to be obtained at high speeds. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-16 

z. RUNBACK ICE: Ice formed from the freezing or refreezing of water 

leaving an area on an aircraft surface that is above freezing and flowing 

downwind to an area that is sufficiently cooled for freezing to take place. This 

ice type is frequently associated as an unwanted product of thermal deicing 

systems. 

aa. SIMULATED ICE: Ice shapes that are fabricated from wood, epoxy, or 

other materials by any construction technique. 

ab. STATIC AIR TEMPERATURE: The air temperature as would be 

measured by a temperature sensor not in motion with respect to that air. This 

temperature is also referred to in other documents as “outside air 

temperature,” “true outside temperature,” or “ambient temperature.” 

ac. SUBSTANTIATED VISUAL CUE: Ice accretion on a reference surface 

that has been demonstrated by testing to correlate with ice accretion on a 

monitored surface. 

ad. SUPERCOOLED LARGE DROPS (SLD): Supercooled liquid water that 

includes freezing rain or freezing drizzle. 

ae. SUPERCOOLED WATER: Liquid water at a temperature below the 

freezing point of 0°C. 

af. FZDZ/G: A drop diameter distribution that has an MVD greater 

than 40 μm and includes freezing drizzle and drops with diameters less 

than 100μm (defined by 14 CFR 25, Appendix X). 

ag. FZDZ/L: A drop diameter distribution that has an MVD less than

40 μm and includes freezing drizzle and drops with diameters less than 

100μm (defined by 14 CFR 25, Appendix X). 

ah. FZRA/G: A drop diameter distribution that has an MVD greater 

than 40 μm and includes freezing rain, freezing drizzle and drops with 

diameters less than 100μm (defined by 14 CFR 25, Appendix X). 

ai. FZRA/L: A drop diameter distribution that has an MVD less than 40 

μm and includes freezing rain, freezing drizzle and drops with 

diameters less than 100μm (defined by 14 CFR 25, Appendix X). 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-17 

3. ICING ENVELOPES

a. Background. The Appendix C icing envelopes are unchanged by the 

addition of Appendix X. See AC20-73A and DOT/FAA/CT-88/8-1 "Aircraft Icing 

Handbook" for detail on the Appendix C envelopes. Appendix X was defined to 

provide a representative icing environment for supercooled large drops (SLD), 

which include freezing drizzle and freezing rain conditions. 

Section 25.1420 requires that the airplane operate safely in the SLD icing 

conditions defined in Part 25, Appendix X. When complying with § 25.1420(a)(1) 

the applicant must provide a method to detect that the airplane is operating in 

Appendix X; and following detection, the airplane must be capable of operating 

safely while exiting all icing conditions. When complying with § 25.1420(a)(2), 

certification may be requested for portions of Appendix X, such as for freezing 

drizzle only or for specific phases of flight. Following detection of conditions that 

exceed the selected portion of Appendix X, the airplane must be capable of 

operating safely while exiting all icing conditions. Certification for a portion of 

Appendix X (§ 25.1420(a)(2)) or none of Appendix X (§ 25.1420(a)(1)) requires 

substantiated methods be provided to alert flight crews when those portions of 

Appendix X are exceeded (§ 25.1420(a)(2)) or Appendix X conditions are 

encountered (§ 25.1420(a)(1)). Certification to Appendix X (§25.1420(a)(3)) 

requires that the airplane operate safely throughout Appendix X. 

The four spectra with associated LWC limits were developed to describe the 

entire Appendix X environment as it could be determined by all the 

measurements available. Initial certifications to a portion of Appendix X will likely 

include all of freezing drizzle or all of freezing rain, or be restricted by phase of 

flight, provided acceptable means are made available to the flight crew to 

distinguish the portion of Appendix X for which the aircraft is not approved. 

Certification for a portion of Appendix X allows latitude for certification with a 

range of techniques; for example, limiting the range of liquid water contents. In 

doing these type of limited certifications, ice shapes will be developed for the 

portion of the envelope for which the aircraft is approved as well as for detecting 

and exiting icing conditions beyond the selected portion. 

The ice shapes developed for the approved portion of the envelope should 

account for the icing conditions in terms of drop distribution and water content 

and consider the proposed method for identifying the icing conditions that require 

exiting. The definition of the certificated portion of Appendix X should be based 

on measured characteristics of the selected icing environment and consistent 

with the methods used for developing Appendix X. A detailed report on the 

development of Appendix X is available from the FAA Technical Center, 

reference report DOT/FAA/AR-04/7, dated xx/xx/xx. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-18 

SLD icing conditions are defined by Appendix X, including freezing drizzle and 

freezing rain. The two types of freezing precipitation are further divided into 

conditions in which the drop median volume diameters are either less than or 

greater than the 40 μm, the largest mean effective diameter of Appendix C 

continuous maximum icing conditions. Appendix X consists of measured data 

that was divided into droplet distributions within these four icing conditions. 

These distributions were averaged to produce the representative distributions for 

each condition. 

The icing conditions data were divided as follows: 

1. Measured icing conditions with an MVD less than 40 μm and a maximum 

drop size from 100 μm to 500 μm were classified as FZDZ/L 

2. Measured icing conditions with an MVD greater than 40 μm and a 

maximum drop size from 100 μm to 500 μm were classified as FZDZ/G 

3. Measured icing conditions with an MVD less than 40 μm and a maximum 

drop size greater than 500 μm were classified as FZRA/L 

4. Measured icing conditions with an MVD greater than 40 μm and a 

maximum drop size greater than 500 μm were classified as FZRA/G 

The data measured to produce Appendix X was taken in wintertime stratus cloud 

formations. As such, the Appendix X conditions are most closely associated with 

the stratus conditions used to define the existing Appendix C, Continuous 

Maximum conditions. Based on the methods used for Appendix X development, 

the boundary between Appendix C and Appendix X is Continuous Maximum icing 

conditions with maximum drop sizes below approximately 100μm. Use of only 

stratiform cloud data is considered acceptable since stratiform clouds 

predominate during prolonged freezing precipitation conditions. See FAA 

Technical Center report DOT/FAA/AR-04/7, dated xx/xx/xx for more information 

on the development of Appendix X. 

The water content versus drop size relationships defined in 14 CFR 25, Appendix 

C figures 1 and 4 are defined in terms of Mean Effective Drop diameter. By 

definition, this acknowledges the distributed nature of droplet sizes around a 

volume median value of assumed drop size distribution. However, Part 25 does 

not require specific distributions that must be considered for Appendix C icing 

conditions. As referenced in DOT/FAA/CT-88/8-1 "Aircraft Icing Handbook" and 

AC20-73A, single mode distributions such as Langmuir distributions have been 

used to represent the range of drop sizes and associated fractional water 

contents in a specific Appendix C icing condition. 

The distributions assumed for Appendix C icing conditions are inappropriate for 

freezing precipitation icing conditions since these icing conditions often consist of 

small and large drops that are bi-modally distributed. Langmuir type distributions 

cannot capture the bi-modal characteristic. Consequently, the distributions of 

drop sizes are defined as part of Appendix X. The need to include the 

distributions comes from the larger amount of mass in the larger drop diameters 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-19 

of Appendix X. The water mass of the larger drops affects the water catch on 

airplane components, the drop impingement, icing limits, and the ice buildup 

shape. Ice accretions resulting from the water catch must be considered relative 

to their effects on safe flight of the airplane. 

The distribution of freezing precipitation drop diameters varies greatly. 

Subdividing freezing drizzle and freezing rain allowed selection of four average 

drop diameter distributions that are representative of the total range of measured 

drop distributions. These four distributions are shown as cumulative mass 

distributions in Appendix X. 

 Also, the maximum values of freezing drizzle and freezing rain liquid water 

content decrease with decreasing temperature as defined in Appendix X, figures 

1 and 4, respectively. Following the same standard as Appendix C, the values of 

total liquid water are selected at the standard icing condition horizontal extent of 

17.4 nautical miles (20 statute miles). 

The maximum vertical extent of 12,000 feet for freezing drizzle includes the 

cloud layer and any drizzle precipitation below the cloud. The maximum altitude 

of freezing drizzle is the same as that of an Appendix C continuous maximum 

icing cloud, 22,000 feet (Appendix X, figure 3). The temperatures are as cold as 

-13° F (-25°C). 

The maximum vertical extent of freezing rain is 7,000 feet. The maximum 

altitude of freezing rain is 12,000 ft. The temperatures are as cold as 8.6° F (-

13°C). 

The United States Standard Atmosphere temperature lapse rate with altitude is 

not appropriate for freezing drizzle and freezing rain. Typically, freezing rain is 

associated with temperature inversions and freezing drizzle is associated with 

other deviations from the standard lapse rate. In generating ice shapes for 

vertical transits through freezing drizzle and freezing rain, the critical 

temperature should be determined and used for the duration of the vertical 

exposure. 

The liquid water contents of continuous freezing drizzle and freezing rain 

conditions decrease with increasing horizontal extents. Horizontal extents of 

exposures in FZDZ and FZRA can be anywhere from a brief encounter to an 

extended encounter. The use of the Appendix X f-factor is the same as for the 

Appendix C f-factor, which is discussed in Appendix N of AC 20-73A. 

b. Use of Icing Envelopes. The use of Appendix C is addressed in AC 20-

73A and is unchanged by the addition of Appendix X. Appendix X is 

designed to be similar to Appendix C and is used in much the same manner. 

The principal differences between the use of Appendix X relative to Appendix 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-20 

C are the need to consider four icing conditions rather than two when 

determining critical icing conditions and the need to address drop size 

distributions (DSD). 

Applications of DSDs typically require a bin tabulation of the proportion of 

mass (liquid water content) relative to drop diameter. Tables 1 and 2 

represent 10-bin tabulations for the cumulative distributions in Appendix X. 

The mass proportions for the bins were selected to provide reasonable 

resolution of the upper range of the distributions. The shaded columns (a) 

and (b) in the tables contain the values that typically would be input to ice 

accretion computer codes. For some simulation techniques, different 

methods of segregating the bins may be appropriate. Different methods 

should consider the impingement characteristics of the geometry relative to 

drop size. 

Table 1. FZDZ distributions represented using 10 bins. 

 FZDZ/L (MVD < 40 μm) FZDZ/G (MVD > 40 μm) 

(a) (b) (b) 

Bin 

Proportio

n of Mass 

Left 

Boundary 

Point 

(μm) 

Massweighted 

Midpoint 

(μm) 

Right 

Boundary 

Point 

(μm) 

Left 

Boundary 

Point 

(μm) 

Massweighted 

Midpoint 

(μm) 

Right 

Boundary 

Point 

(μm) 

1 0.100 1 9 10 1 11 15 

2 0.200 10 13 16 15 22 36 

3 0.200 16 18 20 36 64 106 

4 0.200 20 23 27 106 150 197 

5 0.100 27 29 33 197 221 247 

6 0.050 33 36 40 247 261 276 

7 0.050 40 48 66 276 292 309 

8 0.050 66 97 137 309 330 354 

9 0.025 137 163 202 354 370 390 

10 0.025 202 252 388 390 417 473 

Note: DSDs in one-micrometer-drop bin resolution are available in electronic 

files from the William J. Hughes Technical Center. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-21 

Table 2. FZRA distributions represented using 10 bins. 

 FZRA/L (MVD < 40 μm) FZRA/G (MVD > 40 μm) 

(a) (b) (b) 

Bin 

Proportio

n of Mass 

Left 

Boundary 

Point 

(μm) 

Massweighted 

Midpoint 

(μm) 

Right 

Boundary 

Point 

(μm) 

Left 

Boundary 

Point 

(μm) 

Massweighted 

Midpoint 

(μm) 

Right 

Boundary 

Point 

(μm) 

1 0.100 1 7 8 1 9 14 

2 0.200 8 10 12 14 35 236 

3 0.200 12 15 19 236 399 526 

4 0.200 19 25 53 526 645 765 

5 0.100 53 255 468 765 833 912 

6 0.050 468 534 595 912 957 1008 

7 0.050 595 655 718 1008 1066 1129 

8 0.050 718 792 883 1129 1197 1288 

9 0.025 883 942 1034 1288 1345 1420 

10 0.025 1034 1191 1553 1420 1545 2228 

Note: DSDs in one-micrometer-drop bin resolution are available in electronic 

files from the William J. Hughes Technical Center. 

 c. Effects of Appendix X icing conditions. One of the significant effects of 

Appendix X icing conditions on aircraft icing physics is the inertia effect of droplet 

size. When compared with the Langmuir distributions associated with Appendix 

C cloud drop sizes, the Appendix X drop size distributions have a larger 

percentage of the total water mass in large drop sizes. The mass of the larger 

drop sizes has sufficient inertia to overcome the local airflow effect and tend 

towards straight-line trajectories. The collection efficiency and the impingement 

limits are both influenced by the inertia between the aircraft flow field and 

supercooled water droplets as well as the splashing and breakup of the larger 

drops. In general, smaller drop sizes tend to follow the local streamlines of air as 

they flow around the airframe. Also, behavior of the water drops are influenced 

by the scale of the object being considered. Increasing the scale of the object 

increases the disturbance in flow ahead of the object, which in turn increases the 

ability of the airflow disturbance to influence the trajectory of the supercooled 

water droplets (Both Appendix C and Appendix X). This results in decreasing 

collection efficiency with increases in scale. 

The Appendix X icing conditions contain larger drop sizes, have a greater 

percentage of total liquid water content in the larger drops, and different liquid 

water contents than the icing conditions of Appendix C. These differences may 

affect the water collection efficiency and result in impingement limits that are 

further aft than those determined for Appendix C icing conditions. For more 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-22 

discussion of the effects of droplet size and trajectory influence, see FAA 

Technical Report DOT/FAA/CT-88/8-1, and AC 20-73A. 

An additional effect of the supercooled large drop environment is the variability in 

the distribution of drop sizes. Ice shapes developed from Appendix C icing 

conditions sometimes assume uniform drop size distributions with all drops 

diameters at the MVD (mono-dispersed) because the predicted main ice shapes 

are not highly sensitive to commonly assumed distributions (such as Langmuir 

distributions). However, the large variations of drop sizes and the larger 

percentage of total water contained in the larger drops within Appendix X tends 

to magnify the effects of distributed drop sizes. The distribution of water drops, 

or more directly, the percentage of water content contained within certain drop 

sizes, can affect shapes and runback ice characteristics. 

d. Primary Areas of Interest for Appendix X. The primary areas of interest 

specific to supercooled large droplet conditions include: 

(1) Ice shapes on protected areas of lifting surfaces. Appendix X icing 

conditions may result in a ridge of ice or ice roughness aft of the protected 

areas.. On aircraft with reversible flight controls, a ridge of ice may result in 

uncommanded deflections of the control surfaces, which may result in an aircraft 

upset. Aircraft, independent of the flight control system, may also incur reduced 

maximum lift and reach the reduced maximum lift at lower angles of attack. 

Without adjustments to the stall protection system activation schedules, the 

Appendix X ice buildups can also result in an aircraft upset or stall. 

 (2) Ice shapes on unprotected areas of lifting surfaces. Ice shapes 

resulting from Appendix X icing conditions can be more extensive than those 

from Appendix C icing conditions. When portions of the airfoil leading edge are 

unprotected, there is potential to affect the performance and handling qualities. 

(3) Ice shapes on unprotected airframe regions. Areas of the airframe, 

which do not accrete ice under Appendix C conditions, should be examined 

relative to the increased impingement or accretion areas in and potentially larger 

ice shapes. Issues such as windshield visibility, air data sensor locations, and 

potential airflow disturbance (for example on airplane nose) from Appendix X 

accretions that could influence the performance of the air data sensors should be 

evaluated. 

 (4) Drag and Power effects. Aircraft may exhibit reduced performance in 

supercooled large drop conditions due to increased drag and reduced lift due to 

ice accretions. These accretions may extend further aft on protected areas, 

extend beyond the protected areas, or occur in areas not typically protected (e.g. 

extended accretions on fuselage and lower surface of the wing). The ice 

accretions may be rough and cause large local drag increases. Additionally, 

thrust effects such those due to bleed penalties from system operations or 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-23 

degradation of thrust from powerplant losses (e.g. propeller icing) should be 

addressed. See AC 25.21-1 for more information regarding the evaluation of 

potential effects on airplane performance. 

(5) Engine Considerations. Airframe and air induction system 

components ice accumulations resulting from Appendix X icing conditions should 

be evaluated for potential ingestion of the ice by the engine (relative to the 

requirements of §§ 33.68 and 33.77) and potential blockage effects. 

4. CERTIFICATION PLAN. At the start of the design and development effort, 

the applicant should submit a certification plan to the cognizant FAA Aircraft 

Certification Office (ACO) for approval. The certification plan should include the 

following basic information: 

 a. A general airplane description that includes dimensions, airworthiness 

limitations, and other data that may be relevant to certification of IPS. 

 b. IPS description. 

 c. A compliance checklist that addresses each section of part 25 applicable 

to the product. 

 d. Identification of the certification methods for each applicable section of 

part 25, including a description of analyses and tests, or references to similarity 

of designs, that the applicant intends for certification of the IPS. These methods 

of showing compliance should be agreed upon between the applicant and the 

FAA early in the certification program prior to conducting certification tests. 

 e. A failure hazard assessment to determine the criticality of the system. 

 f. If the ice protection or detection systems contain software, software 

plans, as described in RTCA DO-178B (or another acceptable means of 

compliance for software). 

 g. Projected schedules of design, analyses, testing, and reporting. 

 h. A list of any anomalous 14 CFR part 33 icing certification test results 

(relative to the requirements of §§ 33.68 and 33.77), if completed, that will 

require special operating procedures. 

 i. If the IPS or detection systems contain complex electronic hardware 

(such as programmable logic devices (PLD) or application specific integrated 

circuits (ASIC)), plans for providing a level of design assurance of these devices 

commensurate with their potential contribution to aircraft hazards and system 

failures, which could result from electronic hardware faults or malfunctions. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-24 

5. ANALYSES. The applicant should prepare analyses to substantiate 

decisions involving the application of selected ice protection equipment. Such 

analyses should clearly state the basic protection required and the assumptions 

made, and delineate the methods of analysis used. All analyses should be 

validated by tests or should have been validated by the applicant on a previous 

program. To utilize a previously validated methodology, the applicant should 

substantiate that the methodology is applicable to the new program. 

Analytical simulation methods include impingement and accretion models based 

on computational fluid dynamics. These methods are typically used to evaluate 

unprotected and protected areas to determine the potential ice accretions. 

Analytical simulation provides a method of accounting for the variability in droplet 

distributions. In addition, analytical simulation provides the capability to examine 

impingement relative to visual icing cues and analyze the location of detection 

devices for detrimental local flow effects. Thermal characteristics of IPSs relative 

to icing conditions may also be evaluated using computational simulation. 

During the substantiation process of icing simulation tools, cross comparisons 

should be made with natural icing flight test data, tanker test data, icing tunnel 

data and analytical codes to show that simulating supercooled large droplets 

results in equivalent or conservative results. The results of icing tunnel tests, 

tanker tests and codes should be compared to the results of natural icing flight 

tests in Appendix C conditions that are required in § 25.1419. Engineering 

judgment is required in evaluating the results of these comparisons, as most 

simulation tools are steady-state simulations that are being contrasted with 

variable natural icing conditions. See AC 20-73A for additional guidance relative 

to substantiation of the icing simulation tools. The guidance in this paragraph 

applies to airframe, engines, and propeller icing tools. 

 a. Analysis of Areas and Components to be Protected. The applicant 

should examine those areas listed below in evaluating the ability of the aircraft to 

safely operate in the icing conditions defined in Appendix C and the relevant icing 

conditions of Appendix X and in determining which components will be protected. 

An applicant may determine that protection is not required for one or more of 

these areas or components. If so, the applicant should include supporting data 

and rationale in the analyses for allowing those areas or components to be 

unprotected. The applicant should show that the lack of protection does not 

adversely affect the handling characteristics or performance of the airplane, as 

required by § 25.21(g), nor the operation and functioning of affected systems and 

equipment (e.g. pitot probes). 

 (1) Leading edges of wings, winglets, and wing struts; horizontal and 

vertical stabilizer; canards; and other lifting surfaces. 

 (2) Leading edges of control surface balance areas, if not shielded. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-25 

 (3) Accessory cooling air intakes that face the airstream and/or could 

otherwise become restricted due to ice accretion. Inlets (including NACA inlets) 

that are in shadow zones for Appendix C, may accrete ice in the Appendix X 

conditions. 

 (4) Antennas and masts. 

 (5) External tanks. 

 (6) External hinges, tracks, door handles, and entry steps. 

 (7) Instruments including pitot tube (and mast), static and dynamic ports, 

angle-of-attack sensor, and stall warning. 

 (8) Forward fuselage nose cone and radome. 

 (9) Windshields and cockpit side windows. 

 (10) Landing gear. 

 (11) Retractable forward landing lights. 

 (12) Ram air turbines. 

 (13) Ice detection lights, if required for compliance with § 25.1403. 

 (14) Vortex generators installed on lifting surfaces and the fuselage, and 

stall improvement devices such as strips, vortilons, and fences. 

 (15) Any structure that extends into the free stream such as cameras, 

camera mounts, and video equipment. 

Note 1: Ice protection of fuel tank vents, propellers, and engine inlet cowls is 

addressed by part 25, subpart E – Powerplant. 

 b. Flutter Analysis. Advisory Circular (AC) 25.629-1A, “Flutter 

Substantiation of Transport Category Airplanes,” provides guidance for showing 

compliance with § 25.629. The flutter analyses should reflect any mass 

accumulations on unprotected and protected surfaces from exposure to 

Appendices C and, as appropriate, X icing conditions, including any accretions 

that could develop on control surfaces. Section 25.629(d)(3) requires the 

consideration of inadvertent encounters with icing for airplanes not certificated for 

flight in icing conditions. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-26 

 c. Power Sources. The applicant should evaluate the power sources in the 

IPS design (e.g., electrical, bleed air, and pneumatic sources). An electrical load 

analysis or test should be conducted on each power source to determine that it is 

adequate to supply the IPS plus all other essential electrical loads throughout the 

airplane flight envelope under conditions requiring operation of the IPS. The 

effect of an IPS component failure on power availability to other essential loads 

should be evaluated in accordance with § 25.1309. All power sources that affect 

engine or engine IPSs for multiengine airplanes must comply with the engine 

isolation requirements of § 25.903(b). 

 d. Failure Analyses. 

 (1) Failure Analysis for § 25.1419 (Appendix C Conditions). Advisory 

Circular (AC) 25.1309-1A, “System Design Analysis,” provides guidance for 

demonstrating compliance with the requirements of § 25.1309. Substantiation of 

the hazard classification of IPS failure conditions is typically accomplished 

through analyses and/or testing. The failure analysis should include the potential 

contribution of complex electronic hardware faults and malfunctions to system 

failure conditions, and classify the hardware assurance level appropriately, see 

RTCA/DO-254, "Design Assurance Guidance for Airborne Electronic Hardware." 

The probability of encountering Appendix C icing is considered one for 

quantitative analysis. 

 (2) Failure analysis for § 25.1420 (Appendix X Conditions). The 

application of the system safety principles of § 25.1309 is helpful in determining 

the necessity of any system requirements to address potential hazards due to the 

Appendix X environment. The following information provides guidance relative to 

the application of these principles to the Appendix X conditions. In showing 

compliance to § 25.1309, the following may be used: 

 (a) Hazard classification. The process of assessing a hazard 

classification in showing compliance to § 25.1309 is typically a combination of 

quantitative and qualitative factors. Assessing the severity of the hazard of an 

unannunciated encounter with Appendix X conditions for a new aircraft model 

design is no different. If the design is new and novel and has little similarity to 

previous designs, a hazard classification based on past experiences may not be 

appropriate. However, if the new design is derivative in nature, the assessment 

can consider the specific icing event history of similarly designed aircraft, as well 

as the icing event history of all conventional design aircraft. When assessing the 

similarity to previous designs, the specific effects of supercooled large droplet 

icing should be considered (see paragraph 3.c, “Effects of Appendix X Icing 

Conditions”). 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-27 

The following is a qualitative analysis that may be used for determining the 

hazard classification associated with an unannunciated encounter with Appendix 

X icing conditions for aircraft that are shown to be similar to previous designs 

with respect to Appendix X icing effects: 

In accordance with the principles of § 25.1309, service history, design and 

installation appraisals may be used in support of hazard classifications. If a new 

or derivative aircraft has similar design features to a previously certificated 

airplane, this service history may be appropriate to support the determination of 

the hazard classification. 

While definitive statistics are not available, a historical perspective can provide 

some guidance. Many aircraft have been unknowingly exposed to supercooled 

large droplet conditions. The exposure interval can vary from a brief exposure 

(such as a vertical transition through a cloud) to a more sustained condition (such 

as a hold). The severity of the conditions in terms of water content can vary 

significantly. Therefore, not all encounters with supercooled large droplet 

conditions may result in a catastrophic event. 

For aircraft that are not similar to a previous design, an assessment of the hazard 

classification may require more analysis or testing to assess the hazard. One 

method of assessing the hazard would be to consider the effects of ice 

accumulations similar to those expected for aircraft to be certified under the 

provisions of §25.1420. These ice shapes may be defined from a combination of 

analysis, tanker or icing wind tunnel testing. The aerodynamic effects of such 

shapes could be evaluated with wind tunnel testing or potentially, computational 

fluid dynamics. See AC 20-73A for additional guidance on assessing the 

aerodynamic effects of ice buildups. The hazard classification typically takes 

place early in a certification program. Therefore, a conservative assessment of 

the hazard may be required until sufficient supporting data is available to reduce 

the hazard classification. 

 (b) Probability of Encountering Appendix X. The appendix C conditions 

were determined on the 99 percentile probability of exceeding the icing 

conditions (Ref: NACA TN 2738, “A Probability Analysis of the Meteorological 

Factors Conducive to Aircraft Icing in the United States”). In other words, the 

probability of encountering icing outside of Appendix C droplet conditions is on 

the order of 10-2. Climatology studies for North America indicate that the annual 

average frequency of occurrence of SLD range from 0.4% to 5% of the time in a 

100 km radius column over various stations. Determining the actual occurrence 

of SLD in the atmosphere is difficult using the data sets available. It is 

recognized that there are large geographical differences and changes with 

seasons that influence the probability of SLD occurrence. However, to a first 

approximation, the probability of occurrence of SLD for any particular location in 

North America, representing the altitude ranges between 0 and 15,000 ft, which 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-28 

airplanes normally encounter upon takeoff and landing, is typically 1 to 5 percent 

over a winter season for a large portion of the continent. For operations above 

15,000 ft, the occurrence of SLD during the winter is less frequent. These 

numbers do not account for the full probability of an aircraft encountering 

supercooled large drops within the 100 km radius of the column. Based on this 

information, on an annual basis the average probability of 1 x 10-2 per flight hour 

may be assumed for encountering appendix X conditions within a quantitative 

analysis. The probability should not be reduced based on phase of flight. 

 (c) Numerical safety analysis. For the purposes of a numerical safety 

analysis, the applicant may combine the probability of equipment failure with the 

probability, defined above, of encountering appendix X icing conditions. 

Therefore, if the applicant can support a hazard level of “hazardous” and 

considering the above probability of encountering the specified supercooled large 

droplet conditions (10-2), it follows that the probability of an unannunciated 

equipment failure should be less than 1 x 10-5. 

 (d) Assessment of visual cues. Typical system safety analyses do not 

address the probability of crew actions such as observation of a visual cue prior 

to performing a specified action. As advised in AC 25.1309-1A, quantitative 

assessments of crew errors are not considered feasible. When visual cues are 

the method of detecting Appendix X conditions to determine when to exit, an 

assessment of the appropriateness and reasonableness of the specific cues 

should be performed (see paragraph 8.b.(2).(b) of this AC for additional 

guidance). Reasonable tasks are those for which full credit can be taken 

because they can realistically be anticipated to be performed correctly when they 

are required. The task of visually detecting Appendix X conditions should be 

assessed to determine if it could be performed when required. In part, this is 

coincident with the substantiation of the specific cues as described in paragraph 

13.f. The workload of the visual detection method should be considered in 

combination with the operational workload during applicable phases of flight. It 

may be assumed that the flight crew is already aware that the aircraft has 

encountered icing and the assessment of appropriateness and reasonableness 

of the task is limited to identifying Appendix X accumulations that require exiting 

from icing conditions. 

 e. Similarity Analyses. 

 (1) In the case of certification based on similarities to other type 

certificated airplanes previously approved for flight in icing conditions, the 

applicant should specify the airplane model and the component to which the 

reference applies. Specific similarities should be shown in the areas of physical, 

functional, thermodynamic, pneumatic, aerodynamic, and environmental 

exposure. Analyses should be conducted to show that the component 

installation, operation, and the effect on the airplane’s performance and handling 

are equivalent to that of the previously approved configuration. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-29 

 (2) Similarity requires an evaluation of both the system and installation 

differences. The differences should be evaluated for their effect on the 

functionality of the IPS and on safe flight in icing. If there is uncertainty about the 

effects of the differences, additional tests and/or analyses should be conducted 

as necessary and appropriate to resolve the open issues. 

 (3) Section 14 CFR 25.1419(b) requires flight testing in measured 

natural icing conditions. However, flight test data from previous certification 

programs may be used to show compliance with 14 CFR 25.1419(b) if it can be 

shown that the data is applicable to the airplane in question. If there is 

uncertainty about the similarity analysis, the manufacturer should conduct flight 

tests in measured natural icing conditions for compliance with § 25.1419(b). 

 (4) On derivative airplane designs, similarity to previous type designs that 

have shown compliance to § 25.1420 can be used as a method of compliance if 

the effects of differences are substantiated. Natural ice flight-testing may not be 

required for design shown to be similar. As a minimum, the following differences 

should be addressed: 

 1. Airfoil size, shape, and angle of attack 

 2. IPS design 

 3. Operating altitude, airspeed 

 4. Center of gravity 

 5. Flight control system 

 6. Engine and propeller operation 

Note: The applicant must possess all the data to substantiate compliance with 

applicable regulations, including the data from past certifications upon which the 

similarity analysis is based. 

 f. Impingement Limit Analyses. The applicant should prepare a droplet 

trajectory and impingement analysis of the wing, horizontal and vertical 

stabilizers, propellers, and any other critical surfaces upon which ice may 

accrete. This analysis should consider the various airplane operational 

configurations, phases of flight, and the associated angles of attack. The 

analysis should establish the upper and lower aft droplet impingement limits that 

can then be used to establish the aft ice formation limit and the relationship to the 

IPS coverage. 

g. Ice Shedding Analyses. Airframe ice shedding may damage or erode 

engine or powerplant components as well as lifting, stabilizing, and flight control 

surface leading edges. Fan and compressor blades, impeller vanes, inlet 

screens and ducts, as well as propellers, are examples of powerplant 

components subject to damage from shedding ice. For fuselage-mounted 

turbojet engines (and pusher propellers that are very close to the fuselage and 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-30 

well aft of the airplane's nose), ice shedding from the forward fuselage and from 

the wings may cause significant damage. Ice shedding from components, 

including antennas, of the airplane should cause no damage to engines and 

propellers that would adversely affect engine operation or cause a serious loss of 

power or thrust (compliance to 14 CFR 25.1093). Consideration should also be 

given to airplane damage that can occur due to ice shedding from the propellers. 

Control surfaces such as elevators, ailerons, flaps, and spoilers are also subject 

to damage, especially those constructed of thin metallic, nonmetallic, or 

composite materials. Currently available trajectory and impingement analyses 

may not adequately predict such damage. Unpredictable ice shedding paths 

from forward areas such as radomes and forward wings (canards) have been 

found to negate the results of these analyses. For this reason, a damage 

analysis should consider that the most critical ice shapes will shed and impact 

the areas of concern. 

h. Propeller deicing analysis. A propeller deicing analysis should be 

accomplished. In addition to intercycle ice, the applicant should account for the 

effect of ice accretions aft of the protected leading edge on propeller efficiency 

and airplane performance. 

i. Pitot Probe Ice Protection. Compliance to the TSO qualification standard 

for electrically heated pitot probes (TSO-C16) is not sufficient by itself in 

demonstrating compliance to the installation requirements of §§ 25.1309(a), 

25.1323, 25.1326, 25.1419, and 25.1420. Section 25.1309(a) requires that the 

system must perform its intended function under any foreseeable operating 

condition. Sections 25.1419 and 25.1420 require that an airplane certificated for 

flight in icing must be able to safely operate in part 25 Appendix C and Appendix 

X icing conditions, respectively. It is unlikely that the conditions of Appendices C 

or X that are critical to the air data system equipment will be encountered during 

flight tests. Consequently, certification programs should supplement the icing 

flight tests with icing tunnel test and/or tanker test data, as necessary. In-service 

experience during severe atmospheric conditions has shown that pitot tubes 

qualified to the older standards have resulted in airspeed fluctuations and even 

loss of indicated airspeed. As these components must perform their intended 

function under any foreseeable operating condition, they should be qualified to 

the Continuous and Intermittent maximum icing conditions defined in FAR 25, 

Appendix C, and to Appendix X icing conditions. 

(1) TSO C16, Air-speed tubes (electrically heated), requires compliance 

to the performance specifications of SAE Aerospace Standard AS393. SAE 

AS393A includes a test to demonstrate deicing and anti-icing capability, but only 

temperature and airspeed are specified. Liquid water content is not specified but 

it influences heat requirements. Although functioning of pitot probes are 

evaluated in natural icing conditions during certification test programs, there is no 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-31 

requirement to flight test at the Appendix C or X icing limits because the low 

probability of finding those conditions imposes a burden. The airframe 

manufacturer is responsible for showing the pitot heat is adequate for the 

Appendix C and Appendix X icing conditions. If not obtained in flight test, 

analysis or icing tunnel test data should be submitted. 

(2) Although Appendices C and X of part 25 only consider the liquid 

water content of icing conditions, recent cloud characterization research has 

indicated that approximately 40 percent of icing condition events consist of liquid 

water drops and ice crystals (mixed-phase icing conditions). Also, glaciated 

atmospheric conditions are encountered during aircraft operations. The ice 

crystal environment may be more critical than liquid water for thermal systems 

since more energy is required to evaporate the ice crystals. Recently, some 

aircraft manufacturers and foreign certification authorities have required pitot and 

pitot-static probes to be tested in ice crystal and mixed-phase icing conditions 

along with supercooled liquid water conditions. As a result, some pitot tube 

manufacturers now use the icing environment of British Specification (BS) 

2G.135, “Specification for Electrically-Heated Pitot and Pitot-Static Pressure 

Heads,” in addition to the requirements of TSO. Even though the part 25 

regulations only address liquid water, it is good design practice to ensure the 

pitot heat is sufficient for the ice crystal and mixed-phase conditions of BS 

2G.135. 

j. Stall Warning System Ice Protection. Compliance to the TSO qualification 

standard for stall warning instruments (TSO-C54) is not sufficient by itself in 

demonstrating compliance to the installation requirements of 

§§ 25.1309(a), 25.1419, and 25.1420. 

(1) TSO-C54, “Stall Warning Instruments”, requires compliance to the 

performance specifications of SAE Aerospace Standard AS403A with some 

exceptions and additions. As in AS393A, the requirements include a test to 

demonstrate deicing and anti-icing capability, but only temperature and airspeed 

are specified. The precipitation test conditions of AS-403A include moderate 

icing conditions for Type II instruments. However, "moderate" is not defined. 

The same comments from 5.i.(1) apply. The airframe manufacturer is 

responsible for showing the stall warning heat is adequate for Appendix C and 

the applicable X icing conditions. 

(2) The same comments from 5.i.(2) apply. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-32 

 k. Runback Ice. Water not evaporated by thermal ice protection systems 

and unfrozen water during near-freezing conditions (or when the freezing 

fraction is less than one) may run aft and form runback ice. The resulting 

runback ice can capture additional mass through direct impingement. Computer 

codes may be unable to estimate the characteristics (rivulets or thin layers) of 

the runback water or the resultant ice shapes; however, some codes may be 

able to estimate the mass of the runback ice. Therefore, runback ice should be 

determined experimentally. Runback ice should be considered when 

determining critical ice shapes. Simulated runback ice shapes may be used 

when evaluating the effects of critical ice shapes. Consideration should be 

given to potential hazards resulting from the shedding of runback ice. 

6. SIMULATED ICE SHAPES and ROUGHNESS AC 25.21-1 contains 

guidance on the icing exposure during various phases of flight that should be 

considered when determining simulated ice shapes and surface roughness. The 

shape and surface roughness of the ice shapes should be developed and 

substantiated using acceptable methods. Common practices include: use of 

computer codes for droplet impingement limits and ice shape predictions (see 

SAE ARP5903), flight in measured natural icing conditions, icing wind tunnel 

tests (see SAE ARP5905), and flight in a measured controlled simulated icing 

cloud (e.g., behind an icing tanker, see SAE ARP5904). 

 

a. During holding conditions it should be assumed that the airplane will 

remain in a rectangular “race track” pattern, with all turns being made within 

the 17.4 nautical mile icing cloud. Therefore, no horizontal extent correction 

should be used for this analysis. 

b. The applicant should substantiate the drop diameter distribution (mean 

effective, median volume, spectra), LWC, and temperature that result in the 

formation of an ice shape that is critical to the airplane’s performance and 

handling qualities. 

c. The ice roughness should be based on icing tunnel, natural icing, or 

tanker testing. 

 d. See AC 20-73A for more information on ice shapes. 

7. COMPLIANCE TESTS (Flight/Simulation) . The following paragraphs 

address the major methods normally used to show compliance with the 

requirements of §§ 25.1419 and 25.1420. These requirements state that the 

airplane must be able to safely operate in the continuous maximum and 

intermittent maximum icing conditions described in Appendix C and the 

appropriate icing conditions described in Appendix X of part 25. The airplane 

should be shown to comply with the certification requirements when all IPSs are 

installed and functioning. This can normally be accomplished by performing tests 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-33 

at those conditions found to be most critical to basic airplane aerodynamics, IPS 

design, and powerplant functions. All IPS equipment should perform its intended 

function throughout the entire operating envelope. 

The primary purposes of flight testing are to determine that the IPS is acceptably 

effective and performs its intended functions during flight as predicted by analysis 

or ground testing and to evaluate any degradation in performance and flying 

qualities. Performance and handling qualities requirements are identified in § 

25.21(g). Flight tests to show compliance with these requirements are 

addressed in AC 25.21-1. Flight testing also verifies the adequacy of the 

flightcrew procedures and limitations for the use of the IPS in normal, abnormal, 

and emergency conditions. 

Flight testing is also used to confirm that the powerplant installation as a whole 

(engine, inlet, anti-ice system, etc.) performs satisfactorily while in icing 

conditions. In addition, the APU should be evaluated for performance in icing 

conditions if it performs essential functions. In accordance with part 33, engine 

icing certification results should also be carefully reviewed prior to any part 25 

engine or airframe icing flight tests in order to determine if there were any 

anomalous results that will require special airplane/engine operating procedures. 

Both 14 CFR §§ 25.1419 and 25.1420 require the use of one or more methods of 

compliance. It is common to use a combination of methods to adequately 

represent the conditions and determine the degradation effects with sufficient 

confidence to show compliance. 

 a. Dry Air Ground Tests. Depending upon the details of the IPS design, 

some preliminary ground tests of the equipment may be warranted to verify the 

basic function of each item. Quantitative data may be obtained, as necessary, to 

verify the system designs. These data include operating temperatures of thermal 

devices, deicing fluid flow rates and flow patterns for deicing liquid devices, and 

operating pressures and rates of inflation of pneumatic components. 

 b. Dry Air Flight Tests with Ice Protection Equipment Operating. Initial dry 

air flight tests are usually the first flight tests conducted to evaluate the airplane 

with the IPS operating. The initial dry air tests are conducted to verify that the 

IPS does not affect the flying qualities of the airplane in clear air and to obtain a 

thermal profile of an operating thermal IPS. Several commonly used IPSs and 

components are discussed below to illustrate typical dry air flight test practices. 

Other types of equipment should be evaluated as their specific design dictates. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-34 

 (1) Thermal Ice Protection Leading Edge Systems. Dry air flight tests 

are conducted to verify the system design parameters and thermal performance 

analyses. 

 (a) Normally, the system components are instrumented to 

measure the anti-icing mass flow rate or energy input (for electrical systems), 

supply air temperature, and the surface temperatures. The dry air test plan 

generally includes the airplane operating conditions such as climb, holding, and 

descent phases of a normal flight profile. Since the presence of moisture can 

affect the surface temperatures, the tests should be conducted where no visible 

moisture is present. 

 (b) The measurements of supply air mass flow rate, 

energy input and air temperature allow the determination of heat available to the 

system. The adequacy of the IPS then can be demonstrated through a 

comparison of the measured data to the theoretical analysis over Appendix C 

and either Appendix X or the portion of Appendix X proposed for compliance with 

§ 25.1420(a)(2). The surface temperatures measured in the dry air, for example, 

can be useful in extrapolating the maximum possible leading edge surface 

temperature in-flight, the heat transfer characteristics of the system, and the 

thermal energy available for the IPS. The supply air temperatures or energy 

input also may be used to verify that the materials of the IPS were appropriately 

chosen for the thermal environment. 

 (2) Pneumatic Leading Edge Boots. Tests should demonstrate a rise 

and decrease in operating pressures, which results in the effective removal of 

ice. This pressure rise time, as well as the maximum operating pressure for each 

boot, should be evaluated throughout the altitude range defined in Appendix C 

and X. Boots should be operated in flight at the minimum envelope temperature 

(-22 o

F) of Appendix C Continuous Maximum and Appendix X, to demonstrate 

adequate performance throughout the entire flight envelope and to demonstrate 

that no damage occurs during inflation and deflation. The appropriate speed and 

temperature limitation (if any) on boot activation should be included in the AFM. 

The inflation of the boots should have no significant effect on airplane 

performance and handling qualities. 

 (3) Electrically Heated Propeller Boots. For compliance with the 

provisions of §§ 25.901(c), 25.903(b), 25.929, and 25.1419(c): 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-35 

 (a) When flight testing in dry air, the following system parameters 

should be monitored to confirm proper function. It is suggested that system 

current, brush block voltage (i.e., between each input brush and the ground 

brush) and system duty-cycles be monitored to ensure that adequate power is 

applied to the deicers. Surface temperature measurements should be made 

during dry air tests. These surface temperature measurements are useful for 

correlating analytically predicted dry air temperatures with actual temperatures, 

and as a general indicator that the system is functioning and that each deicer is 

heating. 

 (b) The system operation should be checked throughout the full 

r.p.m. and propeller cyclic pitch range expected during flight in icing. Any 

significant vibrations should be investigated. 

 (c) Consideration should be given to the maximum temperatures to 

which a composite propeller blade may be subjected when the deicers are 

energized. It may be useful to monitor deicer bond-side temperatures. When 

performing this evaluation, the most critical conditions should be investigated 

(e.g., airplane on the ground; propellers non-rotating) on a hot day with the 

system inadvertently energized. 

 (4) Windshield Anti-Ice. Section 25.773(b)(1)(ii) requires that the 

airplane must have a means to maintain a clear portion of the windshield in icing 

conditions specified in §§ 25.1419 and 25.1420. Dry air flight tests should be 

conducted to verify the thermal analysis. Both inner and outer windshield surface 

temperature measurements of the protected area may be needed to verify the 

thermal analysis. Thermal analysis should substantiate that the windshield 

surface temperature is sufficient to maintain anti-icing capability without causing 

structural damage to the windshield. An evaluation of the visibility, including 

distortion effects through the protected area, should be made for both day and 

night operations. In addition, the size and location of the protected area should 

be reviewed for adequate visibility, especially during the approach and landing 

phases of flight. 

 (5) Pitot-Static and Static Pressure Sources. Section 25.1323(e) 

requires a heated pitot tube or an equivalent means of preventing malfunction 

due to icing for each airspeed indicating system. Section 25.1325(b) requires 

that static pressure ports be designed and located such that the correlation 

between air pressure in the static pressure system and true ambient atmospheric 

static pressure is not affected when the airplane encounters icing conditions 

defined in Appendices C and Appendix X. Section 25.1326 requires an 

indication system be provided to indicate to the flightcrew when a pitot heating 

system is not operating. An acceptable indication system may consist of 

separate lights or crew alert indication on an electronic display for each pitot 

source. Additional guidance on an acceptable means of compliance with 

§ 25.1326 is provided in AC 25-11. Surface temperature measurements are 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-36 

typically made for air-data instruments, such as pitot tubes, pitot-static pressure 

probes, and angle-of-attack probes (if ice protected), during icing wind tunnel 

tests to verify thermal analyses. The acceptability of the installed air data 

instrument ice protection should be evaluated during natural or artificial icing 

tests. 

 (6) Bleed Air Systems. The effects of any bleed air extraction on engine 

and airplane performance should be examined and included in the AFM 

performance data. The surface heat distribution analysis should be verified for 

varying flight conditions including climb, cruise, hold, and descent. Temperature 

measurements may be necessary to verify the thermal analyses. In accordance 

with the provisions of § 25.939(a), the effects of the maximum bleed air for ice 

protection should have no detrimental effect on engine operation throughout the 

engine’s power range. 

c. Dry air flight tests with predicted simulated ice shapes and roughness. The 

primary function of dry air flight tests with simulated ice shapes is to demonstrate 

the ability of the airplane to operate safely with critical ice shapes determined in 

Appendix C and appropriate Appendix X icing conditions. Scale effects and 

scaled roughness effects should be substantiated using acceptable methods. 

The specific flight tests used to evaluate airplane performance and handling 

qualities are addressed in AC 25.21-1. In addition, the effects due to ice 

accretion on locations forward of pitot-static sensors, angle of attack sensors, 

and static pressure ports should be assessed. 

For failure conditions of the IPS that are not extremely improbable, validation 

testing may be required (e.g. flight tests, flight simulator, wind tunnel test, 

validated CFD) to demonstrate that the effect on safety of flight (as measured by 

degradation in flight characteristics) is commensurate with the failure probability. 

Dry air flight tests with predicted critical failed IPS ice shapes, which may include 

asymmetric ice shapes, may be used to demonstrate acceptable operational 

safety. 

 d. Icing Flight Tests. Flight tests in measured natural icing and the use of 

artificial icing tools, such as icing tankers, are normally employed to demonstrate 

that the IPS performs during flight as predicted by analysis or other testing. They 

are also used to confirm analyses used in developing the various components 

(e.g., ice detectors) and ice shapes. In the case of natural icing, testing should 

be accomplished within the icing conditions of part 25, Appendix C and, if 

necessary, Appendix X, to corroborate the general nature of the effects on 

airplane handling characteristics and performance determined with simulated ice 

shapes (see AC 25.21-1), as well as to qualitatively assess the analytically 

predicted location and general physical characteristics of the ice accretions. If 

necessary, there should be a means to measure and record ice accumulations 

and impingement limits. They can be approximated by various means, e.g., a 

rod mounted on the airfoil and black paint on the airfoil. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-37 

 (1) Instrumentation. Sufficient instrumentation should be planned to allow 

documentation of important airplane, system and component parameters, and 

icing conditions encountered. The following parameters should be considered: 

 (a) Altitude 

 (b) Airspeed 

 (c) Engine power level or speed 

 (d) Propeller speed and pitch if applicable 

 (e) Temperatures that could be affected by ice protection equipment 

or ice accumulation or that are necessary for validation of analyses, such as: 

 1. Static air 

 2. Engine component 

 3. Electrical generation equipment 

 4. Surface 

 5. Structure integrity 

 (f) Liquid water content. The liquid water content should be 

measured over the complete water drop size distribution. 

 (g) Median volume drop diameter and drop diameter spectra. When 

measurement of the icing environment drop diameter is necessary, 

instrumentation used for measuring drop sizes should be appropriate for the 

Appendix C and Appendix X icing conditions. For aircraft to be certified to a 

portion or all of Appendix X, measurement and recording of drop diameter 

spectra should be accomplished. 

Microscopic measurement of drop impact craters on a gelatin oil or soot slide 

exposed to an icing cloud may be an impractical method for measuring a median 

volume drop diameter because of the splashing and bouncing characteristics of 

large drops. Also, use of gelatin oil or soot slides is impractical for recording the 

drop diameter spectra of Appendix X type clouds. Median volume diameters and 

drop diameter distributions are more practically obtained using more 

sophisticated equipment, such as laser-based drop measuring and recording 

instruments, or their equivalent. Depending on the drop size measurement 

capability of the instruments, more than one instrument may be required to 

measure the expected range of drop diameters. Large drop instrumentation on 

the test aircraft may not be required if testing is performed with a calibrated icing 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-38 

tanker. Some icing conditions may contain mixed-phase icing (liquid plus ice). 

The instrumentation should provide the capability to be able to distinguish 

between liquid drops and ice crystals. The equipment should be calibrated and 

properly maintained to obtain quality data. 

 (2) Artificial Icing. Flight testing in artificial icing environments, i.e., 

behind icing tankers, represents one way to predict ice protection capabilities of 

individual elements of the ice protection equipment and determine local ice 

shapes. Due to limited cross-sectional area of the plume, testing is usually 

limited to components such as, heated pitot tubes, antennas, air inlets including 

engine induction air inlets, empennage, airfoil sections, and windshields. 

Calibration and verification of the icing cloud produced by the icing tanker should 

be accomplished as necessary to meet the test objectives. SAE ARP5904 

provides recommended practices for using tanker icing simulations. 

The use of an icing tanker can provide high confidence in local icing effects. 

However, it may be difficult to obtain small drop sizes with some spray nozzles. 

Therefore, these methods could produce larger ice build-ups and different ice 

shapes than those observed in natural icing conditions within the icing envelope 

of part 25, Appendix C. This technique can be used in a similar manner to the 

icing tunnel testing with respect to ice shape development. The plume may be of 

sufficient size that it could be applied to sections of the airframe to examine any 

potential hinge moment or CLmax effects due to ice accretions behind protected 

areas for Appendix C or Appendix X icing conditions. This method also has the 

advantage of being able to combine the effects of thermal systems (such as 

runback) with direct accretion to simulate the resulting ice accumulations. 

Tanker simulation could also be used to estimate accretion areas that could be 

used in drag estimates from extended accretion regions (such as ice accretions 

extending beyond a radome) during exposures to Appendix X icing conditions. 

Atmospheric effects such as humidity and drop residence time (time required to 

bring the drop to static temperature) should be considered in this type of testing. 

 (3) Appendix C Natural Icing Flight Testing. 

 (a) Section 25.1419(b) requires measured natural icing flight tests. 

Flight tests in measured natural icing conditions are intended to verify the ice 

protection analysis, to check for icing anomalies, and to demonstrate that the IPS 

and its components function as intended. Advisory Circular 20-73A provides 

additional information that is useful for planning a natural icing flight test program. 

Sufficient exposures to icing conditions should be obtained to allow extrapolation 

to the envelope critical conditions by analysis. Test data obtained during these 

exposures may be used to validate the analytical methods used and the results 

of any preceding artificial icing tests. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-39 

 (b) Past experience indicates that flight testing in natural intermittent 

maximum icing conditions may be accompanied by lightning strikes, severe 

turbulence and possible hail encounters that could extensively damage the test 

airplane. When design analyses show that the critical ice protection design 

points (i.e., heat loads, critical ice shapes, accumulation, and accumulation rates, 

etc.) are adequate under these conditions, and sufficient ground or flight test data 

exist to verify the analysis, the flight test in intermittent maximum icing conditions 

may not be necessary. 

 (c) Flight testing in natural icing conditions should also be used to verify 

AFM procedures for activation of the IPS, including recognition and delay times 

associated with IPS activation and verify the analytically predicted location and 

general physical characteristics of the ice accretions. Critical ice accumulations 

should be observed, where possible, and sufficient data taken to allow correlation 

with dry air testing. Remotely located cameras either on the test airplane or on a 

chase airplane have been used to document ice accumulations on areas that 

cannot be seen from the test airplane’s flightdeck or cabin. 

 (d) For an airplane with a thermal deicing system as the IPS, the 

applicant should demonstrate the effectiveness of the deicing operation either in 

artificial icing conditions or during a natural icing flight test certification program. 

The tests usually encompass the measurements of the surface temperature time 

history. This time history includes the time the system is activated, the time the 

surface reaches an effective temperature, and the time the majority of ice is shed 

from the leading edge. The data should be recorded in the flight test report. 

(4) Appendix X Natural Icing Flight Testing. Section 25.1420(a) requires that the 

airplane operate safely in the SLD icing conditions defined in Part 25, Appendix 

X. 

 

 (a) Unless shown by other means, such as discussed in paragraph 

7.d.(4)(b), flight testing in measured natural Appendix X icing conditions may be 

necessary to: 

 1. Verify the general physical characteristics and location of the 

simulated ice shapes utilized for dry-air testing, and in particular, their effects on 

airplane handling characteristics. 

 2. Determine if ice accretes on areas not predicted to accrete ice; 

 3. Verify adequate performance of ice detectors or visual cues 

 4. Conduct the performance and handling quality tests as outlined in 

AC 25.21(g). 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-40 

 5. Evaluate the effects of ice accretion not normally evaluated with 

simulated ice shapes (propeller, antennas, spinners, etc.) and evaluate operation 

of any critical aircraft system or component after exposure to Appendix X 

conditions. 

 (b) Flight testing in natural Appendix X icing conditions should not be 

necessary if: 

 1. The design analyses show that the critical ice protection design 

points (i.e., heat loads, critical ice shapes for performance and handling qualities, 

accumulation, and accumulation rates, etc.) are adequate under the conditions of 

Appendix X and various airplane operational configurations; and 

 2. The analyses performed for item 1. are accomplished with 

methods that have been validated for Appendix X icing conditions by; i) at least 

two different methods of predicting Appendix X ice accretions should be shown to 

provide similar results (ice accretion thickness, location) One method should be 

either an icing wind tunnel or icing tanker test; or; ii) Similarity analysis may be 

used if the methods used on similar designs were validated as in (i); and 

3. Adequate analyses and/or tests are accomplished if there is a 

need to evaluate more than one airplane component simultaneously. As 

examples, the evaluation of airplane performance with propeller and airframe ice 

accretion, asymmetric ice accretions due to propeller wash, engine performance 

with inlet (including cooling) ice accretions, or stall warning and characteristics 

with ice accretion that affects air data used by stall protection systems. 

(c) The necessity of flight testing in natural Appendix X icing is reduced 

for aircraft limiting to exiting from Appendix X conditions per § 25.1420(a)(1) due 

to reduced exposure to such conditions. 

(d) Flight testing in natural Appendix X icing conditions should be 

accomplished for airplane derivatives whose ancestor airplanes have a service 

record that includes a pattern of accidents or incidents due to inflight encounters 

with Appendix X conditions. 

 e. Fluid Anti-Icing/Deicing Systems. Flight testing should include 

evaluation of fluid flow paths to confirm that adequate and uniform fluid 

distribution over the protected surfaces is achieved. Fluid flow paths should be 

determined when the fluid is mixed with impinging water during system operation. 

A means of indicating fluid flow rates, fluid quantity remaining, etc., should be 

evaluated to determine that the indicators are plainly visible to the pilot and that 

the indications provided can be effectively read. A shutoff valve should be 

provided in systems using flammable fluids. The fluid anti-icing/deicing systems 

may be used to protect propellers and windshields as well as leading edges of 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-41 

airfoils. The fluid for windshield fluid anti-ice systems should be tested to 

demonstrate that it does not become opaque at low temperatures. The AFM 

should include information so the flightcrew will know how long it will take to 

deplete the amount of fluid remaining in the reservoir. 

f. Icing Wind Tunnel Tests. Icing wind tunnels provide the ability to 

simulate natural icing conditions in a controlled environment and a variety of 

conditions within both Appendix C and Appendix X freezing drizzle. This type 

testing has been used to evaluate ice shapes on unprotected areas as well as 

the performance of ice protection systems, such as pneumatic deicing or thermal 

systems, and any resulting protected area accretions, such as intercycle, residual 

and thermal runback. Aerodynamic effects (such as hinge moments) may also 

be evaluated in an icing tunnel. Testing in an icing wind tunnel has the ability to 

combine the performance of the IPS, including runback ice, with any potential ice 

accretions behind the protected areas due to Appendix X icing conditions. Icing 

tunnels and scale models may be used, with appropriate scaling corrections, to 

examine impingement limits relative to fuselages or windscreens with scale 

models relative to Appendix C and Appendix X icing conditions. While scaling 

icing shapes is still an emerging area of technology, scaling of droplet inertia 

effects and impingement limits are in a more mature state. This technique may 

be used to examine visual icing cues, validate location of detection devices, and 

determine total accretion areas that could be used in drag estimates (if required). 

g. Dry-air wind tunnel tests. Dry-air wind tunnel testing has long been 

used to determine ice protection requirements. Ice shapes defined by 

computation, icing tunnels or icing tankers can be used. Scale aerodynamic and 

roughness effects should be substantiated using methods found acceptable to 

the authorities. 

8. CERTIFICATION TO § 25.1420. Section 25.1420 requires that the 

airplane operate safely in the supercooled large droplet (SLD) icing conditions 

defined in Part 25, Appendix X. 

Interim SLD tools and test techniques have been developed and are expected to 

be improved and validated. When 14 CFR Part 25.1419 was issued in 1965, the 

capabilities for simulating icing conditions in laboratories and in flight, as well as 

the analyses used to predict ice shapes, were rudimentary or did not exist; thus, 

reliance was placed upon conservative use of then-existing icing simulation 

methods, engineering judgment, and flight testing in natural icing conditions to 

demonstrate compliance with icing requirements. The interim SLD tools and test 

techniques are an extension of current Appendix C methods, but are not 

developed to the same level as current Appendix C tools. Progress has been 

made in freezing drizzle simulation both through calibration of existing icing wind 

tunnels and refinements of drop impingement and ice accretion computer codes. 

However, extension of the progress made in freezing drizzle to the freezing rain 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-42 

regime has not been accomplished. Due to these limitations with freezing drizzle 

and freezing rain methods, reliance on available simulation methods and 

engineering judgment will be required for finding compliance with 25.1420. 

Paragraph 7(d)(4) of this AC discusses the use of simulation in lieu of natural 

icing flight testing in SLD. 

 a. Certification to § 25.1420(a)(3). For compliance with § 25.1420(a)(3), if 

the AFM performance data reflects the most critical ice accretion (Appendix C 

and Appendix X) and no special normal or abnormal procedures are required in 

Appendix X conditions, then a means to indicate when the aircraft has 

encountered Appendix X icing conditions is not required. 

b. Certification to §§ 25.1420(a)(1) and 25.1420(a)(2). As an alternative 

to the requirements of § 25.1420(a)(3), applicants have the option of complying 

with §§ 25.1420(a)(1) or (a)(2), which allows an aircraft to be certified to a limit 

ranging from Appendix C only to a portion of Appendix X. The boundary may be 

in terms of any parameters that defines Appendix X and could include phase of 

flight limits, such as takeoff or holding, in Appendix X or a portion of Appendix X. 

For example, an airplane may be certificated to takeoff in portions of Appendix X 

but not certificated to hold in those same conditions. Substantiated means must 

be provided to inform flight crews when the selected icing conditions boundary is 

exceeded. Compliance with § 25.21(g) for exiting the restricted Appendix X icing 

conditions must be shown. The ice shapes to be tested are those that represent 

the critical icing conditions within Appendix X during recognition and subsequent 

exit from icing conditions. Methods of defining the selected Appendix X icing 

conditions boundary should be considered early in the certification process, with 

concurrence from the appropriate certification authority. See paragraphs 15 of 

this AC for specific guidance on AFM limitations and operating procedures. 

(1). Selection of Appendix X Boundary. As discussed in paragraph 3.c., “Effects 

of Appendix X Icing Conditions,” the physics of ice accretion is complex. 

However, some generalizations can be made relative to the effects of 

supercooled water drop impingement and ice limits. By definition, the modified 

inertia parameter is a measure of a water drop's inertia relative to an object such 

as an airfoil. (See FAA Technical Report DOT/FAA/CT-88/8-1, “Aircraft Icing 

Handbook” for detailed discussion on the icing physics process.) The modified 

inertia parameter is a dimensionless parameter that relates the major influences 

on the trajectory of a water drop. These influences are drop size, drop density, 

local air density, drag forces on a droplet, aircraft velocity, and aircraft scale, and 

the drop's drag if it's shape is distorted from a sphere by the airflow disturbances 

caused by the object. Examination of the modified inertia parameter is of interest 

for the inertia of Appendix X large drops in that increases in the modified inertia 

parameter typically result in increased collection efficiency and further-aft 

impingement limits. Also, as discussed in paragraph 3.c of this AC, other 

influences that affect the object's water collection efficiency, drop impingement 

and icing limits, ice shape, and runback ice include splashing of the large water 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-43 

drops and break-up of the large water drops when forces on the distorted drop 

exceeds the surface tension required to hold the drop together. These latter 

influences become more evident for the larger drops defined in Appendix X. 

Drop size distributions and the related distribution of water mass are important 

considerations for water catch, impingement and ice limits as well as runback ice. 

Consideration of these influences allows comparison of critical icing conditions 

that can be correlated to ice accretions in aft locations, such as behind protected 

areas. Conditions with equivalent modified inertia parameters result in similar 

impingement characteristics. 

The modified inertia parameter is commonly used in icing wind tunnel testing to 

compare high altitude flight conditions to the available low altitude test conditions. 

To simulate the impingement effects of high altitudes and true airspeeds, larger 

dropsizes are used at the lower altitude test conditions. As an example, an IPS 

designed to protect against 50μm drop impingement at 22,000 ft and 200 KCAS 

(290 KTAS) would be equivalently protected against 76 μm drops at sea level 

and 200 KCAS (190 KTAS) due to an equivalent modified inertia parameter (ref. 

6 foot chord). 

 (a) Based on these physical processes, selection of an Appendix X icing 

Conditions boundary based on drop size is difficult to implement. Consideration 

of the drop inertia effects throughout the flight envelope will likely be required. 

While technology does not currently exist to discriminate icing conditions based 

on specific drop sizes (exclusive of inertia effects), methods may be developed in 

the future to certify in such a manner. 

 (b) Alternate methods of defining a selected Appendix X icing conditions 

boundary may also be considered. For example, it may be possible to develop 

an exit cue such that unrestricted flight in Appendix X freezing drizzle is 

approved; however, exit is required from Appendix X freezing rain. 

 (c) See Appendix A of this AC for an example of a certification effort 

considering a certification to Appendix C only (§ 25.1420(a)(1)) based on droplet 

size discrimination. 

2. Means for Detecting Appendix X boundaries. The means for determining 

whether the selected Appendix X boundary icing conditions boundary has been 

exceeded can be substantiated visual cues, an ice detection system, or an 

aerodynamic performance monitor. 

 (a) Ice detection systems and aerodynamic performance monitors are 

discussed in paragraph 11.e of this AC. For compliance with §25.1420(a)(1) it is 

acceptable to use an ice detection system that detects accretions behind the 

aircraft’s protected areas. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-44 

 (b) Substantiated visual cues can range from direct observation of ice 

accretions aft of the airplane’s protected surfaces to observation of ice accretions 

on reference surfaces. The cue should not require the flightcrew to judge the ice 

to be a specific thickness or size. Examples of visual means are: 

• accretions forming on the side windshields, 

• accretions forming on the sides of nacelles, 

• accretions forming on propeller spinners aft of a reference point, 

• accretions forming on radomes aft of a reference point, and/or 

• accretions forming aft of the protected surfaces 

 (i) The visual cues should be developed with the following considerations: 

• Visual cues should be within the flightcrew’s vision scan area while 

seated and performing their normal duties. 

• The visual cues should be visible during all modes of operation 

(day, night) without the use of a handheld flashlight. 

 (ii) During the certification process, the applicant should verify the ability 

of the crew to observe the visual cues or reference surfaces. The visual cues 

should be evaluated from the most adverse flightcrew seat locations in 

combination with the range of flightcrew heights, within the approved range of 

eye reference point locations, if available. A visual cue is required for both the 

left and right seats. If a single visual cue is used, it should be visible from either 

seat. Consideration should be given to the difficulty of observing clear ice. The 

adequacy of the detection method should be evaluated in all expected flight 

conditions. Night evaluations can be done with simulated accretions to assess 

visibility in and out of cloud. Methods used to substantiate visual cues should be 

agreed to with appropriate certification authorities. 

 (iii) Such visual cues should be validated by testing in measured natural 

icing, or otherwise validated through icing tanker testing or potentially through 

icing wind tunnel tests. The cues should consider the drop distributions of 

Appendix X, to establish that these cues would be appropriate in the restricted 

appendix X icing conditions. If a reference surface is used, the applicant should 

validate that it correlates with ice accumulation on the critical surfaces. 

 (iv) Appendix X and AC 25.21-1 should be reviewed for guidance on 

the time required to detect visual ice cues for the ice detection system to activate. 

 

9. PNEUMATIC DEICER BOOTS. Many existing AFM's specify a minimum 

ice accumulation thickness prior to activation of the deicer boot system. 

However, the accident and incident history has shown that it is difficult for 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-45 

flightcrews to use visual means to determine the thickness of an ice accretion. 

The AFM procedure for boot operation should be to activate the boots at the first 

sign of ice if an advisory ice detector is installed and not wait for a specific 

amount of ice to accumulate. If an ice detector is not installed the AFM 

procedure for boot operation should be to activate the boots based on visible 

moisture and temperature. 

In addition, for applicants that choose to recommend an observable ice 

accumulation prior to boot activation, this pre-activation ice accretion must be 

considered when determining critical ice accretions for performance, stability, 

control, and stall testing.

Note: Activation of the deicing boots typically results in one cycle of inflation and 

deflation of all boots, but not necessarily at the same time. Some systems are 

designed such that a complete cycle of the boots does not occur (i.e., some 

boots are not inflated) if the IPS is “selected off” before the cycle is complete. 

For these systems, the AFM should include information to warn the flightcrew to 

select the IPS on for at least one complete cycle of the deicing boots and allow 

completion of any subsequent cycle previously started. This note is equally 

applicable to other deicing systems that similarly do not activate all portions of 

the deicing systems simultaneously. 

10. EMERGENCY AND ABNORMAL OPERATING CONDITIONS. Flight 

tests should be conducted to demonstrate continued safe flight and landing, 

using appropriate AFM abnormal and/or emergency operating procedures, 

following failures of the IPS. These demonstrations should be conducted with 

anticipated ice accumulation on normally protected surfaces. See Appendix C, 

part 2 and AC 25.21-1 for sub-part B testing with failure ice shapes. 

11. ICE AND ICING CONDITIONS DETECTION SYSTEMS. Sections 

25.1419(e), (f), (g), and (h) specify requirements regarding the activation of the 

IPS. These requirements are only applicable to Appendix C icing conditions. 

Section 25.1420(c) requires compliance with § 25.1419(e), (f), (g), and (h) for the 

selected portion or all of Appendix X as applicable. Sections 25.1420(a)(1) and 

(a)(2) require a means to alert the flight crew that they are in Appendix X icing 

conditions or are in Appendix X icing conditions from which they must exit, 

respectively. 

a. COMPLIANCE WITH § 25.1419(e)(1) and (e)(2). 

These sections of the rule provide alternatives to the operation of the IPS based 

on icing conditions defined in § 25.1419(e)(3). These alternatives require either 

a primary ice detection system, or substantiated visual cues and an advisory ice 

detection system. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-46 

(1) Ice Detectors 

(a) Primary ice detector. A primary ice detector must either alert the flight crew 

to operate the IPS using AFM procedures or automatically activate the IPS prior 

to an unsafe accumulation of ice on the airframe, engine components, or engine 

air inlets. The primary ice detection system must perform its intended function for 

the airplane configurations, phases of flight, and within icing envelopes of 

Appendix C and Appendix X. Design of the primary ice detector system should 

account for reasonable time delays in the activation of the IPS, and should 

recognize the effects of intercycle ice on deicer surfaces if the ice detector is a 

component of an automatic deicing system. Laboratory tests should 

demonstrate the ability of the ice detector to function properly within the entire 

required icing conditions and airplane operating envelope. Approval of the 

primary ice detector should include measured natural icing flight tests to verify 

analyses and laboratory test results, as well as to verify that the ice detector 

system performs its intended function. 

The primary ice detection system may incorporate an ice accretion detector 

and/or an icing conditions detector. The use of the icing conditions detector may 

be required if the ice detector fails to detect ice at low freezing fractions at the ice 

detection sensor, see AC20-73A for details. 

(b) Advisory ice detector. The advisory ice detector, in conjunction with another 

cue, such as visible ice accretion on referenced or monitored surfaces, should 

advise the flight crew to initiate operation of the IPS using AFM procedures. An 

advisory system is not the prime means used to determine if the IPS should be 

activated. When there is an advisory system installed on an airplane, the flight 

crew has primary responsibility for determining when the IPS should be 

activated. Analyses and tests similar to those performed for a primary ice 

detector should be performed for an advisory ice detector to understand its 

characteristics, limitations, and installation. 

(c) Ice Detector System Performance When Installed. The applicant should 

accomplish a droplet impingement analysis and/or tests to ensure that the ice 

detector(s) are properly located. It should be shown that under the various 

airplane operational configurations, phases of flight, and associated angles of 

attack, that the ice detector is exposed to free-stream water droplets. The ice 

detector should be located on the airframe surface where the sensor is 

adequately exposed to the icing environment. Flow field and boundary-layer 

analyses of candidate installation positions should be accomplished to ensure 

that the ice detector sensor is not shielded from impinging Appendix C and 

Appendix X water drops. The ice detection system should be shown to operate 

in the range of conditions defined by Appendix C and the applicable portion of 

Appendix X. Sections 25.1419 and 25.1420 also require a combination of tests 

and analysis to demonstrate the performance of the ice detector and the system 

as installed on the airplane. This could include icing tunnel and icing tanker tests 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-47 

to evaluate the ice detector performance. Droplet impingement analysis may be 

used in determining that the ice detector functions properly over the droplet 

range of Appendix C and the applicable portions of Appendix X when validated 

through natural or artificial icing tests (e.g. tanker, icing tunnel). It should be 

demonstrated that the airplane can be safely operated with the ice accretions 

formed at the time the ice protection system becomes effective, following 

activation of the ice detector. The detector and its installation should minimize 

nuisance warnings. 

(d) Ice Detector System Safety Considerations. The applicant should consult 

AC 25.1309-1A for guidance on compliance with § 25.1309. In accordance with 

the AC, the applicant should accomplish a functional hazard assessment to 

determine the hazard level associated with failure of the ice detection system. 

The unannunciated failure of a primary ice detection system is assumed a 

catastrophic failure condition, unless the characteristics of the airplane in icing 

conditions without activation of the airframe ice protection system(s) are 

demonstrated to result in a less severe hazard category. If visual cues are 

primary, failure of an advisory ice detection system is considered to be minor. 

(2) Visual Cues. Visual cues can either be direct observation of ice accretions 

on the airplane’s protected surfaces to observation of ice accretions on reference 

surfaces. Examples of visual means could be: 

• accretions forming on the windshield wiper posts (bolt or blade), 

• accretions forming on propeller spinners, 

• accretions forming on radomes, 

• accretions on the protected surfaces 

If accretions on the protected surfaces cannot be observed, consideration should 

be given to providing a reference surface. 

 (a) Field of View. The visual cues should be developed with the 

following considerations: 

 (i) Visual cues should be within the flightcrew’s vision scan area 

while seated and performing their normal duties. 

 (ii) The visual cues should be visible during all modes of operation 

(day, night, in cloud). 

 (b) Substantiation. During the certification process, the applicant 

should substantiate the ability of the crew to observe the visual cues or reference 

surfaces. The visual cues should be evaluated from the most adverse flightcrew 

seat locations in combination with the range of flightcrew heights, within the 

approved range of eye reference point locations, if available. A visual cue is 
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required for both the left and right seats. If a single visual cue is used, it should 

be visible from either seat. Consideration should be given to the difficulty of 

observing clear ice. If a reference surface is used, the applicant should validate 

that it correlates with ice accumulation on the airframe’s protected areas. Such 

visual cues should be validated by testing in measured natural icing. 

b. COMPLIANCE WITH § 25.1419(e)(3).

This paragraph of §25.1419 provides an alternative to the primary ice detection 

system and the visual cues plus advisory ice detection system as defined in 

paragraph §§ 25.1419(e)(1) and 25.1419(e)(2). This alternative requires the 

operation of the IPS when the airplane is in conditions conducive to airframe 

icing during all phases of flight. 

 (1) Temperature Cue. The temperature cue used in combination with 

visible moisture, should be calibrated and be readily available to the 

flightcrew. A minimum temperature limitation may be required on some types 

of systems due to equipment temperature limitations (such as elastomer 

pneumatic de-ice boot systems). 

 (2) Airplane Flight Manual (AFM). The limitations section of the AFM 

should identify specific static or total air temperature and visible moisture 

conditions which must be considered as conditions conducive to airframe 

icing and should specify that the IPS must be operated when these conditions 

are encountered. 

c. COMPLIANCE WITH § 25.1419(f). 

This paragraph of § 25.1419 states that the requirements of §§ 25.1419(e)(1), 

25.1419(e)(2), and 25.1419(e)(3) are applicable to all phases of flight unless it 

can be shown that the IPS need not be operated. To substantiate that the IPS 

need not be operated during certain phases of flight, the ice accretions that form 

during these phases should be considered when establishing the airplane is able 

to safely operate in the continuous maximum and intermittent maximum icing 

conditions of appendix C and the applicable portions of Appendix X. 

d. COMPLIANCE WITH § 25.1419(g). 

This paragraph of § 25.1419 requires that after the initial activation of the IPS: 

(a) the IPS must operate continuously, or 

(b) the airplane must be equipped with a system that automatically cycles 

the IPS, or 

 (c) an ice detection system must be provided to alert the flightcrew each 

time the IPS must be cycled. 

Some examples of systems which automatically cycle the IPS are: 
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• A system that senses ice accretion on a detector and correlates it to ice 

accretion on a protected surface. This system then cycles the IPS at a 

predetermined condition. 

• A system which cycles the IPS based on the use of a timer. Such a 

system may have more than one cycling time. 

• A system that directly senses the ice thickness on a protected surface and 

cycles the IPS. 

A common attribute of the above systems is that the pilot is not required to 

manually cycle the IPS after initial activation. 

Some examples of an ice detection system which alerts the flight crew each time 

the IPS must be cycled could be the same as the automatic systems as 

discussed above, except that the system alerts the crew each time the IPS must 

be manually cycled. The use of a timer (without ice sensing capability) to alert 

the flight crew to manually cycle the deicers does not meet the requirements of 

§25.1419(g)(3). For a system that employs the use of a timer, it should be 

substantiated that the airplane can safely operate with the ice accretions that will 

form between the time the deicing cycle is completed and the next cycle is 

initiated. For systems that have more than one cycle time it should be 

substantiated that the flight crew is able to determine which cycle time is 

appropriate. The ice shedding effectiveness of the selected means for cycling 

the ice protection system should be evaluated during testing in natural icing 

conditions and any inter-cycle and runback ice should be considered when 

showing compliance with § 25.21(g). See paragraphs 11.a.(1) of this AC for 

guidance on installed ice detector performance and safety considerations. 

e. Compliance with § 25.1420(a)(1) or (a)(2). These paragraphs of § 25.1420 

require that a means be provided to flight crew to detect when the selected 

portion of Appendix X icing conditions is exceeded. Means for determining when 

the selected portion of Appendix X icing conditions is exceeded may include 

visual means, ice detectors or an airplane performance monitor. 

(1) Ice Detectors. An ice detector system installed for compliance with 

§ 25.1420(a)(1) or (a)(2) is intended to determine when the conditions have 

reached the boundary of Appendix X for which the airplane has been 

demonstrated for safe operations. The applicant should accomplish a droplet 

impingement analysis and/or tests to ensure that the ice detector is properly 

located to function over the aircraft operational conditions and in Appendix X 

icing conditions. Analysis may be used in determining that the ice detector is 

located properly to function over the droplet range of Appendix X when validated 

considering methods described in SAE ARP 5903. It should be ensured that the 

system minimizes nuisance warnings when operating in icing conditions. 

The low probability of finding conditions conducive to ice accumulation in 

Appendix X may make natural icing flight tests difficult as a means of 
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demonstrating that the system functions in conditions exceeding appendix C. 

The applicant may use flight tests of the airplane under simulated icing conditions 

(icing tanker) or icing wind tunnel tests of a representative airfoil section and ice 

detector to demonstrate the proper functioning of the system and to correlate the 

signals provided by the detectors and the actual ice accretion on the surface. 

(2) Aerodynamic Performance Monitor (APM). A crew alerting system could be 

developed using pressure probes and signal processors to quantify pressure 

fluctuations in the flow field from contamination over the wing surface. The 

technology exists but a full development is necessary before incorporating into 

the crew warning system.

12. PERFORM INTENDED FUNCTION IN ICING. All systems and components 

of the basic airplane should continue to function as intended when operating in 

an icing environment including, for example: 

 a. Engines and Equipment. Engines and equipment (such as 

generators and alternators operating under maximum ice protection load) should 

be monitored during icing tests for adequate cooling (§ 25.1041) and be found 

acceptable for this operation (§§ 25.1093 and 25.1301). 

 b. Engine Alternate Induction Air Sources. Engine alternate induction 

air sources should remain functional in an icing environment. 

 c. Fuel System Venting. Fuel system venting should not be adversely 

affected by ice accumulation. 

 d. Landing Gear. A retractable landing gear should operate as intended 

following an icing encounter. Gear retraction should not result in an unsafe 

indication because of ice accretion. 

 e. Stall Warning. Ice could form on stall warning and angle-of-attack 

sensors if these devices are not protected. Therefore, the sensors’ functions 

should be evaluated to ensure operation in the icing conditions of Appendices C 

and, as appropriate, X. The activation points of artificial stall warning and stall 

identification systems, if installed, may need to be reset for operations in icing 

conditions to provide adequate stall warning margins and to prevent inadvertent 

stalling or loss of control, respectively. 

 f. Ice Detection Cues. Ice detection cues that the pilot relies on for 

timely operation of ice protection equipment should be evaluated in anticipated 

flight attitudes. (See paragraph 8.b.(2) for guidance on substantiated visual cues 

for compliance with §§ 25.1420(a)(1) or 25.1420(a)(2). Also, see paragraph 

11.a.(2) of this AC for guidance on substantiated visual cues for compliance with 

§ 25.1419(e)(2).) 
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 g. Primary and Secondary Flight Control Surfaces. Primary and 

secondary flight control surfaces should remain operational after exposure to 

icing conditions. It should be demonstrated that aerodynamic balance surfaces 

are not subject to icing throughout the airplane's operating envelope (weight, 

center of gravity, and speed) or that any ice accumulation on these surfaces 

does not interfere with or limit actuation of the control for these surfaces, 

including retraction of flaps for a safe go around from the landing configuration. 

 h. Ram Air Turbine. The ram air turbine should remain operational. It 

does not need to be tested in natural icing conditions if tested in the icing wind 

tunnel. 

 i. Pilot Compartment View. In support of compliance with § 25.773, 

Pilot compartment view, any obstruction of the pilots’ view due to ice 

accumulation should be assessed. 

13. ICE INSPECTION LIGHT(S). Unless operations at night in icing conditions 

are prohibited by an operating limitation, § 25.1403 requires that a means be 

provided for illuminating or otherwise determining the formation of ice on the 

parts of the wings that are critical from the standpoint of ice accumulations. 

 a. If the wings cannot be observed by the flightcrew, one acceptable means 

of compliance with this regulation would be the installation of an ice evidence 

probe located in a position where the flightcrew can observe the ice 

accumulation. Formation of ice on the device should be shown to precede or 

occur simultaneously with the formation of ice on the wings. Consideration 

should be given to the need for illumination of the ice evidence probe. 

 b. Ice inspection lights should be evaluated both in and out of clouds during 

night flight to determine that adequate illumination of the component of interest is 

available without excessive glare, reflections, or other distractions to the 

flightcrew. These tests may be conveniently accomplished both in and out of 

clouds during the airplane certification flight tests. Typically, airplane-mounted 

illumination has been used as a means of compliance with this regulation. Use 

of a hand-held flashlight has not been considered acceptable due to the 

associated workload. The appropriate manual should identify the ice 

characteristics the flightcrew is expected to observe and the flightcrew action 

associated with the observation. 

14. CAUTION INFORMATION. Section 25.1419(c) requires that caution 

information be provided to alert the flightcrew when the IPS is not functioning 

normally. Caution information should be provided if the failure condition of the 

IPS can result in a hazardous airplane condition. It should be assumed that icing 

conditions exist during the failure event. The decision to provide the caution 

information should not be based on the numerical probability of the failure event 

(i.e., even if a numerical probability analysis indicates the system failure is 
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improbable or extremely improbable, the caution information should be provided 

if the failure could result in a hazardous condition). Caution information related to 

§25.1420 must comply with §25.1309(c). 

 a. The placement of the sensor(s) used to identify a failure condition should 

be evaluated to ensure that the sensors are properly located to obtain accurate 

data on the failure of the IPS. 

 b. The applicant should submit data to substantiate that the indication 

system does not give a false indication that the system is functioning normally. 

For example, if a pneumatic deicing system (boots) requires a specific minimum 

pressure and pressure rise rate to adequately shed ice, then caution information 

should be provided if the minimum pressure and pressure rise rate are not 

attained. If this caution information is not provided, the flightcrew may 

erroneously believe that the boots are operating normally when, in fact, the boots 

might not be inflating with sufficient pressure or rate of inflation to adequately 

shed the ice. The need for caution information should also be considered for the 

case of ice forming in the pneumatic system that can result in low pneumatic boot 

pressures or an inadequate pressure rise rate. 

15. AIRPLANE FLIGHT MANUAL (AFM). The AFM should provide the pilot with 

the information needed to operate the IPS, including the following: 

 a. Operating Limitations Section. 

 (1) Establishment of any systems limitations when operating in icing 

conditions. These limitations should address airframe, engine, and equipment 

ice protection systems, and other systems that may need to be specifically 

configured for operation in icing conditions. Appropriate icing conditions 

definitions may need to be provided to ensure proper operation of the related 

IPS. 

 (2) Limitations on operating time for ice protection equipment, if these 

limitations are based on fluid anti-icing/deicing systems capacities and flow rates. 

 (3) Airplane speed limitations (if any) and minimum temperature for 

deicing boot operation for airplanes equipped with boots. 

 (4) Environmental limitations for equipment operations as applicable 

(e.g., minimum temperature for boot operation or maximum altitude for boot 

operation). 

 (5) Minimum engine speed if the airframe IPS does not function properly 

below this speed. 

 (6) A list of required placards. 
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 (7) If compliance is based on §§ 25.1420(a)(1) or 25.1420(a)(2), the 

Limitations section must include a requirement that the flightcrew exit all 

(Appendix X and C) icing conditions immediately upon recognition that the 

Appendix X icing conditions exceed the certification boundary. For certification of 

detection and exit of Appendix X in accordance with § 25.1420(a)(1) and 

§25.1420(a)(2) the Limitations section should contain appropriate limits that 

consider the certification assumptions with respect to portion (phase of flight, 

freezing drizzle, freezing rain, etc.) 

 b. Operating Procedures Section. 

 (1) Section 25.1585 requires that operating procedures be provided in 

the AFM for: (1) normal procedures peculiar to the particular type or model 

encountered in connection with routine operations, (2) non-normal procedures for 

malfunction cases and failure conditions involving the use of special systems or 

the alternative use of regular systems, and (3) emergency procedures for 

foreseeable but unusual situations in which immediate and precise action by the 

crew may be expected to substantially reduce the risk of catastrophe. These 

procedures should include any preflight action necessary to minimize the 

potential of enroute emergencies associated with the IPS. The system 

components should be described with sufficient clarity and depth that the pilot 

can understand their function. Unless flightcrew actions are accepted as normal 

airmanship, the appropriate procedures should be included in the FAA-approved 

AFM, AFM revision, or AFM supplement. These procedures should include 

proper pilot response to cockpit warnings, a means to identify system failures, 

and the use of the system(s) in a safe manner. 

 (2) Procedures should be provided to optimize airplane operation during 

penetration of icing conditions, including climb, holding, and approach 

configurations and speeds. The AFM should define when the ice protection 

equipment should be activated consistent with the system operation during 

certification. 

 (3) Emergency or abnormal procedures, including procedures to be 

followed when IPSs fail and/or warning or monitor alerts occur, should be 

provided. 

 (4) For fluid anti-icing/deicing systems, information and method(s) for 

determining the remaining flight operation time should be provided. 

 (5) For airplanes that cannot supply adequate electrical power for all 

systems at low engine speeds, load-shedding instructions should be provided to 

the pilot for approach and landing in icing conditions if pilot action is required. 

 (6) For compliance with §§ 25.1420(a)(1) or 25.1420(a)(2): 
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 (a) The method used to determine when icing conditions must be 

exited, must be provided, as well as appropriate failure indications and crew 

procedures. 

 (b) The operating procedures section should contain guidance 

relative to exiting the icing environment. 

 (c) For certification of certain phases of flight to Appendix X, 

information defining these restricted phases should be provided. This 

information should include any aircraft configurations associated with the 

restricted phases of flight such as flap position, gear extension, or airspeed. 

(7) The method for determining when the IPS must be activated must be 

provided. Information should be provided to indicate that a de-icing 

system should not be de-activated until the completion of an entire deicing cycle after leaving icing conditions. An anti-icing system should not 

be de-activated before leaving icing conditions. 

(8) The following statement should be included: 

Convective clouds, above the freezing altitude, that are vigorously growing 

should be avoided since they may contain icing conditions exceeding 

those for which the aircraft has been certified. 

16. GUIDANCE FOR AMENDED TCs, AND STCs

General guidance for amended TCs, and STCs can be found in AC21.101-

1/ACJxxx. As stated in its “APPLICABILITY” section, the guidance in this AC 

applies to any STC or amended TC on an airplane for which the applicant seeks 

approval under the provisions of §§ 25.1419 and 25.1420. Increases in gross 

weight, increases in engine power, and propeller changes could affect approval 

in icing and these areas should be evaluated using this AC as the method of 

compliance. An applicant wishing to use an alternate means of compliance 

(AMOC) needs to consult the cognizant Aircraft Certification Office. 

For additional guidance, refer to Appendix B of this AC. 
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Appendix A

Example of Certification to §25.1420(a)(1) using Visual Cues.

To give guidance on the use of Appendix C as a boundary and the determination 

of substantiated visual cues, the following hypothetical example is used.

The aircraft is a transport category, turbofan aircraft. It is a derivative of an 

existing type. The existing aircraft has already been type certificated for flight in 

icing including §25.1419 (Appendix C). The changes in the derivation did not 

alter the original §25.1419 compliance. The derivative aircraft design is 

characterized by:

• Cruise altitude above the Appendix C envelope.

• Wing design does not include leading edge devices.

• Ice protection over the full span of wing and tailplane.

• Reversible flight controls on the roll and pitch axes.

AC 25.21-1 contains guidance on compliance with 14 CFR 25, sub-part B 

requirements and AC 20-147 contains guidance for engine and engine 

installations. All relevant certification requirements must be met. However for 

this example only the following aspects will be discussed.

Impingement aft of the protected areas of lifting surfaces.

The aircraft has ice protection over the full span of the wing and horizontal 

stabilizer with minimal unprotected areas. Since this is a derivative aircraft, the 

system has already been evaluated and certificated for Appendix C conditions. 

The aircraft does have reversible flight controls on the roll and pitch axis. An 

assessment of hinge moment and lift reduction effects is required. See AC 

25.21-1 for details on performance and handling qualities guidance.

Impingement aft of the engine inlet protected surfaces. Ice accumulation aft of 

the engine inlet protected surfaces should be evaluated relative to ingestion of 

the ice by the engine. For the example, the cumulative mass of this ice is less 

than that used for demonstrating compliance with § 33.77. Therefore, compliance 

with 25.1093 will be found based on this known ingestion standard. Refer to AC 

20-147 for further information on engine and engine installation certification.

Ice shapes on unprotected areas of lifting surfaces

The aircraft is designed with full span ice protection on the wing and horizontal 

stabilizer. As such, the unprotected areas (wing tip, stabilizer tip) are a small 

percentage of the effective airfoil areas. Aerodynamic effects due to the effects 

of Appendix X accretions on the minimal exposure area is unlikely to be critical, 

but should be investigated. See AC 25.21-1 for details on performance and 

handling qualities guidance.

Ice shapes on unprotected airframe regions

Other areas of the airframe (which do not accrete ice under Appendix C 
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IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

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conditions) should be examined relative to the increased impingement and 

runback on accretion areas related to Appendix X. Issues such as windshield 

visibility, static port location and potential airflow disturbance from Appendix X 

accretions that could influence sensors (TAT, pitot, AOA) should be examined. 

Use of computational fluid dynamic analysis, or icing impingement analysis (icing 

wind tunnel) combined with a satisfactory field service history on derivative 

models may be sufficient to alleviate any potential issues.

Air data sensors

Ice protection on air data sensors should be examined to ensure adequate 

protection for exposure to Appendix X icing conditions.

Design Substantiation

A review of the impingement analysis and runback ice data versus protected 

area for the wing and horizontal stabilizer indicates that the wing is the critical 

surface for formation of a potential ridge behind the protected areas in front of the 

ailerons. The horizontal stabilizer has farther aft protection limits and has 

sufficient margin with contamination aft of the protected areas. Again, it is 

assumed that the horizontal stabilizer would be addressed, but will not be 

presented in this example.

Determination of Exit Cues

The aircraft-operating envelope encompasses all of the Appendix X icing 

conditions. Climbs and descents should be examined, but due to the transient 

nature of these encounters, they may not produce the critical accretions for 

Appendix X. 

The recommended procedure for holding in icing conditions should be 

considered (e.g., whether flaps or landing gear extended). AFM recommended 

procedures would include this operating information. The case to be evaluated

will consider flaps up, gear up configuration. Since ice accretions aft of the 

protected areas on the upper wing surface are the primary concern in this 

example, an analysis of the operational conditions will be made to determine the 

critical flight conditions for such accretions. The conditions considered as critical 

were determined to be the following:

Highest holding altitude within the conditions of Appendix X that maximizes water 

catch (typically a function of ram air effects and LWC content at defined static 

temperatures) Highest true airspeed expected during holding conditions 

(typically high weight hold speed; produces maximum water catch aft of 

protected areas upper surface.)

Note: Other operating conditions should be considered as required to 

determine the critical conditions. 

Through a combination of impingement analysis, analytical ice shape prediction 

and icing wind tunnel testing, it has been determined that there is potential for an 
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IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

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ice ridge to begin to accumulate on the upper wing surface under these operating 

conditions when there is significant water content in drop sizes above a specific 

drop size and liquid water content. As such, this will be proposed as a condition 

to determine when icing conditions must be exited. It should be noted that this 

dropsize criteria is not a fixed boundary. As an example, at a lower altitude and 

a similar equivalent airspeed (similar AOA), slightly larger dropsizes are required 

to produce a similar accretion of ice behind the protected area. However, this 

combination of drop size, liquid water content, and relative inertia to the aircraft is 

sufficient to potentially produce accumulations behind the protected areas. See 

discussion on the modified inertia parameter in paragraph 8.b(1) of this AC and 

DOT/FAA/CT-88/8-1 Aircraft Icing Handbook, reference section Chap. I, section 

2 and Chapter IV, Section 2.2.2.

Determination of visual exit cue

The target boundary will be icing conditions with significant water content in 

dropsizes greater than the specific drop size determined by analysis and testing 

under the operating conditions of a high altitude, high speed hold. The target 

criterion for a visual exit cue is a surface on the aircraft that is readily visible to 

the flight crew that will indicate water collection in such drop sizes. A CFD 

analysis of the cockpit/fuselage area has determined that there are perceptible 

changes in impingement limits along the side windows with increasing drop 

sizes. The desired visual cue is a specific portion of the windshield as a 

reference surface that can be correlated to ice accumulation on the wing. 

Preliminary impingement analysis indicates that this is feasible as there is 

significant movement of the aft impingement limits on the windshield with 

increasing drop sizes when the specific flight conditions of interest are 

considered. Based on this analysis, a specific location on the aircraft side 

window is proposed as the visible cue to determine when the Appendix X 

conditions that require exiting icing conditions have been encountered.

Definition of critical ice accretion

 It is proposed that the exit cue will be based on the first sign of accretion. It has 

been determined that an accretion of approximately 1/16-inch is required on the 

reference surface for the crew to detect it. This reference-surface accretion is 

then correlated to the specific ice accretion on the monitored surface (behind the 

leading edge) considering the ice accretion corresponding to the first indication 

on the reference surface considering recognition time and time to initiate escape 

as described in AC 25.21-1, Appendix 1. This correlation is based on a 

combination of analysis and icing wind tunnel testing.

This results in accretion of a 1/8-inch ice ridge behind the protected area at the 

specific critical operating condition. The difference in ice thickness is due to 

differences in the local collection efficiencies between the side window and the 

area aft of the leading edge protected surface. Consideration of additional 

accretion during the exit from the icing conditions is required to determine the full 

ice shape to be evaluated for aerodynamic degradation effects.
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IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

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Evaluation of critical ice accretions

Wind tunnel testing of this critical accretion has determined that there are no 

hinge moment issues that result in changes in roll forces. However, there is a 

potential increase in stall speed that must be addressed in the stall warning 

system. Dry air flight-testing is performed using simulated ice shapes. This flighttesting is performed in a built up manner with limited spanwise exposure to 

evaluate the effects with minimal safety risk. The stall warning margins are 

evaluated and result in a modification of the stall warning system to ensure that 

adequate stall margin exists both for the period prior to detection and during the 

exit from all icing conditions. See AC 25.21-1 for details on 14 CFR 25, subpart B compliance.

Validation of the visual cue

The use of the visual cue is to be evaluated and validated through flight-testing 

with an icing tanker. The tanker is equipped to provide a specific MVD with an 

associated droplet distribution. As the tanker cannot simulate the full extent of 

icing conditions within Appendix X, a validation of the specific conditions will be 

made. A fixed nozzle simulating the specific drop size to be used as the exit cue 

will be selected.

Icing tanker testing indicates that the side window does begin to accumulate ice 

at the specific condition considered, which validates the analysis. At slower

speeds, the droplets do not have sufficient inertia to reach the side windows. 

This serves to validate that the visual cue is not overly sensitive and will not 

result in false indications. Tanker testing also showed good correlation to the 

predicted ice shapes validating the ice shape behind the protected areas. The 

location is such that it is adequately illuminated by existing cockpit lighting 

without glare or reflection and is readily observable by crewmembers.

Exiting from restricted Appendix X icing conditions

Flight tests are performed in accordance with 14 CFR § 25.21(g)(2) and (4) to 

show that the airplane can safely exit Appendix X icing conditions. See AC 

25.21-1 for details on performance and handling qualities guidance.
IPHWG Task 2 WG Report - Appendix K 

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Appendix B

GUIDANCE FOR AMENDED TC's, AND STC's

The guidance in this AC applies to any STC or amended TC on an airplane for 

which the applicant seeks approval under the provisions of §§ 25.1419 and 

25.1420. Increases in gross weight, increases in engine power, external 

modifications, and propeller changes could affect approval in icing and these 

areas should be evaluated using this AC as the method of compliance. An 

applicant wishing to use an alternate means of compliance (AMOC) needs to 

consult the cognizant Aircraft Certification Office. 

a. These are some examples of modifications (but not limited to the list) 

for which compliance to icing regulations should be revisited are: 

(1) Engine changes 

(2) Propeller changes 

(3) Engine inlet or accessory inlet changes 

 (4) Antennae installations or other external modifications 

(5) Gross weight increases 

(6) Center of gravity envelope increase 

(7) Flight envelope increase 

(8) Turboprop conversion 

(9) Modifications to lifting surfaces 

(10) Installation of vortex generators 

(11) Modifications to ice protections systems 

Besides the guidance on similarity analysis provided in paragraph 5.e, the 

following guidance on specific modifications should be followed: 

b. Engine changes The effects of increased engine power or thrust on 

airplane handling qualities in icing conditions,§ 25.21(g), should be addressed, 

as well as compliance to engine induction icing regulations, § 25.1093. 

(1) Effects of increased engine power or thrust 

(a) Stall Characteristics, Stability, and Control If there is a 

potential for the increased engine power or thrust to affect 

airplane handling qualities in icing conditions, flight tests with 

ice accretions should be conducted as described in AC 

25.21-1. 

(b) Ice contaminated tailplane stall (ICTS) ICTS should be 

addressed. See AC25-21-1 for compliance materials on tailplane 

stall. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-60 

 (2) Engine induction icing

(a). Similarity - If similarity is used as a method of 

compliance, it should be supplemented with analysis or testing, or both. In 

some cases, an analysis that substantiates similarity, along with an 

installation survey conducted by the engine manufacturer, may be 

sufficient. The analysis should include: 

1. Heat requirements for the inlet lip ice protection, if 

installed, i.e., no increase in speed envelope; 

2. Location of the lip deicer thermostat, if installed; 

3. Inlet material to which the deicer is attached (heat 

sink may be different); 

4. Inlet geometry; 

5. Engine ignition system; and 

6. Engine sensors 

Any differences that cannot be addressed by analysis must be addressed by 

flight testing. The data to be used for similarity, including the data from past 

certification on which the similarity analysis is based must be available to the 

applicant and be provided to the FAA at their request. 

(b) Falling/Blowing Snow and Ground Ice Fog for Turbine 

Engines 

If similarity will be used as a method of compliance, the certification plan should 

state to what similarity will be shown. This is important because similarity may 

be based on a certified installation that does not have falling/blowing snow or 

ground ice fog in its certification basis. Service experience by itself cannot be 

used to show compliance. Compliance is normally shown by testing for 

turboprop installations. With regards to falling and blowing snow, inlets such as 

the oil cooler as well as the induction system are of concern. The certification 

plan should identify how compliance with these sections will be shown. 

(c) Ice Shedding 

Engine compliance data to § 33.77 should be compared between the currently 

installed engine and the proposed engine. If the ice slab used to show 

compliance is smaller for the proposed engine, ice shedding from the airframe 

should be re-addressed. Engine inlet lip ice shedding should be addressed. The 

amount of ice mass that could be shed should be compared to a similar, 

approved installation or to Part 33 engine compliance data. 

(d) IPS Operation 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-61 

Bleed air mass flows, pressures and temperatures of the proposed engine and 

existing, certified engine should be compared. If there is a reduction, the 

effectiveness of the IPSs must be substantiated. 

c. Propeller changes 

Section 25.929 requires a means to prevent or remove hazardous ice 

accumulation on propellers or accessories where ice accumulation would 

jeopardize engine performance. 

The typical analysis report from the deicing boot manufacturer is not sufficient by 

itself to show compliance to § 25.929. The typical report calculates intercycle ice 

thicknesses for various flight and icing conditions, but does not calculate the 

effect on propeller efficiency, which must be done to show no appreciable loss of 

thrust. For Supplemental Type Certificates, it would be acceptable to show that 

intercycle ice is equal to or less than the accretions obtained on the same 

propeller on an airplane that was flight tested in icing conditions and shown to 

have no appreciable loss in thrust. 

The typical deicing boot manufacturer report also contains a caveat that it does 

not address propeller runback ice. Similarity to another propeller that was flight 

tested in icing conditions is usually done to address runback. Similarity would 

include propeller and deicing boot aerodynamic and thermal similarity, deicing 

timer, propeller RPM, and flight conditions. Note that metal and composite 

propellers have different thermal masses. 

As a final qualitative check for both intercycle and runback ice on new airplane 

programs, airplane performance is checked during flight tests in icing conditions. 

The propeller installation, including spinner and cowl geometry, must be 

compared to previously tested installations in icing conditions. Changes that 

could allow moisture to reach the brush blocks must be avoided. 

If the proposed propeller is calculated to have higher efficiency than the existing, 

approved propeller, the guidance under “1. Effects of increased engine power or 

thrust” under “Engine changes” should be followed.

d. Engine inlet or accessory inlet changes

Guidance is provided in the “Engine changes” section above. It should be noted 

that § 25.1093 applies to engine oil and accessory cooling inlets as well as 

induction inlets. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-62 

e. Antennae installations or other external modifications

(1) When antennae, cameras, fairings for such installations, or 

other external installations such as drain masts are installed on aircraft, 

the installer should show the following: 

(a) The predicted ice accretion does not contribute 

significantly to drag; 

(b) There is no ice shedding hazard on downstream 

structure or engines. 

(c) There is no ice related reduction performance of lifting 

surfaces; and 

(d) There is no ice related effect on downstream air data 

sensors or ice detectors; 

(2) A very conservative, simple analysis may be accomplished first 

to show the objectives (a) and (b). If the conservative analysis fails, the 

analysis can be refined to determine if the initial analysis was overly 

conservative. The conservative analysis can assume the following: 

(a) The water catch area is the full frontal area of the 

installation; 

(b) Collection efficiency is one. 

(c) No runback or evaporation of impinging water. 

(d) Assume the shape on blade antenna will be similar to 

airfoils and the shape on low profile antennae will be single horn 

shapes. 

(3) The applicant should determine the critical icing condition. If 

the analysis shows a problem, then one or more of the following can be 

accomplished: 

(a) Determine a less conservative collection efficiency either 

with an ice accretion code or with the FAA "Aircraft Icing 

Handbook" (DOT/FAA/CT-88/8-1); 

(b) Determine less conservative impingement limits by using 

an icing code, which may reduce the collection area; 

(c) Run the full configuration in an icing code to determine if 

the installation is in a shadow zone. 

(d) If drag is a problem, run an ice accretion code to 

determine a less conservative ice shape. 

(4) Flight tests in measured natural icing or with simulated ice 

shapes should be accomplished to determine if there are any detrimental 

effects due to the ice accretions if 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-63 

(a) The installation is upstream of air data sensors or an ice 

detector; or 

(b) The installation is on a lifting surface or 

(c) The installation could create a wake on a lifting surface. 

Note: The one exception to (b) is fairings. An analysis to show the impact on 

maximum lift coefficient, in combination with flight tests with no ice accretions, 

have been accepted in the past. 

f. Gross weight increases

(1) The impingement analysis should cover a weight range that is 

applicable to any increase in gross weight. 

(2) The impingement analysis should also evaluate unprotected 

areas such as fuel vents. 

(3) If there is a potential for the increased gross weight to affect 

airplane handling qualities in icing conditions, flight tests with ice 

accretions should be conducted as described in AC 25.21-1. 

g. Center of gravity envelope increase. Generally, the same guidance 

used for gross weight increases can be used for center of gravity envelope 

increases. The one exception is an increase of forward center of gravity limit on 

airplanes may make an airplane more susceptible to ICTS. This should be 

addressed by flight testing for airplanes with unpowered, reversible elevators or 

for airplanes with propellers. An analysis may be acceptable for other 

configurations. 

h. Flight envelope increase

If an increase in maximum operating altitude is applied for, the applicant should 

demonstrate: 

(1) the IPS operating pressures (for pneumatic systems) or 

temperatures (for thermal air) by dry air testing; and 

(2) stall speeds and characteristics with ice accretions if these are 

shown to be influenced by Mach number 

The effect of increased cruise airspeeds and increased altitudes that could affect 

windshield ice accretion, and adequacy of the windshield heat, should be 

addressed. 
IPHWG Task 2 WG Report - Appendix K 

IPHWG Recommended Advisory Materials§ §25.1419, 25.1420 

12/19/2005 Page K-64 

i. Turboprop conversion

If the IPSs use engine bleed air for operation, the pneumatic lines may 

accumulate more water than the current unmodified type design. This water can 

subsequently refreeze and block the pneumatic lines, resulting in failure of some 

or all zones of the pneumatic system. The applicant needs to show that the 

pneumatic deicing system will continue to function in icing conditions. 

The pneumatic deicer operating pressure may also decrease at lower engine 

RPMs. A minimum engine RPM for acceptable pneumatic operating pressure, 

which should allow for descent, should be established and published in the 

AFM/POH. 

j. Modifications to lifting and control surfaces 

Critical ice accretions (including consideration of pre-activation, intercycle, 

residual, and runback icing) may have to be re-defined, especially if the changes 

affect operational wing angle of attack. Stall strips are good collectors of ice and 

are an example where leading edge ice accretions should be re-defined. If ice 

accretions are changed or the modifications could affect control power or hinge 

moments, AC 25.21-1 provides guidance on flight testing. 

k. Installation of vortex generators

Vortex generators may accrete ice depending on their location. The applicant 

should provide data on expected ice accretions. Flight conditions to consider are 

the hold, descent, and approach. Substantiation on the effects on stall speeds, 

stall characteristics, and stability and control should be provided. 

 l. Modifications to ice protection systems

Critical ice accretions (including consideration of pre-activation, intercycle, 

residual, and runback icing) may have to be re-defined. If ice accretions are 

changed or the modifications could affect control power or hinge moments, flight 

testing with simulated ice accretions should be accomplished. AC 25.21-1 

provides guidance on flight testing. 
IPHWG Task 2 WG Report - Appendix L 

NECCESSITY FOR AN AFM STATEMENT CONCERNING CUMULIFORM 

CLOUD LARGE DROPLET ICING 

12/19/2005 Page L-1 

APPENDIX L - NECCESSITY FOR AN AFM STATEMENT 

CONCERNING CUMULIFORM CLOUD LARGE DROPLET ICING 
IPHWG Task 2 WG Report - Appendix L 

NECCESSITY FOR AN AFM STATEMENT CONCERNING CUMULIFORM 

CLOUD LARGE DROPLET ICING 

12/19/2005 Page L-2 

MINORITY POSITION ON THE NECESSITY OF AN AFM STATEMENT CONCERNING 

CUMULIFORM CLOUD LARGE DROPLET ICING (EXCEEDANCE OF APPENDIX X)

Supported by: ALPA 

The addition of an explicit caution concerning convective cloud icing in future 

aircraft flight manuals is necessary for pilot understanding of aircraft icing 

capability and to increase the probability of avoidance of conditions which may 

exceed Appendix X. 

With Appendix X certification addressing SLD conditions, pilots will expect safety 

in known icing conditions; however, with Appendix X addressing only stratiform 

clouds, that is not necessarily the case. Appendix C certification addresses icing 

in cumuliform clouds, Appendix X does not. There is a body of evidence (photos, 

anecdotal reports, and limited data) of icing that is more intense and severe than 

Appendix C and Appendix X icing. In the modern ATC environment, pilots 

routinely face decisions concerning penetration of cumuliform clouds and 

operations near thunderstorms. While pilots understand that numerous hazards 

are associated with these conditions, without an AFM warning they will not 

understand that icing certification does not provide the same safe operation in 

convective cloud icing as it does in other icing. A warning is necessary to insure 

that this hazard is not dismissed. 

Add the following statement to the IPHWG AC, section 16 a. 

Warning: The large droplet icing conditions that may be 

encountered in or near convective clouds that are vigorously 

growing above the freezing level are not evaluated in aircraft 

certification. Such conditions may result in hazardous degradations 

of aircraft performance and handling characteristics and should be 

avoided. 

{Note: A WARNING is defined as an operating procedure, technique, etc., that 

may result in personal injury or loss of life if not carefully followed. A CAUTION 

is an operating procedure, technique, etc., that may result in damage to 

equipment if not carefully followed. Based on these definitions, a WARNING is 

proposed.} 

This section then reads: 

16. AIRPLANE FLIGHT MANUAL (AFM). The AFM should provide the pilot 

with the information needed to operate the IPS, including the following: 
IPHWG Task 2 WG Report - Appendix L 

NECCESSITY FOR AN AFM STATEMENT CONCERNING CUMULIFORM 

CLOUD LARGE DROPLET ICING 

12/19/2005 Page L-3 

 a. Operating Limitations Section. 

 (9) For all aircraft, a requirement to avoid large drop icing associated with 

convective clouds that are vigorously growing above the freezing level, such 

as: Warning: The large droplet icing conditions that may be encountered in or 

near convective clouds that are vigorously growing above the freezing level 

are not evaluated in aircraft certification. Such conditions may result in 

hazardous degradations of aircraft performance and handling characteristics, 

and should be avoided.

MAJORITY POSITION ON THE NECESSITY OF AN AFM STATEMENT CONCERNING 

CUMULIFORM CLOUD LARGE DROPLET ICING (EXCEEDANCE OF APPENDIX X)

Supported by: Airbus, Boeing, Bombardier, CAA/UK, Cessna, Embraer, 

FAA/FAA Tech Center, MSC, Saab, Transport Canada/TDC 

The majority concurs that there should be information in the AFM to minimize 

exposure of aircraft to large droplet convective cloud icing conditions that are not 

considered during certification. However, the use of a Warning or Caution in the 

Limitations section is not appropriate. For a Warning or a Caution, there should 

be a good means that will allow the flightcrew to follow the instructions. Since all 

vigorously growing convective clouds do not appear as red on the weather radar 

and visual observation is not adequate at night, there is not a good means for the 

flightcrew to consistently avoid these clouds. It would not be prudent to provide a 

Warning or a Caution that could lead the flightcrew to become overly concerned 

about their inability to comply with the instructions. There is not a history of icing 

accidents in convective clouds. Therefore, it is reasonable to place the 

information in the procedures section of the AFM where the flightcrew would 

utilize the information by avoiding these clouds when possible. 

In addition, the majority do not concur that the AFM information is necessary for 

the airplanes that are certificated as detect and exit airplanes because the 

flightcrew will be provided with a means to know when they should exit icing 

conditions. 

 

The AC will be revised to include the following information in the procedures 

section of the AFM: 

“Convective clouds, above the freezing altitude, that are vigorously growing 

should be avoided since they may contain icing conditions exceeding those for 

which the aircraft has been certified.”
IPHWG Task 2 WG Report - Appendix M 

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B 

12/19/2005 Page M-1 

APPENDIX M FTHWG RECOMMENDATIONS FOR 

14 CFR 25 SUB-PART B 
IPHWG Task 2 WG Report - Appendix M 

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B 

12/19/2005 Page M-2 

FTHWG Proposed Requirements for Aeroplane Handling Qualities and 

Performance in Supercooled Large Drop Icing Conditions 

Prepared for the IPHWG 

Introduction

The rules that the FTHWG proposes for performance and handling qualities for 

flight in the icing conditions of Appendix X are based upon those developed for 

flight in Appendix C icing conditions. These proposals address the options 

identified by the IPHWG for CS/FAR 25.1420; for continued operation in the icing 

conditions of Appendix X, operation in a portion of Appendix X and for the 

alternative option of leaving all icing conditions once Appendix X conditions are 

detected ("detect and exit"). 

The FTHWG interpretation of "operating safely"

The FTHWG discussed the interpretation of "operating safely", as in 

CS/FAR 25.1420 proposed by the IPHWG, with respect to the required level of 

safety and the resulting handling qualities and performance levels. 

For CS/FAR 25.1420(a)(2) "operating safely in a portion of Appendix X", the 

FTHWG quickly came to the conclusion that there is a direct analogy with 

operation in Appendix C icing conditions (discussed below). 

For CS/FAR 25/1420(a)(1) "operating safely after encountering Appendix X 

conditions" and 25.1420(a)(1)(ii) "operating safely while exiting all icing 

conditions", the FTHWG considers that these are not failure cases and the 

temptation to link this text with the phrase "continued safe flight and landing", 

commonly used in association with system failure conditions of CS/FAR 25.1309, 

should be resisted. These cases are also discussed below. 

The recognition that the "detect and exit" cases of 25.1420(a)(1) are not failure 

conditions led directly to the decision by the FTHWG to make "detect and exit" 

for Appendix X an integral part of a certification for Appendix C icing conditions 

(discussed later). 

Continued operation in the icing conditions of Appendix X

For continued operation, the FTHWG considers that little change is required from 

the requirements for Appendix C. The list of requirements excepted for flight in 

Appendix C were reviewed and found largely appropriate for Appendix X; the 

only difference is that CS/FAR 25.121(a) "Performance - Take-off landing gear 

extended" is not excepted, as the FTHWG cannot at the moment justify an 
IPHWG Task 2 WG Report - Appendix M 

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B 

12/19/2005 Page M-3 

assumption that the accretion in this flight phase can be assumed insignificant, 

unlike Appendix C. In practice, it is expected that some applicants may prohibit 

take-off in Appendix X conditions. Otherwise, the rationale behind the 

requirements is exactly that applied for Appendix C; that for continued operation 

in icing conditions, there should effectively be no degradation in handling 

qualities and any degradation in performance should be strictly limited by means 

of the same thresholds developed for Appendix C. The proposed requirements 

are thus, with the one exception identified above, identical to those proposed for 

Appendix C. 

The FTHWG also considers that it is worth clarifying that the optional certification 

for continued operation in the supercooled large drop conditions of Appendix X 

cannot be obtained independently of certification for operation in the icing 

conditions of Appendix C. 

Continued operation limited to a portion of Appendix X or to a flight phase

The concept of limiting continued operation to a portion of the Appendix X icing 

conditions is acceptable, provided that there are not too many divisions that 

could lead to significant human factors issues. The FTHWG advised the IPHWG 

at an early stage that a division between freezing rain and freezing drizzle would 

be most practical from an operational standpoint but has accepted a further subdivision to each based on particle size. 

The concept of limiting continued operation in Appendix X to specific flight 

phases, not explicitly stated in CS/FAR 25.1420 but addressed in the IPHWG 

draft AC, is more difficult. In certification there is a general understanding on 

what is intended by reference to the different flight phases in terms of aeroplane 

configuration, speed etc. Operationally, there is not usually such a clear 

distinction. Take-off and final take-off can be considered a distinct flight phase in 

that operation in this phase can be authorised or not and a clear go/no go 

decision can be made on the ground. Otherwise, the only discriminant that can 

sensibly be recognised by the crew is aeroplane configuration. The holding flight 

phase is often not the classic manoeuvre recognised in certification and 

operationally is frequently more akin to low level vectoring. It may, therefore, be 

unrealistic and potentially unsafe to consider holding, approach and landing as 

separate flight phases. 

To meet the IPHWG request, the proposed Subpart B rules allow a limitation on 

operation in Appendix X to a "clearly defined flight phase or atmospheric 

condition" but this requires interpretation in advisory material, a suitably realistic 

ice accretion scenario and a convincing case from any applicant. 
IPHWG Task 2 WG Report - Appendix M 

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B 

12/19/2005 Page M-4 

"Detect and exit" from the icing conditions of Appendix X into clear air

The situation regarding the "detect and exit" option is much more complex. One 

of the key issues is the definition of the appropriate scenarios to be considered 

and this is given in the proposed Appendix X. It is worth noting that the FTHWG 

has followed the advice of the IPHWG in assuming that the aeroplane leaves all 

icing conditions after encountering Appendix X icing conditions and does not 

continue in Appendix C icing conditions. 

"Detect and exit" Appendix X certification integral with Appendix C certification

The "detect and exit" cases of 25.1420(a)(1) are normal flight conditions arising 

from an inadvertent encounter with Appendix X icing conditions. Hence, the 

FTHWG considers it appropriate to make "detect and exit" for Appendix X an 

integral part of any icing certification for Appendix C icing conditions i.e. if an 

Appendix C icing certification is sought, then "detect and exit" for Appendix X 

must also be addressed. 

Performance and handling qualities for "detect and exit"

Regarding specifically handling qualities and performance, the discussions in the 

FTHWG addressed the following issues: 

• Performance – The FTHWG assumes that the take-off case need not be 

considered for a "detect and exit" aeroplane. Hence, with the exception of 

stall speeds, landing speeds and distances and landing climb performance, 

the FTHWG considers that performance need not be specifically addressed 

for "detect and exit" certification. 

• Handling qualities – The FTHWG proposes, with one exception, that the 

same handling qualities as for flight in Appendix C icing conditions are 

retained. That exception is a take-off case, CS/FAR 25/143(c)(1), addressing 

controllability following engine failure at V2. No justification for a relaxed 

standard of handling qualities could be identified, bearing in mind that "detect 

and exit" is not a failure case and that the aeroplane, once clear of all icing 

conditions but retaining the ice accretion, may continue in clear air for some 

time. 

• Roll controllability - The FTHWG reviewed the JAA Interim Policy and the 

FAA ADs published to address roll controllability issues. Several 

manufacturers gave presentations of their experiences in addressing the FAA 

ADs. The FTHWG concluded that the existing requirement is adequate to 

address roll control but there is a need for additional advisory material. 
IPHWG Task 2 WG Report - Appendix M 

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B 

12/19/2005 Page M-5 

• Additional limitations may be imposed after encountering Appendix X 

conditions that may be reflected in the certification flight tests. 

• Demonstration of compliance - AC/AMC 25.21(g) clarifies that many handling 

requirements are only required to be addressed in Appendix C if the 

compliance in non-icing conditions is marginal. It is anticipated that the same 

philosophy will hold for Appendix X. 

• Sandpaper ice, included in the previous Appendix C proposals, does not 

feature in the Appendix X icing environment. 

Entry into icing conditions

• "Prior to normal IPS operation" - The FTHWG presumes that entry directly 

into Appendix X conditions from clear air cannot be ruled out. Hence the 

minimal set of "prior to normal IPS operation" handling requirements 

developed for Appendix C is proposed also for Appendix X. In practice, it 

may be possible to propose one envelope accretion covering both cases. 

• "Prior to detection of Appendix X" – The FTHWG considers that there is a 

need for a set of minimal requirements to address the ice accreted before 

crew recognition of accretion in Appendix X conditions. The same 

requirements as for "prior to normal IPS operation" are proposed but to be 

related to the means of detection of Appendix X. In practice, it may be 

possible in this case also to combine with the "prior to normal IPS operation" 

accretions. 

Summary of proposed Subpart B requirements for "detect and exit"

A summary of the Subpart B requirements (as amended by the 25.21(g) 

Appendix C proposals of NPA 16/2004) applicable to a "detect and exit" 

certification is given below. 
IPHWG Task 2 WG Report - Appendix M 

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B 

12/19/2005 Page M-6 

Subpart B 

Requirements* 

for D+E App X 

Title Comments 

 

Exclude: Retain: 

General 

 25X20 Applicability 

 25.21 Proof of compliance 

No impact in practice 

 25.23 Load distribution limits 

 25.25 Weight limits 

 25.27 Centre of gravity limits 

No changes in load 

distribution and weight and 

c.g. limits allowed in 

Appendix X 

 25.29 Empty weight and 

corresponding centre of 

gravity 

 25.31 Removable ballast 

 25.33 Propeller speed and pitch 

limits 

No impact in practice 

Performance 

 25.101 General 

 25.103 Stall speed Necessary for HQ 

25.105 Take-off 

25.107 Take-off speeds 

25.109 Accelerate-stop distance 

25.111 Take-off path 

25.113 Take-off distance and takeoff run 

25.115 Take-off flight path 

No take-off performance 

required for D+E App X 

 25.117 Climb: general 

 25.119 Landing climb: all-engines 

operating 

25.121 Climb: One-engineinoperative 

25.123 En-route flight paths 

No en-route performance 

required for D+E App X 

 25.125 Landing 
IPHWG Task 2 WG Report - Appendix M 

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B 

12/19/2005 Page M-7 

Subpart B 

Requirements* 

for D+E App X 

Title Comments 

 

Exclude: Retain: 

Controllability and Manoeuvrability 

25.143(b) 

(1),(2), 

(c)(1) 

25.143 

not 

(b)(1),(2), 

(c)(1) 

General Pushover manoeuvre to 

same standard as App C. 

25.143(c)(1) is a take-off 

case 

 25.145 Longitudinal control 

 25.147 Directional and lateral 

control 

25.149 Minimum control speed Engine failure for approach, 

landing and go-around 

addressed qualitatively in 

25.143(c)(2)&(3) 

Trim 

 25.161 Trim 

Stability 

 25.171 General 

 25.173 Static longitudinal stability 

 25.175 Demonstration of static 

longitudinal stability 

 25.177 Static directional and lateral 

stability 

 25.181 Dynamic stability 

Stalls 

25.201(c) 

(2) 

25.201, 

not (c)(2) 

Stall demonstration 

 25.203 Stall characteristics 

25.207(c) 

and (d) 

25.207(a)

,(b),(e),(f) 

Stall warning Same means and margin of 

stall warning required as for 

App C. 

Option for further stick 

shaker advance for App X 

requested by manufacturers 

(in A(M)C). 
IPHWG Task 2 WG Report - Appendix M 

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B 

12/19/2005 Page M-8 

Subpart B 

Requirements* 

for D+E App X 

Title Comments 

 

Exclude: Retain: 

Ground Handling Characteristics 

 25.231 Longitudinal stability and 

control 

 25.233 Directional stability and 

control 

 25.235 Taxying condition 

 25.237 Wind velocities 

Miscellaneous Flight Requirements 

25.251(b) 

thru (e) 

25.251(a) Vibration and buffeting 

 25.253 High-speed characteristics 

 25.255 Out-of-trim characteristics 

* Based on CS-25 as amended by NPA 16/2004 
IPHWG Task 2 WG Report - Appendix M 

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B 

12/19/2005 Page M-9 

Proposed Amendment to CS 25.21(g) Rule to Accommodate Appendix X 

Based on the proposals of EASA NPA 16/2004 

Introduction 

The proposed requirements addressing flight in the icing conditions of 

Appendix C (EASA NPA 16/2004 and the corresponding FAA NPRM) require 

revision to accommodate the proposed Appendix X icing environment. 

This paper uses the NPA 16/2004 circulated for comment by EASA as a basis. 

Proposed changes to CS 25.21(g) are identified by italic font. 

Identification of Amendments 

Deletions from and additions to the current CS-25 text to accommodate flight in 

the icing conditions of Appendix C are shown by strike-through and bolding 

respectively. 

To accommodate the incorporation of Appendix X, further suggested changes to 

the proposals are shown by italic font. 

 "...." is used as an ellipsis (with leading and/or trailing text to aid reference) to 

indicate unchanged text that, for clarity, is not repeated. 

Proposal 1 

CS 25.21 Proof of compliance 

Introduce CS 25.21(g) to read: 

"(g) The requirements of this subpart associated with icing conditions 

apply only if certification for flight in icing conditions is desired. If 

certification for flight in icing conditions is desired, the following 

requirements also apply (see AMC 25.21(g)): 

(1) Unless otherwise prescribed, compliance with each requirement of 

this subpart, except CS 25.121(a), 25.123(c), 25.143(b)(1) and (2), 25.149, 

25.201(c)(2), 25.207(c) and (d), and 25.251(b) through (e), must be shown 

using the ice accretions defined in part II of Appendix C, assuming

normal operation of the aeroplane and its ice protection system in 
IPHWG Task 2 WG Report - Appendix M 

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B 

12/19/2005 Page M-10 

accordance with the operating limitations and operating procedures 

provided in the aeroplane Flight Manual. 

(2) Unless otherwise prescribed, compliance with each requirement of 

this subpart, except CS 25.105, 25.107, 25.109, 25.111, 25.113, 25.115, 

25.121, 25.123, 25.143(b)(1), (b)(2) and (c)(1), 25.149, 25.201(c)(2), 

25.207(c) and (d), and 25.251(b) through (e), must be shown using the ice 

accretions defined in part II(b)(2) of Appendix X, assuming normal 

operation of the aeroplane and its ice protection system in accordance 

with the operating limitations and operating procedures provided in the 

aeroplane Flight Manual. 

(3) The aeroplane must meet the requirements of CS 25.143(j) and 

25.207(h) after entry into icing conditions and before the ice protection 

system has been activated and is performing its intended function. 

Compliance must be shown using the ice accretions defined in (i) part 

II(e) of Appendix C and (ii) part (II)(b)(1)(h) of Appendix X. 

(4) The aeroplane must meet the requirements of CS 25.143(k) and 

25.207(i) after entry into the supercooled large drop icing conditions of 

Appendix X and before the detection of those conditions. Compliance 

must be shown using the ice accretions defined in part II(b)(2)(e) of 

Appendix X. 

 (5) If, additionally, certification for flight in any portion (whether a 

clearly defined flight phase or atmospheric condition) of the supercooled 

large drop icing conditions of Appendix X is desired, unless otherwise 

prescribed, each requirement of this subpart, except CS 25.123(c), 

25.143(b)(1) and (2), 25.149, 25.201(c)(2), 25.207(c) and (d), and 25.251(b) 

through (e), must be met in those icing conditions. Compliance must be 

shown using the ice accretions defined in part II(b)(1) of Appendix X, 

assuming normal operation of the aeroplane and its ice protection 

system in accordance with the operating limitations and operating 

procedures provided in the aeroplane Flight Manual. 

(6) No changes in the load distribution limits of CS 25.23, the weight 

limits of CS 25.25 (except where limited by performance requirements of 

this subpart), and the centre of gravity limits of CS 25.27, from those for 

non-icing conditions, are allowed for flight in icing conditions or with ice 

accretion."

Proposal 2 

Amend CS 25.103 Stall Speed to read: 
IPHWG Task 2 WG Report - Appendix M 

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B 

12/19/2005 Page M-11 

"(a) The reference stall speed VSR .... 

(b) VCLMAX is determined with: 

(1) Engines idling, or, if that resultant thrust causes an appreciable 

decrease in stall speed, not more than zero thrust at the stall speed; 

(2) Propeller pitch controls (if applicable) in the take-off position; 

(3) The aeroplane in other respects (such as flaps, and landing gear, and 

ice accretions) in the condition existing in the test or performance standard in 

which VSR is being used; 

(4) The weight used when VSR is being used as a factor to determine 

compliance with a required performance standard; 

(5) The centre of gravity position that results in the highest value of 

reference stall speed; and 

(6) The aeroplane trimmed for straight flight at a speed selected by the 

applicant, but not less than 1.13 VSR and not greater than 1.3 VSR.

(c) Starting from .... 

(d) In addition to ...." 

Proposal 3 

Amend CS 25.105 Take-off to read: 

"(a) The take-off speeds described in CS 25.107, the accelerate-stop distance 

described in CS 25.109, the take-off path described in CS 25.111, and the takeoff distance and take-off run described in CS 25.113, and the net take-off flight 

path described in CS 25.115, must be determined -

(1) At each weight, altitude, and ambient temperature within the 

operational limits selected by the applicant; and

(2) In the selected configuration for take-off.

in the selected configuration for take-off at each weight, altitude, and 

ambient temperature within the operational limits selected by the applicant 

- 

(1) In non-icing conditions; and 
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(2) In icing conditions, if in the configuration of CS 25.121(b) with the 

applicable “Take-off Ice” accretion specified by CS 25.21(g): 

(i) The stall speed at maximum take-off weight exceeds that in nonicing conditions by more than the greater of 5.6 km/h (3 kt) CAS or 3% 

VSR; or 

(ii) The degradation of the gradient of climb determined in 

accordance with CS 25.121(b) is greater than one-half of the applicable 

actual-to-net take-off flight path gradient reduction defined in CS 

25.115(b). 

 (b) No take-off made ..... 

(c) The take-off data must be based on ..... 

(d) The take-off data must include ....." 

Proposal 4 

CS 25.107 Take-off speeds 

Amend CS 25.107(c), 25.107(g) and add a new CS 25.107(h) as follows: 

"(c) V2, in terms of calibrated airspeed, must be selected by the applicant to 

provide at least the gradient of climb required by CS 25.121(b) but may not be 

less than - 

(1) V2MIN; 

(2) VR plus the speed increment attained (in accordance with CS 

25.111(c)(2)) before reaching a height of 35 ft above the take-off surface; and 

(3) A speed that provides the manoeuvring capability specified in CS 

25.143(h). 

(d) .... 

(e) .... 

(f) .... 

(g) VFTO, in terms of calibrated airspeed, must be selected by the applicant to 

provide at least the gradient of climb required by CS 25.121(c), but may not be 

less than - 
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(1) 1.18 VSR; and 

(2) A speed that provides the manoeuvring capability specified in CS 

25.143(h). 

(h) In determining the take-off speeds V1, VR, and V2 for flight in icing 

conditions, the values of VMCG, VMC, and VMU determined for non-icing 

conditions may be used."

Proposal 5 

Amend CS 25.111 Take-off path to read: 

"(a) The take-off path .... 

(b) During the acceleration .... 

(c) During the take-off path determination in accordance with sub-paragraphs 

(a) and (b) of this paragraph - 

(1) The slope .... 

(2) The aeroplane .... 

(3) At each point ...., 

(i) 1.2% .... 

(ii) 1.5% .... 

(iii) 1.7% ...., and

(4) The aeroplane configuration may not be changed, except for gear 

retraction and automatic propeller feathering, and no change in power or 

thrust that requires action by the pilot may be made until the aeroplane is 

122m (400 ft) above the take-off surface; and 

(5) If CS 25.105(a)(2) requires the take-off path to be determined for 

flight in icing conditions, the airborne part of the take-off must be based 

on the aeroplane drag: 

(i) With the applicable “Take-off Ice” accretion specified by CS 

25.21(g), from a height of 11m (35 ft) above the take-off surface up to 
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FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B 

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the point where the aeroplane is 122m (400 ft) above the take-off 

surface; and 

(ii) With the applicable “Final Take-off Ice” accretion specified by 

CS 25.21(g), from the point where the aeroplane is 122m (400 ft) above 

the take-off surface to the end of the take-off path. 

(d) The take-off path .... 

(e) Not required for CS-25."

Proposal 6 

Amend CS 25.119 Landing climb: All-engines-operating to read: 

"In the landing configuration, the steady gradient of climb may not be less than 

3.2%, with -

(a) the engines at the power or thrust that is available 8 seconds after 

initiation of movement of the power or thrust controls from the minimum flight idle 

to the go-around power or thrust setting (see AMC 25.119(a)) - and

(b) A climb speed which is -

(1) Not less than -

 (i) 1·08 VSR for aeroplanes with four engines on which the 

application of power results in a significant reduction in stalling speed; or

(ii) 1·13 VSR for all other aeroplanes;

(2) Not less than VMCL; and

(3) Not greater than VREF.

(a) In non-icing conditions, with a climb speed of VREF determined in 

accordance with CS 25.125(b)(2)(i), 

(b) In icing conditions with the applicable “Landing Ice” accretion 

specified by CS 25.21(g), and with a climb speed of VREF determined in 

accordance with CS 25.125(b)(2)(ii)." 

Proposal 7 

Amend CS 25.121 Climb: One-engine-inoperative to read: 

"(a) Take-off; landing gear extended. .... 
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(b) Take-off; landing gear retracted. In the take-off configuration existing at 

the point of the flight path at which the landing gear is fully retracted, and in the 

configuration used in CS 25.111 but without ground effect,:

(1) The steady gradient of climb may not be less than 2.4% for two-engined 

aeroplanes, 2.7% for three-engined aeroplanes, and 3.0% for four-engined 

aeroplanes, at V2 and with - 

(i) The critical engine inoperative, the remaining engines at the take-off 

power or thrust available at the time the landing gear is fully retracted, 

determined under CS 25.111, unless there is a more critical power 

operating condition existing later along the flight path but before the point 

where the aeroplane reaches a height of 122m (400 ft) above the take-off 

surface (see AMC 25.121(b)(1)(i)); and 

(ii) The weight equal to the weight existing when the aeroplane’s landing 

gear is fully retracted, determined under CS 25.111. 

(2) The requirements of sub-paragraph (b)(1) of this paragraph must 

be met: 

(i) In non-icing conditions; and 

(ii) In icing conditions with the applicable “Take-off Ice” accretion 

specified by CS 25.21(g), if in the configuration of CS 25.121(b) with 

the “Take-off Ice” accretion: 

(A) The stall speed at maximum take-off weight exceeds that in 

non-icing conditions by more than the greater of 5.6 km/h (3 kt) CAS 

or 3% VSR; or 

(B) The degradation of the gradient of climb determined in 

accordance with CS 25.121(b) is greater than one-half of the 

applicable actual-to-net take-off flight path gradient reduction 

defined in CS 25.115(b). 

(c) Final take-off. In the en-route configuration at the end of the take-off path 

determined in accordance with CS 25.111,:

(1) The steady gradient of climb may not be less than 1.2% for two-engined 

aeroplanes, 1.5% for three-engined aeroplanes, and 1.7% for four-engined 

aeroplanes, at VFTO and with - 

(i) The critical engine inoperative and the remaining engines at the 

available maximum continuous power or thrust; and 
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(ii) The weight equal to the weight existing at the end of the take-off path, 

determined under CS 25.111. 

(2) The requirements of sub-paragraph (c)(1) of this paragraph must 

be met: 

(i) In non-icing conditions; and

(ii) In icing conditions with the applicable “Final Take-off Ice” 

accretion specified by CS 25.21(g), if in the configuration of 

CS 25.121(b) with the “Take-off Ice” accretion: 

(A) The stall speed at maximum take-off weight exceeds that in 

non-icing conditions by more than the greater of 5.6 km/h (3 kt) CAS 

or 3% VSR; or 

(B) The degradation of the gradient of climb determined in 

accordance with CS 25.121(b) is greater than one-half of the 

applicable actual-to-net take-off flight path gradient reduction 

defined in CS 25.115(b). 

(d) Approach. In a configuration corresponding to the normal all-enginesoperating procedure in which VSR for this configuration does not exceed 110% of 

the VSR for the related all-engines-operating landing configuration,:

(1) The steady gradient of climb may not be less than 2.1% for two-engined 

aeroplanes, 2.4% for three-engined aeroplanes, and 2.7% for four-engined 

aeroplanes, with - 

(i) The critical engine inoperative, the remaining engines at the goaround power or thrust setting; 

(ii) The maximum landing weight; 

(iii) A climb speed established in connection with normal landing 

procedures, but not exceeding 1.4 VSR: and 

(iv) Landing gear retracted. 

(2) The requirements of sub-paragraph (d)(1) of this paragraph must 

be met: 

(i) In non-icing conditions, 
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(ii) In icing conditions with the applicable “Holding Ice” accretion 

specified by CS 25.21(g); the climb speed selected for non-icing 

conditions may be used if the climb speed for icing conditions, 

computed in accordance with sub-paragraph (d)(1)(iii) of this 

paragraph, does not exceed that for non-icing conditions by more 

than the greater of 5.6 km/h (3 kt) CAS or 3%. 

Proposal 8 

Amend CS 25.123 En-route flight paths to read: 

"(a) For the en-route configuration, the flight paths prescribed in subparagraphs (b) and (c) of this paragraph must be determined at each weight, 

altitude, and ambient temperature, within the operating limits established for the 

aeroplane. The variation of weight along the flight path, accounting for the 

progressive consumption of fuel and oil by the operating engines, may be 

included in the computation. The flight paths must be determined at a any 

selected speed not less than VFTO, with - 

(1) The most unfavourable centre of gravity; 

(2) The critical engines inoperative; 

(3) The remaining engines at the available maximum continuous power or 

thrust, and 

(4) The means for controlling the engine-cooling air supply in the position 

that provides adequate cooling in the hot-day condition; 

(b) The one-engine-inoperative net flight path data must represent the actual 

climb performance diminished by a gradient of climb of 1.1% for two-engined 

aeroplanes, 1.4% for three-engined aeroplanes, and 1.6% for four-engined 

aeroplanes. -

(1) In non-icing conditions, 

(2) In icing conditions with the applicable “En-route Ice” accretion 

specified by CS 25.21(g), if:

(i) 1.18VSR with the “En-route Ice” accretion exceeds the en-route 

speed selected in non-icing conditions by more than the greater of 5.6 

km/h (3 kt) CAS or 3% VSR, or 
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(ii) The degradation of the gradient of climb is greater than one-half 

of the applicable actual-to-net flight path reduction defined in subparagraph (b) of this paragraph. 

(c) For three- or four-engined ...." 

Proposal 9 

Amend CS 25.125 Landing to read: 

"(a) The horizontal distance necessary to land and to come to a complete 

stop from a point 15m (50 ft) above the landing surface must be determined (for 

standard temperatures, at each weight, altitude and wind within the operational 

limits established by the applicant for the aeroplane) - as follows: 

(1) In non-icing conditions, 

(2) In icing conditions with the applicable “Landing Ice” accretion 

specified by CS 25.21(g) if VREF in icing conditions is greater than VREF in 

non-icing conditions by more than 9.3 km/h (5 kt) CAS.

(b) In determining the distance in (a): 

(1) The aeroplane must be in the landing configuration. 

(2) A stabilised approach, with a calibrated airspeed of not less than VREF, 

must be maintained down to the 15m (50 ft) height. 

(i) In non-icing conditions, VREF may not be less than: 

(A) 1.23VSR0; 

(B) VMCL established under CS 25.149(f); and 

(C) A speed that provides the manoeuvring capability specified in 

CS 25.143(h). 

(ii) In icing conditions, VREF may not be less than: 

(A) The speed determined in sub-paragraph (b)(2)(i) of this 

paragraph; 

(B) 1.23 VSR0 with the applicable "Landing Ice" accretion 

specified by CS 25.21(g) if that speed exceeds VREF selected in nonicing conditions by more than 9.3 km/h (5 kt) CAS; and 
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(C) A speed that provides the manoeuvring capability specified 

in CS 25.143(h).

(3) Changes in configuration,....(See AMC 25.125(b)(3).) 

(4) The landing must.... 

(5) The landings may.... 

(c) The landing distance....(See AMC 25.125(c).) In addition – 

(1) The pressures.... 

(2) The brakes....(see AMC 25.125(c)(2); and

(d) Not required for CS-25.

(e) Not required for CS-25.

(f) The landing distance data.....

(g) If any device....."

Proposal 10 

Amend CS 25.143 Controllability and Manoeuvrability - General to read: 

"(a) (See AMC 25.143(a).) The aeroplane must .... 

(b) (See AMC 25.143(b).) It must be possible .... 

(c) It must be shown that the aeroplane is safely controllable and 

manoeuvrable with the critical ice accretion appropriate to the phase of 

flight specified by CS 25.21(g), and with the critical engine inoperative and 

its propeller (if applicable) in the minimum drag position: 

(1) At the minimum V2 for take-off; 

(2) During an approach and go-around; and 

(3) During an approach and landing. 

(d) The following table prescribes, for conventional wheel type controls, the 

maximum control forces permitted during the testing required by sub-paragraphs 
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(a) and (b) through (c) of this paragraph (See AMC 25.143(d)): ....[Table 

unchanged] 

(e) Approved operating procedures .... that are prescribed in sub-paragraph 

(d) of this paragraph. The aeroplane .... 

(f) When demonstrating compliance .... that are prescribed in sub-paragraph 

(d) of this paragraph, the aeroplane.... 

(g) When manoeuvring .... (see AMC No 1 to CS 25.143(g)), and must .... 

over-controlling. (See AMC No 2 to CS 25.143(g)). 

(h) (See AMC 25.143(h)). The manoeuvring capabilities ....[Table 

unchanged] 

(i) When demonstrating compliance with CS 25.143 in icing conditions - 

(1) Controllability must be demonstrated with the ice accretion 

specified by CS 25.21(g) that is most critical for the particular flight 

phase. For aeroplanes with unpowered elevator controls, the 

“Sandpaper Ice” of Appendix C must also be considered in determining 

the critical ice accretion; 

(2) It must be shown that a push force is required throughout a 

pushover manoeuvre down to zero g or the lowest load factor obtainable 

if limited by elevator power. It must be possible to promptly recover 

from the manoeuvre without exceeding 222 N (50 pounds) pull control 

force; and (Based on FAA/JAA/ALPA position in original FTHWG 

Appendix C report) 

(2) The aeroplane must be controllable in a pushover manoeuvre 

down to zero g, or the lowest load factor obtainable if limited by elevator 

power. It must be shown that a push force is required throughout the 

manoeuvre down to 0.25g. It must be possible to promptly recover from 

the manoeuvre without exceeding 50 pounds pull control force. (TC 

position in original FTHWG Appendix C report) 

(2) The aeroplane must be controllable in a pushover manoeuvre 

down to zero g, or the lowest load factor obtainable if limited by elevator 

power. It must be shown that a push force is required throughout the 

manoeuvre down to 0.5g. It must be possible to promptly recover from 

the manoeuvre without exceeding 50 pounds pull control force. 

(Manufacturers' position in original FTHWG Appendix C report) 
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(3) Changes in longitudinal control force to maintain speed with 

increasing sideslip angle must be progressive with no reversals or 

unacceptable discontinuities. (Based on FAA/JAA/TC/ALPA position in 

original FTHWG Appendix C report) 

 (3) Advisory material only (Manufacturers' position in original 

FTHWG Appendix C report) 

(j) For flight in icing conditions before the ice protection system has 

been activated and is performing its intended function, the following apply: 

(1) If activation of normal operation of any ice protection system is 

dependent upon visual recognition of a specified ice accretion on a 

reference surface, the requirements of CS 25.143 are applicable with the 

ice accretions specified by CS 25.21(g). (Based on position of group less 

ALPA in original FTHWG Appendix C report) 

(1) If activation of normal operation of any ice protection system is 

dependent upon visual recognition of ice accretion, the requirements of 

JAR 25.143 are applicable with the ice accretion defined in Appendix C, 

Part II(e). (ALPA's position in original FTHWG Appendix C report) 

(2) If activation of normal operation of any ice protection system is 

dependent upon means of recognition other than that defined in subparagraph (j)(1) of this paragraph, it must be shown that the aeroplane is 

controllable in a pull-up manoeuvre up to 1.5g and there is no 

longitudinal control force reversal during a pushover manoeuvre down 

to 0.5g with the ice accretions specified by CS 25.21(g).

(k) For flight in icing conditions after entry in the icing conditions of 

Appendix X but before detection of those conditions, the following apply: 

(1) If detection of Appendix X icing conditions is dependent upon 

visual recognition of a specified ice accretion on a reference surface, the 

requirements of CS 25.143 are applicable with the ice accretions 

specified by CS 25.21(g). 

(2) If detection of Appendix X icing conditions is by an approved ice 

detection system, it must be shown that the aeroplane is controllable in 

a pull-up manoeuvre up to 1.5g and there is no longitudinal control force 

reversal during a pushover manoeuvre down to 0.5g with the ice 

accretions specified by CS 25.21(g)."
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Proposal 11 

Amend CS 25.207 Stall warning to read: 

"(a) Stall warning with .... 

(b) The warning must be furnished either through the inherent aerodynamic 

qualities of the aeroplane or by a device that will give clearly distinguishable 

indications under expected conditions of flight. However, a visual stall warning 

device that requires the attention of the crew within the cockpit is not acceptable 

by itself. If a warning device is used, it must provide a warning in each of the 

aeroplane configurations prescribed in sub-paragraph (a) of this paragraph at the 

speed prescribed in sub-paragraphs (c) and (d) of this paragraph. Except for 

the stall warning prior to normal operation of the ice protection system 

prescribed in sub-paragraph (h)(2) of this paragraph, the stall warning for 

flight in icing conditions prescribed in sub-paragraph (e) of this paragraph 

must be provided by the same means as the stall warning for flight in nonicing conditions. (See AMC 25.207(b).) 

(c) When the speed is reduced .... 

(d) In addition to the requirement ....

(e) In icing conditions, when the speed is reduced at decelerations of up 

to 0.5 m/s2 (one knot per second), the stall warning margin in straight and 

turning flight must be sufficient to allow the pilot to prevent stalling (as 

defined in CS 25.201(d)) when recovery, using the same test technique as 

for the non-contaminated aeroplane, is initiated not less than 3 seconds 

after the onset of stall warning, with - 

(1) The applicable “Holding Ice” accretion specified by CS 25.21(g) for 

the en-route, holding, approach, landing, and go-around high-lift 

configurations; and 

(2) The more critical of the applicable “Take-off Ice” and “Final Takeoff Ice” accretions specified by CS 25.21(g) for each high-lift 

configuration used in the take-off phase. 

(f) The stall warning margin must be sufficient to allow the pilot to prevent 

stalling (as defined in CS 25.201(d)) when recovery is initiated not less than one 

second after the onset of stall warning in slow-down turns with at least 1.5g load 

factor normal to the flight path and airspeed deceleration rates of at least 1m/s2

(2 knots per second), with the flaps and landing gear in any normal position, with 

the aeroplane trimmed for straight flight at a speed of 1.3 VSR, and with the power 

or thrust necessary to maintain level flight at 1.3 VSR. When demonstrating 

compliance with this sub-paragraph with ice accretions, the same test 
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FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B 

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technique as for the aeroplane without ice accretions must be used for 

recovery. 

(g) Stall warning must .... 

(h) For flight in icing conditions prior to activation of normal operation 

of the ice protection system, the following apply: 

(1) If activation of normal operation of any ice protection system is 

dependent upon visual recognition of a specified ice accretion on a 

reference surface, the requirements of this paragraph except subparagraphs (c) and (d) are applicable with the ice accretions specified by 

CS 25.21(g). (Based on position of group less ALPA in original FTHWG 

Appendix C report) 

(1) If activation of normal operation of any ice protection system is 

dependent upon visual recognition of ice accretion, the requirements of 

this paragraph except sub-paragraphs (c) and (d) are applicable with the 

ice accretion defined in Appendix C, Part II(e). (ALPA's position in 

original FTHWG Appendix C report) 

(2) If activation of normal operation of any ice protection system is 

dependent upon means of recognition other than that defined in subparagraph (h)(1) of this paragraph: 

(i) If stall warning is provided by the same means as for flight in nonicing conditions, when the speed is reduced at rates not exceeding 0.5 

m/s2 (one knot per second), the stall warning margin in straight and 

turning flight must be sufficient to allow the pilot to prevent stalling, 

using the same test technique as for the non-contaminated aeroplane,

without encountering any adverse characteristics when recovery is 

initiated not less than 1 second after the onset of stall warning, with 

the ice accretions specified by CS 25.21(g). 

(ii) If stall warning is provided by a different means than for flight in 

non-icing conditions, when the speed is reduced at rates not 

exceeding 0.5 m/s2

 (one knot per second), the stall warning in straight 

and turning flight must be sufficient to allow the pilot to prevent 

stalling, using the same test technique as for the non-contaminated 

aeroplane, without encountering any adverse characteristics when 

recovery is initiated not less than 3 seconds after the onset of stall 

warning, with the ice accretions specified by CS 25.21(g). Additionally, 

compliance with CS 25.203 must be shown using the demonstration 

means prescribed by CS 25.201, except that the 1.5 m/s2

 (3 kt) per 
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second airspeed deceleration rates of CS 25.201(c)(2) need not be 

demonstrated. 

(i) For flight in icing conditions after entry in the icing conditions of 

Appendix X but before detection of those conditions, the following apply: 

(1) If detection of Appendix X icing conditions is dependent upon 

visual recognition of a specified ice accretion on a reference surface, the 

requirements of this paragraph except sub-paragraphs (c) and (d) are 

applicable with the ice accretions specified by CS 25.21(g). 

 (2) If detection of Appendix X icing conditions is by an approved ice 

detection system: 

(i) If stall warning is provided by the same means as for flight in 

non-icing conditions, when the speed is reduced at rates not 

exceeding 0.5 m/s2

 (one knot per second), the stall warning margin in 

straight and turning flight must be sufficient to allow the pilot to 

prevent stalling, using the same test technique as for the noncontaminated aeroplane, without encountering any adverse 

characteristics when recovery is initiated not less than 1 second after 

the onset of stall warning, with the ice accretions specified by CS 

25.21(g). 

(ii) If stall warning is provided by a different means than for flight in 

non-icing conditions, when the speed is reduced at rates not 

exceeding 0.5 m/s2 (one knot per second), the stall warning in straight 

and turning flight must be sufficient to allow the pilot to prevent 

stalling, using the same test technique as for the non-contaminated 

aeroplane, without encountering any adverse characteristics when 

recovery is initiated not less than 3 seconds after the onset of stall 

warning, with the ice accretions specified by CS 25.21(g). 

Additionally, compliance with CS 25.203 must be shown using the 

demonstration means prescribed by CS 25.201, except that the 1.5 

m/s2 (3 kt) per second airspeed deceleration rates of CS 25.201(c)(2) 

need not be demonstrated."

Proposal 12 

Amend CS 25.237 Wind velocities to read: 

"(a) __The following applies:

(1) A 90° cross component of wind velocity, demonstrated to be safe for 

take-off and landing, must be established for dry runways and must be at least 
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37 km/h (20 kt) or 0.2 VSR0, whichever is greater, except that it need not 

exceed 46 km/h (25 kt). 

(2) The crosswind component for take-off established without ice 

accretions is considered valid with ice accretions. 

(3) The landing crosswind component must be established for: 

(i) Non-icing conditions, and 

(ii) Icing conditions with the applicable “Landing Ice” accretion 

specified by CS 25.21(g). 

(b) Not required for CS-25."

Proposal 13 

Amend CS.253 High-speed characteristics to read: 

"(a) Speed increase and recovery characteristics. .... 

 (b) Maximum speed for stability characteristics, VFC/MFC. VFC/MFC is the 

maximum speed at which the requirements of CS 25.143(g), 25.147(e), 

25.175(b)(1), 25.177(a) through (c), and 25.181 must be met with wing-flaps 

and landing gear retracted. Except as noted in CS 25.253, it may not be less 

than a speed midway between VMO/MMO and VDF/MDF, except that, for altitudes 

where Mach number is the limiting factor, MFC need not exceed the Mach number 

at which effective speed warning occurs. 

(c) The maximum speed for stability characteristics with the ice 

accretions specified by CS 25.21(g), at which the requirements of 

CS 25.143(g), 25.147(e), 25.175(b)(1), 25.177 and 25.181 must be met, is 

the lower of: 

 (1) 556 km/h (300 knots) CAS, 

 (2) VFC, or 

 (3) A speed at which it is demonstrated that the airframe will be 

free of ice accretion due to the effects of increased dynamic 

pressure." 
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Proposal 14 

Amend CS 25.941(c) to read: 

"(c) In showing compliance with sub-paragraph (b) of this paragraph, the pilot 

strength required may not exceed the limits set forth in CS 25.143(d) subject to 

the conditions set forth in sub-paragraphs (e) and (f) of CS 25.143." 

Proposal 15 

Amend CS 25.1419 Ice protection to read: 

"If certification for flight in icing conditions is desired, the aeroplane must be able 

to safely operate in the continuous maximum and intermittent maximum icing 

conditions of Appendix C. To establish this that the aeroplane can operate 

within the continuous maximum and intermittent maximum conditions of 

Appendix C –

(a) An analysis .... 

(b) To verify .... 

(c) Caution information .... 

(d) For turbine engine powered aeroplanes ...." 

Proposal 16 

Amend Appendix C to CS-25 to read: 

"Part I - Atmospheric Icing Conditions 

(a) Continuous Maximum Icing. ....

(b) Intermittent Maximum Icing. ....

(c) Take-off maximum icing. The maximum intensity of atmospheric 

icing conditions for take-off (take-off maximum icing) is defined by the 

cloud liquid water content of 0.35 g/m3

, the mean effective diameter of the 

cloud droplets of 20 microns, and the ambient air temperature at ground 

level of -9 degrees C. The take-off maximum icing conditions extend from 

ground level to a height of 457m (1500 ft) above the level of the take-off 

surface. 
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FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B 

12/19/2005 Page M-27 

Part II - Airframe Ice Accretions for Showing Compliance with Subpart B 

(a) Ice accretions - General. CS 25.21(g) states that in the icing 

conditions of Appendix C the applicable requirements of subpart B must be 

met (except as specified otherwise). The most critical ice accretion in 

terms of handling characteristics and/or performance for each flight phase 

must be determined, taking into consideration the atmospheric conditions 

of part I of this Appendix, and the flight conditions (e.g. configuration, 

speed, angle-of-attack, and altitude). The following ice accretions must be 

determined: (Based on position of group less ALPA in original FTHWG 

Appendix C report) 

(a) Ice accretions - General. JAR 25.21(g) states that in the icing 

conditions of Appendix C the applicable requirements of subpart B must be 

met (except as specified otherwise). The most critical ice accretion in 

terms of handling characteristics and/or performance for each flight phase 

must be determined, taking into consideration the atmospheric conditions 

of part I of this Appendix, and all flight conditions within the operational 

limits of the aeroplane (e.g. configuration, configuration changes, speed, 

angle-of-attack, and altitude). The following ice accretions must be 

determined: (ALPA's position in original FTHWG Appendix C report) 

 (1) Take-off Ice is the most critical ice accretion on unprotected 

surfaces, and any ice accretion on the protected surfaces appropriate to 

normal ice protection system operation, occurring between lift-off and 

122m (400 ft) above the take-off surface, assuming accretion starts at liftoff in the take-off maximum icing conditions of Part I, paragraph (c) of 

this Appendix. 

(2) Final Take-off Ice is the most critical ice accretion on unprotected 

surfaces, and any ice accretion on the protected surfaces appropriate to 

normal ice protection system operation, between 122m (400 ft) and 457m 

(1500 ft) above the take-off surface, assuming accretion starts at lift-off 

in the take-off maximum icing conditions of Part I, paragraph (c) of this 

Appendix. 

(3) En-route Ice is the critical ice accretion on the unprotected 

surfaces, and any ice accretion on the protected surfaces appropriate to 

normal ice protection system operation, during the en-route phase. At 

the applicant’s option, Holding Ice may be used in showing compliance 

with requirements that specify En-route Ice. 

(4) Holding Ice is the critical ice accretion on the unprotected 

surfaces, and any ice accretion on the protected surfaces appropriate to 

normal ice protection system operation, during the holding flight phase. 
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(5) Landing Ice is normally Holding Ice, unless modified by ice 

protection system operation during the landing phase. 

(6) Sandpaper Ice is a thin, rough layer of ice. 

(b) In order to reduce the number of ice accretions to be considered 

when demonstrating compliance with the requirements of CS 25.21(g): 

(1) The more critical of Take-off Ice and Final Take-off Ice may be 

used throughout the take-off phase. 

(2) Holding Ice may be used for the en-route, holding, approach, 

landing and go-around flight phases. 

(3) Holding Ice may also be used for the take-off phase provided it is 

shown to be more conservative than Take-off Ice and Final Take-off Ice.

(c) The ice accretion that has the most adverse effect on handling 

characteristics may be used for performance tests provided any 

difference in performance is conservatively taken into account. 

(d) Ice accretions for the take-off phase. For both unprotected and 

protected parts, the ice accretion may be determined by calculation, 

assuming the take-off maximum icing conditions defined in Appendix C, 

and that: 

(1) Aerofoils, control surfaces and, if applicable, propellers are free 

from frost, snow, or ice at the start of the take-off, 

(2) The ice accretion starts at lift-off, 

(3) The critical ratio of thrust/power-to-weight, 

(4) Failure of the critical engine occurs at VEF, and 

(5) Crew activation of the ice protection system is in accordance with 

an AFM procedure, except that after commencement of the take-off roll 

no crew action to activate the ice protection system should be assumed 

to occur until the aeroplane is 122m (400 ft) above the take-off surface. 

(e) Ice accretion prior to normal system operation. The ice accretion 

prior to normal system operation is the ice accretion formed on the 

unprotected and normally protected surfaces prior to activation and 

effective operation of any ice protection system in continuous maximum 

atmospheric icing conditions. 
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FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B 

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Proposal 17 

Renumber AMC 25.119(a) as AMC 25.119 and amend references to CS 

25.119(a) accordingly. 

Proposal 18 

Renumber AMC 25.121(b)(1) as AMC 25.121(b)(1)(i) and amend references to 

CS 25.121(b)(1) accordingly. 

Proposal 19 

Renumber AMC 25.125(a)(3) as AMC 25.125(b)(3) and amend references to 

CS 25.125(a)(3) accordingly. 

Proposal 20 

Renumber AMC 25.125(b) as AMC 25.125(c) and amend references to 

CS 25.125(b) accordingly. 

Proposal 21 

Renumber AMC 25.125(b)(2) as AMC 25.125(c)(2) and amend references to 

CS 25.125(b)(2) accordingly. 

Proposal 22 

Renumber AMC 25.143(c) as AMC 25.143(d) and amend references to 

CS 25.143(c) accordingly. 

Proposal 23 

Renumber AMCs No 1 and No 2 to CS 25.143(f) as AMCs No 1 and No 2 to CS 

25.143(g) and amend references to CS 25.143(f) accordingly, and to 

CS 25.143(c) as above. 
IPHWG Task 2 WG Report - Appendix M 

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B 

12/19/2005 Page M-30 

Proposal 24 

Renumber AMC 25.143(g) as AMC 25.143(h) and amend references to 

CS 25.143(g) accordingly. 

Proposal 25 

Amend the cross-references in paragraphs 4.3(a) and 5.3.1(b) of AMC 25.1329 

to refer to CS 25.143(d). 
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APPENDIX N - FTHWG RECOMMENDATIONS FOR ADVISORY 

MATERIAL 
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Note: Bold and italic fonts (and strikeouts) in this Appendix indicate changes relative to 

previously submitted ACJ 25.21(g) which was initially drafted to address Appendix C 

icing conditions only. The changes are intended to address Appendix X icing conditions 

and to consolidate performance and handling quality information that was previously 

contained in AC 25.1419-1A. 
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ACJ 25.21(g) Performance and Handling Characteristics in Icing Conditions 

Contained in Appendix C, Part 25 (Acceptable Means of Compliance)

(see JAR 25.21(g)) 

Table of Contents 

Para. Title 

1 Purpose 

2 Related Requirements 

3 Reserved 

4 Requirements and Guidance 

4.1 General 

4.2 Proof of Compliance (JAR 25.21(g)) 

4.3 Propeller Speed and Pitch Limits (JAR 25.33) 

4.4 Performance - General (JAR 25.101) 

4.5 Stall Speed (JAR 25.103) 

4.6 Failure Conditions (JAR 25.1309) 

4.7 Flight-related Systems 

4.8 Aeroplane Flight Manual (JAR 25.1581) 

5 Acceptable Means of Compliance - General 

5.1 General 

5.2 Flight Testing 

5.3 Wind Tunnel Testing and Analysis 

5.4 Engineering Simulator Testing and Analysis 

5.5 Engineering Analysis 

5.6 Ancestor Aeroplane Analysis 

6 Acceptable Means of Compliance - Flight Test Programme 

6.1 General 

6.2 Stall Speed (JAR 25.103) 

6.3 Accelerate-stop Distance (JAR 25.109) 

6.4 Take-off Path (JAR 25.111) 

6.5 Landing Climb: All-engines-operating (JAR 25.119) 

6.6 Climb: One-engine-inoperative (JAR 25.121) 

6.7 En-route Flight Path (JAR 25.123) 

6.8 Landing (JAR 25.125) 
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6.9 Controllability and Manoeuvrability - General (JAR 25.143) 

6.10 Longitudinal Control (JAR 25.145) 

6.11 Directional and Lateral Control (JAR 25.147) 

6.12 Trim (JAR 25.161) 

6.13 Stability - General (JAR 25.171) 

6.14 Demonstration of Static Longitudinal Stability (JAR 25.175) 

6.15 Static Directional and Lateral Stability (JAR 25.177) 

6.16 Dynamic Stability (JAR 25.181) 

6.17 Stall Demonstration (JAR 25.201) 

6.18 Stall Warning (JAR 25.207) 

6.19 Wind Velocities (JAR 25.237) 

6.20 Vibration and Buffeting (JAR 25.251) 

6.21 Natural Icing Conditions 

6.22 Failure Conditions (JAR 25.1309) 

A1 Appendix 1 - Airframe Ice Accretion 

A1.1 General 

A1.2 Operative Ice Protection System 

A1.3 Ice Protection System Failure Cases 

A1.4 Additional Guidance for Appendix X Ice Accretions 

A2 Appendix 2 - Artificial Ice Shapes 

A2.1 General 

A2.2 Shape and Texture of Artificial Ice 

A2.3 "Sandpaper Ice" 

A3 Appendix 3 - Design Features 

A3.1 Aeroplane Configuration and Ancestry 

A3.2 Wing 

A3.3 Empennage 

A3.4 Aerodynamic Balancing of Flight Control Surfaces 

A3.5 Ice Protection/Detection System 
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1 Purpose. 

1.1 This ACJ describes an acceptable means for showing compliance with the 

requirements related to performance and handling characteristics of Large Aeroplanes 

as affected by flight in the icing conditions that are defined in Appendixes C and X to 

JAR-25. The means of compliance described in this ACJ is intended to provide 

guidance to supplement the engineering and operational judgement that should form the 

basis of any compliance findings relative to handling characteristics and performance in 

Appendix C icing conditions. 

1.2 The guidance information is presented in sections 4 to 6 and three 

appendices. 

1.3 Section 4 explains the various performance and handling requirements in 

relation to the flight conditions that are relevant for determining the shape and texture 

of ice accretions for the aeroplane in the atmospheric icing conditions of JAR-25, 

Appendixes C and X. 

1.4 Section 5 describes acceptable methods and procedures that an applicant 

may use to show that an aeroplane meets these requirements. Depending on the 

design features of a specific aeroplane as discussed in Appendix 3 of this ACJ, its 

similarity to other types or models, and the service history of those types or models, 

some judgement will often be necessary for determining that any particular method 

or procedure is adequate for showing compliance with a particular requirement. 

1.5 Section 6 provides an acceptable flight test programme where flight testing 

is selected by the applicant and agreed by the Authority as being the primary means 

of compliance. 

1.6 The three appendices provide additional reference material associated with 

ice accretion, artificial ice shapes, and aeroplane design features. 

2 Related Requirements. The following paragraphs of JAR-25 are related to the 

guidance in this ACJ: 

• JAR 25.21 (Proof of compliance) 

• JAR 25.103 (Stall speed) 

• JAR 25.105 (Takeoff) 

• JAR 25.107 (Takeoff speeds) 

• JAR 25.111 (Takeoff path) 

• JAR 25.119 (Landing climb) 
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• JAR 25.121 (Climb: One-engine-inoperative) 

• JAR 25.123 (En-route flight paths) 

• JAR 25.125 (Landing) 

• JAR 25.143 (Controllability and Manoeuvrability - General) 

• JAR 25.207 (Stall warning) 

• JAR 25.237 (Wind velocities) 

• JAR 25.253 (High-speed characteristics) 

• JAR 25.1309 (Equipment, systems, and installations) 

• JAR 25.1419 (Ice protection) 

• JAR 25.1420 (Supercooled large drop icing conditions) 

• JAR 25.1581 (Aeroplane Flight Manual) 

• JAR-25, Appendix C 

• JAR-25, Appendix X 

3 Reserved.

4 Requirements and Guidance. 

4.1 General. This section provides guidance for showing compliance with Subpart 

B requirements for flight in the icing conditions of Appendixes C and X to JAR-25.

4.1.1 Operating rules for commercial operation of large aeroplanes 

(e.g. JAR-OPS 1.345) do not allow a take-off to be commenced unless the external 

surfaces are clear of any deposit (ice or other contaminant) that might adversely 

affect the performance and/or controllability of the aeroplane except as permitted 

in the Aeroplane Flight Manual require that the aeroplane is free of any significant ice 

contamination at the beginning of the take-off roll due to application of appropriate ice 

removal and ice protection procedures during flight preparation on the ground.

4.1.2 Appendixes C and X to JAR-25 define the ice accretions to be used in 

showing compliance with JAR 25.21(g). Appendix 1 of this ACJ provides details on 

ice accretions, including accounting for delay in the operation of the ice protection 

system and consideration of ice detection systems. 
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4.1.3 Certification experience has shown that it is not necessary to consider ice 

accumulation on the propeller, induction system or engine components of an 

inoperative engine for handling qualities substantiation. Similarly, the mass of the ice 

need not normally be considered. 

4.1.4 Flight in icing conditions includes operation of the aeroplane after leaving the 

icing conditions, but with ice accretion remaining on the critical surfaces of the 

aeroplane. 

4.1.5 Ice contaminated tailplane stall (ICTS) is a phenomenon that occurs due 

to airflow separation on the lower surface of the tailplane. This can occur if the 

angle-of-attack of the horizontal tailplane exceeds the stall angle-of-attack, which 

can be reduced by even small quantities of ice on the tailplane leading edge. The 

increase in tailplane angle-of-attack can result from aeroplane configuration (e.g., 

increased flap extension increasing the downwash angle or trim required for the 

centre-of-gravity position) and flight conditions (e.g., a high approach speed 

resulting in an increased flap downwash angle, gusts, manoeuvring, or changes 

to engine power setting). ICTS is characterized by a reduction or loss of pitch 

control or stability while operating in, or after recently departing from, icing 

conditions. For aeroplanes with unpowered longitudinal control systems, the 

pressure differential between the upper and lower surfaces of the stalled tailplane 

may result in a high elevator hinge moment, forcing the elevator trailing edge 

down. This elevator hinge moment reversal can be of sufficient magnitude to 

draw the control column forward with a level of force that is beyond the combined 

efforts of the flightcrew to overcome. On some aeroplanes, ICTS has been caused 

by a lateral flow component coming off of the vertical stabilizer, as may occur in 

sideslip conditions or due to a gust with a lateral component. Aerodynamic 

effects of reduced tailplane lift should be considered for all aeroplanes, including 

those with powered controls. Aeroplanes susceptible to this phenomenon are 

those having a near zero or negative tailplane stall margin with tailplane icing 

contamination. An acceptable flight test procedure for determining susceptibility 

to ICTS is presented in paragraph 6.9.3 of this ACJ. 

4.1.6 Supercooled large drop icing conditions or runback ice can result in the 

formation of a ridge of ice aft of the protected area on the wing upper surface, 

leading to separated airflow over the aileron. Ice induced airflow separation 

upstream of the aileron can have a significant effect on the aileron hinge moment. 

Depending on the extent of the separated flow and the flight control system 

design, ice accretion upstream of the aileron may lead to aileron hinge moment 

reversal, reduced aileron effectiveness, and aileron control reversal. Although 

aeroplanes with de-icing boots and unpowered aileron controls are most 

susceptible to this problem, all aeroplanes should be evaluated for roll control 

capability in icing conditions. Acceptable flight test procedures for checking roll 

control capability are presented in paragraphs 6.9.2(e), 6.15, and 6.17.2(e) of this 
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ACJ and consist of bank-to-bank roll manoeuvres, steady heading sideslips, and 

stall tests. 

4.1.7 JAR 25.21(g) requires that for an aeroplane to be certified for flight in 

icing conditions, the aeroplane must be able to meet certain performance and 

handling quality requirements of JAR-25 subpart B while operating in the 

atmospheric icing environment defined in Appendix C to JAR-25. 

4.1.8 JAR 25.21(g) also requires compliance with certain subpart B 

performance and handling qualities requirements with ice accretions on the 

aeroplane that are defined in Appendix X to JAR-25. The only subpart B 

performance requirements that must be met for an encounter with supercooled 

large drop atmospheric icing conditions beyond those for which the airplane is 

certified for are: (1) stall speed (JAR 25.103), landing climb (JAR 25.119), and 

landing (JAR 25.125). Relative to the other performance requirements, it is 

assumed that the existing requirements for Appendix C ice accretions will provide 

adequate performance capability to allow a safe exit from all icing conditions after 

an encounter with supercooled large drop atmospheric icing conditions beyond 

those for which the airplane is certified. 

4.1.9 For Appendix X atmospheric icing conditions, applicants can choose to 

either: (1) not seek approval for flight in Appendix X conditions; (2) seek approval 

for flight in only a portion of the Appendix X icing conditions; or (3) seek approval 

for flight throughout the entire Appendix X atmospheric icing envelope. 

Applicants may also choose to certify for flight in Appendix X (or a portion of 

Appendix X) icing conditions in some flight phases and not in others. For any 

portion of Appendix X for which the aeroplane is not certified for flight, the 

aeroplane must be capable of encountering those conditions, and then safely 

exiting all icing conditions. 

4.1.10 If the aeroplane is not to be certified for flight throughout the entire 

atmospheric icing envelope of Appendix X, there must be a means to indicate 

when the aeroplane has encountered icing conditions beyond those for which it 

has been certified. See ACJ 25.1419/1420 for guidance on acceptable means of 

detecting and indicating when the aeroplane has encountered icing conditions 

beyond those for which it has been certified. Procedures should be provided in 

the Aeroplane Flight Manual to enable a safe exit from all icing conditions after an 

encounter with icing conditions beyond those for which the aeroplane is certified. 

4.1.11 To certify the aeroplane for operations in Appendix X icing conditions 

only for certain flight phase(s), the flight phase(s) for which operation is approved 

should be defined such that a flightcrew can easily determine whether the 

aeroplane is operating inside or outside of its certified icing envelope. The critical 

ice accretion or accretions used to show compliance with the applicable Subpart 

B requirements should cover the range of airplane configurations, operating 

speeds, angles-of-attack, and engine thrust or power settings that may be 
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encountered during that phase of flight (not just at the conditions specified in 

Subpart B requirements), and should include an adequate time in the icing 

conditions to address operational variabilities. For the ice accretion scenarios 

defined in paragraph A.1.4.3.3 of Appendix 1 of this AC, the applicable flight 

phases are: take-off (including the ground roll, take-off and final take-off 

segments), en-route, holding, and approach/landing (including both the approach 

and landing segments). 

4.1.12 The ice accretions used to show compliance with the applicable subpart 

B regulations should be consistent with the extent of the desired certification for 

flight in icing conditions. Appendixes C and X to JAR-25 define the ice 

accretions, as a function of flight phase, that must be considered for certification 

approval for flight in those respective icing conditions. In order to reduce the 

number of ice accretions used for demonstrating compliance, any of the 

applicable ice accretions (or a composite accretion representing a combination of 

accretions) may be used to show compliance with a particular Subpart B 

requirement if that accretion is either the ice accretion identified in the 

requirement or is shown to be more conservative than the ice accretion identified 

in the requirement. In addition, the ice accretion that has the most adverse effect 

on handling characteristics may be used for compliance with the airplane 

performance requirements if any difference in performance is conservatively 

taken into account. The ice accretion(s) used to show compliance should 

consider the speeds, configurations (including configuration changes), angles of 

attack, power or thrust settings, etc. for the flight phases and icing conditions that 

they are intended to cover. For example, if certification for flight in the 

supercooled large drop icing conditions of Appendix X is desired in addition to 

the icing conditions of Appendix C, compliance with the applicable subpart B 

requirements may be shown using the most critical of the Appendix C and 

Appendix X ice accretions. 

4.2 Proof of Compliance (JAR 25.21(g)).

4.2.1 Demonstration of compliance with certification requirements for flight in icing 

conditions may be accomplished by any of the means discussed in paragraph 5.1 of this 

ACJ. 

4.2.2 Certification experience has shown that aeroplanes of conventional design do 

not require additional detailed substantiation of compliance with the requirements of the 

following paragraphs of JAR-25 for flight in icing conditions or with ice accretions: 

25.23, 

25.25, 

25.27, 

25.29, 

25.31, 

25.231, 
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25.233, 

25.235, 

25.253(a) and (b), and 

25.255 

4.2.3 Where normal operation of the ice protection system results in changing the 

stall warning system and/or stall identification system activation settings, it is acceptable 

to establish a procedure to return to the non icing settings when it can be demonstrated 

that the critical wing surfaces are free of ice accretion. 

4.3 Propeller Speed and Pitch Limits (JAR 25.33). Certification experience has 

shown that it may be necessary to impose additional propeller speed limits for 

operations in icing conditions. 

4.4 Performance - General (JAR 25.101).

4.4.1 The propulsive power or thrust available for each flight condition must be 

appropriate to the aeroplane operating limitations and normal procedures for flight in 

icing conditions. In general, it is acceptable to determine the propulsive power or 

thrust available by suitable analysis, substantiated when required by appropriate 

flight tests (e.g. when determining the power or thrust available after 8 seconds for 

JAR 25.119). The following aspects should be considered: 

a. Operation of induction system ice protection. 

b. Operation of propeller ice protection. 

c. Operation of engine ice protection. 

d. Operation of airframe ice protection system. 

4.4.2 The following should be considered when determining the change in 

performance due to flight in icing conditions: 

a. Thrust loss due to ice accretion on propulsion system components with normal 

operation of the ice protection system, including engine induction system and/or engine 

components, and propeller spinner and blades. 

b. The incremental airframe drag due to ice accretion with normal operation of the 

ice protection system. 

c. Changes in operating speeds due to flight in icing conditions. 
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4.4.3 Certification experience has shown that any increment in drag (or 

decrement in thrust) due to the effects of ice accumulation on the landing gear, 

propeller, induction system and engine components may be determined by a suitable 

analysis. 

4.4.4 Apart from the use of appropriate speed adjustments to account for 

operation in icing conditions, any changes in the procedures established for take-off, 

balked landing, and missed approaches should be agreed with the Authority. 

4.4.5 Performance associated with flight in icing conditions is applicable after exiting 

icing conditions until the aeroplane critical surfaces are free of ice accretion and the ice 

protection systems are selected “Off.” 

4.5 Stall speed (JAR 25.103). Certification experience has shown that for 

aeroplanes of conventional design it is not necessary to make a separate 

determination of the effects of Mach number on stall speeds for the aeroplane with 

ice accretions.

4.6 Failure Conditions (JAR 25.1309). 

4.6.1 The failure modes of the ice protection system and the resulting effects on 

aeroplane handling and performance should be analysed in accordance with JAR 

25.1309. In determining the probability of a failure condition, it should be assumed 

that the probability of entering the atmospheric icing conditions covered by 

Appendix C to JAR-25, is one. As explained in ACJ 25.1419/1420, on an annual 

basis, the average probability of 1 x 10-2 per flight hour may be assumed for 

encountering Appendix X conditions within a quantitative analysis. This 

probability should not be reduced based on phase of flight. The "Failure Ice" 

configuration accretion is defined in Appendix 1, paragraph A1.3. 

4.6.2 For probable failure conditions that are not annunciated to the flight crew, 

the guidance in this ACJ for a normal condition is applicable with the "Failure Ice" 

configuration. 

4.6.3 For probable failure conditions that are annunciated to the flight crew, with 

an associated procedure that does not require the aeroplane to exit icing conditions, 

the guidance in this ACJ for a normal condition is applicable with the "Failure Ice" 

configuration. 

4.6.4 For probable failure conditions that are annunciated to the flight crew, with 

an associated operating procedure that requires the aeroplane to leave the icing 

conditions as soon as practicable, it should be shown that the aeroplane is capable 

of continued safe flight and landing with the “Failure Ice" configuration. The 

operating procedures and related speeds should provide an adequate operating 
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envelope and acceptable performance and handling characteristics to ensure 

continued safe flight and landing. 

4.6.5 For failure conditions that are improbable but not extremely improbable, the 

analysis and substantiation of continued safe flight and landing, in accordance with 

JAR 25.1309, should take into consideration whether annunciation of the failure is 

provided and the associated operating procedures and speeds to be used following 

the failure condition. 

4.7 Flight-related Systems. In general, systems aspects are covered by the 

applicable systems and equipment requirements in other subparts of JAR-25, and 

associated guidance material. However, certification experience has shown that 

other flight related systems aspects should be considered when determining 

compliance with the flight requirements of subpart B. For example, the following 

aspects may be relevant: 

a. The ice protection systems may not anti-ice or de-ice properly at all power 

or thrust settings. This may result in a minimum power or thrust setting for operation 

in icing conditions which affects descent and/or approach capability. 

b. Ice blockage of control surface gaps and/or freezing of seals causing 

increased control forces, control restrictions or blockage. 

c. Airspeed, altitude and/or angle of attack sensing errors due to ice accretion 

forward of the sensors (e.g. radome ice). Dynamic pressure ("q") operated feel 

systems using separate sensors also may be affected. 

d. Ice blockage of unprotected inlets and vents that may affect the propulsive 

thrust available, aerodynamic drag, powerplant control, or flight control. 

e. Operation of stall warning and stall identification reset features for flight in 

icing conditions, including the effects of failure to operate. 

f. Operation of icing condition sensors, ice accretion sensors, and automatic 

or manual activation of ice protection systems. 

g. Flight guidance and automatic flight control systems operation. See ACJ 

25.1329-XX, “Flight guidance system,” for guidance on compliance with 

§ 25.1329 for flight in icing conditions, including stall and maneuverability 

demonstrations with the airplane under flight guidance system control. 

h. Installed thrust. This includes operation of ice protection systems when 

establishing acceptable power or thrust setting procedures, control, stability, lapse 

rates, rotor speed margins, temperature margins, Automatic Reserve Power (ARP) 

operation, and power or thrust lever angle functions. 
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4.8 Aeroplane Flight Manual (JAR 25.1581). 

4.8.1 Limitations.

4.8.1.1 Where limitations are required to ensure safe operation in icing conditions, 

these limitations should be stated in the Aeroplane Flight Manual (AFM).

4.8.1.2 Performance limitations should be presented for flight in icing 

conditions that reflect any effects on lift, drag, thrust, and operating speeds 

due to operating in icing conditions. These limitations may be presented in the 

Performance Information Section of the AFM and included as limitations by 

specific reference in the Limitations Section of the AFM. 

4.8.1.3 Any airspeed limitations associated with flight in icing conditions 

should be presented, such as the minimum airspeed that should be maintained 

for each normal airplane configuration in icing conditions. 

4.8.1.4 The Limitations section of the AFM should include, as applicable, a 

statement similar to the following: “In icing conditions the aeroplane must be 

operated, and its ice protection systems used, as described in the operating 

procedures section of this manual. Where specific operational speeds and 

performance information have been established for such conditions, this information 

must be used." 

4.8.1.5 The “kinds of operation” portion of the AFM Limitations section should 

include, as applicable, a statement similar to the following, clearly stating the 

extent of the certification for flight in icing conditions: 

“This aircraft has been certified for operations under freezing drizzle or freezing 

rain icing conditions (as appropriate). If these conditions are anticipated at 

takeoff [for aeroplanes approved to take off in these conditions,] any relevant 

limitations or operating procedures for operating on a contaminated runway 

should also be observed.” 

4.8.1.6 For aeroplanes not certified to operate throughout the atmospheric icing 

envelope of Appendix X for every flight phase, the Limitations section of the AFM 

should also identify the means for detecting when the certified icing conditions 

have been exceeded and that intentional flight, including take off and landing, into 

these conditions is prohibited. A requirement to exit all icing conditions must be 

included if the uncertified portion of Appendix X is encountered. The statements 

in the Limitations section should generally be concise with additional information 

provided in the AFM and Flightcrew Operating Manual (FCOM) procedure 

sections. See ACJ 25.1419/1420 for guidance on acceptable means for identifying 

when the icing conditions exceed the icing conditions for which the aeroplane has 

been certified. Examples of AFM and FCOM wording are provided in Appendix 4 

of this AC for some of the possible certification options. (Note: This example 
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AFM wording only addresses the extent of certification for Appendix X icing 

conditions. Other limitations and procedural information as noted under 

paragraphs 4.8.1 and 4.8.2 of this AC should be included as well.) 

4.8.2 Operating Procedures.

4.8.2.1 AFM operating procedures for flight in icing conditions should include 

normal operation of the aeroplane including operation of the ice protection system 

and operation of the aeroplane following ice protection system failures. Any changes 

in procedures for other aeroplane system failures that affect the capability of the 

aeroplane to operate in icing conditions should be included. 

4.8.2.2 Normal operating procedures provided in the AFM should reflect the 

procedures used to certify the aeroplane for flight in icing conditions. This includes 

configurations, speeds, ice protection system operation, power plant and systems 

operation, for take-off, climb, cruise, descent, holding, go-around, and landing. For 

airplanes not certified for flight in the entire supercooled large drop 

atmospheric icing conditions defined in Appendix X to JAR-25, procedures 

should be provided for safely exiting icing conditions that exceed those for 

which the airplane is certified. Examples of acceptable procedures statements 

are provided in Appendix 4 of this AC for some of the possible certification 

options. 

4.8.2.3 Abnormal operating procedures should include the procedures to be 

followed in the event of annunciated ice protection system failures and suspected 

unannunciated failures. Any changes to other abnormal procedures contained in the 

AFM, due to flight in icing conditions, should also be included. 

4.8.3 Performance Information. Performance information, derived in accordance 

with subpart B of JAR-25, must be provided in the AFM for all relevant phases of 

flight. 

5 Acceptable Means of Compliance - General.

5.1 General.

5.1.1 This section describes acceptable methods and procedures that an 

applicant may use to show that an aeroplane meets the performance and handling 

requirements of subpart B in the atmospheric icing conditions of Appendix C to JAR25. 

5.1.2 Compliance with JAR 25.21(g) should be shown by one or more of the 

methods listed in this section. 
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5.1.3 The compliance process should address all phases of flight, including takeoff, climb, cruise, holding, descent, landing, and go-around as appropriate to the 

aeroplane type, considering its typical operating regime and the extent of its 

certification approval for operation in Appendix X.

5.1.4 The design features included in Appendix 3 of this ACJ should be 

considered when determining the extent of the substantiation programme. 

5.1.5 Appropriate means for showing compliance include the actions and items 

listed in Table 1. These are explained in more detail in the following sections of this 

ACJ. 

TABLE 1: Means for Showing Compliance 

Flight Testing Flight testing in dry air using artificial ice 

shapes or with ice shapes created in natural 

icing conditions. 

Wind Tunnel Testing and 

Analysis 

An analysis of results from wind tunnel tests 

with artificial or actual ice shapes. 

Engineering Simulator Testing 

and Analysis 

An analysis of results from engineering 

simulator tests. 

Engineering Analysis An analysis which may include the results 

from executing an agreed computer code. 

Ancestor Aeroplane Analysis An analysis of results from a closely related 

ancestor aeroplane. 

5.1.6 Various factors that affect ice accretion on the airframe with an operative 

ice protection system and with ice protection system failures are discussed in 

Appendix 1 of this ACJ. 

5.1.7 An acceptable methodology to obtain agreement on the artificial ice shapes is 

given in Appendix 2 of this ACJ. That appendix also provides the different types of 

artificial ice shapes to be considered. 
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5.2 Flight Testing.

5.2.1 General. 

5.2.1.1 The extent of the flight test programme should consider the results obtained 

with the non-contaminated aeroplane and the design features of the aeroplane as 

discussed in Appendix 3 of this ACJ. 

5.2.1.2 It is not necessary to repeat an extensive performance and flight 

characteristics test programme on an aeroplane with ice accretion. A suitable 

programme that is sufficient to demonstrate compliance with the requirements can 

be established from experience with aeroplanes of similar size, and from review of 

the ice protection system design, control system design, wing design, horizontal and 

vertical stabiliser design, performance characteristics, and handling characteristics of 

the non-contaminated aeroplane. In particular, it is not necessary to investigate all 

weight and centre of gravity combinations when results from the non-contaminated 

aeroplane clearly indicate the most critical combination to be tested. It is not 

necessary to investigate the flight characteristics of the aeroplane at high altitude 

(i.e. above the upper limit specified in Appendixes C and X to JAR-25). An 

acceptable flight test programme is provided in section 6 of this ACJ. 

5.2.1.3 Certification experience has shown that tests are usually necessary to 

evaluate the consequences of ice protection system failures on handling 

characteristics and performance and to demonstrate continued safe flight and 

landing. 

5.2.2 Flight Testing Using Approved Artificial Ice Shapes. 

5.2.2.1 The performance and handling tests may be based on flight testing in dry 

air using artificial ice shapes that have been agreed with the Authority. 

5.2.2.2 Additional limited flight tests should be conducted in natural icing 

conditions, which are discussed in paragraph 5.2.3, below. 

 

5.2.3 Flight Testing In Natural Icing Conditions.

5.2.3.1 Where flight testing in natural atmospheric icing conditions is the primary means 

of compliance, the conditions should be measured and recorded. The tests should 

ensure good coverage of Appendixes C and X (as appropriate for the extent of the 

certification approval for operation in Appendix X) conditions and, in particular, the 

critical conditions. The conditions for accreting ice (including the icing atmosphere, 

configuration, speed and duration of exposure) should be agreed with the Authority. 

5.2.3.2 Where flight testing with artificial ice shapes is the primary means of 

compliance, additional limited flight tests should be conducted in natural icing 

conditions. The objective of these tests is to corroborate the handling characteristics 
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and performance results obtained in flight testing with artificial ice shapes. As such, 

it is not necessary to measure the atmospheric characteristics (i.e. liquid water 

content (LWC) and median volumetric diameter (MVD)) of the flight test icing 

conditions. For some derivative aeroplanes with similar aerodynamic characteristics 

as the ancestor, it may not be necessary to carry out additional flight tests in natural 

icing conditions if such tests have been already performed with the ancestor. 

Depending on the extent of certification being sought for flight in the icing 

conditions of Appendix X and the means for showing compliance with the 

applicable aeroplane performance and handling characteristics requirements, 

it may also may not be necessary to conduct flight tests in the natural icing 

conditions of Appendix X. See ACJ 25.1419/1420 and paragraph 6.21.1.2 of 

this ACJ for guidance on when flight testing in the natural atmospheric icing 

conditions of Appendix X is necessary. 

5.3 Wind Tunnel Testing and Analysis. Analysis of the results of dry air wind 

tunnel testing of models with artificial ice shapes, as defined in Appendixes C and X

Part II of Appendix C JAR-25, may be used to substantiate the performance and 

handling characteristics. 

5.4 Engineering Simulator Testing and Analysis. The results of an engineering 

simulator analysis of an aeroplane that includes the effects of the ice accretions as 

defined in Appendixes C and X to Part II of Appendix C to JAR-25 may be used to 

substantiate the handling characteristics. The data used to model the effects of ice 

accretions for the engineering simulator may be based on results of dry air wind tunnel 

tests, flight tests, computational analysis, and engineering judgement. 

5.5 Engineering Analysis. An engineering analysis that includes the effects of the 

ice accretions as defined in Appendixes C and X Part II of Appendix C to JAR-25 may 

be used to substantiate the performance and handling characteristics. The effects of the 

ice shapes used in this analysis may be determined by an analysis of the results of dry 

air wind tunnel tests, flight tests, computational analysis, engineering simulator analysis, 

and engineering judgement. 

5.6 Ancestor Aeroplane Analysis.

5.6.1 An ancestor aeroplane analysis that includes the effect of the ice accretions 

as defined in Appendixes C and X Part II of Appendix C to JAR-25 may be used to 

substantiate the performance and handling characteristics. This analysis should 

consider the similarity of the configuration, operating envelope, performance and 

handling characteristics, and ice protection system of the ancestor aeroplane. 

5.6.2 The analysis may include flight test data, dry air wind tunnel test data, icing 

tunnel test data, engineering simulator analysis, service history, and engineering 

judgement. 
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6 Acceptable Means of Compliance - Flight Test Programme.

6.1 General.

6.1.1 This section provides an acceptable flight test programme where flight 

testing is selected by the applicant and agreed by the Authority as being the primary 

means for showing compliance. 

6.1.2 Where an alternate means of compliance is proposed for a specific 

paragraph in this section, it should enable compliance to be shown with at least the 

same degree of confidence as flight test would provide (see JAR 25.21(a)(1)). 

6.1.3 Ice accretions for each flight phase are defined in Appendixes C and X 

to JAR-25. Additional guidance for determining the applicable ice accretions is 

provided in Appendix 1 to this ACJ. 

6.1.4 This test programme is based on the assumption that the applicant will 

choose to use "Holding Ice" for the majority of the testing on the basis that this is the 

most conservative shape if “Holding Ice” is found to be the most conservative 

ice shape for all phases of flight. Where this is not so, the In general, the

applicant may choose to use either an the specific ice shape accretion appropriate 

to the particular phase of flight and icing conditions for which certification is 

desired (i.e., Appendix C or Appendix X to JAR-25), or an accretion that is 

shown to be conservative for the particular application. In addition, the ice 

accretion that has the most adverse effect on handling characteristics may be 

used for airplane performance tests provided any difference in performance is 

conservatively taken into account. 

6.2 Stall Speed (JAR 25.103).

6.2.1 The stall speed for intermediate high lift configurations can normally be 

obtained by interpolation. However if a stall identification system (e.g. stick pusher) 

firing point is set as a function of the high lift configuration and/or the firing point is 

reset for icing conditions, or if significant configuration changes occur with extension 

of trailing edge flaps (such as wing leading edge high-lift device position movement), 

additional tests may be necessary. 

6.2.2 Acceptable Test Programme. The following represents an acceptable test 

programme subject to the provisions outlined above: 

a. Forward centre of gravity position appropriate to the configuration. 

b. Normal stall test altitude. 
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c. In the configurations listed below, trim the aeroplane at an initial speed of 

1.13 to 1.30 VSR. Decrease speed until an acceptable stall identification is obtained. 

i. High lift devices retracted configuration, "Final Take-off Ice." 

ii. High lift devices retracted configuration, "En-route Ice." 

iii. Holding configuration, "Holding Ice." 

iv. Lowest lift take-off configuration, "Holding Ice." 

v. Highest lift take-off configuration, "Take-off Ice." 

vi. Highest lift landing configuration, "Holding Ice." 

6.3 Accelerate-stop Distance (JAR 25.109). The effect of any increase in V1 due 

to take-off in icing conditions may be determined by a suitable analysis. 

6.4 Take-off Path (JAR 25.111). If VSR in the configuration defined by JAR 

25.121(b) with the “Takeoff Ice" accretion defined in Appendix C to JAR-25 exceeds VSR

for the same configuration without ice accretions by more than the greater of 3 knots or 

3%, the take-off demonstrations should be repeated to substantiate the speed schedule 

and distances for take-off in icing conditions. The effect of the take-off speed increase, 

thrust loss, and drag increase on the take-off path may be determined by a suitable 

analysis. 

6.5 Landing Climb: All-engines-operating (JAR 25.119). Acceptable Test 

Programme. The following represents an acceptable test programme: 

a. "Holding Ice." 

b. Forward centre of gravity position appropriate to the configuration. 

c. Highest lift landing configuration, landing climb speed no greater than VREF.

d. Stabilise at the specified speed and conduct 2 climbs or drag polar checks 

as agreed with the Authority. 

6.6 Climb: One-engine-inoperative (JAR 25.121). Acceptable Test Programme. 

The following represents an acceptable test programme: 

a. Forward centre of gravity position appropriate to the configuration. 
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b. In the configurations listed below, stabilise the aeroplane at the specified 

speed with one engine inoperative (or simulated inoperative if all effects can be 

taken into account) and conduct 2 climbs in each configuration or drag polar checks 

substantiated for the asymmetric drag increment as agreed with the Authority. 

i. High lift devices retracted configuration, final take-off climb speed, "Final Takeoff Ice." 

ii. Lowest lift take-off configuration, landing gear retracted, V2 climb speed, "Takeoff Ice." 

iii. Approach configuration appropriate to the highest lift landing configuration, 

landing gear retracted, approach climb speed, "Holding Ice." 

6.7 En-route Flight Path (JAR 25.123). Acceptable Test Programme. The 

following represents an acceptable test programme: 

a. "En-route Ice." 

b. Forward centre of gravity position appropriate to the configuration. 

c. En-route configuration and climb speed. 

d. Stabilise at the specified speed with one engine inoperative (or simulated 

inoperative if all effects can be taken into account) and conduct 2 climbs or drag 

polar checks substantiated for the asymmetric drag increment as agreed with the 

Authority. 

6.8 Landing (JAR 25.125). The effect of landing speed increase on the landing 

distance may be determined by a suitable analysis. 

6.9 Controllability and Manoeuvrability - General (JAR 25.143 and 25.177).

6.9.1 A qualitative and quantitative evaluation is usually necessary to evaluate 

the aeroplane's controllability and manoeuvrability. In the case of marginal 

compliance, or the force limits or stick force per g limits of JAR 25.143 being 

approached, additional substantiation may be necessary to establish compliance. In 

general, it is not necessary to consider separately the ice accretion appropriate to 

take-off and en-route because the "Holding Ice" is usually the most critical. 
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6.9.2 General Controllability and Manoeuvrability. The following represents an 

acceptable test programme for general controllability and manoeuvrability, subject to 

the provisions outlined above: 

a. "Holding Ice." 

b. Medium to light weight, aft centre of gravity position, symmetric fuel loading. 

c. In the configurations listed in Table 2, trim at the specified speeds and 

conduct the following manoeuvres: 

i. 30° banked turns left and right with rapid reversals;

i. Pull up to 1.5g (except that this may be limited to 1.3g at VREF), and 

pushover to 0.5g (except that the pushover is not required at VMO and VFE); and 

ii. Deploy and retract deceleration devices. 

TABLE 2: Trim Speeds 

Configuration Trim Speed 

High lift devices retracted 

configuration: 

• 1.3 VSR, and 

• VMO or 250 KIAS, whichever 

is less 

Lowest lift takeoff configuration: • 1.3 VSR, and 

• VFE or 250 KIAS, whichever 

is less 

Highest lift landing configuration: • VREF, and 

• VFE or 250 KIAS, whichever 

is less. 

d. Lowest lift take-off configuration: At the greater of 1.13 VSR or V2 MIN, with 

one engine inoperative (simulated), conduct 30° banked turns left and right with 

normal turn reversals and, in wings-level flight, a 5 knot speed decrease and 

increase. 

e. Holding configuration, maximum landing weight, minimum holding 

speed (highest expected holding angle-of-attack) and highest lift landing 

configuration, maximum landing weight, forward c.g, VREF (highest expected 

landing approach angle-of-attack): Trim the aeroplane in level flight, establish 

a 30° bank level turn. Using a step input of approximately 1/3 full lateral 

control deflection, roll the aeroplane in the other direction. When the airplane 

reaches approximately 20° bank in the opposite direction, apply the same 

lateral control input in the opposite direction. Release input and recover as the 
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aeroplane passes a wings level attitude. Repeat the test procedure with 2/3 

and then full lateral control deflection unless the roll rate is judged to be 

excessive. It should be possible to readily arrest and reverse the roll rate 

using only lateral control input, and the lateral control force should not reverse 

with increasing control deflection. 

f. Conduct an approach and go-around with all engines operating using the 

recommended procedure. 

g. Conduct an approach and go-around with one engine inoperative 

(simulated) using the recommended procedure. 

h. Conduct an approach and landing using the recommended procedure. In 

addition satisfactory controllability should be demonstrated during a landing at VREF

minus 5 knots. These tests should be done at heavy weight and forward centre of 

gravity. 

i. Conduct an approach and landing with one engine inoperative (simulated) 

using the recommended procedure. 

6.9.3 Low g Manoeuvres and Sideslips. The following represents an acceptable 

test programme for compliance with controllability requirements in low g manoeuvres 

and in sideslips. 

JAA/FAA/ALPA Position 

6.9.3.1 It should be shown that a push force is required throughout a pushover 

manoeuvre down to zero g or the lowest load obtainable if limited by elevator power. 

It should be possible to promptly recover from the manoeuvre without exceeding 50 

pounds pull control force. 

Industry Position 

6.9.3.1 For pushover manoeuvres, it should be shown that the aeroplane is 

controllable down to zero g or the lowest load factor obtainable if limited by elevator 

power. It should be shown that a push force is required down to 0.5 g load factor, 

and that it is possible to promptly recover from the manoeuvre without exceeding 50 

pounds pull control force. 

TC Position 

6.9.3.1 For pushover manoeuvres, it should be shown that the aeroplane is 

controllable down to zero g or the lowest load factor obtainable if limited by elevator 

power. It should be shown that a push force is required down to 0.25 g load factor, 

and that it is possible to promptly recover from the manoeuvre without exceeding 50 

pounds pull control force. 
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6.9.3.2 For sideslips, changes in longitudinal control force to maintain speed with 

increasing sideslip should be progressive, with no reversals or unacceptable 

discontinuities (see paragraph 6.15.1 of this ACJ). 

6.9.3.3 The test manoeuvres described in paragraphs 6.9.3.1 and 6.9.3.2, above, 

should be conducted using the following configurations and procedures: 

a. "Holding Ice." For aeroplanes with unpowered elevators, these tests should 

also be performed with "Sandpaper Ice." 

b. Medium to light weight, the most critical of aft or forward centre of gravity 

position, symmetric fuel loading. 

c. In the configurations listed below, with the aeroplane in trim, or as nearly as 

possible in trim, at the specified trim speed, perform a continuous manoeuvre 

(without changing trim) to reach zero g normal load factor or, if limited by elevator 

control authority, the lowest load factor obtainable at the target speed. 

i. Highest lift landing configuration at idle power or thrust, and the more 

critical of: 

 - Trim speed 1.23 VSR, target speed not more than 1.23 VSR, or 

 - Trim speed VFE, target speed not less than VFE - 20 knots. 

ii. Highest lift landing configuration at go-around power or thrust, and the more 

critical of: 

 - Trim speed 1.23 VSR, target speed not more than 1.23 VSR, or 

 - Trim speed VFE, target speed not less than VFE - 20 knots. 

iii. Conduct steady heading sideslips to full rudder authority, 180 lb. rudder force 

or full lateral control authority (whichever comes first), with highest lift landing 

configuration, trim speed 1.23 VSR, and power or thrust for -3° flight path angle. 

6.9.4 Controllability prior to Normal Operation of the Ice Protection System and prior 

to Detection of Appendix X icing conditions. The following represents an acceptable 

test programme for compliance with the applicable controllability requirements with the 

ice accretion prior to normal operation of the ice protection system. 

6.9.4.1 Where the ice protection system is activated as described in paragraph 

A1.2.3.3.a of Appendix 1 of this ACJ, paragraphs 6.9.1, 6.9.2 and 6.9.3 of this ACJ are 

applicable with the ice accretion prior to normal system operation. When the primary 

means of detecting Appendix X icing conditions depends on the flight crew to 

recognize visual cues of Appendix X icing, paragraphs 6.9.1, 6.9.2 and 6.9.3 of this 
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ACJ are also applicable using the ice accretion prior to detection of Appendix X 

icing conditions.

6.9.4.2 Where the ice protection system is activated as described in paragraphs 

A1.2.3.3.b,c,d or e of Appendix 1 of this ACJ, it is acceptable to demonstrate adequate 

controllability with the ice accretion prior to normal system operation, as follows: 

a. In the configurations listed below, trim the aeroplane at the specified speed. 

Conduct pull up to 1.5g and pushover to 0.5g without longitudinal control force 

reversal. 

i. High lift devices retracted configuration (or holding configuration if different), 

holding speed, power or thrust for level flight. 

ii. Landing configuration, VREF for non-icing conditions, power or thrust for 

landing approach (limit pull up to stall warning). 

6.9.4.3 When the primary means of detecting Appendix X icing conditions is 

an approved ice detection system, controllability prior to detection of 

Appendix X icing conditions should be assessed using the criteria of 

paragraph 6.9.4.2 with the ice accretion prior to detection of the Appendix X 

icing conditions. 

6.10 Longitudinal Control (JAR 25.145).

6.10.1 No specific quantitative evaluations are required for demonstrating 

compliance with JAR 25.145(b) and (c). Qualitative evaluations should be combined 

with the other testing. The results from the non-contaminated aeroplane tests should 

be reviewed to determine whether there are any cases where there was marginal 

compliance. If so, these cases should be repeated with ice. 

6.10.2 Acceptable Test Programme. The following represents an acceptable test 

programme for compliance with JAR 25.145(a): 

a. "Holding ice." 

b. Medium to light weight, aft centre of gravity position, symmetric fuel loading. 

c. In the configurations listed below, trim the aeroplane at 1.3 VSR. Reduce 

speed using elevator control to stall warning plus one second and demonstrate 

prompt recovery to the trim speed using elevator control. 

i. High lift devices retracted configuration, maximum continuous power or 

thrust. 
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ii. Maximum lift landing configuration, maximum continuous power or thrust. 

6.11 Directional and Lateral Control (JAR 25.147). Qualitative evaluations should 

be combined with the other testing. The results from the non-contaminated aeroplane 

tests should be reviewed to determine whether there are any cases where there was 

marginal compliance. If so, these cases should be repeated with ice. 

6.12 Trim (JAR 25.161). Qualitative evaluations should be combined with the other 

testing. The results from the non-contaminated aeroplane tests should be reviewed to 

determine whether there are any cases where there was marginal compliance. If so, 

these cases should be repeated with ice. 

6.13 Stability - General (JAR 25.171). Qualitative evaluations should be combined 

with the other testing. Any tendency to change speed when trimmed or requirement for 

frequent trim inputs should be specifically investigated. 

6.14 Demonstration of Static Longitudinal Stability (JAR 25.175).

6.14.1 Each of the following cases should be tested. In general, it is not 

necessary to test the cruise configuration at low speed (JAR 25.175(b)(2)) or the 

cruise configuration with landing gear extended (JAR 25.175(b)(3)); nor is it 

necessary to test at high altitude. Although the maximum speed for substantiation of 

stability characteristics is the lower of 300 knots CAS or VFC (JAR 25.253c), the 

maximum speed for demonstration can be limited to 280 knots CAS, provided that 

the stick force gradient can be satisfactorily extrapolated to 300 knots CAS or VFC

(e.g. there is no gradient decrease with increasing speed). 

6.14.2 Acceptable Test Programme. The following represents an acceptable test 

programme for demonstration of static longitudinal stability: 

a. "Holding Ice." 

b. High landing weight, aft centre of gravity position, symmetric fuel loading. 

c. In the configurations listed below, trim the aeroplane at the specified speed. 

The power or thrust should be set and stability demonstrated over the speed ranges 

as stated in JAR 25.175(a) through (d), as applicable. 

i. Climb: With high lift devices retracted, trim at 1.3 VSR.

ii. Cruise: With high lift devices retracted, trim at VMO or 250 knots CAS, 

whichever is lower. 
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iii. Approach: With the high lift devices in the approach position appropriate to 

the highest lift landing configuration, trim at 1.3 VSR.

iv. Landing: With the highest lift landing configuration, trim at 1.3VSR. 

6.15 Static Directional and Lateral Stability (JAR 25.177).

6.15.1 Compliance should be demonstrated using steady heading sideslips to 

show compliance with directional and lateral stability. The maximum sideslip angles 

obtained should be recorded and may be used to substantiate a crosswind value for 

landing (see paragraph 6.19 of this ACJ). Directional and lateral control 

movements and forces must be substantially proportional to the angle of 

sideslip without snatching (i.e. sudden, sharp oscillations or reversals in 

control force).

6.15.2 Acceptable Test Programme. The following represents an acceptable test 

programme for static directional and lateral stability: 

a. "Holding Ice." 

b. Medium to light weight, aft centre of gravity position, symmetric fuel loading. 

c. In the configurations listed below, trim the aeroplane at the specified speed 

and conduct steady heading sideslips to full rudder authority, 180 lb. rudder pedal 

force, or full lateral control authority, whichever comes first. 

i. High lift devices retracted configuration: Trim at best rate-of-climb speed, 

but need not be less than 1.3 VSR.

ii. Lowest lift take-off configuration: Trim at the all-engines-operating initial 

climb speed. 

iii. Highest lift landing configuration: Trim at VREF. 

6.16 Dynamic Stability (JAR 25.181). Provided that there are no marginal 

compliance aspects with the non-contaminated aeroplane, it is not necessary to 

demonstrate dynamic stability in specific tests. Qualitative evaluations should be 

combined with the other testing. Any tendency to sustain oscillations in turbulence or 

difficulty in achieving precise attitude control should be investigated. 

6.17 Stall Demonstration (JAR 25.201).
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6.17.1 Sufficient stall testing should be conducted to demonstrate that the stall 

characteristics comply with the requirements. In general, it is not necessary to 

conduct a stall programme which encompasses all weights, centre of gravity 

positions (including lateral asymmetry), altitudes, high lift configurations, deceleration 

device configurations, straight and turning flight stalls, power off and power on stalls. 

Based on a review of the stall characteristics of the non-contaminated aeroplane, a 

reduced test matrix can be established. However, additional testing may be 

necessary if: 

• the stall characteristics with ice accretion show a significant difference from 

the non-contaminated aeroplane, 

• testing indicates marginal compliance, or 

• a stall identification system (e.g. stick pusher) is required to be reset for icing 

conditions. 

6.17.2 Acceptable Test Programme. The following represents an acceptable test 

programme subject to the provisions outlined above. Turning flight stalls at 

decelerations greater than 1 knot/sec are not required. 

a. "Holding Ice." 

b. Medium to light weight, aft centre of gravity position, symmetric fuel loading. 

c. Normal stall test altitude. 

d. In the configurations listed below, trim the aeroplane at the same initial stall 

speed factor used for stall speed determination. For power-on stalls, use the power 

setting as defined in JAR 25.201(a)(2) but with ice accretions on the aeroplane. 

Decrease speed to stall identification and recover using the same test technique as 

for the non-contaminated aeroplane. 

i. High lift devices retracted configuration: Straight/Power Off, Straight/Power 

On, Turning/Power Off, Turning/Power On. 

ii. Lowest lift take-off configuration: Straight/Power On, Turning/Power Off. 

iii. Highest lift take-off configuration: Straight/Power Off, Turning/Power On. 

iv. Highest lift landing configuration: Straight/Power Off, Straight/Power On, 

Turning/Power Off, Turning/Power On. 
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e. For the configurations listed in paragraph 6.17.2(d)(i) and (iv) (and any 

other configuration if deemed more critical), in 1 knot/second deceleration 

rates down to stall warning with wings level and power off, roll the aeroplane 

left and right up to 10 degrees of bank using lateral control. 

6.18 Stall Warning (JAR 25.207).

6.18.1 Stall warning should be assessed in conjunction with stall speed testing and 

stall characteristics testing (JAR 25.103, JAR 25.203 and paragraphs 6.2 and 6.17 of 

this ACJ, respectively) and in tests with faster entry rates. 

6.18.2 Normal Ice Protection System Operation. The following represents an 

acceptable test programme for stall warning in slow down turns of at least 1.5g and 

at entry rates of at least 2 knot/sec: 

a. "Holding Ice." 

b. Medium to light weight, aft centre of gravity position, symmetric fuel loading. 

c. Normal stall test altitude. 

d. In the configurations listed below, trim the aeroplane at 1.3 VSR with the 

power or thrust necessary to maintain straight level flight. Maintain the trim power or 

thrust during the test demonstrations. Increase speed as necessary prior to 

establishing at least 1.5g and a deceleration of at least 2 knot/sec. Decrease speed 

until 1 sec after stall warning and recover using the same test technique as for the 

non-contaminated aeroplane. 

i. High lift devices retracted configuration; 

ii. Lowest lift take-off configuration; and 

iii. Highest lift landing configuration. 

6.18.3 Ice Accretion Prior to Normal System Operation and prior to Detection of 

Appendix X icing conditions (for airplanes not certified for flight throughout 

the icing conditions of Appendix X). The following represent acceptable means 

for evaluating stall warning margin with the applicable ice accretion prior to normal 

operation of the ice protection system.

6.18.3.1 Where the ice protection system is activated as described in paragraph 

A1.2.3.3.a, of Appendix 1 of this ACJ, paragraphs 6.18.1 and 6.18.2 of this ACJ are 

applicable with the ice accretion prior to normal system operation. When the primary 

means of detecting Appendix X icing conditions depends on the flight crew to 

recognize visual cues of Appendix X icing, paragraphs 6.18.1 and 6.18.2 of this 

ACJ are also applicable with the ice accretion existing prior to detection of 
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Appendix X icing conditions. For assessing compliance with the ice accretion 

prior to detection of Appendix X icing conditions, it is assumed that the aeroplane 

has been in icing conditions long enough for the ice protection system to have 

been activated. Therefore, any changes to stall warning or stall identification 

system settings resulting from activation of the ice protection system are 

assumed to have taken place. 

6.18.3.2 Where the ice protection system is activated as described in paragraphs 

A1.2.3.3.b,c,d or e of Appendix 1 of this ACJ, it is acceptable to demonstrate 

adequate stall warning with the ice accretion prior to normal system operation, as 

follows:

a. In the configurations listed below, trim the aeroplane at 1.3 VSR. 

 

i. High lift devices retracted configuration: Straight/Power Off. 

i. Landing configuration: Straight/Power Off. 

b. At decelerations of up to 1 knot per second, reduce the speed to stall warning 

plus 1 second, and demonstrate that stalling can be prevented using the same test 

technique as for the non-contaminated aeroplane, without encountering any adverse 

characteristics (e.g., a rapid roll-off). As required by JAR 25.207(h)(2)(ii), where stall 

warning is provided by a different means than for the aeroplane without ice accretion, 

the stall characteristics must be satisfactory and the delay must be at least 3 seconds. 

6.18.3.3 When the primary means of detecting Appendix X icing conditions is 

an approved ice detection system, the stall warning margin prior to detection 

of Appendix X icing conditions should be assessed using the criteria of 

paragraph 6.18.3.2 with the ice accretion prior to detection of the Appendix X 

icing conditions. For assessing compliance with the ice accretion prior to 

detection of Appendix X icing conditions, it is assumed that the aeroplane has 

been in icing conditions long enough for the ice protection system to have 

been activated. Therefore, any changes to stall warning or stall identification 

system settings resulting from activation of the ice protection system are 

assumed to have taken place. 

6.19 Wind Velocities (JAR 25.237).

 

6.19.1 Crosswind landings with "Landing Ice" should be evaluated on an 

opportunity basis. 

 
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6.19.2 The results of the steady heading sideslip tests with “Landing Ice” may be 

used to establish the safe cross wind component. If the flight test data show that the 

maximum sideslip angle demonstrated is similar to that demonstrated with the noncontaminated aeroplane, and the flight characteristics (e.g. control forces and 

deflections) are similar, then the non-contaminated aeroplane crosswind component 

is considered valid. 

 

6.19.3 If the results of the comparison discussed in paragraph 6.19.2, above, are 

not clearly similar, and in the absence of a more rational analysis, a conservative 

analysis based on the results of the steady heading sideslip tests may be used to 

establish the safe crosswind component. The crosswind value may be estimated 

from: 

 VCW = VREF x sin (sideslip angle) / 1.5 

 where: 

VCW is the crosswind component, 

VREF is the landing reference speed appropriate to a minimum 

landing weight, and 

sideslip angle is that demonstrated at VREF (see paragraph 6.15 of this 

ACJ). 

6.20 Vibration and Buffeting (JAR 25.251).

6.20.1 Qualitative evaluations should be combined with the other testing, including 

speeds up to the maximum speed obtained in the longitudinal stability tests (see 

paragraph 6.14 of this ACJ). 

6.20.2 It is also necessary to demonstrate that the aeroplane is free from harmful 

vibration due to residual ice accumulation. This may be done in conjunction with the 

natural icing tests. 

6.20.3 An aeroplane with pneumatic de-icing boots should be evaluated to 

VDF/MDF with the de-icing boots operating and not operating. It is not necessary to 

do this demonstration with ice accretion. 

6.21 Natural Icing Conditions. 

6.21.1 General.
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6.21.1.1 Whether the flight testing has been performed with artificial ice shapes

accretions or in natural icing conditions, additional limited flight testing described 

in this section should be conducted in the natural atmospheric icing conditions of

specified in Appendix C, and, if necessary, Appendix X to JAR-25. Where 

flight testing with artificial ice shapes accretions is the primary means for showing 

compliance, the objective of the tests described in this section is to corroborate the 

handling characteristics and performance results obtained in flight testing with 

artificial ice shapes accretions. At least a qualitative assessment should be 

made that the artificial ice accretions are conservative relative to the ice 

accretions obtained in natural atmospheric icing conditions, and to confirm 

that ice does not accrete in unexpected places.

6.21.1.2 ACJ 25.1419/1420 provides guidance on when flight testing in the 

natural atmospheric icing conditions of Appendix X is necessary. 

6.21.1.3 It is acceptable for some ice to be shed during the testing due to air loads 

or wing flexure, etc. However, an attempt should be made to accomplish the test 

manoeuvres as soon as possible after exiting the icing cloud to minimise the 

atmospheric influences on ice shedding. 

6.21.1.4 During any of the manoeuvres specified in paragraph 6.21.2, below, the 

behaviour of the aeroplane should be consistent with that obtained with artificial ice 

shapes. There should be no unusual control responses or uncommanded 

aeroplane motions. Additionally, during the level turns and bank-to-bank rolls, 

there should be no buffeting or stall warning. 

6.21.2 Ice Accretion/Manoeuvres. 

6.21.2.1 Holding scenario.

a. The manoeuvres specified in Table 3, below, should be carried out with the 

following ice accretions defined in Appendix C and, if applicable, Appendix X to 

JAR-25, representative of normal operation of the ice protection system: 

i. On unprotected parts: For the icing conditions defined in Appendix C to 

JAR-25, a thickness of 3 inches on those parts of the aerofoil where the collection 

efficiency is highest should be the objective. (A thickness of 2 inches is normally a 

minimum value, unless a lesser value is agreed by the Authority.) 

ii. On protected parts: The ice accretion thickness should be that resulting from 

normal operation of the ice protection system. 

b. For aeroplanes with control surfaces that may be susceptible to jamming due 

to ice accretion (e.g. elevator horns exposed to the air flow), the holding speed that is 

critical with respect to this ice accretion should be used. 
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TABLE 3: Holding Scenario - Manoeuvres 

Configuration c.g. Trim speed Manoeuvre 

Flaps up, gear up Optional 

(aft 

range) 

Holding • Level, 40° banked turn, 

• Bank-to-bank rapid roll, 30° - 30°, 

• Speedbrake extension, retraction, 

• Full straight stall. 

Flaps in 

intermediate 

positions, gear up 

Optional 

(aft 

range) 

1.3 VSR Deceleration to stall warning. 

Landing flaps, gear 

down 

Optional 

(aft 

range) 

VREF • Level, 40° banked turn, 

• Bank-to-bank rapid roll, 30° - 30°, 

• Speedbrake extension, retraction (if 

approved), 

• Full straight stall. 

6.21.2.2 Approach/Landing Scenario. The manoeuvres specified in Table 4, 

below, should be carried out with successive accretions in different configurations 

on unprotected surfaces. Each test condition should be accomplished with the ice 

accretion that exists at that point. The final ice accretion (Test Condition 3) 

represents the sum of the amounts that would accrete during a normal descent 

from holding to landing in icing conditions. 
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TABLE 4: Approach/Landing Scenario - Manoeuvres 

Test 

Conditio

n 

Ice accretion 

thickness (*) 

Configuration c.g. Trim 

speed 

Manoeuvre 

_ First 0.5 in. Flaps up, 

gear up 

Optional 

(aft 

range) 

Holding No specific test 

1 

Additional 

0.25 in. 

(0.75 in. 

total) 

First 

intermediate 

flaps, gear 

up 

Optional 

(aft 

range) 

Holding • Level 40° banked 

turn, 

• Bank-to-bank rapid 

roll, 30°- 30°, 

• Speed brake 

extension and 

retraction (if 

approved), 

• Deceleration to stall 

warning. 

2 

Additional 

0.25 in. 

(1.00 in. 

total) 

Further 

intermediate 

flaps, gear 

up (as 

applicable) 

Optional 

(aft 

range) 

1.3 VSR • Bank-to-bank rapid 

roll, 30° - 30°, 

• Speed brake 

extension and 

retraction (if 

approved), 

• Deceleration to stall 

warning. 

3 

Additional 

0.25 in. 

(1.25 in. 

total) 

Landing 

flaps, gear 

down 

Optional 

(aft 

range) 

VREF • Bank-to-bank rapid 

roll, 30° - 30°, 

• Speed brake 

extension and 

retraction (if 

approved), 

• Bank to 40°, 

• Full straight stall. 

(*) The indicated thickness is that obtained on the parts of the unprotected aerofoil with 

the highest collection efficiency. 

6.21.3 For aeroplanes with unpowered elevator controls, in the absence of an 

agreed substantiation of the criticality of the artificial ice shape used to demonstrate 

compliance with the controllability requirement, the pushover test of paragraph 

6.9.3 should be repeated with a thin accretion of natural ice. 
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6.21.4 Existing propeller speed limits or, if required, revised propeller speed limits for 

flight in icing, should be verified by flight tests in the natural icing conditions of

Appendix C, and, if applicable, Appendix X. 

6.22 Failure Conditions (JAR 25.1309).

6.22.1 For failure conditions which are annunciated to the flight crew, credit may 

be taken for the established operating procedures following the failure. 

6.22.2 Acceptable Test Programme. In addition to a general qualitative 

evaluation, the following test programme (modified as necessary to reflect the 

specific operating procedures) should be carried out for the most critical probable 

failure condition where the associated procedure requires the aeroplane to exit the 

icing condition: 

a. The ice accretion is defined as a combination of the following: 

i. On the unprotected surfaces - the “Holding ice” accretion described in 

paragraph A1.2.1 of this ACJ; 

ii. On the normally protected surfaces that are no longer protected - the “Failure 

ice” accretion described in paragraph A1.3.2 of this AC; and 

iii. On the normally protected surfaces that are still functioning following the 

segmental failure of a cyclical de-ice system – the ice accretion that will form during the 

rest time of the de-ice system following the critical failure condition. 

b. Medium to light weight, aft centre of gravity position, symmetric fuel loading. 

c. In the configurations listed below, trim the aeroplane at the specified speed. 

Conduct 30° banked turns left and right with normal reversals. Conduct pull up to 

1.5g and pushover to 0.5g. 

i. High lift devices retracted configuration (or holding configuration if different): 

Holding speed, power or thrust for level flight. In addition, deploy and retract 

deceleration devices. 

ii. Approach configuration: Approach speed, power or thrust for level flight. 

iii. Landing configuration: Landing speed, power or thrust for landing approach 

(limit pull up to 1.3g). In addition, conduct steady heading sideslips to angle of 

sideslip appropriate to type and landing procedure. 

d. In the configurations listed below, trim the aeroplane at estimated 1.3 VSR. 

Decrease speed to stall warning plus 1 second, and demonstrate prompt recovery 
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using the same test technique as for the non-contaminated aeroplane. Natural stall 

warning is acceptable for the failure case. 

 

i. High lift devices retracted configuration: Straight/Power Off. 

ii. Landing configuration: Straight/Power Off. 

e. Conduct an approach and go-around with all engines operating using the 

recommended procedure. 

f. Conduct an approach and landing with all engines operating (unless the 

one-engine-inoperative condition results in a more critical probable failure condition) 

using the recommended procedure. 

6.22.3 For improbable failure conditions, a flight test may be required to 

demonstrate that the effect on safety of flight (as measured by degradation in flight 

characteristics) is commensurate with the failure probability or to verify the results of 

analyses and/or wind tunnel tests. The extent of any required flight test should be 

similar to that described in paragraph 6.22.2, above, or as agreed with the Authority 

for the specific failure condition. 
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Appendix 1 - Airframe Ice Accretion 

A1.1 General. The most critical ice accretion in terms of handling characteristics 

and/or performance for each flight phase should be determined. The parameters to be 

considered include: 

• the flight conditions (e.g. aeroplane configuration, speed, angle of attack, altitude) and 

• the atmospheric icing conditions (i.e., Appendix C or Appendix X to JAR-25) for 

which certification is desired of Appendix C to JAR-25 (e.g. temperature, liquid 

water content, mean effective drop diameter). 

A1.1.1 In accordance with JAR 25.1420(a)(1), any aeroplane certified for flight in 

icing conditions must, as a minimum, be capable of safely operating: (1) in the 

atmospheric icing conditions of Appendix C to JAR-25, and (2) after encountering 

the atmospheric icing conditions of Appendix X and subsequently exiting all icing 

conditions. Therefore, certification for flight in icing conditions must consider at 

least the following ice accretions: (a) operations in Appendix C icing conditions, 

and (b) detecting and exiting Appendix X icing conditions. JAR 25.21(g) specifies 

which subpart B airplane performance and handling characteristics requirements 

apply for each of these ice accretions. 

A1.1.2 In accordance with JAR 25.1420(a)(2), an aeroplane may also be certified 

for operation in a portion of the atmospheric icing conditions of Appendix X to 

JAR-25. In that case, the aeroplane must also be capable of operating safely after 

encountering, and while exiting, the atmospheric icing conditions in the portion of 

Appendix X for which operation is not approved. Certification for flight in a 

portion of Appendix X must consider the following ice accretions: (a) operations 

in Appendix C icing conditions, (b) operations in the Appendix X icing conditions 

for which approval is sought, and (c) detecting and exiting the Appendix X icing 

conditions in the portion of Appendix X beyond those for which approval is 

sought. JAR 25.21(g) specifies which subpart B airplane performance and 

handling characteristics requirements apply for each of these ice accretions. 

A1.1.3 In accordance with JAR 25.1420(a)(3), in addition to certification for flight 

in Appendix C conditions, an aeroplane may be certified for operation throughout 

the atmospheric icing conditions of Appendix X to JAR-25. Certification for flight 

throughout the atmospheric icing conditions of Appendix X must consider the 

following ice accretions: (a) operations in Appendix C icing conditions, and 

(b) operations in Appendix X icing conditions. JAR 25.21(g) specifies which 

subpart B airplane performance and handling characteristics requirements apply 

for each of these ice accretions. 

A1.1.4 In order to reduce the number of ice accretions used for demonstrating 

compliance, any of the applicable ice accretions (or a composite accretion 

representing a combination of accretions) may be used to show compliance with 
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a particular Subpart B requirement if that accretion is either the ice accretion 

identified in the requirement or is shown to be more conservative than the ice 

accretion identified in the requirement. In addition, the ice accretion that has the 

most adverse effect on handling characteristics may be used for compliance with 

the airplane performance requirements if any difference in performance is 

conservatively taken into account. The ice accretion(s) used to show compliance 

should consider the speeds, configurations (including configuration changes), 

angles of attack, power or thrust settings, etc. for the flight phases and icing 

conditions that they are intended to cover. 

A1.2 Operative Ice Protection System.

A1.2.1 All flight phases except take-off.

A1.2.1.1 For unprotected parts, the ice accretion to be considered should be 

determined in accordance with Appendixes C and X to JAR-25 JAR 25.1419.

A1.2.1.2 Unprotected parts consist of the unprotected aerofoil leading edges and all 

unprotected airframe parts on which ice may accrete. The effect of ice accretion on 

protuberances such as antennae or flap hinge fairings need not normally be 

investigated. However aeroplanes that are characterised by unusual unprotected 

airframe protuberances, e.g. fixed landing gear, large engine pylons, or exposed 

control surface horns or winglets, etc., may experience significant additional effects, 

which should therefore be taken into consideration. 

As provided by FTHWG 

A1.2.1.3 For "Holding Ice," certification experience for Appendix C icing 

conditions has shown that the amount of ice on the most critical unprotected main 

aerofoil surface (e.g. wing, horizontal or vertical stabilisers) to be considered need 

not exceed a pinnacle height of typically 3 inches (75 mm) in a plane in the direction 

of flight. For other unprotected main surfaces an analysis may be performed to 

determine the maximum ice accretion associated with this maximum pinnacle height. 

In the absence of such an acceptable analysis a uniform pinnacle height of 3 inches 

(75 mm) should be assumed. The shape and texture of the ice are important and 

should be agreed with the Authority. 

As proposed by IPHWG 

A1.2.1.3 For unprotected surfaces an analysis may be performed to determine 

the maximum ice accretion. Assume a 45 minute hold, no reduction for cloud 

horizontal extent. It is allowable to truncate the pinnacle height of 3 inches (75 

mm) if sufficient service history exists on similar ice protection system 

designs. The shape and texture of the ice are important and should be agreed 

with the Authority. 
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Discussion: 

This issue has been debated extensively in both the FTHWG and IPHWG 

without resolution. The FTHWG provided language is essentially unchanged 

from the draft 25.21(g) materials relative to Appendix C. The FTHWG could not 

achieve consensus in the time available and elected to leave the language 

unchanged. 

The IPHWG is not in agreement with the draft 25.21(g) materials with respect to 

using a 3" criterion as the primary means of determining ice shapes. Ice shape 

size varies with geometry, which can lead to ice shapes less than or greater 

than the 3" ice shape. The time base criteria allows for differing ice shapes 

based on geometry, yet retains a consistent level of safety (with respect to 

exposure time). The proposed language is offered as a compromise in that the 

3" criterion can be used if sufficient justification exists. In addition, since this 

language resides in the advisory materials, alternate methods of compliance 

are possible provided the same level of safety is achieved. 

A1.2.1.4 There is no comparable certification experience to that identified in 

paragraph A1.2.1.3 for Appendix X icing conditions. See paragraphs A1.4.2.3 

and A1.4.3.3 for scenarios that define how to determine the applicable ice 

accretion for the holding flight phase in Appendix X conditions. The total time 

in icing conditions in these scenarios should be 45 minutes, and no reduction 

should be taken for cloud horizontal extent. 

A1.2.1.5 For protected parts, the ice protection systems are normally assumed to be 

operative. However, the applicant should consider the effect of ice accretion on the 

protected surfaces that results from: 

a. The rest time of a de-icing cycle. Performance may be established on the 

basis of a representative intercycle ice accretion for normal operation of the de-icing 

system (consideration should also be given to the effects of any residual ice accretion 

that is not shed.) The average drag increment determined over the de-icing cycle may 

be used for performance calculations. 

b. Runback ice which occurs on or downstream of the protected surface. 

c. Ice accretion prior to normal operation of the ice protection system (see 

paragraph A1.2.3, below). 

A1.2.2 Take-off phase.

A1.2.2.1 For both unprotected and protected parts, the ice accretion identified in 

Appendixes C and X Appendix C to JAR-25 for the take-off phase may be determined 
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by calculation, assuming that the Takeoff Maximum icing conditions defined in Appendix 

C exist, and:

• aerofoils, control surfaces and, if applicable, propellers are free from frost, snow, or 

ice at the start of the take-off; 

• the ice accretion starts at lift-off; 

• the critical ratio of thrust/power-to-weight; 

• failure of the critical engine occurs at VEF; and 

• flight crew activation of the ice protection system in accordance with an AFM 

procedure, except that after commencement of the take-off roll no flight crew action to 

activate the ice protection system should be assumed to occur until the aeroplane is 

400 ft above the take-off surface. 

A1.2.2.2 The ice accretions identified in Appendixes C and X to JAR-25 for the takeoff phase are: 

• "Take-off ice": The most critical ice accretion between lift-off and 400 ft above the 

takeoff surface, assuming accretion starts at lift-off in the icing environment. 

• "Final Take-off ice": The most critical ice accretion between 400 ft and the height 

at which the transition to the en-route configuration and speed is completed, 

or 1500 ft above the take-off surface, whichever is higher, assuming accretion 

starts at lift-off in the icing environment. 

A1.2.3 Ice accretion prior to normal system operation.

A1.2.3.1 Ice protection systems are normally operated as anti-icing systems (i.e. 

designed to prevent ice accretion on the protected surface) or de-icing systems (i.e. 

designed to remove ice from the protected surface). In some cases, systems may be 

operated as anti-icing or de-icing systems depending on the phase of flight. Operation 

of ice protection systems can also include a resetting of stall warning and/or stall 

identification system (e.g. stick pusher) activation thresholds. 

A1.2.3.2 The aeroplane Flight Manual contains the operating limitations and operating 

procedures established by the applicant. Since ice protection systems are normally only 

operated when icing conditions are encountered or when airframe ice is detected, 

means of flight crew determination of icing conditions and/or airframe ice should be 

considered in determining the ice accretion prior to normal system operation. This 

includes the ice accretion appropriate to the specified means of identification of icing 

conditions and an additional ice accretion, represented by a time in the icing 

conditions before the ice protection system is activated. In accordance with 
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§ 25.21(g)(3), compliance must be shown with ice accretions corresponding to 

both the Continuous Maximum icing conditions of Appendix C and the icing 

conditions of Appendix X (or to reduce the number of ice accretions tested, the 

most critical of the two ice accretions if the most critical ice accretion can be 

determined). This additional ice accretion is to account for flight crew delay in either 

identifying the conditions and activating the ice protection systems (see paragraphs 

A1.2.3.3(a), (b) and (c) below), or activating the ice protection system following 

indication from an ice detection system (see paragraph A1.2.3.3 (d) below). In addition 

the system response time should be considered. System response time is defined as 

the time interval between activation of the ice protection system and the performance of 

its intended function (e.g. for a thermal ice protection system, the time to heat the 

surface and remove the ice). 

A1.2.3.3 The means of detecting that the aeroplane is in icing conditions should 

be the same for all icing conditions for which certification is desired, regardless of 

whether the icing conditions are within Appendix C or Appendix X. 

A1.2.3.4 The following examples indicate the ice accretion to be considered on the 

unprotected and normally protected aerodynamic surfaces: 

FTHWG less ALPA Position 

a. If activation of normal operation of any ice protection system is dependent 

on visual recognition of a specified ice accretion on a reference surface (e.g. ice 

accretion probe, wing leading edge), the ice accretion should not be less than that 

corresponding to the ice accretion on the reference surface taking into account 

probable flight crew delays in recognition of the specified ice accretion and operation 

of the system, determined as follows: 

i. the specified accretion, plus 

ii. the ice accretion equivalent to thirty seconds of operation in icing

conditions (the Continuous Maximum icing conditions of Appendix C, Part I(a) and 

the icing conditions of Appendix X), plus 

iii. the ice accretion during the system response time. 

b. If activation of normal operation of any ice protection system is dependent 

on visual recognition of the first indication of ice accretion on a reference surface 

(e.g. ice accretion probe), the ice accretion should not be less than that 

corresponding to the ice accretion on the reference surface taking into account 

probable flight crew delays in recognition of the ice accreted and operation of the 

system, determined as follows: 

i. the ice accretion corresponding to first indication on the reference surface, 

plus 
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ii. the ice accretion equivalent to thirty seconds of operation in icing 

conditions (the Continuous Maximum icing conditions of Appendix C, Part I(a) and 

the icing conditions of Appendix X), plus 

iii. the ice accretion during the system response time. 

ALPA Position

a. If normal operation of any ice protection system is dependent on visual 

recognition of ice accretion (e.g. ice accretion probe, wing leading edge), the ice 

accretion should not be less than that corresponding to the ice accretion on the 

reference surface taking into account probable flight crew delays in recognition of the 

specified ice accretion and operation of the system, determined as follows:

i. the specified accretion, plus 

ii. the ice accretion equivalent to two minutes of operation in the Continuous 

Maximum icing conditions of Appendix C, Part I(a), plus 

iii. the ice accretion during the system response time. 

b. [RESERVED]

c. If activation of normal operation of any ice protection system is dependent 

upon pilot identification of icing conditions (as defined by an appropriate static or total 

air temperature and visible moisture conditions), the ice accretion should not be less 

than that corresponding to the ice accreted during probable crew delays in 

recognition of icing conditions and operation of the system, determined as follows: 

i. the ice accretion equivalent to thirty seconds of operation in icing 

conditions (the Continuous Maximum icing conditions of Appendix C, Part I(a) and 

the icing conditions of Appendix X), plus 

ii. the ice accretion during the system response time. 

d. If activation of normal operation of any ice protection system is dependent 

on pilot action following indication from an ice detection system, the ice accretion 

should not be less than that corresponding to the ice accreted prior to indication from 

the ice detection system, plus that accreted due to probable flight crew delays in 

activating the ice protection system and operation of the system, determined as 

follows: 

i. the ice accretion corresponding to the time between entry into the icing 

conditions and indication from the ice detection system, plus 

 
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ii. the ice accretion equivalent to ten seconds of operation in icing conditions 

(the Continuous Maximum icing conditions of Appendix C, Part I(a) and the icing 

conditions of Appendix X), plus 

iii. the ice accretion during the system response time. 

e. If activation of normal operation of any ice protection system is automatic 

following indication from an ice detection system, the ice accretion should not be less 

than that corresponding to the ice accreted prior to indication from the ice protection 

system and operation of the system, determined as follows: 

i. the ice accretion on the protected surfaces corresponding to the time 

between entry into the icing conditions and activation of the system, plus 

ii. the ice accretion during the system response time. 

A1.3 Ice Protection System Failure Cases. 

A1.3.1 Unprotected parts. The same accretion as in paragraph A1.2.1 is 

applicable. 

A1.3.2 Protected parts following system failure. "Failure Ice" is defined as follows: 

A1.3.2.1 In the case where the failure condition is not annunciated, the ice accretion 

on normally protected parts where the ice protection system has failed should be the 

same as the accretion specified for unprotected parts. 

A1.3.2.2 In the case where the failure condition is annunciated and the associated 

procedure does not require the aeroplane to exit icing conditions, the ice accretion 

on normally protected parts where the ice protection system has failed should be the 

same as the accretion specified for unprotected parts. 

A1.3.2.3 In the case where the failure condition is annunciated and the associated 

procedure requires the aeroplane to exit icing conditions as soon as possible, the ice 

accretion on normally protected parts where the ice protection has failed, should be 

taken as one-half of the accretion specified for unprotected parts unless another 

value is agreed by the Authority. 

A1.4 Additional Guidance for Appendix X Ice Accretions. 

A1.4.1 Pre-detection ice. Pre-detection ice refers to the ice accretion existing at 

the time the flightcrew becomes aware that they are in Appendix X icing 

conditions, they have initiated the appropriate action(s) identified by aeroplane 

Flight Manual limitations or procedures, and any resulting systems changes (e.g., 

reset of stall warning or stick pusher activation points) are functional. 
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A1.4.1.1 Both direct entry into Appendix X icing conditions and entry from flight in 

Appendix C icing conditions should be considered. The aeroplane Flight Manual 

limitations or procedures may restrict the operating envelope (e.g., speeds, 

configurations), alter the operation of the ice protection system, change stall 

warning and stall identification system activation points, or for aeroplanes not 

certified for flight in those icing conditions, direct the flightcrew to exit all icing 

conditions. 

A1.4.1.2 The time that it takes to detect Appendix X icing conditions that exceed 

those for which the airplane is certified should be based on the means of 

detection. Guidance for certification of the detection means is provided in ACJ 

25.1419/1420. In general, it is expected that the time to detect exceedance icing 

conditions may be significantly longer for a detection means that relies on the 

flightcrew to see and recognize a visual icing cue than for an ice detection system 

that provides an attention-getting alert to the flightcrew. 

A1.4.1.3 Visual detection requires time for accumulation on the reference 

surface(s) of enough ice to be reliably identified by either pilot in all atmospheric 

and lighting conditions and consideration of the time between pilot scans of the 

reference surface(s).

a. The amount of ice for reliable identification is a function of the 

distinguishing characteristics of the ice (e.g., size, shape, contrast from the 

surface feature that it is present on), the distance from the pilots (e.g., windshield 

vs. engine vs. wingtip), and the relative viewing angle (location with respect to the 

pilots’ primary fields of view). 

b. Pilot scan time of the reference surface(s) will be influenced by many 

factors. Some factors include phase of flight, workload, frequency of occurrence 

of Appendix X conditions, pilot awareness of the possibility of SLD conditions, 

and ease of seeing the reference surface(s). The infrequency of Appendix X 

conditions (approximately 1 in 100 to 1 in 1000, on average in all worldwide icing 

encounters) and the high workload associated with some phases of flight in 

instrument conditions (e.g. approach and landing) justify using a conservative 

estimate for the time between pilot scans.

c. In the absence of specific studies or tests validating visual detection times, 

the following times should be used for visual detection of exceedance icing 

conditions following accumulation of enough ice to be reliably identified by either 

pilot in all atmospheric and lighting conditions. For a visual reference located on 

or immediately outside a cockpit window (e.g., ice accretions on side windows, 

windshield wipers, or icing probe near the windows) - 3 minutes. For a visual 

reference located on a wing, wing mounted engine, or wing tip – 5 minutes.
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A1.4.2 Ice accretions for encounters with Appendix X beyond those for which 

the aeroplane is certified to operate. 

A1.4.2.1 These ice accretions are to be used to evaluate compliance with the 

applicable Subpart B requirements for operating safely after encountering 

Appendix X atmospheric icing conditions for which the aeroplane is not approved, 

and then safely exiting all icing conditions. 

A1.4.2.2 These ice accretions apply when the aeroplane is not certified for 

flight in any portion of Appendix X atmospheric icing conditions, when the 

aeroplane is certified for flight in only a portion of Appendix X, and for any flight 

phase for which the aeroplane is not certified for flight throughout the Appendix X 

icing envelope. 

A1.4.2.3 The following table shows the scenarios to be used for determining 

the ice accretions for encounters with Appendix X beyond those for which the 

aeroplane is certified to operate (i.e., detecting and exiting those conditions): 

Flight Phase/Condition Appendix X Detect-and-Exit Ice Accretion

Ground Roll No accretion 

Take-off No accretion1

Final Take-off No accretion1 

En-route En-route Detect-and-Exit Ice 

Combination of: (1) Either Appendix C en-route ice or 

Appendix X en-route ice for which the airplane is 

approved, whichever is applicable, (2) pre-detection 

ice, (3) accretion from one standard cloud horizontal 

extent (17.4 nautical miles) in Appendix X conditions, 

and (4) accretion from one standard cloud horizontal 

extent (17.4 nautical miles) in Appendix C continuous 

maximum icing conditions.

Holding Holding Detect-and-Exit Ice 

Combination of: (1) Either Appendix C holding ice or 

Appendix X holding ice for which the airplane is 

approved, whichever is applicable, (2) pre-detection 

ice, (3) accretion from one standard cloud horizontal 

extent (17.4 nautical miles) in Appendix X conditions, 

and (4) accretion from one standard cloud horizontal 

extent (17.4 nautical miles) in Appendix C continuous 

maximum icing conditions. The total time in icing 

conditions need not exceed 45 minutes. 
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Flight Phase/Condition Appendix X Detect-and-Exit Ice Accretion

Approach Approach Detect-and-Exit Ice 

The more critical of holding detect-and-exit ice and the 

combination of: (1) Ice accreted during a descent in 

the cruise configuration from the maximum vertical 

extent of the Appendix C maximum continuous or the 

Appendix X icing environment for which the airplane 

is approved, whichever is applicable, to 2,000 feet 

above the landing surface, where transition to the 

approach configuration is made, (2) pre-detection ice, 

and (3) ice accreted at 2,000 feet above the landing 

surface while transiting one standard cloud horizontal 

extent (17.4 nautical miles) in Appendix X conditions 

and one standard cloud horizontal extent (17.4 

nautical miles) in Appendix C continuous maximum 

icing conditions. 

Landing Landing Detect-and-Exit Ice 

The more critical of holding detect-and-exit ice and the 

combination of: (1) Either Appendix C or Appendix X 

approach and landing ice for which the airplane is 

approved, whichever is applicable, (2) pre-detection 

ice, and (3) ice accreted during an exit manoeuvre 

beginning with the minimum climb gradient specified 

in § 25.119 from a height of 200 feet above the landing 

surface and transiting through one standard cloud 

horizontal extent (17.4 nautical miles) in Appendix X 

conditions, and one standard cloud horizontal extent 

(17.4 nautical miles) in Appendix C continuous 

maximum icing conditions. 

For the purposes of defining the landing detect-andexit ice shape, the Appendix C approach and landing 

ice is defined as the ice accreted during a descent in 

the cruise configuration from the maximum vertical 

extent of the Appendix C maximum continuous icing 

environment to 2,000 feet above the landing surface, 

transitioning to the approach configuration and 

manoeuvring for 15 minutes at 2,000 feet above the 

landing surface, a descent from 2,000 feet to 200 feet 

above the landing surface with a transition to the 

landing configuration. 
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Flight Phase/Condition Appendix X Detect-and-Exit Ice Accretion

Pre-Activation Pre-Activation Ice 

Ice accreted on the protected and unprotected 

surfaces during the time it takes for icing conditions 

(either Appendix C or Appendix X) to be detected, the 

ice protection system activated, and the ice protection 

system to become fully effective in performing its 

intended function. 

Pre-Detection Pre-Detection of Appendix X Ice 

Ice accreted on the protected and unprotected 

surfaces during the time it takes to detect and identify 

Appendix X conditions (based on the method of 

detection) beyond those for which the aeroplane is 

certified to operate, and the time for the flight crew to 

refer to and act on procedures, including coordinating 

with Air Traffic Control, to exit the icing conditions 

beyond those for which the aeroplane is certified to 

operate. A minimum time period of two minutes 

should be used as the time needed for the flight crew 

to refer to and act on the procedures to exit the icing 

conditions after the Appendix X icing conditions are 

recognized. 

Failure Failure Ice 

No accretion2 

Notes: 

1

 T/O is not permitted when Appendix X conditions beyond those for which the 

aeroplane is certified to operate exist in the vicinity of departure airport. 

2

 Unnecessary to consider unintentional encounter with Appendix X icing 

conditions beyond those for which the aeroplane is certified to operate with a 

failed ice protection system. 

A1.4.3 Ice accretions for encounters with Appendix X atmospheric icing 

conditions in which the aeroplane is certified to operate. 

A1.4.3.1 These ice accretions are to be used to evaluate compliance with the 

applicable Subpart B requirements for operating safely in the Appendix X 

atmospheric icing conditions for which the aeroplane is approved. 

A1.4.3.2 The ice accretions should include consideration of combinations of 

Appendix C and Appendix X icing conditions within the scenarios defined in 

paragraph A4.1.3.3. For example, flight in Appendix X conditions may result in ice 

accreting, and potentially forming a ridge, behind a protected surface. Once this 

accretion site has been established, flight in Appendix C icing conditions for the 

remaining portion of the applicable flight phase scenario may result in a more 
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critical additional accretion than would occur for continued flight in Appendix X 

icing conditions. 

A1.4.3.3 The following table shows the scenarios to be used for determining 

the ice accretions for certification for flight in the icing conditions of Appendix X 

to JAR-25: 

Flight 

Phase/Condition

Appendix X Ice Accretion

Ground Roll No accretion 

Take-off Take-off Ice 

Ice accretion occurring between lift-off and 400 feet 

above the takeoff surface assuming ice accretion starts 

at lift-off. 

Final Take-off Final Take-off Ice 

Ice accretion occurring between 400 ft and the height at 

which the transition to the en-route configuration and 

speed is completed, or 1500 ft above the take-off 

surface, whichever is higher, assuming ice accretion 

starts at lift-off. 

En-route En-route Ice 

Ice accreted during the en-route phase of flight. 

Holding Holding Ice 

Ice accreted during a 45 minute hold with no reduction 

for horizontal cloud extent (i.e., the hold is conducted 

entirely within the 17.4 nautical mile standard cloud 

extent). 

Approach Approach Ice 

Most critical ice accretion of: (1) Ice accreted during a 

descent in the cruise configuration from the maximum 

vertical extent of the Appendix X icing environment to 

2,000 feet above the landing surface, followed by 

transition to the approach configuration and 

manoeuvring for 15 minutes at 2,000 feet above the 

landing surface; and (2) Holding ice (if the airplane is 

certified for holding in Appendix X conditions). 
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Flight 

Phase/Condition

Appendix X Ice Accretion

Landing Landing Ice

Most critical ice accretion of: (1) Approach ice plus 

descent from 2,000 feet above the landing surface to 200 

feet above the landing surface with a transition to the 

landing configuration, followed by a go-around 

manoeuvre beginning with the minimum climb gradient 

specified in § 25.119 from 200 feet to 2,000 feet above the 

landing surface, holding for 15 minutes at 2,000 feet 

above the landing surface in the approach configuration, 

and a descent to the landing surface in the landing 

configuration; and (2) Holding ice (if the airplane is 

certified for holding in Appendix X conditions). 

Pre-Activation Pre-Activation Ice 

Ice accreted during the time for the flightcrew to 

recognize icing conditions and activate the ice 

protection system, plus the time for the ice protection 

system to perform its intended function. 

Pre-Detection Pre-Detection Ice 

Ice accreted during the time for the flightcrew to detect 

Appendix X conditions, refer to and initiate procedures, 

and any time for systems to perform their intended 

functions (if applicable). Pre-detection ice need not be 

considered if there are no specific crew actions or 

systems changes associated with flight in Appendix X 

conditions. 

Failure Failure Ice 

Same criteria as for Appendix C, but in Appendix X 

conditions. 
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Appendix 2 - Artificial Ice Shapes 

A2.1 General.

A2.1.1 The artificial ice shapes used for flight testing should be those which have 

the most adverse effects on handling characteristics. If analytical data show that 

other reasonably expected ice shapes could be generated which could produce 

higher performance decrements, then the ice shape having the most adverse effect 

on handling characteristics may be used for performance tests provided that any 

difference in performance can be conservatively taken into account. 

A2.1.2 The artificial shapes should be representative of natural icing conditions in 

terms of location, general shape, thickness and texture. Following determination of 

the form and surface texture of the ice shape under paragraph A2.2, a surface 

roughness for the shape should be agreed with the Authority as being representative 

of natural ice accretion. 

A2.1.3 "Sandpaper Ice" is addressed in paragraph A2.3. 

A2.2 Shape and Texture of Artificial Ice. 

A2.2.1 The shape and texture of the artificial ice should be established and 

substantiated by agreed methods. Common practices include: 

• use of computer codes, 

• flight in measured natural icing conditions, 

• icing wind tunnel tests, and 

• flight in a controlled simulated icing cloud (e.g. from an icing tanker). 

A2.2.2 In the absence of another agreed definition of texture the following may be 

used: 

A2.2.2.1 For small amounts of ice (for example the amount of ice accreted during a deicing system rest time), the roughness should be typically: 

• roughness height: 1 mm 

• particle density: 8 to 10/cm2 

A2.2.2.2 For large amounts of ice (for example on an unprotected, exposed surface), 

the roughness should be typically: 

• roughness height: 3 mm 

• particle density: 8 to 10/cm2 
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A2.3 "Sandpaper Ice."

A2.3.1 "Sandpaper Ice" is the most critical thin, rough layer of ice resulting from 

exposure to the atmospheric icing conditions of Appendix C to JAR-25. 

Sandpaper Ice is not relevant to the supercooled large drop conditions defined 

in Appendix X to JAR-25. Any representation of "Sandpaper Ice" (e.g. 

carborundum paper no. 40) should be agreed by the Authority. 

A2.3.2 The spanwise and chordwise coverage should be consistent with the areas 

of ice accretion determined for the conditions of JAR-25, Appendix C except that, for 

the zero g pushover manoeuvre of paragraph 6.9.3 of this ACJ, the "Sandpaper Ice" 

may be restricted to the horizontal stabiliser if this can be shown to be conservative. 
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Appendix 3 - Design Features 

A3.1 Aeroplane Configuration and Ancestry . An important design feature of an 

overall aeroplane configuration that can affect performance, controllability and 

manoeuvrability is its size. In addition, the safety record of the aeroplane's closelyrelated ancestors may be taken into consideration. 

A3.1.1 Size. The size of an aeroplane determines the sensitivity of its flight 

characteristics to ice thickness and roughness. The relative effect of a given ice 

height (or ice roughness height) decreases as aeroplane size increases. 

A3.1.2 Ancestors. If a closely related ancestor aeroplane was certified for flight in 

icing conditions, its safety record may be used to evaluate its general arrangement 

and systems integration. 

A3.2 Wing. Design features of a wing that can affect performance, controllability, 

and manoeuvrability include aerofoil type, leading edge devices and stall protection 

devices. 

A3.2.1 Aerofoil. Aerofoils with significant natural laminar flow when noncontaminated may show large changes in lift and drag with ice. Conventional 

aerofoils operating at high Reynolds numbers make the transition to turbulent flow 

near the leading edge when non-contaminated, thus reducing the adverse effects of 

the ice. 

A3.2.2 Leading Edge Device. The presence of a leading edge device (such as a 

slat) reduces the percentage decrease in CLMAX due to ice by increasing the overall 

level of CL. Gapping the slat may improve the situation further. Leading edge 

devices can also reduce the loss in angle of attack at stall due to ice. 

A3.2.3 Stall Protection Device. An aeroplane with an automatic slat-gapping 

device may generate a greater CLMAX with ice than the certified CLMAX with the slat 

sealed and a non-contaminated leading edge. This may provide effective protection 

against degradation in stall performance or characteristics. 

A3.2.4 Lateral Control. The effectiveness of the lateral control system in icing 

conditions can be evaluated by comparison with closely related ancestor aeroplanes. 

A3.3 Empennage. The effects of size and aerofoil type also apply to the horizontal 

and vertical tails. Other design features include tailplane sizing philosophy, aerofoil 

design, trimmable stabiliser, and control surface actuation. Since tails are usually not 

equipped with leading edge devices, the effects of ice on tail aerodynamics are similar to 

those on a wing with no leading edge devices. However, these effects usually result in 
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changes to aeroplane handling and/or control characteristics rather than degraded 

performance. 

A3.3.1 Tail Sizing. The effect on aeroplane handling characteristics depends on 

the tailplane design philosophy. The tailplane may be designed and sized to provide 

full functionality in icing conditions without ice protection, or it may be designed with 

a de-icing or anti-icing system. 

A3.3.2 Horizontal Stabiliser Design. Cambered aerofoils and trimmable stabilisers 

may reduce the susceptibility and consequences of elevator hinge moment reversal 

due to ice-induced tailplane stall. 

A3.3.3 Control Surface Actuation. Hydraulically powered irreversible elevator 

controls are not affected by ice-induced aerodynamic hinge moment reversal. 

A3.3.4 Control Surface Size. For mechanical elevator controls, the size of the 

surface significantly affects the control force due to an ice-induced aerodynamic 

hinge moment reversal. Small surfaces are less susceptible to control difficulties for 

given hinge moment coefficients. 

A3.3.5 Vertical Stabiliser Design. The effectiveness of the vertical stabiliser in 

icing conditions can be evaluated by comparison with closely-related ancestor 

aeroplanes. 

A3.4 Aerodynamic Balancing of Flight Control Surfaces. The aerodynamic balance 

of unpowered or boosted reversible flight control surfaces is an important design feature 

to consider. The design should be carefully evaluated to account for the effects of ice 

accretion on flight control system hinge moment characteristics. Closely balanced 

controls may be vulnerable to overbalance in icing. The effect of ice in front of the 

control surface, or on the surface, may upset the balance of hinge moments leading to 

either increased positive force gradients or negative force gradients. 

A3.4.1 This feature is particularly important with respect to lateral flight control 

systems when large aileron hinge moments are balanced by equally large hinge 

moments on the opposite aileron. Any asymmetric disturbance in flow which affects this 

critical balance can lead to a sudden uncommanded deflection of the control. This auto 

deflection, in extreme cases, may be to the control stops. 

A3.5 Ice Protection/Detection System. The ice protection/detection system design 

philosophy may include design features that reduce the ice accretion on the wing and/or 

tailplane. 

 
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A3.5.1 Wing Ice Protection/Detection. An ice detection system that activates a 

wing de-icing system may ensure that there is no significant ice accretion on wings 

that are susceptible to performance losses with small amounts of ice. 

A3.5.1.1 If the entire wing leading edge is not protected, the part that is protected 

may be selected to provide good handling characteristics at stall, with an acceptable 

performance degradation. 

A3.5.2 Tail Ice Protection/Detection. An ice detection system may activate a 

tailplane de-icing system on aeroplanes that do not have visible cues for system 

operation. 

A3.5.2.1 An ice protection system on the unshielded aerodynamic balances of 

aeroplanes with unpowered reversible controls can reduce the risk of ice-induced 

aerodynamic hinge moment reversal. 
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Appendix 4 – Examples of Aeroplane Flight Manual limitations and operating 

procedures for operations in supercooled large drop icing conditions. 

A4.1 Approved for Appendix C icing conditions only (no Appendix X) 

A4.1.1 AFM Limitations 

Intentional flight, including takeoff and landing, into freezing drizzle or freezing 

rain conditions is prohibited. If freezing drizzle or freezing rain conditions are 

encountered, or if {insert cue description here}, immediately request priority 

handling from air traffic control to facilitate a route or altitude change to exit all 

icing conditions. Stay clear of all icing conditions for the remainder of the flight, 

including landing, unless it can be determined that ice accretions no longer 

remain on the airframe. 

A4.1.2 AFM Operating procedures (normal procedures section) 

Freezing drizzle and freezing rain conditions are severe icing conditions for this 

aeroplane. Intentional flight, including takeoff and landing, into freezing drizzle or 

freezing rain conditions is prohibited. A flight delay or diversion to an alternate 

airport is required if these conditions exist at the departure or destination 

airports. 

{insert cue description here} is one indication of severe icing for this aeroplane. If 

severe icing is encountered, immediately request priority handling from air traffic 

control to facilitate a route or altitude change to exit all icing conditions. Stay 

clear of all icing conditions for the remainder of the flight, including landing, 

unless it can be determined that ice accretions no longer remain on the airframe. 

A4.1.3 Flightcrew Operating Manual operating procedures 

Warning: Severe icing may result from environmental conditions outside of those 

for which the airplane is certified. Intentional flight into severe icing conditions 

may result in ice build-up on protected surfaces exceeding the capability of the 

ice protection system, or may result in ice forming aft of the protected surfaces. 

This ice may not be shed when using the ice protection systems, and may 

seriously degrade the performance and controllability of the aeroplane. 

Operations in icing conditions were evaluated as part of the certification process 

for this aeroplane. Freezing drizzle and freezing rain conditions were not 

evaluated and are considered severe icing conditions for this aeroplane. 

Intentional flight, including takeoff and landing, into freezing drizzle or freezing 

rain conditions is prohibited. A flight delay or diversion to an alternate airport is 

required if these conditions exist at the departure or destination airports. {insert 

cue description here}is an indication of severe icing conditions that exceed those 
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for which the airplane is certified. If severe icing is encountered, immediately 

request priority handling from air traffic control to facilitate a route or altitude 

change to exit all icing conditions. Stay clear of all icing conditions for the 

remainder of the flight, including landing, unless it can be determined that ice 

accretions no longer remain on the airframe. 

A4.2 Approved for Appendix C and freezing drizzle conditions of Appendix X 

(i.e., not approved for freezing rain) 

A4.2.1 AFM Limitations 

Intentional flight, including takeoff and landing, into freezing rain conditions is 

prohibited. If freezing rain conditions are encountered, or if {insert cue 

description here}, immediately request priority handling from air traffic control to 

facilitate a route or altitude change to exit all icing conditions. Stay clear of all 

icing conditions for the remainder of the flight, including landing, unless it can be 

determined that ice accretions no longer remain on the airframe. 

A4.2.2 AFM Operating procedures (normal procedures section) 

Freezing rain conditions are severe icing conditions for this aeroplane. 

Intentional flight, including takeoff and landing, into freezing rain conditions is 

prohibited. A flight delay or diversion to an alternate airport is required if these 

conditions exist at the departure or destination airports. 

{insert cue description here} is one indication of severe icing for this aeroplane. If 

severe icing is encountered, immediately request priority handling from air traffic 

control to facilitate a route or altitude change to exit all icing conditions. Stay 

clear of all icing conditions for the remainder of the flight, including landing, 

unless it can be determined that ice accretions no longer remain on the airframe. 

A4.2.3 Flightcrew Operating Manual operating procedures 

Warning: Severe icing may result from environmental conditions outside of those 

for which the aeroplane is certified. Intentional flight into severe icing conditions 

may result in ice build-up on protected surfaces exceeding the capability of the 

ice protection system, or may result in ice forming aft of the protected surfaces. 

This ice may not be shed when using the ice protection systems, and may 

seriously degrade the performance and controllability of the aeroplane. 

Operations in icing conditions were evaluated as part of the certification process 

for this aeroplane. Freezing rain conditions were not evaluated and are 

considered severe icing conditions for this aeroplane. 

Intentional flight, including takeoff and landing, into freezing rain conditions is 

prohibited. A flight delay or diversion to an alternate airport is required if these 
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conditions exist at the departure or destination airports. {insert cue description 

here}is an indication of severe icing conditions that exceed those for which the 

airplane is certified. If severe icing is encountered, immediately request priority 

handling from air traffic control to facilitate a route or altitude change to exit all 

icing conditions. Stay clear of all icing conditions for the remainder of the flight, 

including landing, unless it can be determined that ice accretions no longer 

remain on the airframe. 

A4.3 Approved for Appendix C, and approved for Appendix X except for en route 

and holding 

A4.3.1 AFM Limitations 

Intentional holding or en route flight into freezing drizzle or freezing rain 

conditions is prohibited. If freezing drizzle or freezing rain conditions are 

encountered during a hold (in any aeroplane configuration) or in the en route 

phase of flight (climb, cruise, or descent with high lift devices and gear retracted), 

or if {insert cue description here}, immediately request priority handling from air 

traffic control to facilitate a route or altitude change to exit all icing conditions. 

Stay clear of all icing conditions for the remainder of the flight, including landing, 

unless it can be determined that ice accretions no longer remain on the airframe. 

A4.3.2 AFM Operating procedures (normal procedures section) 

Freezing drizzle and freezing rain conditions encountered during a hold (in any 

aeroplane configuration) or in the en route phase of flight (climb, cruise, or 

descent with high lift devices and gear retracted) are severe icing conditions for 

this aeroplane. Intentional holding or en route flight into freezing drizzle or 

freezing rain conditions is prohibited. 

{insert cue description here} is one indication of severe icing for this aeroplane. If 

severe icing is encountered, immediately request priority handling from air traffic 

control to facilitate a route or altitude change to exit all icing conditions. Stay 

clear of all icing conditions for the remainder of the flight, including landing, 

unless it can be determined that ice accretions no longer remain on the airframe. 

A4.3.3 Flightcrew Operating Manual operating procedures 

Warning: Severe icing may result from environmental conditions outside of those 

for which the airplane is certified. Intentional flight into severe icing conditions 

may result in ice build-up on protected surfaces exceeding the capability of the 

ice protection system, or may result in ice forming aft of the protected surfaces. 

This ice may not be shed when using the ice protection systems, and may 

seriously degrade the performance and controllability of the airplane. 

Operations in icing conditions were evaluated as part of the certification process 
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for this aeroplane. En route (climb, cruise, and descent with high left devices and 

gear retracted) and holding flight (in any aeroplane configuration) in freezing 

drizzle and freezing rain conditions were not evaluated and are considered severe 

icing conditions for this aeroplane. 

Intentional holding or en route flight into freezing drizzle or freezing rain 

conditions is prohibited. {insert cue description here}is an indication of severe 

icing conditions that exceed those for which the airplane is certified. If severe 

icing is encountered, immediately request priority handling from air traffic control 

to facilitate a route or altitude change to exit all icing conditions. Stay clear of all 

icing conditions for the remainder of the flight, including landing, unless it can be 

determined that ice accretions no longer remain on the airframe. 

A4.4 Approved for a portion of Appendix X icing conditions 

A4.4.1 AFM Limitations 

Intentional flight, including takeoff and landing, into {insert pilot usable 

description here} conditions is prohibited. If {insert pilot usable description here} 

conditions are encountered, or if {insert cue description here}, immediately 

request priority handling from air traffic control to facilitate a route or altitude 

change to exit all icing conditions. Stay clear of all icing conditions for the 

remainder of the flight, including landing, unless it can be determined that ice 

accretions no longer remain on the airframe. 

A4.4.2 AFM Operating procedures (normal procedures section) 

{insert pilot usable description here} are severe icing conditions for this 

aeroplane. Intentional flight, including takeoff and landing, into {insert pilot 

usable description here} conditions is prohibited. A flight delay or diversion to an 

alternate airport is required if these conditions exist at the departure or 

destination airports. 

{insert cue description here} is one indication of severe icing for this aeroplane. If 

severe icing is encountered, immediately request priority handling from air traffic 

control to facilitate a route or altitude change to exit all icing conditions. Stay 

clear of all icing conditions for the remainder of the flight, including landing, 

unless it can be determined that ice accretions no longer remain on the airframe. 
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A4.4.3 Flightcrew Operating Manual operating procedures 

Warning: Severe icing may result from environmental conditions outside of those 

for which the airplane is certified. Intentional flight into severe icing conditions 

may result in ice build-up on protected surfaces exceeding the capability of the 

ice protection system, or may result in ice forming aft of the protected surfaces. 

This ice may not be shed when using the ice protection systems, and may 

seriously degrade the performance and controllability of the airplane. 

Operations in icing conditions were evaluated as part of the certification process 

for this aeroplane. {insert pilot usable description here} were not evaluated and 

are considered severe icing conditions for this aeroplane. 

Intentional flight, including takeoff and landing, into {insert pilot usable 

description here} is prohibited. A flight delay or diversion to an alternate airport 

is required if these conditions exist at the departure or destination airports. 

{insert cue description here}is an indication of severe icing conditions that 

exceed those for which the airplane is certified. If severe icing is encountered, 

immediately request priority handling from air traffic control to facilitate a route or 

altitude change to exit all icing conditions. Remain clear of all icing conditions for 

the remainder of the flight, including landing, unless it can be determined that ice 

accretions no longer remain on the airframe. 
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APPENDIX O - ENGINE SUB-GROUP RECOMENDATIONS FOR 

ADVISORY MATERIALS (REVISION TO AC 20-147) 
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Subject: TURBOJET, TURBOPROP, AND 

TURBOFAN ENGINE INDUCTION SYSTEM 

ICING AND ICE INGESTION 

Date: 2/02/04 

Initiated By: AIR100 

AC No: 20-147 

Change: 

1. PURPOSE. This advisory circular (AC) provides guidance and acceptable 

methods, but not the only methods, for demonstrating compliance with the 

applicable engine induction system icing and engine ice ingestion requirements. 

These requirements are applicable to the Federal Aviation Regulations, parts 23, 

25, and 33 of Title 14 of the Code of Federal Regulations (14 CFR parts 23, 25, 

and 33). The primary purpose of this AC is to prevent inconsistencies when 

installing a part 33 certified engine in a part 23 or 25 aircraft. The guidance in 

this AC is not intended to address turboshaft engine installations, or the rotary 

wing aircraft they are installed on. Due to the complexity that those aircraft and 

installations pose for icing, AC 20-73, Aircraft Icing Protection, is considered the 

primary AC for those installations. Further, this AC also provides guidance 

materials on mixed phase (supercooled liquid drops and ice particles) and ice 

crystal icing conditions (ice particles only), and a discussion on an acceptable 

means of demonstrating compliance in such icing conditions. While these 

guidelines are not mandatory, they are historically based and are derived from 

extensive Federal Aviation Administration (FAA) and industry experience in 

determining compliance with the relevant regulations. This AC does take 

precedence over the engine and engine installation icing guidance in AC 20-73. 

It is important to note that AC 20-73 does contain useful information on the 

understanding and characterization of the icing environment. Additionally, AC 

23-16, Powerplant Guide for Certification of Part 23 Airplanes, does take 

precedence over the engine installation guidance provided in this AC, as one 

method of compliance to part 23 regulations. 
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2. APPLICABILITY.

 a. The guidance provided in this document is directed to engine 

manufacturers, modifiers, foreign regulatory authorities, FAA engine type 

certification engineers and their designees. 

 b. This material is neither mandatory nor regulatory in nature and does not 

constitute a regulation. It describes acceptable means, but not the only means, 

for demonstrating compliance with the applicable regulations. The FAA will 

consider other methods of demonstrating compliance that an applicant may elect 

to present. Terms such as “should,” “shall,” “may,” and “must” are used only in 

the sense of ensuring applicability of this particular method of compliance when 

the acceptable method of compliance in this document is used. While these 

guidelines are not mandatory, they are derived from extensive FAA and industry 

experience in determining compliance with the relevant regulations. 

Alternatively, if the FAA becomes aware of circumstances that convince us that 

following this AC would not result in compliance with the applicable regulations, 

we will not be bound by the terms of this AC, and we may require additional 

substantiation as the basis for finding compliance. 

 c. This material does not change, create any additional, authorize changes in, 

or permit deviations from existing regulatory requirements. 

3. RELATED REGULATIONS. 

 a. Part 23, Airworthiness Standards: Normal, Utility, Acrobatic, and 

Commuter Category Airplanes, §§ 23.901(d)(2), 23.1093, and 23.1419. 

 b. Part 25, Airworthiness Standards: Transport Category Airplanes, §§ 

25.1093 and 25.1419. 

 c. Part 33, Airworthiness Standards: Aircraft Engines, §§ 33.68, 33.77(c), 

33.77(e), 33.89(b), and § 33.78, Rain and Hail Ingestion. 

4. RELATED READING MATERIAL (Latest Revisions). 

 a. AC 20-73, Aircraft Ice Protection. 

 b. AC 33-2B, Aircraft Engine Type Certification Handbook. 

 c. FAA Report No. FAA-RD-77-78, Engineering Summary of Powerplant Icing 

Technical Data, July 1977. 

 d. AC 23-16, Powerplant Guide for Certification of Part 23 Airplanes. 
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5. BACKGROUND. 

 a. The induction system icing requirements of parts 33, 23, and 25 are 

intended to provide protection for anticipated flight into icing conditions with no 

adverse effect on engine operation or serious loss of power or thrust. Propulsion 

systems certified under these requirements and operated in accordance with the 

airplane flight manual, have generally demonstrated safe operation when 

exposed to natural icing environments. This AC will supersede the engine and 

induction system icing guidance contained in AC 20-73 and AC 33-2 with regard 

to turbojet, turboprop and turbofan engine icing and engine installation icing 

approvals. Bear in mind AC 20-73 does contain additional guidance on 

turboshaft engines and installations. The test conditions called out in Part 33.68 

are intended as standardized engine icing certification test conditions. These 

standard conditions, in conjunction with any design-specific critical test points, 

should be used together with any additional conditions that the Administrator 

determines to be critical. These standard test conditions have been determined, 

through more than 30 years of certification experience, to provide an adequate 

and consistent basis for engine icing certification with good service experience. 

The successful demonstration of the test conditions outlined in the regulation is 

intended to address many potential engine power conditions, aircraft flight 

conditions, and environmental conditions that could otherwise prove to be costly 

and difficult to realistically test. Service experience, now in the hundreds of 

millions of hours, has also shown a long success record when using these test 

points to cover unknown environmental or operational factors. Finally, one 

should be aware that in Appendix C and Appendix X of part 25 and Appendix D 

of part 33, the environmental threat is considered probable, and therefore likely 

to occur. In comparison, this occurrence rate is far more probable than the 

remote threat posed by the rain and hail environmental threat in Appendix B of 

part 33. Therefore, a direct comparison of guidance between the acceptable test 

outcomes of the icing certification requirements and the rain and hail certification 

requirements is not appropriate. Again, it should also be recognized that 

although Appendix C, Appendix X and Appendix D are the certification 

standards, it is still possible to have in-service icing conditions that are more 

severe than the Appendix C, Appendix X and Appendix D conditions. The 

Appendix C and X conditions were designed to include 99 percent of icing 

conditions. Evaluation of icing data has indicated that the probability of 

encountering icing LWC levels outside of Appendix C droplet conditions is on the 

order of 10-21

. The applicant may assume this probability for encountering the 

large droplet conditions conducive to ice accumulation aft of the airframe’s 

protected areas. It should be considered as an average probability throughout 

the flight. Often, experience has shown that the actual icing environment in 

nature can be a combination of conditions, such as a continuous maximum cloud 

followed by a intermittent maximum cloud followed by a continuous maximum 

 

1

 Note that exceedance of Appendix C LWC & MVD and operating below Appendix C temperature 

simultaneously is on the order of 10-3. 
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cloud, and so on. While technology efforts need to be completed regarding the 

analysis and testing requirements for compliance with Appendix D conditions, the 

allowance for comparative evaluation under the rule enables application of the 

rule in advance of completing that work. 

6. DEFINITIONS. The following are defined for the purpose of this AC 

 a. Auto-recovery systems. Auto-recovery systems typically include autorelight systems, stall recovery systems, or any other engine system intended to 

recover the operability of an engine following a flameout, surge, stall, or a 

combination of these. 

 b. Freezing fraction. The fraction of impinging water that freezes on impact 

 c. Ice formations. Ice formations resulting from the impact of supercooled 

water droplets on propulsion system surfaces are classified as follows: 

 (1) Glaze ice. A clear, hard ice, which forms at temperatures close to (but 

below) freezing, in air with high liquid water content and large droplet sizes. 

Droplets impacting the surface do not freeze immediately, but run back along the 

surface until freezing occurs. Glaze ice typically has a non-aerodynamic shape 

and is more susceptible to aerodynamic forces that result in shedding. Glaze ice 

typically has both a lower freezing fraction and lower adhesive properties than 

rime ice. Glaze ice is often a concern for static hardware while rime ice is often a 

concern for rotating hardware. 

 (2) Rime ice. A milky, white ice which forms at low temperatures, in air with 

low liquid water content and small droplet sizes. Rime ice typically forms in an 

aerodynamic shape, on both rotating and static engine hardware. The freezing 

fraction is high for rime ice, typically approaching a value of 1.0. Rime ice typically 

has greater adhesion properties than glaze ice but often a lower density. 

Adhesion properties increase with lower temperature up to a test point where no 

additional adhesion is gained with additional lower temperature. 

 (3) Mixed or intermediate ice. A combination of glaze and rime ice which 

forms with rime patches slightly aft of the glaze ice portions. This ice forms at 

temperatures, liquid water content, and droplet sizes between those that produce 

rime and glaze ice. 

 d. Ice shed cycles. The time period required to build up and then shed ice on 

a propulsion system surface for a given power and icing condition. A shed cycle 

can be identified through visual means (for example, high-speed camera which 

should view fan and booster or low pressure compressor (LPC) inlet guide vane 
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components), and engine instrumentation (such as, vibration pickups, 

temperature probes, speed pickups, and so on). 

 e. Icing condition. A meteorological condition defined by the following 

parameters: 

 (1) Liquid Water Content (LWC). Concentration of liquid water in air, 

typically expressed in grams of water per cubic meter of air. 

(2) Median Volume Diameter (MVD) – The drop diameter which divides the 

total water volume present in a droplet distribution in half, i.e. half the 

water volume is in larger drops and half the volume in smaller drops. 

(Note: MVD is equivalent to MED of Appendix C for an assumed 

Langmuir type droplet distribution.) 

(3) Median Mass Dimension (MMD) - The particle size (sphere of equivalent 

mass) which divides the total ice mass present in an ice particle 

distribution in half; i.e. half the ice mass is in larger particles and half the 

ice mass is in smaller particles. 

 (4) Temperature. The total temperature associated with the icing cloud 

environment. Appendix C, Appendix X and Appendix D temperatures are static 

ambient temperatures. When a critical test point analysis is conducted, the local 

total temperatures at the engine inlet should be considered, based on applying 

the Appendix C, Appendix X and Appendix D static temperatures and assumed 

flight Mach number. 

 f. Power loss instabilities. Engine operating anomalies such as nonrecoverable or repeating surge, stall, rollback, or flameout, can result in engine 

power or thrust cycling. 

 g. Scoop factor (concentration factor). The ratio of nacelle inlet highlight area 

(AH) to the area of the captured air stream tube (AC) [scoop factor = AH/AC]. The 

highlight area is defined as the area bounded by the leading edge of the nacelle 

inlet. Scoop factor potentially concentrates liquid water available for ice 

formation in the inlet and additionally in the low-pressure compressor or engine 

core as a function of aircraft forward airspeed and engine power condition. The 

scoop factor affect depends on the droplet diameter, the simulated airspeed and 

the engine power level as well as the geometry and size of the engine 

 h. Serious loss of power or thrust. Engine operating anomalies such as nonrecoverable or repeating surge, stall, rollback or flameout, which can result in 

noticeable engine power or thrust loss. The FAA (that is, the Engine and 

Propeller Directorate, the Transport Airplane Directorate, and the Small Airplane 

Directorate) expects there will not be a noticeable power or thrust loss. This is 
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especially important when considering that icing encounters are considered a 

frequent event, and multiple encounters for each flight is a reasonable 

assumption. The word “noticeable”, as used above, refers to flight crews tactile 

feel during the event, or the use of typical engine test instrumentation, or flight 

deck instrumentation (such as, N1, N2, vibes, exhaust gas temp). 

 i. Steady Operation. During icing testing, the engine should demonstrate 

steady, reliable, and smooth operation while sitting on test point (during multiple 

build or shed cycles, if ice is accreting), as well as during throttle transients. The 

term “steadily” is intended to address both stabilized ice accretions and stabilized 

engine operation. Ice accretions are considered stabilized when either ice is not 

forming on any engine parts, or the accreting ice has demonstrated a regular 

shed cycle when viewed by a video camera or instrumentation indication. Engine 

operation is considered stabilized when the measured engine parameters are not 

changing, or a regular, repeatable shed cycle has been demonstrated through 

the recording of measured engine parameters. The applicant should determine 

what parameters need to be monitored to determine steady operation of the 

engine during the icing test. Variations in measured parameters are acceptable 

during the performance of the ice test, as long as the long-term trend (typically 

the duration of several shed cycles) is stable and not trending upwards or 

downwards. 

 j. Sustained power loss. A permanent reduction in power or thrust at the 

engine’s primary power set parameter (for example, fan rotor speed, engine 

pressure ratio). A sustained measurable power loss is considered a “severe 

power loss” in the context of the icing requirement. Power or thrust losses that 

are not sustained are temporary in nature and may be related to the effects of 

ingesting super-cooled water or ice particles, or possibly the effects of ice 

accumulation or ice shedding. The engine’s momentary response during 

shedding may be from the thermodynamic engine response to the ice ingestion. 

 k. Water impingement rate. The rate (gm/Sq. m/min) at which a portion of the 

surface area of a solid object is impacted by the water droplets in a moving air 

stream. 

 l. Descent Idle Engine RPM. The engine RPM for this phase of flight is the 

altitude- dependent minimum flight idle generally in effect from the top of descent 

(TOD) to the point at which the approach phase of flight begins (typically defined 

when flaps are deployed) and idle RPM increases to reflect the flight control 

requirements during approach. 

m. Span. The length of an engine compressor airfoil (rotor or stator) 

measured from the flowpath ID to the flowpath OD 
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n. Momentary power loss. A reduction in engine power or thrust associated 

with a transient event (e.g. ice shedding). 

o. Temporary power loss. A reduction in engine power or thrust occurring 

during an icing encounter due to ice accretion within the flowpath. 

p. Maximum continuous. Engine rating required by 14 CFR Part 33.7 (c)(1)(i) 

and documented in the engine type certificate data sheet, intended as the 

maximum power for continuous operation. 

q. Takeoff power. Engine rating required by 14 CFR Part 33.7 (c)(1)(iii) and 

documented in the engine type certificate data sheet. 

7. DISCUSSION. The induction system icing requirements of §§ 23.1093, 

25.1093, and 33.68 are intended to provide protection for flight into icing 

conditions with no adverse effect on engine operation or sustained loss of power. 

An icing encounter, including a prolonged encounter, should not be of 

consequence to the crew, and should not invalidate the engine's compliance with 

other part 33 requirements (for example, §§ 33.15, 33.19, 33.63, 33.65, 33.75, 

33.77, 33.83, 33.89(b), and the like). The engine should have sufficient durability 

to operate through prolonged or repeated environmental encounters, such as 

icing, without special operational or maintenance interventions. Operational 

procedures to assist ice shedding, such as throttle manipulation, should not be 

relied on, or be required to comply with parts 23, 25, and 33 in-flight icing 

requirements. It is acceptable to provide engine throttle manipulation (for 

example, power run-ups to shed ice) instructions to shed accumulated ice during 

ground operations. These instructions will be used as a recommendation for inservice ground operation, although they would be mandatory if they were utilized 

during the ground icing compliance demonstration of §§ 33.68(b), 23.1093(b)(2), 

and 25.1093(b)(2). The applicant should provide instrumentation and video or 

photographic coverage to supplement test results under §§ 33.68, 23.1093, and 

25.1093. The applicant should determine the parameters, which may include 

both visual and instrumented indications that need to be monitored. The 

demonstration should include stable build or shed cycles (that is, steady 

operation) with either no ice buildup or no additional ice buildup on the engine or 

inlet. Normal engine control system responses during the ice accumulation 

process (for example, isochronous control response to accreting ice) are 

considered acceptable as long as there are no power losses. At the conclusion 

of the test point, during the acceleration to takeoff power, the measured 

parameters should demonstrate a smooth and steady acceleration characteristic, 

unless the applicant can provide an acceptable justification for a performance 

change while on test point (for example, thermodynamic engine response to 

shed ice ingestion is acceptable). Close coordination is necessary by all parties 

to ensure that test plans are in reasonable bounds for the anticipated use of the 
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airplane. The body of this AC is arranged in three sections corresponding to the 

applicable parts (§§ 33.68 and 33.89(b); 33.77; and 23.1093 and 25.1093). 

 a. Mixed Phase or Ice crystal Icing Conditions. Mixed phase icing conditions 

occur when supercooled liquid water droplets and ice particles coexist in a cloud, 

often around the outskirts of a deep convective cloud formation. Ice crystal icing 

condition exists when all of the liquid water particles in the cloud have frozen into 

ice particles. In the past, susceptibility of turbine engines to mixed phase or ice 

crystal conditions has been considered minimal with the possible exception of 

two design features. They are (1) pronounced inlet bends (such as particleseparator inlets), or inlet flow reversals, where inlet flow can stagnate and 

accumulate ice, and (2) high solidity, high turning front stage compressor stators 

that can be susceptible to non-aerodynamic ice buildup on the stator air foils 

resulting in core airflow blockage. More recently, however, there have been 

service events due to icing within the engine core stream due to mixed phase/ice 

crystal conditions. Adverse effects of this type of icing condition on the engine 

are uncommanded rollback of power or flameout with occasional core hardware 

damages. The root cause of these events can be traced to ice buildup within the 

core flow paths of the affected engines. In general, these events occur near 

convective clouds at ambient temperature warmer than the ISA standard 

atmosphere and outside of the FAR Part 25 Appendix C and Appendix X icing 

envelopes. FAR Part 33 Appendix D has been established to include this icing 

envelope. Malfunctioning of the Total Air Temperature (TAT) probe has been 

noted during many of these engine icing events and is a known indicator of ice 

crystals. Ice accumulation within the TAT probe would produce a false signal of 

air temperature stabilizing at the freezing point. Additionally, there have been 

cases of icing encounters in mixed phase icing conditions where ice detectors 

have not detected ice formation. Ice detection systems should be evaluated for 

these conditions. Guidelines to comply with the certification requirements for 

mixed phase or ice crystal icing are discussed in this AC. 

 b. Auto-recovery systems. The use of auto-recovery systems is acceptable 

for certain engine certification testing. The FAA supports the use of autorecovery systems, or other protective engine systems or devices, while in 

service, and allows the use of auto-recovery systems during ice slab ingestion 

certification testing as defined in § 33.77. Generally, compliance with §§ 33.68 

and 33.77 requires a demonstration that no flameout, sustained power loss, 

surge or stall, or rundown is evident. Although ignition systems have generally 

been found to be reliable for auto-relight use after certain ice ingestion or 

accretion induced flameouts (§ 33.77, 25.1093), the auto-relight system should 

not be relied on during typical icing encounters (§ 33.68). Auto-recovery systems 

are regarded as only back-up devices, and should not be routinely needed. In 

addition, auto recovery systems are not considered the primary protection for 

continued safe engine operation during normal ice sheds or accumulations while 

operating in typical icing conditions. Details will be provided later in this AC 
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about the use of auto-recovery systems when demonstrating compliance to §§ 

33.68 and 33.77. 

 c. Use of Cloud Extent Factors for § 33.68. In Appendix C, Appendix X and 

Appendix D, a cloud extent is the distance vertically (vertical extent) or 

horizontally (horizontal extent) that a cloud extends. Vertical extent is normally 

measured in feet while horizontal extent is measured in nautical miles. The cloud 

extent factor is a dimensionless number, which relates the length of a cloud to an 

average LWC across the cloud. These relationships described within Appendix 

C, Appendix X and Appendix D should be used in the critical point analysis 

(CPA) for assessing the probability of occurrence of icing conditions during 

various aircraft mission and performance analyses. These factors are applicable 

to airframe flight profiles where the straight-line flight portion of the evaluations 

may use the cloud extent factor included in Appendix C in Figures 3 and 6, 

Appendix X in Figure 7 and Appendix D in Figure D-3, as applicable. However, 

engines and induction systems being evaluated under § 33.68 have historically 

not been limited to or evaluated against a specific aircraft flight profile when 

considering icing environments. Instead, under § 33.68, they are evaluated for 

unlimited operation in icing. It is emphasized that the criteria in Appendix C 

represent those icing conditions that could result from encounters with 

supercooled clouds. Other conditions that may contribute to aircraft engine icing 

conditions include freezing precipitation (that is, rain and drizzle) covered by 

Appendix X. Ice crystals, and mixed conditions (meaning, mixture of 

supercooled water droplets and ice crystals) are defined in Appendix D. To 

account for the differences described above, multiple clouds should be assumed, 

with an extent factor equal to 1.0, as actual cloud extent is not a consideration for 

engine operations, particularly in an aircraft hold pattern. This approach will 

assure unlimited engine and induction system operation within the atmospheric 

conditions defined by Appendix C, Appendix X and Appendix D, and as 

experience indicates, in actual icing environments that include Appendix C, 

Appendix X and Appendix D in its entirety. The cloud horizontal extent factor 

was not intended to be used to limit the severity of exposure to icing conditions, 

where it is reasonable to assume the aircraft will be required to operate in that 

condition (for example, the holding pattern which may require repeated passes 

through a severe icing environment, or continuously remain in that severe 

environment). As a general rule, engines and induction systems should be 

shown to operate continuously in icing without regard to time in icing conditions. 

The cloud extent factor defined in Appendix D relies primarily on limited data 

derived from Reference 1. The data covered several geographical regions 

including eastern Asia where ice crystal-related service events are prevalent. 
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SECTION 1. INDUCTION SYSTEM ICING (§§ 33.68 and 33.89(b))

8. CRITICAL POINT ANALYSIS (CPA). Compliance with the requirements of § 

33.68 includes identifying, through analysis, the critical operating test points for 

icing within the declared operating envelope of the engine. The CPA should 

include a range of possible combinations of icing conditions. This range should 

relate to Appendix C and X of part 25, Appendix D of part 33, aircraft speed 

range, and engine powers as defined by the engine manufacturer, and prolonged 

operation in icing (for example, in-flight hold pattern), or repeat icing encounters. 

The CPA should be validated by empirical test data. This analysis should 

consider both critical ice accumulation conditions (that is, rime ice and glaze ice), 

environmental and engine operational effects on accumulation, accretion 

locations, as well as the most critical engine operating conditions for ice shed 

and ingestion. Some manufacturers have included within their CPA, a best 

practice of including conditions that may be outside of Appendix C, Appendix X 

and Appendix D, that have been identified through service difficulty of other 

engine models. Often the CPA is supplemented with development test data (for 

example, wet and dry testing with thermocouple components). The methodology 

used to calculate ice accretions should account for freezing fraction and pertinent 

aerodynamic effects. For example, water ingestion into fan inlet and core inlet, 

water impingement rates for critical surfaces, forward aircraft air speed effects, 

engine configuration effects such as inter-compressor bleed, and altitude effects 

such as bypass ratio effects. This should be in conjunction with an energy 

balance of critical engine surfaces, for example, latent heat and heat of fusion 

effects, metal-to-ice heat transfer effects, and ice insulating effects. For anti-iced 

parts, the critical test point should be determined from energy balance 

calculations of required heat loads encompassing the range of possible 

combinations of icing condition and engine power. In instances of low freezing 

fraction in glaze ice conditions, additional complexities arise from assessing the 

effects of non-aerodynamic ice formations and their shedding. Federal Aviation 

Administration (FAA) Report No. FAA-RD-77-78, Engineering Summary of 

Powerplant Icing Technical Data, provides additional guidance on performing a 

critical test point icing analysis. 

 a. Test versus Analysis. The CPA was not meant to replace testing, but 

instead provides a means to predict other critical test points with equivalent level 

of safety to replace or supplement the standard certification test points (§ 

33.68C) . The FAA maintains this view of the CPA complementing the standard 

table test points provides an adequate methodology to cover all icing conditions. 

The FAA recognizes an improvement in the fidelity of analysis tools that are 

available today. However, based on experience with the various applicants, the 

FAA believes that the CPA is best utilized as a method to predict the critical icing 

conditions for a given design, and then use these conditions in conjunction with 

the standardized certification test points for certification compliance test 
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purposes. It should be noted that FAA concurrence with a type certificate 

holders generic CPA method does not automatically constitute FAA acceptance 

of the resulting critical icing test points for future certification projects. The 

content of an icing certification program for any given certification project will be 

evaluated on a case-by-case basis. Some engine manufacturers have had 

difficulty in consistently achieving the agreed on test points that are necessary for 

compliance demonstration, due to facility or weather limitations. This difficulty is 

based on test facility limitations that can be expensive and often impractical to 

overcome. To directly address this repeated shortfall, the FAA has identified 

several national and international icing resources for accomplishing § 33.68 

compliance testing. These resources include the Air Force’s McKinley Climatic 

Lab (Florida), the Air Force’s AEDC (Tennessee), and the Canadian 

government’s NRC (Ottawa), to name a few. There are additional test facilities 

that are suited for component or model testing. 

 b. Elements of CPA. The CPA should address, at minimum, the following 

icing issues: 

 (1) Ice shed damage. Ice accretion on engine surfaces (for example, 

blades, vanes, sensors) may eventually shed. The shed ice can subsequently 

cause engine damage if it impacts an engine surface with sufficient mass and 

velocity. 

 (2) Fan module. Acoustic panels, fan rub strips, and fan blade tips are 

susceptible to ice shed from inlet sensor(s), spinner, and fan blade root. The 

effects of ice density, hardness, and adhesion strength should be assessed to 

realistic flight conditions. The ice shed cycle for rotating surfaces, such as fan 

blades, is strongly influenced by rotor speed and the adhesive strength of the ice 

to the surface. The adhesive strength of ice generally increases with decreasing 

surface temperature. The ice thickness and rotor speed at the time of the shed 

defines the impact threat. In determining the critical conditions for fan module 

damage, surface temperature, exposure time, and rotor speed are important 

considerations in addition to more typical parameters, such as icing condition and 

scoop factor In particular, extended operation in a holding condition in very cold 

continuous maximum icing conditions will maximize the adhesion of ice on 

rotating fan components. 

 (3) Compressor damage. A common damage scenario in turbofan 

engines, is the accretion of glaze (non-aerodynamic) ice formations on static 

components (for example, sensors, vanes, and bleed ducts upstream of the 

compressor) which when it sheds, results in damage. This type of damage 

generally occurs on the first blade set in the high-pressure compressor 

(intermediate pressure compressor for three spool engines). Establishing the 

critical conditions for these glaze ice accretions requires careful consideration as 

they occur at specific limited conditions of low freezing fractions over a range of 
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local mach numbers and air densities. The critical conditions may not occur 

during any of the power settings recommended by this AC (that is, flight-idle, 50-

percent and 75-percent of maximum continuous or 100-percent maximum 

continuous). 

Any engine damage that results from ice testing should be evaluated against the 

possibility of multiple occurrences, since icing is a common environmental 

condition. 

 (4) Engine operability and compressor rematch. Ice shed from upstream 

components may enter the core compressor. The presence of ice or water from 

melted ice in the gas path may cause the engine to assume new operating 

conditions (that is, engine component cycle rematch). The engine should be 

capable of accelerating from minimum flight-idle and ground idle to takeoff 

power, at any icing condition, without power loss, or instability (surge or stall). 

Ice sheds should not result in flameout, rollback, or surge. Any anomalous 

engine behavior should be raised to the cognizant Aircraft Certification Office 

(ACO) for evaluation and if found acceptable, should be documented in the 

engine’s installation manual. The applicant should consider as part of the CPA 

both engine accelerations and decelerations relative to operability challenges. 

Critical test point testing should demonstrate those conditions where minimum 

operability margin is expected. 

 (5) Core and Booster ice blockage. Ice accretion on internal engine vanes 

due to the presence of glaze ice accretions may affect flow capacity and rematch 

of the engine cycle and should be considered in the CPA. At engine powers that 

can sustain flight, ice accretion should be reconciled through a demonstration of 

several ice build shed cycles to demonstrate no adverse operating effects of 

either the ice builds or sheds. 

 (6) Sensor fouling. Ice accretion and blockage of control sensors can 

result in erroneous engine pressure and temperature measurements. A power 

loss or power loss instability could result if these measurements are used by the 

engine control systems control law to establish power or thrust ratings, or to 

schedule other systems required to operate the engine (for example, variable 

stator vanes). Critical sensors should be designed to operate without accreting 

ice sufficient to cause an erroneous measurement that would result in an 

unacceptable operating characteristic. Additionally, ice accretion on upstream 

sensors can shed and cause engine damage to downstream rotating hardware. 

9. TEST POINT(S) SELECTION. Test points selected for supercooled droplet 

environment must address the FAR Part 25 Appendix C and X icing envelopes. 

Test points for the mixed phase and ice crystal icing environment must be 

representative of the meteorological conditions defined in FAR Part 33 Appendix 

D. Typically, the supercooled droplet test points include those described in the 

rule and any additional test points identified as part of the CPA. The applicant 
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should consider pertinent service experience as well as the anticipated use of the 

aircraft when selecting critical icing test points. The following should be 

considered when constructing an icing test matrix: 

 a. Section 33.68 Acceptable Means of Compliance. The engine should be 

capable of operating acceptably under the meteorological conditions of Appendix 

C and Appendix X over the engine-operating envelope, as described in § 

33.68(B), and under conditions of ground fog (§ 33.68(D)). Experience has 

indicated that testing to the conditions specified in the § 33.68(C) tables have 

been a successful means of showing compliance, if used in conjunction with the 

critical conditions determined in the CPA. § 33.68(C) does allow for the 

elimination of specific standard certification test points if the applicant chooses to 

run a similar CPA test condition that can be shown to be more severe in terms of 

ice accretion mass at critical engine locations or to produce an equivalent level of 

safety. Similar is defined as being at essentially the same level of engine low 

rotor RPM, the same type of ice accretion (glaze or rime), and for the same or 

longer period of icing exposure. 

Specifically for the icing conditions defined in the Table 33.68-1 of Part 33:

• The LWC for all table points represent ambient icing conditions for an 

open inlet ground test facility or equivalently within the inlet duct of a direct 

connect test facility. Consideration of local enrichment for flight effects as 

would normally be done for a CPA point LWC is not necessary. 

• Conditions 1 and 2. Operate the engine steadily (see definitions for 

“steady operation”) under rime and glaze icing conditions for at least 10 

minutes each, typically during separate tests, at maximum continuous 

(M.C.), 75-percent of M.C., 50-percent of M.C., and for 10 minutes at a 

flight-idle setting. Each separate test is followed by an acceleration to 

takeoff power. If ice is still building up at the end of 10 minutes at the 

three higher power settings, continue running until the engine 

demonstrates stabilized operation (meaning, stabilized building and 

shedding is demonstrated or the engine will no longer operate 

satisfactorily). 

• Conditions1 and 2 of Table 33.68-1, are intended to partially cover a 

widely bounded test matrix of environmental and engine operating 

conditions to be used when showing compliance with §33.68. This test 

matrix includes power settings from idle to takeoff during exposure to 

conditions typical of high altitude where rime ice formations occur, as well 

as conditions typical of low altitude where glaze ice formations often 

occur. Icing conditions are normally run for a minimum of 10 minutes for 

all engine powers that can sustain level flight, or longer if the natural 

engine ice shed cycle is not established. Special consideration and tests 
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should be conducted to adequately substantiate engine inlet screens and 

inlet air passages that might accumulate snow or ice due to restrictions or 

contours. Also, engines that employ an inlet de-ice system that may have 

some unheated inlet surfaces may need to be run longer. At idle descent 

power the required time period is limited to 10 minutes. The rime icing 

condition should be run at an engine speed associated with high altitude 

top of descent operation. The glaze icing condition should be run at the 

minimum engine speed associated with lower altitudes at the end of the 

descent phase of flight. 

• Conditions 3 and 4 (Holding phase). This test, when performed as part of 

a part 33 demonstration, is intended to be applicable to part 33 engine 

components. Both the engine (§ 33.68) and aircraft inlet induction system 

(§§ 23.1093 and 25.1093) should operate safely in an in-flight holding 

phase without a time limit. The test program for turbofan and turboprop 

applications should include test points (for example, icing condition and 

power setting) to address the effects of prolonged exposure in icing 

conditions typical of in-flight holding patterns. Condition 4 in table 33.68-1 

represents a rime icing condition that is typically encountered on transport 

category airplanes. Condition 3 in table 33.68-1 represents a mixed rime 

and glaze icing condition that was originally derived from the JAR-E ice 

requirements of LWC, temperature and droplet size. The engine and inlet 

should be capable of prolonged exposure to the conditions specified in 

table 33.68-1. A 45-minute test exposure followed by acceleration to 

takeoff power will typically demonstrate several ice shed cycles and should 

normally be sufficient to assess compliance for the engine. 

• For all conditions. Engine operation in these icing conditions should be 

reliable, uninterrupted, and without any significant adverse effects, and 

should include the ability to continue in operation and accelerate and 

decelerate. Some power reduction is acceptable at idle power settings due 

to the cycle effects of pumping ice and water, but all other operation should 

be unaffected. 

• While at flight-idle engine speed and above, for engines with icing 

protection systems (including systems for probes), stabilize the engine for 

at least 2 minutes in the icing atmosphere with these protection systems 

off, prior to turning on the icing protection system to simulate the delay 

expected for the pilot to recognize the icing condition. Fully automatic 

systems may use an appropriate delay. Systems that are automatic and 

controlled by the full authority digital electronic/engine control (FADEC) do 

not require the 2-minute delay in ice protection system demonstration. 

Where the engine’s anti-icing system relies on an ice detector to indicate 

the presence of icing conditions, delay in anti-ice selection is likely 
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following failure of the detector, and so delayed selection testing should still 

be demonstrated. 

• The engine must operate “steadily” under the tested icing conditions. The 

term “steadily” is defined in the definitions section and is intended to 

address both stabilized ice accretion and shed cycles during stabilized 

engine operation. Ice accretions are considered stabilized when either ice 

is not forming on any engine parts or the accreting ice has demonstrated a 

regular shed cycle when viewed by a video camera or instrumentation 

indication, or both. Engine operation is considered stabilized when the 

measured engine parameters are not changing, or a regular, repeatable 

shed cycle has been demonstrated through the recording of measured 

engine parameters. 

• Turboprop engines equipped with an inlet screen: The inlet screen 

compliance demonstration for icing including any associated shed cycle 

may be addressed separately from the part 33.68 engine ice compliance 

testing, but then should be noted in the engine installation manual. If not 

addressed under part 33.68, compliance could then be demonstrated 

under the aircraft inlet requirements of 23.1093, 25.1093, 27.1093, or 

29.1093. Special attention should be given to these inlet features, 

including specialized tests or analysis. 

• At the conclusion of each steady state icing test point, the engine should 

be accelerated to takeoff power (throttle movement of 1 second or less) or 

shedding should be induced by other methods, depending on the critical 

shed methodology. The throttle motion should be the most critical when 

considering the ice-shed effects on engine operability. In some cases, a 

quick deceleration before accelerating to takeoff power may be more 

critical to the ice shed effects on engine operation. The applicant should 

assess this effect and account for their assessment in their test proposal. 

 b. Section 33.68(D) – Ground Operation: Section 33.68(D) provides two icing 

test points that represent a typical freezing fog, a supercooled large droplet 

(SLD) test point that represents freezing rain or drizzle, and a falling or blowing 

snow icing encounter during ground operation. The SLD condition is similar to 

freezing fog in terms of ambient temperature range and LWC levels, but the 

larger droplets can notably penetrate further back on the surface of the engine 

spinner with the potential for ice shedding into fan blades. Allowance for analysis 

as means of compliance offers the option of comparing a new engine design for 

spinner icing/de-icing characteristics and fan blade robustness to prior engines 

with good service experience. 

The guidance contained in AC 33.2B, with respect to snow ingestion testing, is 

considered outdated. The ground-fog icing, SLD and falling or blowing snow 
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demonstrations should continue for at least 30 minutes or until stabilized 

operation (see “steady operation” in the definitions section of this AC) is 

demonstrated. If this stabilized operation cannot be demonstrated then periodic 

engine speed run-ups should be demonstrated. These run-ups would then 

become mandatory in the AFM in icing conditions since they were required to 

comply with the icing requirements. Service experience has demonstrated 

compressor damage as a result of exposure to prolonged periods of falling snow 

during ground operation. Based on review of service events, airports have 

continued to operate with falling-snow concentrations that result in 0.25 mile or 

less visibility (about 0.9 gm/m3). While visibility can be a poor indicator of 

precipitation rate (Rasmussen et al., 1999), the following calculation based upon 

equivalent rainfall rate gives a similar result. The maximum precipitation rate for 

moderate snow is 2.5 mm/hr water equivalent. Using a typical fall speed for 

snow of 0.8 m/s, this translates into a snow concentration of 0.9 g/m3 water 

equivalent. From a Transport Canada data set of 338,000 minutes of snowfall 

data, the 95% and 99% values were 2 and 4 mm/hr respectively showing that a 

2.5 mm/hr threshold provides an extreme value. It should be mentioned that 

Holdover Time Tables for de-icing fluids are only good for the extreme values of 

moderate snow which is defined as 2.5 mm/hr (or a visibility of 1⁄4 statute mile). 

Engine core icing service events in snow conditions support the temperature 

range of 26 to 32 deg F (-3 to 0 deg C). Within this range, water content in the 

form of supercooled liquid drops can be considered negligible.. This environment 

is conducive to ice accretion in mid and rear compressor stages of the engine. 

Liquid water icing conditions defined in §33.68 adequately evaluate the effects of 

ice accretion in front sections of the engine. Sections 23.1093(b)(ii) and 

25.1093(b)(ii) require that engines must operate satisfactorily in falling and 

blowing snow throughout the engine power range, within the limitations 

established for the airplane for such operation. 

 c. Ground Idle Demonstrations: Operate steadily at ground idle setting for at 

least 30 minutes under icing conditions of §33.68(D) followed by acceleration to 

takeoff setting. Since a broad temperature range is provided, the applicant 

should identify the most critical temperature, as determined by the CPA, and 

target that range. Turboprop engine inlet screen icing compliance 

demonstrations may be addressed separately from the part 33.68 engine ice 

compliance testing, but then should be noted in the engine installation manual. It 

is recommended that special attention be paid to these inlet features. For large 

bypass fan engines, shedding at this low speed from static surfaces is not likely 

at 30 minutes and beyond. For these engines, this test generally establishes the 

maximum allowable time in the engine operating manual between engine run-ups 

to shed ice. The AFM should specify the maximum demonstrated time between 

run-ups. Demonstration that ice sheds from compressor static surfaces during 

the run-up establishes that the engine can operate continuously (including runups, if necessary) at the ground icing condition, as might well occur at an airport 

where aircraft can be held on the ground several hours in bad weather awaiting 
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takeoff. Alternate run-up procedures that can be shown to provide acceptable 

engine acceleration to takeoff power are also acceptable. 

 d. Mixed phase or ice crystal icing: It has been established that mixed phase 

or ice crystal icing conditions that are hazardous to turbine engines exist around 

the outskirts (anvil) of a deep convective cloud formation as well as near tropical 

storms such as monsoons, hurricanes, and typhoons. A compilation of mixed 

phase/ice crystal icing condition currently available is in Reference 7. This report 

recommends that ice water content up to 1.2 g/m3 could exist in thunderstorm 

anvil cloud above 25,000 feet altitude, but also includes information from ADS-4 

citing total water concentrations (TWC) in the range of 1 to 5 g/m3 in this range. 

The ice particles within this cloud can range up to 10 mm, with highest 

concentrations having a median diameter of less than 200 microns. In order to 

define compliance requirements, it was necessary to define an extension to the 

icing envelopes defined in FAR Part 25 Appendix C and Appendix X. FAR Part 

33 Appendix D has been created for this purpose. TWC levels in Appendix D are 

based upon predicted “adiabatic” condensation rates in convective storm 

updrafts. This has been shown to be a reasonable assumption based on limited 

field trials. Until a more comprehensive mixed phase and ice crystal icing 

envelope is defined, Appendix D should be treated as the critical mixed phase/ice 

crystal icing condition in demonstrating compliance to certification requirements. 

10. TEST SETUP CONSIDERATIONS. The Liquid Water Content (LWC) levels 

defined in Appendix C and Appendix X of Part 25 are intended as supercooled 

droplet ambient icing conditions. Tests may be conducted with a simulated cloud 

outside of the inlet that is ingested into the engine. Under such a test 

environment, the LWC within the inlet ducting may be more or less than the 

engine inlet LWC concentration if the engine were actually installed in an airplane 

and flying through those icing conditions at actual airspeeds. This inlet 

concentration/dilution effect is dependent on droplet size, engine fan speed, and 

simulated forward airspeed. As an example, operation at idle descent power with 

simulated forward airspeed that is less than flight speed might require a 

compensating increase in test level LWC concentration above Appendix C and 

Appendix X conditions. This increase would be greater for larger supercooled 

droplet diameters. Engine size is also a variable that affects the difference in 

LWC inlet concentration between flight conditions and the engine test 

environment, with the greatest potential compensation requirements being for 

small engines. 

Icing tests that provide a simulated icing cloud by direct connection of facility 

piping to the front flange of the engine, where no inlet air spillage is allowed, may 

cause test facility effects that could alter the test parameters (for example, LWC 

and MVD). The applicant must provide the FAA with substantiation that the 

required simulated test conditions adequately simulate an installed engine flying 
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through the Appendix C and X conditions. This substantiation can be in the form 

of direct measurement of LWC within the inlet or by acceptable validated analysis 

of water droplet trajectories for the test setup. In some cases, LWC may need to 

be adjusted to address any effects due to the test setup (for example, nonuniformity across the engine face). 

Flight-testing in mixed phase or ice crystal icing conditions defined in part 33 

Appendix D: Flight-testing is the most accurate method of demonstrating engine 

operation in this icing condition. Two important considerations in flight testing in 

mixed phase or ice crystal icing conditions are the measurement of ambient 

meteorological data and the ability to project the measured engine performance 

to the critical icing point cited in this AC. To address the latter, a fully 

instrumented engine with temperature sensors strategically located in the core 

flow passage is required to collect data during the encounter with this 

environment. Since the meteorological condition encountered may not be as 

high as the critical point level, scaling of the measured data would be required to 

substantiate satisfactory engine operation at the critical point. To support the 

scaling calculation, an accurate measurement of the ambient meteorological 

condition is essential. Past engine flight tests in this environment had 

demonstrated that a combination of liquid water content probe, total water 

content probe, particle sizing and imaging probe, ice detector and TAT probe is 

necessary to fully characterize the ambient environment. Onboard real time 

meteorological data display with GPS positioning capability were also very 

helpful tools in locating the area where high ice crystal concentration exists. This 

allows the pilot to position the aircraft for the test. Since testing around a 

thunderstorm elevates the risk level of a flight test, an on-site rapid data 

reduction capability would give a timely indication if the test objective is met and 

minimizes the number of sorties into this hazardous environment. 

Simulation of the critical mixed phase/ice crystal icing condition in ground facility: 

As indicated in Reference 7, facility simulation of mixed phase/ice crystal icing 

conditions is difficult and not done routinely. It is not known how well any of the 

various methods that have been used actually simulate the natural environment. 

A testing standard for this icing condition is not established. Therefore, using 

ground simulation as a means of demonstrating compliance to the mixed 

phase/ice crystal icing certification requirement will have to be evaluated on a 

case-by-case basis. 

11. TEST RESULTS AND COMPLIANCE ISSUES. During all icing tests 

(supercooled droplet, mixed phase or ice crystal icing), the engine should 

operate without the accumulation of ice, which would adversely affect engine 

operation (for example, flameout, surge, stall, run-down, high vibrations, slow 

acceleration or lack of throttle response) or cause a sustained loss of power or 

thrust. Additionally, the applicant should accurately monitor icing test point 

conditions either through video surveillance or instrumentation and provide the 
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means to identify the source of ice damage, especially in those instances where 

test apparatus may shed ice (for example, icing nozzles, special test 

instrumentation). 

 a. Sustained loss of power or thrust and power loss instabilities. There 

should be no sustained power loss while operating at approved ratings in icing 

conditions. Temporary steady state power losses below the engine power and 

thrust ratings selected in accordance with § 33.8 can be accepted if it is proven 

that there is sufficient margin against any power loss instability, such as rollback, 

surge, stall, high vibes or flameout. Momentary power loss caused by pumping 

or processing of ice debris through the fan and compressor during the ice shed 

ingestion process are generally acceptable. If temporary power loss or 

temporary high vibrations are deemed acceptable to the Administrator, they 

should be documented in the engine installation manual. 

 b. Mechanical Damage. There should be no engine damage as a result of § 

33.68 icing testing. In some circumstances, some limited damage may be 

accepted. The acceptance of any damage must fully account for the cumulative 

damage from repeat encounters, provided the applicant satisfies the following 

criteria: 

 (1) Continued in-service use. Any resultant damage should be shown to 

be acceptable for continued in-service use. 

 (2) Sustained power losses. There should be no resultant sustained power 

loss beyond the nominal accepted level considered to be within measurement 

capability (1.5%); 

 (3) Temporary power loss. Although not generally acceptable, any 

resultant temporary steady state power loss, surge or high vibrations, if found 

acceptable by the Administrator, should be recorded in the installation manual. 

 (4) Validation basis. Analytical tools used to substantiate the criteria for 

determining acceptable damage should be shown to have a sufficient validation 

basis (for example, engine tests, rig tests, service experience) to substantiate the 

accuracy of results or be shown to yield conservative results. 

 (5) Disposition of Damage. Disposition of damage to any engine or engine 

component may not be obtainable solely by comparing the damage against the 

maintenance manual limits. The cumulative damage for repeated encounters 

should be evaluated. 

 (6) Communication of results. The Installation and Operating Manuals 

required by § 33.5 should provide information describing any resultant engine 

condition observed during engine certification icing tests. The engine 
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manufacturer should provide a process to permit disposition of any potential 

damage that could occur during natural icing flight tests conducted to 

demonstrate compliance with §§ 23.1093 or 25.1093, if the installing FAA Aircraft 

Certification Office finds this acceptable. Also, if periodic engine power run-ups 

are necessary to minimize damage from icing during the ground icing operation 

demonstration of § 33.68(b), then this run-up must be documented. 

Documentation must contain a description of the run-up requirements and the 

required run-up intervals and it must be contained in the operating manual and 

airworthiness limitations section of the instructions for continued airworthiness 

(ICA). Any power loss anomalies due to accumulation, shed, runback and the 

like, and their effects on performance and operation should be documented in the 

Installation Manual. Both the engine certifying ACO and the installing ACO 

should carefully consider any high vibrations induced from ice accretions during 

ice testing. This too should be documented as described above. 

 c. Engine systems. It is permissible to use engine systems (that is, 

automatic, engine initiated ice protection systems) to fulfill § 33.68 requirements 

provided that its operation is not expected to result in crew action. Examples of 

engine characteristics that may not be transparent to the flight crew are exhaust 

gas temp fluctuations, or audible surging. 

Any engine control system function which may be acceptable and required for 

engine ice protection certification in part 33 should not create an adverse 

interaction with other aircraft systems, aircraft handling qualities and 

performance, and human factors considerations. Any unacceptable interactions 

with the airplane may result in the engine being deemed un-installable for part 25 

operation. 

Crew interface, uncommanded thrust changes, thrust setting, asymmetric engine 

behavior, pilot workload and appropriate flight deck indication and procedures, 

the effect on airplane handling, pilotability, and human factors are critical issues 

which must be addressed. 

Additionally, any engine system required to show compliance with § 33.68 should 

meet the following requirements: 

 (1) System reliability. Demonstrate the capability of the system for reliably 

sensing the conditions, which enables the function, throughout the operating 

envelope; 

 (2) Dispatch. The function should be available for all dispatchable 

configurations. The system should be configured in its most critical dispatch 

state for certification icing tests; 

 (3) Electronic faults. If the system uses electronics, substantiate that the 

function is not lost due to any single or probable multiple electronic faults; 

 (4) Other environmental testing. The function should not be affected when 

the system and any associated electronic systems are exposed to required 
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operating environments, including high intensity radiated fields (HIRF) and 

lightning; and 

 (5) Power requirements. For those systems that are powered solely with a 

dedicated engine alternator (either directly or using another engine system such 

as full authority digital electronic/engine control (FADEC)), it should be 

demonstrated that over the operating envelope, that the function (that is, sensing 

and performance) is provided at the minimum certified rotor speeds. Minimum 

certified engine speed is the minimum idle rotor speed achievable anywhere in 

the icing envelope. 

 d. Auto-recovery systems. Auto-recovery systems should not be needed 

during § 33.68 testing since these icing conditions are considered to be within the 

engine’s certified operational envelope. The intent of § 33.68 is to certify engines 

that will be able to perform and operate reliably in the icing conditions described 

Appendix C, Appendix X and Appendix D Auto-recovery systems are considered 

to be back-up devices that are only needed following rare ice ingestion events 

that result from icing conditions significantly outside Appendix C, Appendix X and 

Appendix D, and should be communicated to the installing ACO if activation is 

expected or experienced in these rare occasions. Auto-recovery systems are not 

the primary protection for continued safe engine operation during normal ice 

sheds, or accretion while operating in icing conditions described in Appendix C, 

Appendix X and Appendix D. Therefore, it is acceptable to perform § 33.68 

compliance testing with auto-recovery systems enabled, but they should not 

activate throughout the § 33.68 test sequence. Additionally, continuous ignition 

should not be selected during § 33.68 compliance testing. To assure nonactivation of an enabled auto-recovery system, it may be necessary to display an 

instrumented signal that monitors auto-recovery system activation. If activation 

monitoring cannot be accomplished, then disabling of the auto-recovery system 

may be necessary. 

 e. Operating instructions. Any operating procedure (for example, ground runup procedures) required to ensure continued operational compliance with ground 

icing conditions or falling and blowing snow evaluated under § 33.68(d), and 

§§23.1093(b)(ii), or 25.1093(b)(2), should be communicated to the installer in the 

Operating Instructions as a requirement, and should be included in the limitation 

section of the airplane flight manual. It may be necessary to coordinate with the 

installer on these procedures to ensure that they can be effectively implemented 

in-service. 
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 f. Special Considerations for Mixed Phase / Ice Crystals

14-CFR part 33 requirements for the type certification approval of engines 

operating in an ice crystal environment are located in section 33.68 and 

Appendix D of Part 33. These requirements have been developed in response to 

service events. The root cause of these events can be traced to ice buildup 

within the core flow paths of the affected engines. Adverse effects of this type of 

icing condition on the engine include uncommanded rollback of power or 

flameout as well as compressor stall and core hardware damage. This section 

provides guidance to applicants for new engine type approvals to address the 

issue of engine operation in an ice crystal environment defined in Appendix D of 

Part 33. 

Ice crystals have only been recognized as a threat to turbine engines in recent 

years. In response to this recent recognition, the FAA has worked within a crossfunctional industry/governmental group to develop standards to address this 

threat. During this rule and guidance development process, it is noted that ice 

crystal tools and test techniques have not yet been developed and validated, 

thus a phased-in approach has been developed to address this ice crystal issue 

during the conduct of the engine type certification program. 

In the near term, new engines must be shown to address the known field issues 

in ice crystal environments (core damage and engine flameouts). Until ice crystal 

tools and test techniques have been developed and validated, the engine 

manufacturer should use a comparative analysis to specific field events. This 

analysis approach should show that the new engine cycle and/or design features 

will result in acceptable engine operation. 

Acceptable operation would include the absence of rollback, rundown, stall, 

flameout, and unacceptable compressor blade damage as described under 

paragraph (b) of this section. 

Additionally, the manufacturer should show progress towards incorporating 

available technology and be driving towards a thorough substantiation for 

operation in ice crystals. 

Long-term, an acceptable demonstration would include a Critical Point Analysis 

of an ice crystal environment as defined in Appendix D of Part 33. All engine 

power levels including in-flight idle operation should be evaluated in these 

conditions. The critical conditions should be demonstrated to the FAA through a 

combination of testing and validated analysis using the latest tools and 

technology when proposing the compliance methodology. Computational tools 

used in this analysis should be calibrated by either rig calibration test data or 

engine test measurements. 

 
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Comparative Analysis Guidance 

As stated earlier, until ice crystal tools and test techniques have been developed 

and validated, the engine manufacturer should use a comparative analysis to 

specific field events. This analysis must show that the new engine cycle and/or 

design features will result in acceptable engine operation, when subjected to the 

ice crystal environment defined in Appendix D of Part 33. This comparative 

analysis should take into account both suspected susceptible design features as 

well as mitigating design features. 

Susceptible design features could include such features as: 

• Stagnation points which could provide an increased accretion potential. 

• Exposed core entrance (as opposed to hidden core). 

• High turning rates in the inlet, in the booster and core flow path 

(particularly compound turning elements) 

• Protrusions into the core flow path (e.g. bleed door edges and 

measurement probes) 

• Unheated surfaces on booster and front core stages. 

• Narrow vane-to-vane circumferential stator spacing leading to a small 

stator passage hydraulic diameter 

These susceptible design features can be significantly mitigated by one or more 

of the following design features. Mitigating design features could include: 

• Heated surfaces in the fan, booster and forward core compressor stages. 

• Elevated rotor speeds. 

• Hidden core entrance. 

• Low frontal cross-sectional area on flowpath probes. 

• Inter-compressor bleed scheduling to remove both the ice crystal media 

and any up-stream shedding, from the flowpath. 

• Circumferential spacing of stators set to enhance tolerance to ice 

blockage (generally denoted by the hydraulic diameter of the stator 

passage) 

• Core compressor airfoil soft body damage tolerance 

 (i). Alternate method of compliance by similarity to engines proven safe to 

operate in mixed phase or ice crystal icing conditions: Although it has been 

established that mixed phase or ice crystal icing conditions are hazardous to 

turbine engine operation, severe incident involving this type of meteorological 

condition is not common. Many currently certified engine designs have been 

proven by their field service experience to be safe to operate in these conditions. 

New engine design, similar to those proven engines, is allowed to show 

compliance by comparative analysis. 
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Firstly, the baseline (certified) engine should be identified and evidence that this 

engine is safe to operate in mixed phase or glaciated icing condition should be 

supplied by the applicant. This evidence can be field service experience or 

certification test report. Next, the icing pertinent engine features that may 

influence mixed phase or glaciated ice accretion within the target (certifying) 

engine should be compared to the baseline engine to establish that the former is 

less or equally susceptible to icing than the latter. 

Lastly, a critical point analysis (CPA) should be performed to show that the 

operation and flight envelope of the target engine does not make it more 

susceptible to mixed phase or ice crystal icing than the baseline engine. This 

analysis should consider both environmental and engine operational effects on 

accumulation, accretion locations, as well as the most critical engine operating 

conditions for ice shedding. 

If the new engine cycle and/or design features contain innovative design ideas 

such that a comparative analysis with current engines and specific engine events 

is not possible, then the applicant should demonstrate that this innovative feature 

will not be susceptible to the adverse effects of operating in ice crystal 

environment in a two parts process. Part 1 is to document the physical basis on 

why this design should result in acceptable operation in ice crystals. Part 2 is to 

generate physical evidence to substantiate the claims in Part 1. In the near term, 

before ice crystal tools and test techniques are developed and validated, water 

particles may be used in lieu of ice crystals in Part 2 of this process. However, 

data collected with water particles should be corrected to ice crystals 

An example of steps that may be used in this process are: 

- Part 1 

• Discuss consequences of ice accretion 

• Identify design features that promote tolerance to critical ice accretion 

• Discuss consequences of ice crystals on promotion of ice accretion 

-Part 2 

• Identify test article (engine or component) 

• Establish test conditions promoting ice accretion (with allowance for use of 

liquid water in the near term if testing with ice crystals is not feasible) 

• Evaluate tolerance of new design features over a range of test points 

 (ii). Compliance Considerations 

Engine icing events in atmospheric conditions associated with convective clouds 

and a mixed phase/ice crystal environment appear to be associated with 

accretion of ice in regions of the compressor which are further aft than typical for 

supercooled droplet environments. Understanding of this process is limited, 

however. It is expected that, as the ‘state-of-the-art’ progresses in these areas, 
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the applicants can utilize advanced methods to support compliance for engine 

operation in Mixed Phase/Ice crystal icing conditions. 

The accretion process is believed to be a result of ice crystals passing through 

the front compressor stages until the local atmosphere is conducive to forming a 

liquid layer on blade surfaces which then allows impinging crystals to melt and 

accrete on the surface. This is a combination of conductive & convective heat 

transfer due to melting, evaporation and surface contact. The form of ice can be 

rime or glaze, this is dictated by local temperature/pressure conditions and thus a 

function of power setting and ambient atmospheric conditions combined with 

specific design details. The applicant should use FAR33 Appendix D to define a 

range of operation to be assessed for potential accretion sites. 

(iii). Mixed Phase / Ice Crystals Assessment

Applicant should consider the whole icing envelope, to include cruise, hold and 

descent power settings in mixed phase / ice crystal conditions defined in 

Appendix D. There is currently no established critical point analysis in Mixed 

Phase / Ice crystal icing conditions. To aid the applicant, some possible CPA 

point selections are illustrated in this section as an example. Some general 

criteria for selecting points includes: 

• Service history suggests selection of points at high ambient temperatures 

within the Appendix D icing envelope consistent with higher TWC levels . 

Lower temperatures might push the accretion aft, where there may be a 

more critical ice accretion site, even with lower TWCs. Hence, both high 

and low temperatures within the envelope should be evaluated. 

• Power levels with internal total air temperature within the core flow path 

between 0 deg C ( 32 deg F) and approximately 50 deg (120 deg F) to 

promote ice crystal melting. Power level adjusts the accretion site forward 

or aft within the engine. 

• High altitude (low air density) to allow greater ice accretion mass prior to 

shedding from static surfaces. 

The operating points for this evaluation are depicted on the Mixed Phase / Ice 

crystal icing envelope from FAR Part 33 Appendix D in the following two figures. 

They cover the various flight phases including cruise, idle descent and holding. 

The water contents shown in Figure 2 represent the level for the standard 

exposure distance of 17.4 nautical miles and must be adjusted to reflect the 

expected icing exposure period using the distance scale factor from Appendix D. 

Service experience suggests that exposure distances in the range of 20 to 80 

nautical miles are appropriate. 

As noted earlier, ice crystals promote icing at engine sites more rearward and at 

higher local air temperatures than would exist with only supercooled liquid. 

These sites also will have higher air loads which can limit ice accretion due to 
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shedding. Hence, as noted in the general criteria, the CPA should consider not 

only ice accretion, but also the likelihood of ice shedding. 

Icing Envelope Limits

-70

-60

-50

-40

-30

-20

-10

0

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000

Altitude - ft

Ambient Temperature - deg C

Appendix D Cruise

Idle Descent Hold

Figure 1. Appendix D Icing Envelope 
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TWC Levels: Standard Exposure Length of 17.4 Nautical Miles

(Scaled from Adiabatic Lapse from Sea Level @ 90% Relative Humidity) 

Legend : Ambient Temperature

0

1

2

3

4

5

6

0 5 10 15 20 25 30 35 40 45 50

Altitude - K ft

TWC - gms/m3

- 60 C - 50 C - 40 C - 30 C

- 20 C - 10 C 0 C Upper App D B'ndy

Lower App D B'ndy Cruise Idle Descent Holding

Figure 2. Appendix D Icing Total Water Content 

 
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12. APPENDIX X WATER IMPINGEMENT. The water impingement region 

becomes potentially greater for the large water droplets of Appendix X. This 

occurs because larger droplets have greater inertia and follow a more ballistic 

trajectory that is less influenced by local airflow streamlines. A way to illustrate 

this is through the use of a variable known as the modified inertia parameter. 

This parameter is a relationship based upon the momentum of droplet, air 

viscosity and size scale of object (with correction for Reynolds No) and is used to 

correlate the streamline effect on deflecting a droplet from the object. Such a 

correlation is depicted in the next figure for several object shapes. 

Figure 3. Droplet impingement efficiency for different shapes 

Airfoils
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The reduction in impingement efficiency below unity from the correlation indicates the 

degree to which droplets are deflected from the object due to streamlines moving 

around the object. For most shapes a modified inertial parameter of 10 or greater 

implies a ballistic path for the droplets. This observation can be used to illustrate the 

consequences of the larger water droplets defined in Appendix X using the next figure 

with some target objects of interest denoted to show typical size ranges for larger 

transport engines. The shaded area covering the Appendix C droplet size range is seen 

to behave ballistically for small “target” object dimensions typical of engine airfoils, airfoil 

cascades and probes. Consequently, Appendix X will not alter this local behavior 

affecting the surface water impingement. For larger targets, however, one would expect 

greater impingement on surfaces such as the nacelle and engine spinner with the 

potential for water impingement on regions that are unprotected by surface heating 

systems that are designed for Appendix C sized droplets. 

Figure 4. Ballistic boundary as function of target and droplet size 

10 100 1000 10000

Droplet Diameter - microns

Airfoils

Probes

Nacelle 

Lip

Spinner

Inlet 

Diameter

Cascades

.01

.1

1

10

“Target” Dimension - ft

Ballistic (Minimal 

Deflection by 

Streamlines)

Locus of Modified Inertia Parameter = 10

Droplet 

Size 

Range 

of 

Appdx C
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SECTION 2. ICE SLAB INGESTION (§ 33.77)

13. INTENT OF ICE SLAB INGESTION TEST. The intent of the ice slab ingestion test 

is to demonstrate tolerance to ice ingestion resulting from nacelle surfaces and to 

establish limits for ice released from other aircraft surfaces in FAR 25 certification. It is 

intended that the engine manufacturer will conservatively consider the potential 

installation effects of the engine induction system. Also, there should be close 

coordination with the installer to ensure that potential airframe ice accumulation sites 

that can result in ice ingestion by the engine (for example, inboard section of wing for an 

aft fuselage mounted engine) are either demonstrated under § 33.77 or addressed 

under §§ 23.901(d)(2), 23.1093, or 25.1093 (see paragraph within this AC). The 

induction system manufacturer or installer should assess these accumulations in 

accordance with §§ 23.901(d)(2), 23.1093, or 25.1093 and provide pertinent test 

variables to the engine manufacturer for incorporation into a test demonstration in 

accordance with § 33.77. In the case where an application or product inlet has not been 

selected at the time of engine certification, the engine manufacturer should provide all 

pertinent inlet assumptions and test data and results in the engine installation manual 

for use by the future installer. The dimensions of this slab are related to engine size 

(defined by inlet highlight area) based on earlier certification requirements which 

showed satisfactory performance in service. 

14. COMPLIANCE CONSIDERATIONS. Compliance may be demonstrated by the 

standard engine ice slab test or by means of a validated analysis procedure that uses 

equivalent soft body testing. The test demonstration should consider ice slab sizes and 

trajectories aimed at critical engine locations that are based on the ice accretion and 

shed characteristics of the induction system that is likely to be installed on the engine. 

Lacking such specific knowledge, the applicant may select test conditions, which are 

typical of a condition for a representative installation in-service. The ice slab size, 

thickness, and density used in § 33.77 compliance demonstration should be assessed 

for appropriateness against parts 23 and 25 compliance requirements. Minimum ice 

slab density equivalent to a 0.9 specific gravity should be used unless a different value 

is considered more appropriate. See AC 20-73 (latest revision), AC 23.1419-2B, and 

AC 25.1419-1 for more guidance on ice shedding. If it is determined that the ice slab 

size, thickness, and density are appropriate for the engine installation, then the part 33 

test results can often be used by the airframe manufacturer to comply with the natural 

icing flight test requirements. Execution of the ice slab ingestion test typically involves 

targeting the slab to the air stream ahead of the fan at the outer diameter of the inlet 

duct. This is intended to mimic the ice release from the inlet and results in impact on 

the outer diameter of the fan. 

a. Validated Analysis with Equivalent Soft Body Tolerance Testing 

The alternate compliance method allows use of a validated analysis process in 

conjunction with appropriate soft body damage testing. If the applicant satisfies the 
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requirements of this alternate compliance approach, the engine shall be certified for the 

minimum standard ice slab consistent with the engine inlet area as defined in § 33.77. 

It is recognized that alternate soft body damage testing may in some circumstances 

include objects that are larger than the standard ice slab based upon inlet area. 

Certification for larger than the standard ice slab by this method is currently not allowed. 

However, it is expected that, as the experience progresses in these areas, the 

applicants can utilize this method to support compliance for engine operation with larger 

ice slabs. 

Validated Analysis 

A validated analysis must contain sufficient elements to show compliance, these 

elements may include: 

– Full fan blade model utilizing latest techniques such as finite element analysis 

– Blade material properties for yield and/or failure as appropriate 

– Dynamic / time variant capability 

– Thrust variance prediction if required to account for blade damage 

– Appropriate engine and/or component testing with impact in outer 1/3 span location 

to anchor results 

This model may be used alone, or in conjunction with the results of a certification bird 

ingestion test to show compliance. 

The analysis of the ice slab impact on the fan must properly account for critical 

controlling parameters: 

- Relative kinetic energy normal to the leading edge chord 

- Incidence angle – relative slab speed & blade speed 

- Slab dimensions 

- Slab orientation 

Any predicted power loss or blade damage (distortion, cracking, tearing) must be 

assessed against the criteria of paragraph 15(a) below 

The following figures help to describe the contribution of these parameters in 

establishing the threat to the fan blade. 
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Figure 5. Normal component of Kinetic Energy of ice slab 

Figure 6. Ice slab orientation effects 

The relative kinetic energy of the ice slab shall be determined from an assessment of 

flight conditions that control the engine rotor speed and the velocity of the ice slab as 

influenced by the air stream velocity and density at the corresponding “ice slab test 

Dependent on orientation at fan face the “bite” can be 

increased substantially

Load shared by 6-7 blades

Load shared by 3 blades

rotation

slab

Relative KE 

component 

normal to 

leading edge is 

critical
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point”. Engine test results from previous ice slab testing may be provided to support the 

predicted ice slab velocity.

The analysis shall assume the most critical orientation unless the manufacturer can 

substantiate that an alternate ice slab orientation is still conservative relative to ice slab 

testing. 

Ice Slab Break Up 

The analysis shall utilize the largest slab size consistent with a conservative 

assessment of slab “break up” that can occur within the air stream ahead of the fan. 

Historically, the ice slab breaks up into smaller pieces during the ice slab testing. Data 

derived from a number of tests are included in the next figure. On the basis of the data, 

the largest ice piece is typically 1/3 to 1/2 the original size. For compliance purposes, 

the applicant should assume 1/2 of the original slab length unless evidence suggests 

that this is not conservative relative to ice slab testing. 

Largest Ice Piece Size After Breakup

(Initial Ice Piece Length = 12 inches)

0

1

2

3

4

5

6

7

01234567

Ice Width - inch

Ice Length - inch

Piece Size

Figure 7. Ice slab breakup experience 

 

15. TEST RESULTS. Section 33.77(c) requires that the ingestion of ice, under the 

conditions stipulated in § 33.77(e), may not cause a sustained power or thrust loss, or 

require the engine to be shutdown. The following criteria should be met: 
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 a. Sustained power losses. Any fan blade bending or damage should be assessed 

against potential sustained power loss. Compliance requires that permanent power loss 

associated with fan damage from the slab be less than 1.5%. The similarity of fan 

damage type allows the use of other soft-body testing, such as the medium bird 

ingestion test, results for compliance with this requirement. If the other soft-body test 

results in less than 1.5-% permanent power loss and there are no cracks, tears or blade 

piece breakout due to a bird introduced at the outer 33% of fan diameter, the 33.77 

requirement is met. In the event that power loss exceeds 1.5%, the manufacturer must 

provide a validated analysis that shows consistency with the bird test results and 

verifies that the ice slab would produce less than the 1.5-% power loss. It is also 

necessary to demonstrate by test that any cracks, tears or blade piece breakout will not 

result in “unacceptable sustained power or thrust loss” within 100 cycles (considered 

sufficient to allow engine operation to the next scheduled “A” check). Further, any 

damage that results from this test must be documented within the engine installation 

manual. 

 b. Engine operability. Damage should not adversely affect engine operability (that 

is, should not cause surge, flameout, nor prevent transient operation). 

 c. In-service capability. Damage should not result in a failure or a performance loss 

that would prevent continued safe operation for a conservative flight cycle scenario (for 

example, within fly back limits or greater if appropriate testing is done to validate a 

continued period of in-service capability). The period of in-service capability to be 

demonstrated may vary with installation if the damage is not readily evident to the crew 

or visible on preflight inspection (for example, tail mounted positions). 

 d. Other anomalies. Damage should not result in any other anomaly (for example, 

vibration) that may cause the engine to exceed operating or structural limitations. 

 e. Auto-recovery systems. If during § 33.77 ice slab ingestion testing, an engine 

does incur a momentary flameout and auto-relight, then normally the acceptance of that 

test would be predicated on the inclusion of the auto-relight system as being a required 

part of the engines type design, and an additional dispatch criteria would be required, 

where the ignition system must be functional (that is, fully operable) prior to each 

dispatch. The reason for the additional dispatch criteria is to ensure the ignition 

system’s critical relight function is reliably available during the subsequent flight. The 

reason for the allowance of auto-recovery systems during § 33.77 certification testing is 

to account for ice accretion and shedding, as a result of an inadvertent 2-minute delay 

in actuating the anti-icing system, which is considered to be an abnormal operational 

result where mild operability effects (for example, momentary flameout and relight) may 

be accepted. 

16. COMMUNICATION OF TEST RESULTS. The installation and operating 

instructions required by § 33.5 should provide information on the size, thickness, and 

density of the ice slab ingested, any anomalous behavior such as high vibrations and 

any affect on the engines ability to operate at the commanded power setting or rating. 
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The icing certification report should include information regarding ice slab orientation 

and trajectories, slab breakup, impact locations, description of any resultant damage, 

and any other pertinent data defining the engine's capability or response to the ice 

ingestion event. Additionally, if the auto-recovery system is required to comply with § 

33.77, then the functional state of the recovery system (for example, one igniter 

inoperative) becomes a limitation that needs to be communicated to the installer to 

ensure compliance with the delayed activation requirements of §§ 23.1093 or 25.1093. 

SECTION 3. INDUCTION SYSTEM ICING PROTECTION (§§ 23.1093 and 25.1093)

Discussion 

In recent years, applicants have been using the results of FAR33.77 for compliance with 

25.1093 in lieu of a test demonstration of the 2 minute delayed activation of inlet antiice. The 33.77 results are also used to show compliance with 25.1093 for other 

airframe ice sources. This requires close coordination between the engine 

manufacturer, and the airframer to make sure the 33.77 test covers all the potential ice 

sources. Further, this close coordination needs to take place as the engine is certified 

for 33.68 so that the engine can be installed on the airframe – see paragraph 11 (c) of 

this AC for more definition of potential incompatibilities. 

Section 25.1093 (b)(1) Acceptable Means of Compliance 

Engine induction systems have historically not been limited to or evaluated against a 

specific aircraft flight profile when considering icing environments, but instead are 

evaluated for unlimited operation in icing. The cloud horizontal extent factor was not 

intended to be used to limit the severity of exposure to icing conditions where it is 

reasonable to assume the aircraft will be required to operate in that condition. As a 

general rule, engine induction systems should be shown to operate continuously in icing 

without regard to time in icing conditions. The only exception would be for low engine 

power conditions where sustained level flight is not possible. Even then, a conservative 

approach must be used where multiple horizontal and vertical cloud extents in series 

are assumed. 

The applicant must adequately analyze the performance of the engine inlet anti-ice 

system and address the potential ingestion hazard to the engine from any predicted ice 

buildup on the engine inlet, including any runback or lip ice. It must be shown by 

analysis, and verified by test, that the engine inlet anti-ice system provides adequate 

protection under all flight operations. The following conditions must be considered. 

Additional critical points may be identified, depending on the specifics of the 

airplane/engine design. Bleed crossover points need to be reviewed, if applicable. 

If an applicant can show that the inlet anti-ice system performance and the fan blade 

capability are equivalent to previous certification experience, then certification may be 

shown by similarity to previous designs. 
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Inlet Design Point Selection 

If the inlet is evaporative under the critical points in continuous maximum icing 

conditions, and is running wet under intermittent maximum icing conditions, then the 

design is satisfactory. 

Engine inlet anti-ice systems have historically good service experience with systems 

that run wet, when runback ice is evaluated at the conditions defined in Table A at the 

ambient conditions that promote maximum runback. Therefore, inlets that are certified 

for unlimited operation in icing may allow runback ice formation as defined in table A 

during hold, descent and diversions. Analysis must be used to find the critical accretion 

conditions for each of the scenarios in Table A and each must be compared individually 

with the amount of ice the engine has been satisfactorily demonstrated to ingest during 

engine certification (§ 33.77(c)). 

Flight 

Condition 

Descent Hold Straight line flight 

Icing Design 

Condition 

6500 foot 

descent in 

Appendix C 

continuous 

maximum, 

with extent 

factor = 1, 

followed by an 

Appendix C 

intermittent 

maximum 

exposure 

45 minute hold, 

in Appendix C 

continuous 

maximum, with 

an extent factor 

= 1 

45 minute exposure in Appendix 

C continuous maximum with an 

extent factor, followed by an 

Appendix C intermittent maximum 

exposure 

Table A Inlet Lip and Runback Ice 

Specifically for the design conditions defined in Table A: 

Descent: 

If the engine inlet anti-ice system is not fully evaporative during descent in Appendix C 

continuous maximum conditions, the amount of ice accretion (runback and lip) must be 

calculated for the continuous maximum and intermittent maximum icing conditions 

within Appendix C as defined in Table A. Airspeed and scoop factor should be part of 

this assessment. Ingestion of these calculated quantities of ice runback for that 

condition (plus lip ice for this condition, if any) must not result in more ice ingestion 

damage, based on the criteria defined below (size or kinetic energy), than the amount of 

ice the engine has been satisfactorily demonstrated to ingest during engine certification 

(§ 33.77(c)). 

Holding: 

For holding conditions the engine inlet anti-icing system must be capable of safe 

operation in Appendix C for extended airplane operations. The extended airplane 
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holding condition is defined as 45 minutes at the critical accretion ice conditions for the 

continuous maximum icing cloud using a horizontal extent factor of one. . If the inlet is 

evaporative under continuous maximum icing conditions, and is running wet under 

intermittent maximum icing conditions, then the design is satisfactory; this is because 

the descent condition provides less power to the anti-ice system and will always 

become the critical accretion condition in intermittent maximum. If the inlet is running 

wet in a maximum continuous atmospheric condition, then the applicant should 

calculate the amount of ice that would accumulate during the holding conditions defined 

in Table A. Ingestion of these calculated quantities of ice (total amount of ice: runback 

and lip for the condition)) must not result in more ice ingestion damage, based on the 

criteria defined below (size or kinetic energy), than the amount of ice the engine has 

been satisfactorily demonstrated to ingest during engine certification (§ 33.77(c)). 

Straight Line Flight 

For straight line flight (cruise, diversion to alternate airport, etc.) the engine inlet antiicing system must be capable of safe operation in Appendix C for extended airplane 

operations. The straight line flight evaluation must be investigated including the use of 

the extent factor. If the inlet is evaporative under continuous maximum icing conditions, 

and is running wet under intermittent maximum icing conditions, then the design is 

satisfactory. If the inlet is running wet in a maximum continuous atmospheric condition, 

then the applicant should calculate the amount of ice that would accumulate during the 

straight line flight conditions defined in Table A. (continuous maximum exposure 

followed by an intermittent maximum exposure). Ingestion of these calculated 

quantities of ice (total amount of ice: runback and lip for the condition)) must not result 

in more ice ingestion damage, based on the criteria defined below (size or kinetic 

energy), than the amount of ice the engine has been satisfactorily demonstrated to 

ingest during engine certification (§ 33.77(c)). 

2 Minute Delayed Selection of Inlet Anti-ice Accretion Analysis

Inlet lip ice can form as the result of a 2-minute delayed activation of the engine inlet 

anti-ice system. 

The latest FAR33.77 testing criteria were developed to account for historical means of 

compliance for the 2-minute delayed selection of inlet anti-ice. Therefore for traditional 

pitot-style inlets the applicant does not need to consider this scenario if the applicant 

shows compliance with the FAR33.77 (amendment level defining minimum ice slab 

size). For inlet designs other than the traditional pitot style inlets, 2 minute delay 

calculation may be required. The amount of inlet lip ice that forms during the 2-minute 

delayed activation should be calculated using Appendix C continuous maximum 

conditions with an extent factor of one. Of the total lip ice, only the ice on the inner 

barrel side of the stagnation point would be ingested into the engine. Further it may be 

assumed that 1/3 of the inlet perimeter is ingested as one piece, consistent with the 

historical approach taken by the engine manufacturers. Since maximum damage to fan 

would occur at high fan speed, and this critical condition is also when the inlet has the 
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most heat, therefore 2 minutes is a reasonable time to include both pilot reaction to 

conditions and time to shedding after anti-ice has been selected. 

ETOPS 

If certification of ETOPS is desired, the applicant must consider the maximum ETOPS 

diversion scenarios. For example, if a lightweight two-engine diversion at 10,000ft 

following a cabin depressurization puts the airplane into a region that is susceptible to 

ice build-up, then ingestion of the calculated quantities of ice must not result in more ice 

ingestion damage, based on the criteria below (size or kinetic energy) than the amount 

of ice the engine has been satisfactorily demonstrated to ingest during engine 

certification (§ 33.77(c)). 

17. AIRFRAME ICE SOURCES INCLUDING INLET 

It is intended that the engine manufacturer will conservatively consider the potential 

installation effects of the engine induction system. Also, there should be close 

coordination with the installer to ensure that potential airframe ice accumulation sites 

that can result in ice ingestion by the engine (for example, inboard section of wing for an 

aft fuselage mounted engine) are either demonstrated under § 33.77 or addressed 

under §§ 23.901(d)(2), 23.1093, or 25.1093 (see paragraph within this AC). The 

induction system manufacturer or installer should assess these accumulations in 

accordance with §§ 23.901(d)(2), 23.1093, or 25.1093 and provide pertinent test 

variables to the engine manufacturer for incorporation into a test demonstration in 

accordance with § 33.77. In the case where an application or product inlet has not been 

selected at the time of engine certification, the engine manufacturer should provide all 

pertinent inlet assumptions and test data and results in the engine installation manual 

for use by the future installer. 

It is normally sufficient to show that the ice from these sites is smaller in size than the 

FAR 33.77 ice slab. The applicant may also elect to compare the ice on the basis of the 

kinetic energy of the ice slab. 

Kinetic energy may be used as an acceptable method for comparing the airframe ice 

source to the results of the FAR 33.77 test. Any kinetic energy method must be agreed 

to by the applicant’s Aircraft Certification Office. 

 

Wing Sourced Ice for Aft Mounted Engines

Clear ice may occur on the wing upper surfaces when cold-soaked fuel (due to aircraft 

prolonged operation at high altitude) is in contact with the fuel tanks’ upper surfaces, 

and the airplane is exposed to conditions of atmospheric moisture (fog, precipitation, 

and condensation of humid air) at ambient temperatures above freezing. This 

atmospheric moisture, when in contact with cold wing surfaces, may freeze. 

Simultaneous ice shedding from both wings of an airplane with aft-mounted engines has 

resulted in ice ingestion in both engines during takeoff. 

In cases where recommended fuel management procedures allow wing fuel tanks to 

remain basically full until the fuel in fuselage tanks has been consumed, clear ice may 
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form on cold wing surfaces during some operations. Moreover, some past analysis has 

shown the potential for ice formation on the upper surfaces when the wing tanks are 

more than 70% full. Therefore, for some operations fuel in the wing tanks could remain 

above the 70% level for several hours. 

Considering only aircraft with aft-mounted engines, equipped with wing mounted ice 

detection systems, the applicant must demonstrate that any ice mass resulting from 

cold-soaked fuel undetected by the wing ice detection system is not greater than that 

used to demonstrate compliance of the engine induction system to 14 CFR §§ 25.1093. 

Also, it must be demonstrated that shedding of the ice resulting from cold-soaked fuel 

does not result in hazardous engine operation. Consider that a wing mounted ice 

detection system advises the presence of clear ice build-up on the upper surface of the 

wings when the clear ice thickness reaches approximately 0.020 inches. For this 

example, the worst case to consider would be a 0.020 inch thickness of clear ice sheds 

from the wing, and the largest clear ice plate area that will be ingested into an engine is 

the same area as the engine’s inlet highlight area. 

18. COMPLIANCE WITH APPENDIX D of PART 33 

The FAA has conducted mixed phase icing testing on an airfoil, Ref (Q). The results of 

these tests indicate that for unheated aircraft surfaces, a mixed phase environment 

results in the same or less ice accretion than the same total water content of 

supercooled liquid water. The overall power required by the running-wet ice protection 

system was practically unchanged between all-liquid and mixed-phase conditions. 

However, in the running-wet mode of operation, the local power density was much 

higher around the stagnation area in the mixed-phase conditions compared to the 

purely liquid conditions. This is a result of the power required to offset the heat of fusion 

necessary to melt the impacting ice particles that either fully or partially stick to the 

surface. 

This may explain why engines with ‘pitot’ style inlets have not proved to be susceptible 

to mixed phase icing and the Appendix C compliance methods adequately address 

these installations for operation in Appendix D of Part 33. Engines designed with 

reverse flow intakes, or with intakes involving considerable changes in airflow direction, 

must comply with Appendix D of Part 33. Compliance for pitot-style inlets, without 

considerable changes in airflow direction, may be shown through qualitative analysis of 

the design and supported by similarity to previous design that have shown successful 

service history. 

19. COMPLIANCE WITH APPENDIX X of PART 25 

In-flight Exposure to Appendix X of Part 25 Conditions: 

For engine inlets, an assessment of inlet impingement limits, differing catch efficiency, 

distribution effects, and water contents for Appendix X should be accomplished. 

Ground Taxi Exposure to Appendix X of Part 25 Conditions: Service experience 

considered for this advisory material indicates that there are engine fan damage events 
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as a result of exposure to SLD during ground taxi operations. For this reason, an 

additional compliance condition was added to 25.1093 for a 30 minute, idle power 

exposure to SLD on the ground. Consideration should be given to terminal falling 

velocity of SLD (freezing rain, freezing drizzle) in assessing the trajectory relative to the 

protected sections of the inlet. The 100 micron minimum MED was defined as a 

reasonably achievable test condition given current technology. An applicant choosing 

to certify by analysis should evaluate the Appendix X drop sizes up to a maximum of 

3000 microns particle size to find a critical condition. 

20. NATURAL ICING FLIGHT TESTS. Natural icing flight tests are intended to 

demonstrate that each turbine engine is capable of operating throughout the flight 

power range of the engine (including idling), without the accumulation of ice on the 

engine, inlet system components, or airframe components that would have an adverse 

affect on engine operation or cause a serious loss of power or thrust. Based on multiple 

engine natural ice damage and operability events experienced on natural icing flight test 

and in-service airplanes, the FAA requires natural ice encounters for showing 

compliance with §§ 23.1093(b)(1) or 25.1093(b)(1). However, for airplanes that are not 

intended to be certificated for flight in icing conditions, the FAA will accept other 

methods in lieu of natural icing flight tests, such as analysis, ground testing, dry air flight 

testing, and similarity, to show compliance to § 23.1093(b)(1). Aside from the benefit of 

validating the engine inlet icing analysis model, there are several other key issues that 

the natural ice encounter addresses. These evaluations include: (1) The adequacy of 

the flight crew procedures for operation in icing conditions, (2) acceptability of tactile 

inputs to the flight crew as the airplane responds to engine fan blade ice shedding 

during a variety of airplane operating conditions, (3) performance of the engine vibration 

indication system as well as other engine indication systems and, (4) confirmation that 

the powerplant installation as a whole (for example, engine, inlet, anti-ice system) 

performs satisfactorily while in icing conditions. 

a. Identification of ice source. A means should be provided to aid in identifying the 

source of any ice that may be ingested by the engine during the natural icing 

certification testing. Special attention should be given to non-representative ice 

accretions on flight test instrumentation probes or other surfaces forward of the engine 

during prolonged operation in icing conditions. 

 b. Icing test point monitoring. The applicant should provide sufficient monitoring of 

the icing test point condition (that is, LWC, droplet diameter, temperature) against the 

time to ensure that the icing encounter is representative of the Appendix C and 

Appendix X certification icing conditions under part 25 or the Appendix D conditions 

under part 33. 

 

 c. Compliance. Compliance with §§ 23.1093 or 25.1093 is required even if flight into 

icing approval (§§ 23.1419 or 25.1419 compliance) is not obtained. Compliance with 

the natural ice encounter criteria should be proposed by the applicant and agreed to by 

the FAA prior to the test. However, typically an adequate test sequence includes three 
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natural fan ice shed cycles at each of the following conditions (with inlet anti-ice turned 

"on"); descent (flight-idle), holding (power necessary to maintain level flight for a range 

of anticipated airplane gross weight conditions), and maximum climb, unless a more 

critical engine power setting exists. These encounters should be conducted at a steady 

state engine thrust level and although not preferred, sometimes have involved flying 

through the same icing cloud multiple times (lapping) in order for the fan to accumulate 

enough ice for a shed cycle to occur. (Caution is emphasized when flying in natural ice 

conditions.) These fan shed cycles should be due to natural ice accumulation and not 

induced or forced by throttle excursions or manipulations, or both, during each 

condition. It has also been allowed for the airplane to exit the icing conditions between 

each fan shed cycle for the purpose of clearing any other unprotected airplane surfaces 

from ice. To avoid masking any adverse engine operating conditions during the natural 

icing encounter, the test engine’s ignition system should be selected off during the icing 

conditions. This may require pulling several airplane circuit breakers to disable the test 

engine’s auto-ignition or recovery system, or both, and caution should be used and all 

safety precautions exercised. Lastly, based on past experience, it is advisable that the 

applicants establish and gain concurrence with the FAA for engine damage criteria prior 

to conducting the natural ice encounter test. 

21. FALLING AND BLOWING SNOW. Sections 23.1093(b)(ii) and 25.1093(b)(1) and 

(2) require that engines must operate satisfactorily in falling and blowing snow 

throughout the flight power range, and ground idle. The effect of ingesting snow during 

ground operations can and should be evaluated. The applicant must identify and 

evaluate the critical temperature for the configuration proposed. A temperature range 

between 25 and 34 degrees Fahrenheit has been found conducive to the heavy snow 

environment and to providing the “wet sticky snow” which may accumulate on unheated 

surfaces (airframe and engine) subject to impingement. Colder temperatures may be 

critical to some configurations. In these cases, colder exterior surfaces may be 

bypassed, and the snow crystals may stick to partially heated interior inlet surfaces 

where melting and refreezing may occur. Service experience has demonstrated 

compressor damage (see paragraph 8b(3) of this AC) as a result of exposure to 

prolonged periods of falling snow during ground operation. Based on review of service 

events, airports have continued to operate with falling-snow concentrations that result in 

a 0.25 mile or less visibility (about 0.9 gm/m3

, see paragraph 9b of this AC). In-flight 

service experience has also shown that snow can shed from engine or aircraft 

accumulation sites and cause severe operability affects on turbine engines. Therefore, 

all engine inlets, including those with plenum chambers, screens, particle-separators, 

variable geometry, or any other feature, such as an oil cooler, struts or fairings, which 

may provide a potential accumulation site for snow should be evaluated. Also, any 

airframe accumulation sites upstream of the engine inlet should also be considered. 

22. TEST RESULTS. The applicant should carefully consider all evidence of ingestion 

and damage to the engines and their potential sources. If damage is incurred, the 

possible test outcomes include: 
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 a. Acceptable damage. The extent of damage is equivalent to or less than that 

incurred and accepted during engine certification testing. 

 (1) All systems operating normally. The extent of damage is equivalent or less 

than that incurred and accepted during the § 33.68 tests. 

 (2) Delayed activation of induction system anti-icing. If ice ingestion tests under § 

33.77 do not adequately represent the particular airframe installation, then the delayed 

anti-icing system activation test should be considered. (Caution should be used and all 

safety precautions exercised.) For this condition, the acceptance criteria defined in 

paragraphs 13 and 14 of this AC, should be used. The airframe manufacturer still must 

consider all potential ice shedding sites (for example, inboard wing and radome). 

Similar to the accepted compliance of § 33.77 ice slab ingestion tests (outlined in 

section 2 of this AC), the use of engine auto-ignition and recovery systems are allowed 

to show compliance with the delayed activation tests of parts 23 or 25, as long as these 

automatic systems can not be easily turned off by the flight crew (that is, a flight crew 

that inadvertently forgets to turn on the engine anti-ice protection is also likely not to 

have selected any other engine protection features such as continuous ignition, prior to 

entering the inclement weather). It is important to note the difference in anti-iced inlets 

versus de-iced inlets. De-iced inlets produce a cyclic shedding of ice from the engine 

inlet into the engine and typically incorporate, as part of their design, an inlet particleseparator that precludes the ingestion of ice into the core of the engine. It should be 

recognized that an engine auto-recovery system should not be a compensating design 

feature utilized to minimize the negative effects of an inadequate particle-separating 

inlet that is not in full compliance with §§ 23.1093 or 25.1093. 

 b. Damage from testing in non-representative icing conditions. Damage resulting 

from icing test conditions which fall significantly outside Appendix C, Appendix X or 

Appendix D icing envelopes, or when the airplane flight test is conducted in an 

abnormal manner and results in excessive ice shed damage, may be given additional 

consideration relative to compliance with the provisions of either §§ 23.1093 or 25.1093, 

and in some cases may be disregarded. An example of abnormal operation could be 

flying with one engine at idle while the aircraft is operated in level flight. 

c. Unacceptable damage. The icing test conditions were representative of inservice encounters and the resultant airframe or engine ice sheds caused 

damage that exceeds the criteria established in paragraph 11b of this AC. 
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23. CONCLUDING REMARKS. Although applicants may conduct representative tests 

under §§ 33.68, 33.77, 23.1093, and 25.1093, flight test events during airplane 

certification may still occur which appear inconsistent. In all likelihood, those results 

would not be inconsistent when judged in light of the scope, intent, and limitations of the 

certification testing. Only through reliable instrumentation and photographic evidence 

can the icing test disparities be fully understood. Because of the relatively frequent 

encounters with icing conditions in conjunction with the potential impact on safety, the 

FAA takes a conservative approach when accepting icing compliance standards. 

 /s/ 

David W. Hempe 

Manager, Aircraft Engineering Division 
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References 

1. ROYAL AIRCRAFT ESTABLISHMENT (Farnborough) TECHNICAL NOTE No: 

MECH. ENG. 283 THE ANALYSIS OF MEASUREMENTS OF FREE ICE AND 

ICE/WATER CONCENTRATIONS IN THE ATMOSPHERE OF THE 

EQUATORIAL ZONE by Ian I. McNaughton, B.Sc., Dip. R.T.C. 

2. FAA-RD-77-76, ENGINEERING SUMMARY OF POWERPLANT ICING 

TECHNICAL DATA by G. D. Pfeiffer and G. P. Maier 

3. THE ICING OF AN UNHEATED NON-ROTATING CYLINDER IN LIQUID 

WATER DROPLET-ICE CRYSTAL CLOUDS, E. P. Lozowski, J. R. Stallabrass & 

P. P. Hearty, National Research Council Canada Report LTR-LT-96, February, 

1979. 

4. SNOW AND ICE PARTICLE SIZES AND MASS CONCENTRATIONS AT 

ALTITUDES UP TO 9 KM ( 30,000 FT), R. K. Jeck, DOT/FAA/AR-97/66, August, 

1998. 

5. ASSESSMENT OF EFFECTS OF MIXED-PHASE ICING CONDITIONS ON 

THERMAL ICE PROTECTION SYSTEMS, DOT/FAA/AR-03/48, Kamel Al-KHalil, 

MAY 2003. 

6. FURTHER ICING EXPERIMENTS ON AN UNHEATED NON-ROTATING 

CYLINDER, LTR-LT-105, J. R. Stallabrass and P. F. Hearty, November 1979. 

7. DOT/FAA/AR-98/76 REPORT: "MIXED PHASE ICING CONDITION, A 

REVIEW", James T Riley. 

8. SNOW CONCENTRATION MEASUREMENTS AND CORRELATION WITH 

VISIBILITY, J. R. Stallabrass, AGARD Conference Proceedings No. 236 Icing 

Testing for Aircraft Engines.

9. THE ESTIMATION OF SNOWFALL RATE USING VISIBILITY , Rasmussen, 

R.M., J. Vivekanandan, J. Cole, B. Myers, and C. Masters, 1999 : Journal of 

Applied Meteorology, 38, 1542-1563.