# 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

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### 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. BACKG ROUND: .............................................................................................................. 11 a. Safety Issue Addressed/ Statement of th e Problem................................................... 11

b. Current Standards or Means to Addres s.................................................................... 13

2. DISCUSSION of PROP OSAL ........................................................................................ 19 a. Section-by-Section Desc ription of Propos ed Action.................................................. 19 Revised Regul ations ......................................................................................................... 34

b. Alternatives Consid ered............................................................................................... 49

c. Harmonizati on Status................................................................................................... 52

3. COSTS AND OTHER ISSUES THAT MUST BE CONSIDER ED.............................. 54

a. Costs Associated with the Pr oposal............................................................................ 54

b. Othe r Iss ues ............................................................................................................... .. 55

4. ADVISORY MATERIAL................................................................................................... 56

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### Table of Appendices

Appendix A Typical Airworthines s Directive on Seve re Icing .......................... A-1

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

Appendix C JAA Interim Policy on Severe Icing Conditi ons............................ C-1

Appendix D Acceptable complianc e methods for Appendix X Conditions (Natural Ice Flight Te sting Necessi ty) .................................................... D-1 Necessity to Flight Test in Natural Appendix X Condition s.......................... D-2

Initial Authoritie s’ Positi on............................................................................ D-2

Initial Manufactur ers’ Posi tion...................................................................... D-5

Authorities’ Response ................................................................................ D-19 Majority Working Group Positi on ............................................................... D-22 Remaining Minorit y Position ...................................................................... D-24 Majority Response to Remain ing Minority Po sition.................................... D-26

Appendix E Necessity of a Means to Determine Exceedance of Appendix X . E-1 Necessity of a Means to Dete rmine Exceedance of Appendix X................. E-2

Conclusi ons................................................................................................. E-3

Appendix F Rule Applicability based on aircraft desi gn featur es .....................F-1 Position statement supporting exclus ion from §25.1420 for Airplanes with Certain Design Features .........................................................................F-2 Majority Position - Response to Excl usion 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 Certificati on to a Portion of Appendix X ..........G-2 Majority Re sponse.......................................................................................G-3 Minority Position Against Certifying to Freezing Drizzle or Freezing Rain Using a Subset of the Appendix X Cu rves .............................................G-5 Majority Re sponse.......................................................................................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 Re levant to the Proposed Ru le.................I-1 Icing Incident Da tabase Revi ew ....................................................................I-2 Icing Accident Da tabase Revi ew ...................................................................I-3 Minority Position on Appendix I of the IPHWG Working Group Report (Accidents and Incidents Relevant to the Propos ed Rule) .......................I-5 Regional Airline Association Statem ent of Support of Embraer Minority

Position ....................................................................................................I-8 Majority Re sponse.........................................................................................I-8

EHWG/PPIHWG Review of A ccidents and Inci dents ..................................I-11

Appendix J Cost Estimate s ..............................................................................J-1

General Assu mptions ...................................................................................J-2 Example Co sts .............................................................................................J-2 Engine Certificat ion Cost s ............................................................................J-4 Non recurring costs (devel opment, certific ation) ...........................................J-7 Recurring co sts.............................................................................................J-8

Appendix K IPHWG Recomme nded Advisory Materials §§25.1419, 25. 1420 ............................................................................... K-1 1. Backgr ound. ......................................................................................... K-10

2. Definition of Terms ............................................................................... K-13 3. Icing Enve lopes .................................................................................... K-17

4. Certificati on Plan .................................................................................. K-23 5. Analyse s ............................................................................................... K-24

6. Simulated Ice S hapes and Roughne ss ................................................. K-32 7. Compliance Tests (Fli ght/Simulati on) ................................................... K-32

8. Certification To § 25. 1420 ..................................................................... K-41

9. Pneumatic Deic er Boots ....................................................................... K-44 10. Emergency and Abnormal O perating Conditi ons ................................ K-45 11. Ice and Icing Conditions Detection S ystems ....................................... K-45

12. Perform Intended Func tion In Ici ng ..................................................... K- 50 13. Ice Inspection Light(s) ........................................................................ K-51

14. Caution Info rmation ............................................................................ K-51 15. Airplane Flight Manual (AF M) ............................................................. K-52

16. Guidance for Amended TCs, And ST Cs ............................................. K-54 Appendix A - Example of Certification to §25.1420(a)(1) using Visual Cues ............................................................................... K-55 Appendix B - Guidance for Amended TC's a nd STC' s .............................. K- 59

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### Table of Appendices (continued)

Appendix L Necessity for an AFM stat ement concerning cumuliform cloud large droplet ic ing.............................................................................................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 S ub-Part B ..............M-1

Introducti on..................................................................................................M-2 The FTHWG interpretation of "operating sa fely" .........................................M-2 Continued operation in the ici ng conditions of Appendix X .........................M-2 Continued operation limited to a portion of Appendix X or to a flight phas e ............................................................................................M-3 "Detect and exit" from the icing conditi ons of Appendix X into clear air ......M-3 "Detect and exit" Appendix X certif ication integral with Appendix C

certificat ion ............................................................................................M-4 Performance and handling qualitie s 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 RECOMMENDATION S FOR ADVISORY MATERIAL .. N-1

1. Pur pose .................................................................................................. N-5

2. Related R equirement s ............................................................................ N-5

3. Rese rved ................................................................................................ N-6 4. Requirements and Guidanc e .................................................................. N-6 5. Acceptable Means of Co mpliance - Gene ral ........................................ N-14 6. Acceptable Means of Compliance - Flight Test Pr ogramme ................. N-18 A1. Appendix 1 - Airframe Ice Accret ion ................................................... N-36 A2. Appendix 2 - Artifi cial Ice Sh apes ....................................................... N-49 A3. Appendix 3 - Desi gn Features ............................................................ N-51

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### Table of Appendices (continued)

Appendix O EHWG and PPIHWG SUB-GROUP RECOMENDATIONS FOR ADVISORY MATERIALS (Rev ision to AC 20-147) ................................O-1

1. Pur pose .................................................................................................O-2 2. Applicab ility. ...........................................................................................O-3

3. Related R egulation s ..............................................................................O-3 4. Related Reading Materi al (Latest Revi sions). ........................................O-3 5. Backg round. ..........................................................................................O-4

6. Defini tions ..............................................................................................O-5

7. Discu ssion ............................................................................................. O-8 Section 1. Induction System Ic ing (§§ 33.68 And 33.89(B)) ....................O-11

8. Critical Point An alysis (CPA) . .............................................................O-11 9. Test Point(S) Selection. ......................................................................O-13 10. Test Setup C onsiderations ................................................................O-18

11. Test Results And Compliance Iss ues ................................................O-19 12. Appendix X Wate r Impingement ........................................................O-29

Section 2. Ice Slab I ngestion (§ 33. 77) ....................................................O-31

13. Intent Of Ice Sl ab Ingestion Te st .......................................................O-31 14. Compliance Cons iderations ...............................................................O-31

15. Test Re sults .......................................................................................O-34

16. Communication Of Test Result s ........................................................O-35 Section 3. Induction System Icing Protection (§§ 23.1093 And 25.1093) O-36

17. Airframe Ice Source s Including In let ...................................................O-39 18. Compliance with Appen dix D Of Part 33 ............................................O-40 19. Compliance with Appen dix X Of Part 25 ............................................O-40 20. Natural Icing Flight Test s ...................................................................O-41 21. Falling and Blo wing Snow ..................................................................O-42

22. Test Re sults .......................................................................................O-42 23. Concluding Remarks ..........................................................................O-44

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### Table of Figures

Figure 1 - 14 CFR 25, Appendix X, Freezing Drizzle, Liquid Wa ter Content.......22 Figure 2 - 14 CFR 25, Appendix X, Freezing Drizzle, Drop Diameter

Distributio n ...............................................................................................23 Figure 3 - 14 CFR 25, Appendix X, Freezing Drizzle, Temperature

and Altitu de ..............................................................................................24 Figure 4 - 14 CFR 25, Appendix X, Freez ing Rain, Liquid Water Content ..........25 Figure 5 - 14 CFR 25, Appendix X, Freezing Rain, Drop Diameter

Distributio n ...............................................................................................26 Figure 6 - 14 CFR 25, Appendix X, Freezing Rain, Temperature

and Altitu de ..............................................................................................27 Figure 7 - 14 CFR 25, Appendix X, Hori zontal Extent, Freezing Drizzle and Freezing Ra in...........................................................................................28 Figure 8 - 14 CFR Part 33, App. D, Fi gure D-1 Convective Cloud Ice Crystal Envelope ..................................................................................................42 Figure 9 - 14 CFR Part 33, App. D, "F igure D-2" Total Wa ter Content ...............43 Figure 10 - 14 CFR Part 33, App. D, "Figure D-3" Exposure Length

Influence on TWC ....................................................................................44 Figure D-1 - Example Impingem ent Characte ristic ........................................... D-8 Figure D-2 - LWC as a Func tion of Drop Size................................................. D-19 Figure I-1 - Fan Dam age Events Ta lly ..............................................................I-11 Figure I-2 - Rollback/Flameout/Stall and Core Damage Event s Tally ...............I-12 Figure O-1 - Appendix D Ic ing Envel opes.......................................................O-27 Figure O-2 - Appendix D Icing Total Water C ontent........................................O-28 Figure O-3 - Droplet Impingement Effi ciency for Differ ent Shapes .................O-29 Figure O-4 - Ballistic Boundary as Function of Target and Droplet Size.........O-30 Figure O-5 - Normal Component of Kinet ic Energy of Ice Slab.......................O-33 Figure O-6 - Ice Slab Ori entation Effe cts ........................................................O-33 Figure O-7 - Ice Slab Br eakup Experienc e .....................................................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 Sl ab 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 Ti meline ................................. D-16 Table G-1 - Comparison of In-fli ght versus Ground Ob servations ....................G-6 Table I-1 - Applicable Ai rcraft Icing In cidents ......................................................I-3 Table I-2 - Applicable Accidents Of Ai rcraft With Pre-A ppendix –C Icing

Certification Criteria ..................................................................................I-4 Table I-3 - Applicable Aircraft Ac cidents For Aircraft With Appendix C

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

Table J-1 - Exam ple Cost s.................................................................................J-3 Table J-2 - Example Costs/Engi nes...................................................................J-6 Table J-3 - Operat ing Cost s...............................................................................J-9

Table K-1 - FZDZ Distributions R epresented using 10 Bins ........................... K-20

Table K-2 - FZRA Distributions R epresented 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 S howing Compli ance .................................................. N-15

Table N-2 - Trim Speeds................................................................................. N-21 Table N-3 - Holding Scenar io - Manoeuvre s................................................... N-32 Table N-4 - Approach/Landing Sc enario - Manoeuv res.................................. N-33 Table O-A - Inlet Lip and Runback Ic e............................................................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 simu lation 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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12/19/2005 Page 10 ATTENTION : This Working Group Report was written with the assumption that three icing rules, which have been propos ed 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. T he three rules are: §§25.21(g); 25.1419(e), (f), (g), and (h); and 121.321.

This Working Group Report contains refere nces to the Joint Aviation Authorities (JAA). The report was coordinated wit h IPHWG members who participated as representatives of the J AA. Coordination with EASA has not been accomplished.

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12/19/2005 Page 11 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

. S AFETY I SSUE A DDRESSED /S TATEMENT OF THE P ROBLEM

(1) What prompted this rulemaking activity (e.g., accident, accident investigation, NTSB reco mmendation, new technology, service history, etc.)? What focused ou r attention on the issue?

On October 31, 1994, an accident involving an Aerospatiale Model ATR72 series airplane occurred in which icing conditi ons, believed to include freezing drizzle drops, were reported in the area. The FAA, Aerospatia le, the French Direction

Général de l’Aviation Civil e, Bureau Enquete Accident, National Aeronautics and Space Administration, National Transpor tation Safety Board, and others have conducted an extensive investigation of this accident. This investigation led to the conclusion that freezing drizzle condi tions created a ridge of ice on the wing’s upper surface aft of the deicing boots and forw ard of the ailerons. It was further concluded that the ridge of ice resulted in uncommanded roll of the airplane. The atmospheric conditions (freezing drizzl e) 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 certif ication of the airplane. Freezing rain is an atmospheric conditi on 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 reco mmendations to the FAA following the

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

that:

If safe operations in certain icing conditions cannot be demonstrated by the manufacture r, operational limitations should be imposed to prohibit f light in such conditions and flightcrews should be provided wit h the means to positively determine when they are in ici ng conditions that exceed the

limits for aircraft certification

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12/19/2005 Page 12 Another recommendation, A-96-54, states:

Revise the icing criteria pub lished in 14 Code of Federal Regulations (CFR), parts 23 and 25, in light of both recent

research into aircraft ice accr etion under varying conditions of liquid water content, drop size di stribution, and temperature, and recent developments in both the desi gn and use of aircraft. Also, expand the appendix C icing certif ication envelope to include freezing drizzle/freezing rain and mixed water/ice crystal conditions, as necessary.

In response to the NTSB safety reco mmendations, 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 regulati on that requires installation of ice detectors, aerodynamic performance monitors, or another acceptable means to warn fli ghtcrews 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 iss ue to be addressed in this proposal?

The Part 25 and Part 33 airworthiness rule s 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 freezi ng rain), nor does it cover mixed phase and ice crystal icing conditions. The operat ing rules do not prohibit operations in supercooled large drop conditions or mixed phase and ice crystal icing

conditions. The accident and incident histor y 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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12/19/2005 Page 13 required safety margin should be estab lished to help ensure the safety of operations in these conditions.

(4) Why should the r equirement exist?

The proposed rule would expand the certif ication icing environment to include certain freezing drizzle and freezing rain co nditions in which an airplane must be capable of safely operating eit her for the time to exit or without restriction. The proposed rule would require specified sa fety margins when operating in these icing conditions. Additionally the pr oposed 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 co mpanies. 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 re commends that NASA take the necessary steps to ensure their aircraft icing products ar e available to all airplane companies

worldwide.

B

. C URRENT S TANDARDS OR M EANS TO A DDRESS

(1) If regulations currently exist:

(a) What are the current regul ations 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 env ironment 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) par t 25 (§§ 25.21(g),25.773, 25.929, 25.1093, 25.1323,

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12/19/2005 Page 14 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 cont inuous maximum and intermittent maximum icing conditions of 14 CFR part 25, A ppendix C and JAR 25, A ppendix C. Also, icing certification requirements exist for engines in Part 33, sections 33.68 for induction system icing and section 33.77 fo r ice slab ingestion. Some of the other regulations that addr ess flight in icing conditions require the affected equipment operate to various ex tents of the icing conditions defined in Appendix C while others do not specif y the icing conditions that must be considered.

Appendix C characterizes continuous ma ximum and intermittent maximum icing

conditions within stratiform and cumulifo rm clouds. Appendix C defines icing

cloud characteristics in terms of mean e ffective drop diameters, liquid water content, temperature, horiz ontal and vertical extent, and altitude. Freezing drizzle and freezing rain pr ecipitation are not incl uded as these environments typically contain mean effective diamet ers that are larger than the cloud mean effective drop diameters defined in A ppendix C. Consequently, these icing conditions containing freezing drizzle and fr eezing rain are not considered during the certification of the airplane’s ic e protection system, and exposure to these conditions could result in hazardous ice accumulations because the larger diameters typically impinge farther aft on ai rfoil surfaces. Also, mixed phase and ice crystal icing conditions are not currently considered during the certification of the engine, and exposure to these conditi ons could result in hazardous ice accumulations within the engine that c ould 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 Oper ating and Flight Rules”), 14 CFR part 121 (“Operating R equirements: Domestic, Flag, and Supplemental Operations”), and 14 CFR part 135 (“Operati ng Requirements: Commuter and On Demand Operations and Rules Governing Persons on Board Such Aircraft”).

Specifically, § 91.527 (“O perating in icing cond itions”) and § 135.227 (“Icing conditions: Operating limit ations”) address limitations in icing conditions for aircraft operated under these rules.

Specific requirements regar ding exiting hazardous icin g conditions are found in § 121.629(a) (“Operation in icin g conditions”) which states:

No person may dispatch or release an aircraft, continue to operate an aircraft en route, or land an airc raft 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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12/19/2005 Page 15 Also, § 121.341 (“Equipment for operations in icing conditions”) requires the installation of ce rtain types of ice protection equipment and wing illum ination equipment.

14 CFR 121.321 requires airpla nes with less than 60,000 lbs maximum takeoff weight and equipped with reversible flight controls for the pitch and/or roll ax is 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 eit her a substantiated visual cue or

a caution level alert to the fli ghtcrew when the airplane is operated in supercooled large drop c onditions conducive to ice accumulation aft of the airframe’s protected areas.

(b) How have the regulations been appl ied? (What are the current means of compliance?) If there are differenc es 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 r egulations 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 descr ibed 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 regula tions. 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 di scussion on the inadequacies of the existing rules. Additionally, ther e have been documented cases of over 100 ice crystal and mixed phase engine events, wit h six occurrences of multi-engine flameouts, during the peri od 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 aircra ft and two events resulted in an air-turnback.

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12/19/2005 Page 16 (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 F AA relied on issue papers? Special Conditions? Policy statements? Cert ification 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 t he above-mentioned ATR a ccident in 1994, the

FAA issued a series of Airworthiness Dir ectives (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 airp lanes to exit icing when the conditions exceed the capabilities of t he ice protection equipment.

Additionally, JAA interim policy INT/PO L/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 ot her 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 prov iding for means of detection and exiting from freezing drizzle and freezing rain conditions. However, they are not intended to certify an airplane for unrestric ted flight in supercooled large drops or any other conditions which are outsi de of the Appendix C icing envelope.

See Appendices A, B, and C of this repor t for copies of a typical AD, the FAA generic issue paper, and the JAA interim policy.

The FAA has been working with engine manu facturers 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) pr ovides consideration for ice crystal icing conditions, in practice, engine m anufacturers have not provided objective pre-certification evidence of subst antiated operation in these conditions.

(b) Why are those means inadequat e? 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 sa fely in freezing rain and freezing drizzle conditions as well as mixed phase and ice cr ystal 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 airpl anes operated under Part 121 that have

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12/19/2005 Page 17 a MTOW less than 60,000 lbs and are equipped with unpower ed roll or pitch

controls.

The scope of these actions are also limited because they do not address the underlying safety concern of the un known 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 t he addition of part 25 §25.21(g) that defined safe performance and handling qualities fo r flight in part 25 Appendix C icing

conditions.

The ADs are intended to minimize t he 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 icin g environment and the dynamics of ice accretion in such conditi ons 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 protec ted area and forward of the ailerons. Extensive research and analyses have been performed to characterize freezing drizzle and freezing rain icing conditi ons 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 unsaf e operations. Also, other considerations in addition to the roll control anomaly i dentified 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 condi tions (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 haz ards associated with exposure of the identified airplanes to seve re 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 interi m 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 operat ors. Operators of future airplanes limited by the Issue Paper and JAA interim policy, will suffer a commercial disadvantage relative to operator s of airplanes not subjected to these

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 18 requirements when these icing conditions exist. There are no airworthiness requirements, similar to those establis hed 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 P aper 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 meteorol ogical data has allowed a joint approach to developing standards for these icing co nditions. 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 (g laciated ice) addressed under JAR-E 780(d), but with no specific def inition of the environmental threat or related ACJ guidance material defining acceptable means of compliance.

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 19 2. DISCUSSION OF PROPOSAL

This section explains:

• what the proposal would require,

• what effect we intend t he requirement to have, and

• how the proposal addresses the probl ems identified in Background.

• Discuss each requirement separately. Where two or more requirements are very closely related, discuss them together.

• This section also should discuss alte rnatives considered and why each was rejected.

A

. S ECTION

- BY -S ECTION D ESCRIPTION OF P ROPOSED A CTION

(1) What is the proposed action? Is the proposed action to introduce a new regulation, revise the existing regulati on, 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, revi se §§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 requi res “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 app licant 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 wi th 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 r equired” which appear in (b)(3) and (b)(5) will result in the applicant choosing the m eans of compliance that is necessary to support the analysis but the authorities wil l make a finding whether the means of compliance is acceptable.

Additionally note t hat following the proposed 14 CFR § 25.21(g) icing certification performance and handling qua lities requirements in 14 CFR part 25 Appendix C icing conditions, similar requirements ar e proposed for the icing conditions of Appendix X, including defin ing 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 par t II of the proposed Appendix X.

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 20 (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 ici ng 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 enc ountering Appendix X conditions:

(i) There must be a means provided to det ect that the airplane is operating in Appendix X; and

(ii) Following detection, t he 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 det ect that the airplane is operating in

conditions that exceed the sele cted portion of Appendix X; and (ii) Following the ex ceedance 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 ic ing conditions of Appendix X.

(b) To establish that the airplane can oper ate safely as required in paragraph (a), an analysis must be performed to establish that the ice protec tion 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 te sts, or a combinatio n of both, of the components or models of the components. (2) Laboratory dry air or simulated icing test s, 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 airp lane with simulated ice shapes. (5) Flight tests of the airplane in natural icing conditions, measured as required to support the analysis.

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 21 (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 me t 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 wh ich the drop median volume diameter (MVD) is less than or greater than 40 μm, the maximum mean effective drop diameter (MED) of Appendix C conti nuous maximum (stratiform clouds) icing conditions. For Appendix X, supercooled large drop icing conditions consist of freezing drizzle and freezing rain and can c onsist of precipitation in and/or below

stratiform clouds. Part II defines ic e shapes used to show compliance with 14 CFR § 25.21(g) requirements for continuous flight or for fli ght in a portion of Appendix X.

PART I – METEOROLOGY

Appendix X icing conditions are defined by the parameter s of altitude, vertical and horizontal extent, temperature, liquid water content, and water mass

distribution as a function of dr op diameter distribution. a. Freezing Drizzle (Cond itions with spectra maxi mum 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 di stance of 17.4 nautical miles

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 22 4. Total liquid water content (cloud and precipitation):

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

Figure 1 - 14 CFR 25, Appendix X, Freez ing Drizzle, Liquid Water Content

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 23 5. Drop diameter distribution:

Figure 2 - 14 CFR 25, Appendix X, Freez ing Drizzle, Drop Diameter Distribution

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 24 6. Altitude and tem perature envelope:

Figure 3 - 14 CFR 25, Appendix X, Freezin g Drizzle, Temperature and Altitude

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 25 b. Freezing Rain (Conditions with spec tra 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 di stance of 17.4 nautical miles 4. Total liquid water content (cloud and precipitation)

Note: LWC based on horizontal ext ent standard distan ce of 17.4 nm.

Figure 4 - 14 CFR 25, Appendix X, Fr eezing Rain, Liquid Water Content

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 26 5. Drop Diameter Distribution

Figure 5 - 14 CFR 25, Appendix X, Fr eezing Rain, Drop Diameter Distribution

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 27 6. Altitude and tem perature envelope:

Figure 6 - 14 CFR 25, Appendix X, Fr eezing Rain, Temperature and Altitude

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 28 c. Horizontal extent

The liquid water content for freezing dr izzle 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, Hori zontal Extent, Freezing Drizzle and Freezing Rain

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 29 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 Appendi x 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 t he 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 conditi ons 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 accr etion on unprotected surfaces, and any ice accretion on the protected su rfaces 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 mo st 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 a ccretion 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 t he 17.4 nautical miles

cloud horizontal extent standard distanc e during the holding phase of flight.

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 30 (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 par t I of this appendix to 2,000 feet above the landing surface in the crui se configuration, and transition to the approach configuration and maneuv ering 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 surfac e 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 landi ng configuration. (ii) Holding ice as defi ned by Part II b(1)(d).

(g) Ice accretion for the takeoff phase

For both unprotected and protected surf aces, 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, propelle rs 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 V EF , and (v) Crew activation of the ice prot ection system is in accordance with a normal operating procedure provided in the Airplane Flight Manual in accordance with § 25.1581(a)(1), exc ept that after commencement of

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

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

takeoff surface.

(h) Ice accretion befor e normal system operation

Ice accretion prior to normal system operat ion (pre-activation ice) is the ice accretion formed on the unprotected and no rmally protected surfaces before the

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 31 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 becom e 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 ap pendix (total accumulation limited to 45 minutes). If failure of the ice protection system is unannounced or if Airplane Flight Manual procedures do not call fo r exiting icing conditions following an ice protection system failure, the failure ice is the same as the ice accretion normally considered for unprotected surfaces. Fo r airplanes certificated in accordance with 25.1420(a)(1) failure ice need not be considered. Fo r airplanes certificated in accordance with 25.1420(a)(2) failu re 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 accret ions, pre-existing ice accretions may exist from operations in approved icing co nditions prior to encountering the icing conditions requiring an exi t. For aircraft certified under25.1420(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 rout e ice, as defined by paragraph part II b(1)(c) of this appendi x (25.1420(a)(2) certifications) or by paragraph part II (a)(3) of 14 CFR part 25 Appendix C (25.1420(a)(1) ce rtifications), as applicable, plus pre-detection ice, as described by paragr aph 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 envir onment described in part I of this appendix

and one standard cloud horizontal extent (17.4 nautical mile s) 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 paragr aph part II (a)(4) of 14 CFR part 25 Appendix C (25.1420(a)(1) certif ications), as applicable, plus pre-

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 32 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 horizont al extent (17.4 nautical miles) of the ici ng environment described in pa rt 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 condi tions 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 par t I of this appendix to 2,000 feet above the landing surface in the cr uise configuration (25.1420(a)(2) certifications) or descent from t he maximum vertical extent of the maximum continuous icing environment described in part I of 14 CFR part 25 Appendix C (25.1420(a)(1) cert ifications), as applicable, transition to the approach configurati on, 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 horizont al extent (17.4 nautic al miles) of the icing environment described in part I of this appendix and one standard cloud horizontal extent (17.4 nautic al 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 t he 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) ce rtifications), as applicable, plus pre-detection ice, as described by paragraph part II b(2)(e) of this appendix, and the ice accumulated duri ng an exit maneuver, beginning with the minimum climb requirem ents 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 mile s) of the icing environment described in part I of this appendix and one standard cloud horizontal extent (17.4 nautic al miles) of the continuous maximum icing environment described in part I of 14 CFR part 25 Appendix C.

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 33 (ii) Approach & Landing ic e 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 crui se 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 A ppendix 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 a ccretion formed on t he unprotected and normally protected surfaces or on pre-exis ting ice. The pre-detection ice must consider the ice accumulated during t he period required to detect the icing conditions (considering the provided means of detection) followed by two minutes of ice accumulation to represent the peri od for the flight crew to reference and implement prescribed procedures to ex it 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------------- ------------

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 34 R EVISED R EGULATIONS (Note: new words are in bold )

Proposal 28 - Reserved

Proposal 29 - Sec. 25.773 – Pilot compartment view

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

X § 25.1419 if certification with ic e 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) Sect ion 33.68 of this chapter in eff ect on [insert effective date of final rule], or as subsequently amended; or (ii) Co mply with § 33.68 of this chap ter in effect on February 23, 1984, or as subsequently amended before [insert effective date of final rule], unless that engine’s ice accumu lation service history has resulted in an unsafe condition; or (i ii) Comply with § 33.68 of this chapter in e ffect on October 1, 1974, or as subsequently amended prior to February 23, 1984, unless that engine’s ice accumulation service hist ory 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.

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 35 Proposal 32 - Sec. 25.1093 – Induction system icing protection

Revise section (b) Turbine Engines.

Each engine, with all icing pr otection systems operating, must: 1. Operate throughout its flight pow er range, in continuous maximum and intermittent maximum icing conditions as defined in Appendix C, Appendix X of Part 25 of this chapter, and Appe ndix D of Part 33 of this chapter, in falling and blowing snow within the li mitations established for the airplane for such operation, including the mi nimum 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 thru st ; 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, wit hout adverse effect, followe d by an acceleration to takeoff power or thrust. During the id le 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 limita tions section of the Aircra ft 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"

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 36 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 preventi ng 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 pr otection systems operating, must:

A. Operate throughout its flight power range, including the minimum descent idling speeds, in icing condi tions 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 flame out or loss of engine controllability (for example, rollback). The appli cant must account for in-flight ram effects (for example; scoop f actor amplification, water temperature, air density) in any critical point analysis or test demonstration of these flight conditions.

B. Operate throughout its flight po wer range (including minimum descent idling speeds in icing) in cont inuous 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 Crit ical Point Analysis (CPA) that the complete ice envelope has been anal yzed, 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 e ngine 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:

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 37 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 fl ight 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.

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 38 C. In addition to complying with para graph 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-minut es 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 protecti on at its critical condition, without adverse effect, followed by an accelerat ion to takeoff power or thrust. During the idle operation the engine may be run up periodically to a moderate power or thrust setti ng in a manner acceptable to the Administrator. The applicant must document any demonstrated run ups and minimum ambient temperature cap ability during the conduct of icing testing in the engine operating manua l 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 crystal s 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 requirement s of this paragraph shall be demonstrated by engine ice ingest ion test or by validated analysis showing equivalence of other mean s 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 th e 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 re sultant sustained reduc tion in power or thrust greater than those values requir ed by paragraph (c) of this section.

(e) Compliance with paragraph (c) of this section may be shown by engine test under the following inges tion conditions: (1) The minimum ice quantity wi ll 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 Sl ab 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 c onvective storm cloud formations exist within the 14 CFR Part 25 Appendix C Intermittent Maximum Icing envelope (including the extension to -40 de g 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 - ftAmbient Temperature - deg C

Appendix D

Figure 8 - 14 CFR Part 33, App. D, Fi gure 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 t he convective rise of 90% relative humidity air from sea level to higher alti tudes and scaled by a factor of 0.65 to a standard cloud length of 17.4 naut ical 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 ftTWC - 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 dim ension (MMD) range is 50 - 200 microns (equivalent spherical size) based upon m easurements near convective storm

cores.

The TWC can be treated as completely glac iated 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 expos ure distance (horizontal cloud length) of 17.4 nautical miles that must be adjus ted with length of icing exposure. The

assessment from data meas urements 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 MilesTotal Water Content (TWC) Factor

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

TWC

Proposal 38 - Sec. 121.321 – O perations in Icing Conditions

Sec 121.321: After [a date 24 months after the effectiv e date of the final rule], no person may operate an airplane with a maxi mum 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 c hange 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 icin g environment used to evaluate safe operation of the airplane in icing conditi ons. 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 env ironment; and, if necessary, a means to differentiate environmental icing condition s and development of exit procedures.

The proposed 25.1420 extends t he requirements of 25.1419(e), (f), (g), and (h), regarding the activation and operation of ai rframe ice protection systems, to the certificated portion of Appendix X icing c onditions. These requirements are not applicable to non-certificat ed Appendix X icing condi tions because 25.1420(a) requires a method to identify these conditi ons and safely exitin g the conditions.

Current knowledge suggests that lar ge-droplet dynamic phenomena such as breakup and splashing can reduce the am ount of ice that forms. These

mechanisms can contribute to mass leavi ng the airfoil surface after impact and smaller droplets (either from breakup or sp lash-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 ri dges may form due to the impingement and freezing of large drops aft of protected areas . However, there is experimental evidence indica ting that, in some conditions, in particular when the total air temperature is near the freezing poi nt or at low freezing fractions, aft ice accretions may result from runback ic e or ice migration (ice-on-surface movement), as well as from direct drop impingement. Ice migration has been observed in videos of icing tunnel experim ents at total air temperature near the freezing point and is characterized by ice- buildup that initially grows at or nearthe leading edge, then slides aft due to aerodynamic forces overcoming ice-toairfoil adhesion forces. The videos indicate that the ice migrat ion can contribute to the potential for development of ic e ridges behind protected areas. Drop impingement onto such formations woul d result in their enlargement.

If certification for flight in icing is desir ed, 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 s pecific components. Since these rules reference Part 25, Appendix C icing cond itions or generical ly 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 com ponents 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 a nd 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 supercool ed large drop icing conditions described in the proposed part 25 Appendix X are the icing me teorological conditions in which the airplane must either safely operate unrestr icted or safely exit following detection. Appendix X includes supercooled drops lar ger than those considered by current icing regulations. These larger drops will impinge and freeze farther aft on airplane surfaces and may affect the ai rplane’s performance, flying qualities, engine and systems operation. The Appen dix X icing conditions may affect design considerations of airp lane ice protection provisions.

Sections 25.929 and 25.1323 generically refe rence icing instead of specifically mentioning Appendix C. Historically the icing condition s of Appendix C have been considered as applicable to these rules. For clarity, the rules are revised to specifically referenc e 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. Se ction 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. Ai rplanes certificated to a portion of Appendix X must have a method to detect that the airplane is operating

in conditions which exceed the select ed 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 Appendi x X conditions whil e exiting icing. It is not practical to certificate probes, components, or engines to varying portions of Appendix X. Therefore the icing r egulations 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 inte rpretation that the components may be certificated for only the certificat ed portions of Appendix X conditions.

Section 25.773(b) is only applicable if ther e 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 cl arify 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 remo ve the requirement to comply with the rule providing the airplane compli es with certain part 25 regulations.

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 47 Section 121.321 addresses two issues: w hen 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. Se ction 121.321 partially addresses the concern of airplanes operating in ici ng conditions that exceed Appendix C conditions. The part 121 rule is not performance based but instead identifies specific conditions (icing conditions c onducive to ice accretions aft of the protected areas) from which the airplane must exit. Th e rule does not allow the option of substantiating t hat 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 det ection, the airplane must be capable of operating safely while exiting all icing conditions.

When complying with §25.1420(a)(2), certif ication 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. Cert ification for a por tion of Appendix X (§25.1420(a)(2)) or detect and exit fr om 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 A ppendix 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 pr oposed §25.1420 would not need to comply with Part 121.321(c), which requires certain ai rplanes 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 inst alled engine in icing conditions. The reason for the change is that the incident hi story of some aircraft has shown that the current icing environm ent requirements are inadequa te. The effect of the change is to require an evaluation of sa fe operation in the revised icing environment. Engine and engine installati on certification to Appendix X of part 25 and Appendix D of part 33 requires that the plane operate safely throughout these icing envelopes. Acceptable co mpliance 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 fo r installation of pre § 33.68 certification basis engines into new aircraft app lications at the FAA’s discretion.

(4) If not answered already, how will t he 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 re sponse to the question, “Why should the requirement exist?”

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

The proposed action is superior to the cu rrent 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 sa fety issue. However, as noted in section 1.b.(2)(b), there ar e limitations associated with the JAA and FAA actions.

Similar to rulemaking that added §25.21( g) to define airplane performance and handling qualities that ensur e safe flight in part 25 Appendix C icing conditions, appropriate part 25 Subpart B requirem ents have been defined to ensure safe flight in the freezing drizzle and freezi ng 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 la rge drop icing conditions. Subsequently, part 25 Subpart B requirements that ensure safe f light 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 freez ing 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 ce rtified to operate safely while exiting Appendix X.

Preliminary definiti ons of the supercooled large dr op 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 Meteorologica l Service of Canada, the National Aeronautics and Space Administration, t he FAA, and others. Appendix H of this report summarizes the developm ent of the new part 25 Appendix X, and a more detailed description is provi ded in DOT/FAA/AR-04/7.

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 49 B

. A LTERNATIVES C ONSIDERED

(1) What actions did the working group c onsider other than the action proposed? Explain alternative ideas and dissenting opinions.

Alternate Means of Addressing Tasking

Research is ongoing to address avoidanc e operations in supercooled large drops. The use of terminal ar ea radar and sensors for detection and characterization of icing condition s and holding areas is being developed. These have limited capability to determine supercooled large drops in between terminals or holding areas and could not pr ovide 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. Alt hough active, these developments are not mature enough to provide sufficient aircra ft protection over the entire flight operations area.

Additionally, icing diagnostic and predi ctive weather tools have been developed that can provide information on icing and SLD potential, with acceptance of the icing diagnostic tool by the Aviation Weat her Center. Internet access to these experimental tools can be used to provi de flight planning information guidance for avoidance of SLD conditions. However, aircraft safety marg ins 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 thres hold, 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 suppor t a rule based on its technology. However, the rule is written broadly enough to allow the us e of an APM as a means of compliance should the APM’s reach the maturity requi red 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 t he proposed §25.1420 should contain a requirement similar to §25.1419(c). Par agraph 25.1419(c) requi res that caution information, such as an amber caution light or equivalent, must be provided to alert the flightcrew when t he anti-ice or de-ice system is not functioning normally. (Amdt. 25-72, July 20, 1990). In the pream ble to Amendment 25-72 it is stated:

One commenter suggests that the requirement of proposed §25.1419(b)(3) for flightcrew cauti on indication is unnecessary as system failure indication require ments 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 degr ee 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 §25.14 19(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 pr oposal for §25.1420 does not contain the requirement for caution informati on as contained in §25.1419(c).

Necessity to Flight Test in Natural Appendix X Conditions

Another area of extended debat e 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 ce rtification 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 discriminat e between conditions within and outside the certification envelope." This topic was debated within the working group and a

consensus was achieved that a means fo r discrimination of conditions outside the certification envelopes is not nece ssary. 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 suppor ting statement from Cessna, have put forth a Minority Position advocating ex clusion from §25.1420 for aircraft with specific design features. The rationale fo r the position of these manufacturers is that large transport aircraft still in pr oduction have experienced no accidents and few incidents as a result of flying in SLD conditions. Therefor e, the imposition of new, burdensome and costly certificat ion 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 excl uded 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 t he intent of these

manufacturers that all thr ee of these design features would be required to qualify for exclusion. The details of this pos ition 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 submitt ed, and consensus of these positions was not achieved. Both minority positi ons 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 Associ ation, holds a minority view of the applicability of so me of the accidents identifi ed for potential use in the economic analysis. See Appendix I of th is report for the pos ition statements.

Necessity for an AFM Statement Concer ning Cumuliform Cl oud 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 Appe ndix 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 airp lanes 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 airpl anes with the proposed design features will not ensure safe flight for future new and innovative airplane with similar design features. For this reason, airplane ai rworthiness requirements typically do not

include exclusions unless those excl usions are addressed by other more appropriate regulations or if the requirement is not applicable to a specific type of engine or airplane. (Examples of appropria te exclusions include turbojet versus reciprocating engines, and land based versus water based airplanes.) See Appendix F of this repor t 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 Concer ning Cumuliform Cl oud Large Droplet Icing

See Appendix L of this report.

C

. H ARMONIZATION S TATUS

(1) Is the proposed action the same for the FAA and the JAA?

Yes for large drop aspects. Mixed phas ed 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, t he rules are somewhat different. The objective is to maintain equivalency, just as has been the case historically.

(2) If the proposed action differs for t he JAA, explain the proposed JAA action.

No difference is anticipated, but the J AA, 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 main tain 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 r egulation. The more quality information that is supplied, the quicker t he evaluation can be completed.

A

. C OSTS A SSOCIATED WITH THE P ROPOSAL

(1) Who would be affected by the propos ed 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 ai rplane type certificate programs, engine manufacturers of new Part 33 engine type ce rtificate 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 prov ide additional substantiation.

(2) What is the cost im pact of complying with the proposed regulation? Provide any information that will assist in estimati ng the costs (either positive or negative) of the proposed rule. (For example: What are the differences (in general te rms) 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 c an be reported relative to purchase, installation, and maintenance costs?

• In contrast, if the proposed rule relieve s industry of testing or other costs, please provide any known es timate 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, downti me, etc. Include as much detail as possible.)

See Appendix I for the accidents and inciden ts relevant to this proposed rule. See Appendix J for an es timate of the costs.

### Transport Airplane Directorate IPHWG Task 2 Report

12/19/2005 Page 55 B

. O THER I SSUES

(1) Will small businesses be affected? (In general terms, “small businesses” are those employing 1,500 people or le ss. 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 affect ed parties to do any new or additional recordkeeping? If so, explain. [This question relate s 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 m illion or more in one year? [This question relates to the Un funded 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 materi al here (or attach), or

summarize the information it will cont ain, and indicate what form it will be in (e.g., Advisory Circular, Adviso ry 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 replac e 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. Th e 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 prepar ed 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 Re port - Appendix A Typical Airworthiness Directive

12/19/2005 Page A-1 

### APPENDIX A - TYPICAL AIRW ORTHINESS DIRECTIVE ON

SEVERE ICING

### IPHWG Task 2 WG Re port - 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 Ma rk 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 eac h airplane identified in the preceding applicability provision, regardless of whet her 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 repair ed so that the performance of the requirements of this AD is affected, t he owner/operator must request approval for an alternative method of compliance in acco rdance with paragraph (b) of this AD. The request should include an assessment of the effect of the modification, alteration, or repair on the unsafe conditi on 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 a ssociated with operating the airplane in severe icing conditions by provid ing more clearly defined procedures and

limitations associated with such condi tions, accomplish the following: (a) Within 30 days after the effect ive date of this AD, accomplish the requirements of paragraphs (a)(1) and (a)(2) of this AD. NOTE 2: Operators mu st initiate action to not ify 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 Limita tions Section of the AFM. This may be accomplished by inserting a copy of this AD in the AFM.

"WARNING

Severe icing may result from env ironmental 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 ic e build-up on protected surfaces exceeding the capability of the ice protection syst em, or may result in ice forming aft of the protect ed surfaces. This ice may not be shed using the ice protection syst ems, and may seriously degrade the performance and controlla bility of the airplane.

• During flight, severe icing condit ions that exceed those for which the airplane is certificated s hall be determined by the following

visual cues. If one or more of t hese visual cues exists, immediately

### IPHWG Task 2 WG Re port - Appendix A Typical Airworthiness Directive

12/19/2005 Page A-3 request priority handling from Air Tra ffic 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 tact ile cues that indicate adverse changes in handling characteristi cs, use of the autopilot is prohibited when any of the visual cues specified above exist, or when unusual lateral trim requirement s or autopilot trim warnings are encountered while the airplane is in icing conditions.

• All icing detection lights must be oper ative prior to flight into icing conditions at night. [NOTE: This supersedes any relief provided by the Master Minimum Equi pment List (MMEL).]"

(2) Revise the FAA-approved AFM by incorporating the following into the

Procedures Section of the AF M. 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 splatte r 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 th e visual cues specified in the

Limitations Section of the AF M for identifying severe icing conditions are observed, accomplish the following:

### IPHWG Task 2 WG Re port - Appendix A Typical Airworthiness Directive

12/19/2005 Page A-4 • Immediately request priority handl ing from Air Traffic Control to

facilitate a route or an altitude change to exit the severe icing

conditions in order to avoid extend ed 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, hol d 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 exte nded operation in icing conditions. Operation with flaps extended can result in a reduced wing angle-of-attack, with the possibilit y of ice forming on the upper surface further aft on the wing th an 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 conditi ons to Air Traffic Control."

(b) An alternative method of comp liance 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 i ssued in accordance with sections

21.197 and 21.199 of the F ederal Aviation Regulatio ns (14 CFR 21.197 and 21.199) to operate the airpl ane to a location where the requirements of this AD can be accomplished.

### IPHWG Task 2 WG Re port - Appendix B FAA Issue Paper on SLD

12/19/2005 Page B-1 

### APPENDIX B - FAA ISSUE PAPE R ON ROLL CONTROL IN SLD

CONDITIONS

### IPHWG Task 2 WG Re port - 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 cert ification of an airplane with ice protection provisions are defined in o 25.1419. The current regul ation requires that an airplane with ice protection provisions must be able to sa fely operate in the conditions defined in the Federal Aviation Regulations, part 25, appendix C. The regulation is not adequate to address freezing drizzle and freez ing 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 report ed 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 r ange of freezing drizzl e 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 Boar d recommended that the FAA develop a test procedure to identify unsafe ailer on hinge moment char acteristics. 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 us ed in regularly scheduled passenger service equipped with non-powered controls and pneumatic deicing boots.

### IPHWG Task 2 WG Re port - 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 unsaf e condition that may exist on other airplanes. The FAA is particularly c oncerned with airplanes with non-powered flight controls, since non-powered fli ght 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 c ontrol 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 t he 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 μ m (microns), 2. an LWC (liquid water content) of approximately 0. 6 grams per cubic meter 1

, 3. a median volumetric diam eter of approxim ately 170 microns 2

, 4. temperatures near freezing such that runback conditions exist at the stagnation line 3 , and 5. at holding speeds and appr oved 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 wit h 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 dynam ic conditions may be hazardous even though the peak force may be less than the static limit.

c. The tests and analyses in paragraph a. mu st consider the effects of asymmetric shedding of the ice.

1 For this condition, the LWC strongly affects the rate at wh ich the ice feature develops. A higher LWC results in more rapid formation of the ice feature while a lower LWC results in a slower formatio n 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 instrumentat ion, calibration, and data processing methodologies must be acceptable to the FAA.

3 For this test, temperature is a critical factor. Not only is th e temperature critical to the development of the ice shape and dimension, static air temperature excurs ions above freezing, although short in duration, can reverse the ice accretion

process.

### IPHWG Task 2 WG Re port - Appendix B FAA Issue Paper on SLD

12/19/2005 Page B-4 d. There must be a means for the fli ghtcrew 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 info rmation provided in the airplane flight

manual that describes the limitations and pr ocedures to be obser ved while exiting the supercooled large droplet environment. The FAA finds the limitations and procedures contained in AD 96-09-25 ar e 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 light s 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 retr act 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 complianc e with paragraphs a., b., and c. is to perform a high speed taxi test to evaluate control wheel fo rce 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 quar ter-round molding, flat side forward, located on the upper surface of the wing, at the c hord position aft of the active portion of the boots and forw ard of the non-powered roll control surfaces (i.e. ailerons and/or infli ght spoilers) that produces the most

adverse lateral wheel force. 2. Locate this shape in front of t he 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 wit h 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 obt ained from the high speed taxi tests to the maximum speeds expected while in holding conditions. In most

cases the maximum forces will occu r at the maximum angle of attack achieved during the high speed taxi tests. 6. The extrapolated forces ma y not exceed 50 pounds with two hands

available for control.

### IPHWG Task 2 WG Re port - Appendix B FAA Issue Paper on SLD

12/19/2005 Page B-5 7. Airplanes equipped with non-powered in flight spoilers may require tunnel or flight testing to evaluate the effect on airplane control and handling

characteristics.

### IPHWG Task 2 WG Re port - Appendix C JAA Interim Policy on Se vere Icing Conditions

12/19/2005 Page C-1 

### APPENDIX C - JAA INTERIM POLICY ON SEVERE ICING

CONDITIONS

### IPHWG Task 2 WG Re port - Appendix C JAA Interim Policy on Se vere 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 rela ted to loss of aeroplane control have raised the concern that aeropl ane operating in certain me teorological conditions can accrete ice, not only aft of the pr otected areas but also on the underwing airfoil surface. This may result particula rly 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 degr adation resulting

from Ice accretion dow nstream 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 conditi ons 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 JA R-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 follo wing issues must be addressed:

1.) Means for the flight crew to de termine when the aeroplane has inadvertently entered Severe Icing Conditions.

2.) Assessment of the design to ensure t hat the aeroplane can safely exit such environment (susceptibility to loss of c ontrol leading to an unsafe condition).

3.) Crew information that describes the lim itations to be observed while exiting

such environment.

4.) Means for the crew to determi ne when the hazard no longer exists.

### IPHWG Task 2 WG Re port - Appendix C JAA Interim Policy on Se vere 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 characteri stic values of freezing drizzle and freezing rain the under mentioned document may be used.

"REPRESENTATIVE VALUES OF ICIN G 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 extrac ted 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 Re port - Appendix C JAA Interim Policy on Se vere 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 (Freez ing drizzle and freezing rain)

The guidance material outlines an accep table 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 dropl et size diameter, liquid water content, temperature) outside the Appendix C can lead to an unsafe situation and

- Demonstrate by an agreed selection of m eans of compliance (analysis, test in simulated icing conditions, test in dry air with artificial ice shapes, test in measured natural icing conditions,...) t hat 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 adver se effect on handli ng qualities to be flight tested.

The applicant should apply the method consisting in:

- Ice shape determination either by test in simulated Icing Cond ition (tanker test or wind tunnel or analysis) or using t he arbitrary defined Ice shape "Quarter Round/One Inch/high/fl at side forward."

- Wind tunnel test in dry air with the above agreed artificial Ice shape and

location to determine the most critical f light conditions/flight characteristics

effects.

- In flight evaluation with the agreed Artificial Ice Shape using the following

criteria:

### IPHWG Task 2 WG Re port - Appendix C JAA Interim Policy on Se vere Icing Conditions

12/19/2005 Page C-5 While flying the airplane to assess handl ing 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 exce ed 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 a fter establishing the maximum Autopilot coupled bank angle for norma l operation or 40o, whichever is less; or disconnected with the maximu m roll rate is established when changing the direction of bank angle wi th the Autopilot heading control (in both directions); or disconnected at stall warning during a deceleration with Autopilot in attitude hold and headi ng 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 recogni tion point for manoeuvring flight (turns included).

Influence of power setting should be evaluated.

### IPHWG Task 2 WG Re port - 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 CONDITION S (NATURAL ICE FLIGHT TESTING NECESSITY)

### IPHWG Task 2 WG Re port - Appendix D Acceptable Compliance Methods For Appendix X Conditions (Natural Ice Flight Testing Necessity)

12/19/2005 Page D-2 N ECESSITY TO F LIGHT T EST IN N ATURAL A PPENDIX X C ONDITIONS

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 anal ysis must be supplemented by at least one type of test. The proposed rule lists the tests, and natural icing Appendix X flight te sts is one of them. The gr oup 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 confi gurations. After considerable discussion, concerns for requiring flight testing in Appendi x 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 configurati ons or design features that would warrant natural Appendix X flight tests. Guidance on these critical issues

are contained in the draft AC. Th is appendix provides a history of the issue including the background, key point s from both sides of the issue, a Minority Position, and the Majority Response.

I NITIAL A UTHORITIES ’ P OSITION

The initial position of the FAA, J AA, and Transport Canada was that flight testing in measured natu ral 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 va riety of analysis and test methods to optimize the airplane and ice protecti on systems design. For compliance with 14 CFR/JAR 25.1419, the following means are used in addition to analysis and flight in measur ed natural icing conditions:

• Wind Tunnel / Icing Tunnel Tests

• Dry Air and Simulated Ice Shape Flight Tests

• Artificial Icing Flight Tests

• Ancestor Airplane Compar ison (Similarity Analysis)

NOTE: The FTHWG proposed AC 25.21- 1 identifies similar means of showing compliance to t he 25.21(g) requirement.

### IPHWG Task 2 WG Re port - 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 lim itations with r egard to Appendix X icing conditions. For example:

• Engineering tools, such as so ftware for CFD, computer aided design analysis (CAD), aerodyna mics, and drop and ice accretion effects (LEWICE, CANICE, CFD, et c.), 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 effect s. 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 conti nue in Appendix X icing conditions.

• Airborne icing tankers can onl y produce an icing plume of a consistent drop distribution and li quid water content over a limited area. The test area of the pl ume may not adequately expose the areas of the airplane that are of in terest. There are also concerns of low humidity effects, ambient temperature, unif ormity of the plume relative to liquid wa ter 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. Ot herwise, 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 ch eck 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 syst em (IPS) operating as a complete system, not as a series of discrete co mponents. The natural icing flight tests demonstrate that the compo nents 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 Re port - 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” c an 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 condition s 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 blo cked 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 Re port - 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 sign ificant ice in Appendix C can do so in Appendix X. Communica tion and navigation systems would

have to be checked with the ic e accretions on the antennae. Similarly, systems such as the weat her 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 recogn ized as a complex task. Even though the aviation industry has been us ing 14 CFR 25.1419 and Appendix C

standards for decades and the tools used for certification have matured, flight tests in measured natural ic ing conditions are still considered a valuable engineering tool and continue to identify areas of design or operating procedures where improvem ents can be made. The authorities see no reason why Appendix X s hould be treated differently than Appendix C.

I NITIAL M ANUFACTURERS ’ P OSITION

The airplane manufacturers examined the practicality of certification testing in natural SLD conditions. T hey concluded that there are multiple

issues that contribute to the di fficulty and hence the practicality of certification testing in natural SLD c onditions and that natural icing flight tests in Appendix X conditions should not be required.

Encounter Probability

One issue that has been brought up repeat edly in IPHWG deliberations is the probability of encounter ing 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 deliber ated on the current knowledge of the

SLD environment. Co-Chairs for th is working group were George Isaac (MSC) and Richard Jeck (FAA Technica l Center). In the published proceedings of this event, one of t he consensus items was that the definition of SLD was “any droplets larger than 50 microns diameter.”

### IPHWG Task 2 WG Re port - Appendix D Acceptable Compliance Methods For Appendix X Conditions (Natural Ice Flight Testing Necessity)

12/19/2005 Page D-6 As has been previously brought before the working group, this definition does not consider the drop distributi on effects of icing clouds. In a previously present ed technical paper, 4 it was proposed t hat the definition of SLD as drops greater than 50μm s hould stand. However, it was also recommended that agreement should be re ached on the definition of “SLD

conditions”. This new term would address distribution effects and align more closely with today’s Appendi x 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 envel opes with the term “mean effective diameter” (MED). This term 5 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 curr ent 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 per cent 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 an d is a probable icing condition.

However, it is unlikely that an airw orthiness authority would accept a 20μm

MVD encounter as a certification test representative of flight in SLD conditions. As an example, consi der a wing system designed with the aft protection limits set at the 40μm or 50μm impingement limit (mono-distributed as discussed in cu rrent AC20-73 and ACJ25.1419). Using the Langmuir “E” distribution, only 5 percent of the water even has the potential for accreting behind the pr otected area due to the drop size exceeding the 50μm protection limit. The remaining 95 percent of the water (smaller drops) will not have suffi cient 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 Re port - 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 co llection efficiency. The local collection efficiency approaches zero at the aft edges of t he protected area as shown in Figure D-1. Using the example, the local co llection efficiency near the protection limits is less than 1 percent. Assuming the flight conditions noted in the figure, the theoretical maximum a ccumulation potential is approximately 0.012 inches film thickness over a 10-minute exposure. 7 This analysis has some simplifying assumptions 8 but provides an or der of magnitude estimate to help illustrate the point.

The purported purpose of natural ice f light testing evaluations is as

follows:

The primary purposes for fli ght 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

environment 9

and

to consolidate the results obtai ned 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 unlik ely than any meaningf ul evaluation of degradation effects could be performed.

This effect indicates that the r eal acceptance threshold for a SLD certification test would require si gnificantly 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 propos ed in the previously referenced technical paper. 4 The result of this line of reasoning is that the reduced

7 Using maximum accumulation potential meth ods such as Eq. 2-29 and Eq. 2-30 of DOT/FAA/CT-88/8-1, "Aircr aft Icing Handbook".

8 Does not account for any runback, assumes i ce growth normal to surface, assumes clean efficiency characteristics, conversely does no t account for splash effects which would reduce accumulation potential.

9 FAA/AC 25.1419-1, Certification of Transp ort Category Airplanes for Flight in Icing Conditions, 8/18/99.

10 JAA/NPA 25F-219, Flight in Icing Conditi ons Acceptable Handling Characteristics and Performance Effects, May 1996.

### IPHWG Task 2 WG Re port - 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 great ly decrease the probability of encountering such conditions.

Figure D-1 - Example Impingement Characteristic

Previous Testing Experience

It has been reported 11 that previous attempts to perform development and/or certification testing in SLD c onditions were of limited success. The testing involved installation of an ice detector positioned to alert to the presence of ice behind the protected ar eas. 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 reported 12 that three successive winter flight test camp aigns were flown on company-owned aircraft, as well as evaluations of in-service experience, in the US and Europe for this same purpose. The f light tests were unsuccessful for a variety of reasons including, “[t]he rari ty 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 Re port - 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 r eported to the IPHWG that the NASA

SLD data collection for the 1998-1999 f light 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 colle ction. No data was collected due to

either extinction of the conditions bef ore arrival, lack of significant SLD conditions, or data issues due to inst rumentation problem s. 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 resour ces, such as frequent forecasts and in-flight updates through a satellite te lephone. A typical certification program would likely not have this leve l of meteorological support. This would decrease the pr obability of success.

As a practical matter, aircraft certif ication programs would incur a large expense to station an airc raft for an extended peri od to await SLD weather patterns to move into the test region. This expense would not only include aircraft flight and crew costs but w ould also include costs associated with accompanying flight test engineer(s), maintenance personnel, inspection personnel (both company and FAA delegat es) 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 “chas ed” based on available meteorological data (including NCAR products and pilot reports). The ability to forecast

SLD conditions is much less matu re than currently available icing products. Pilot Reports (PIREPS) will be of limited use since there is no requirement to distinguish SLD co nditions from conventional icing encounters. In addition, the succe ss of “chasing” a particular icing condition is heavily influenced by aircraft range, speed, and service ceiling. Due to the tr ansient nature of icing, aircraft with limited range, speed, or service ceiling have a dec reased probability of reaching the reported icing conditions before the conditions dissipate.

These factors provide actual flight te st evidence of the difficulty in finding SLD conditions that would provid e a significant validation of SLD

conditions.

### IPHWG Task 2 WG Re port - 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 dist ributions can provide a substantial amount of water in small drops a nd a relatively few large drops. Conversely, the distribution can prov ide a relatively large population of large drops with virtually no small cloud-sized drops.

Current certification enc ounters 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 unprotec ted surfaces, flight tests should explore stratiform ici ng clouds (Continuous Maximum Fig. 1) for a period of time repr esentative 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 unprotect ed 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 unpr otected 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 A ppendix C (Figure 1) liquid water content envelope with no horizontal extent adjustment. The primary variable sought is water conten t. Temperatures are a function of

water content and altitudes are taken as available considering sufficient ground clearance for safety.

### IPHWG Task 2 WG Re port - 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 ic e shapes due to distribution effects, the presence of large dr ops alone may not provi de adequate confidence to certification engineers t hat the conditions have been validated. This forces the potential candidate for SLD certification to pursue not only water content but also a specific dist ribution, again considering sufficient

altitude for safe evaluation. Given t hat the SLD conditions tend to be lowaltitude events, pursuing water conten t, altitude, and distribution effects reduces the probability of findi ng adequate SLD conditions beyond the limits of practicality. Accounting for th is variation in distribution effect is best suited to analytical methods.

Instrumentation Requirements

Research flights have carried extensive instrumentation for particle classification/sizing. As an exampl e, 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 addi tion to any for water content and temperature measurement requirements. The instrumentation probes require expert attention to maintain calibration and ensur e functionality. The difficulty in maintaining this inst rumentation was a c ontributing factor in the previously reported unsuccessful NASA research flight testing to obtain SLD data. Maintaining th is 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-p rocessing 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 consumin g and has taken months within the research environment to complete sufficient post-flight processing and analysis required to determine the fu ll extent of the conditions. The

aircraft certification environment is a fast-paced, costly environment where it would be extremely difficult to just ify 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 Re port - Appendix D Acceptable Compliance Methods For Appendix X Conditions (Natural Ice Flight Testing Necessity)

12/19/2005 Page D-12 Test aircraft typically require modifi cation to provide the structural support required for particle measurement syst ems for Appendix C certifications. Suspending pylon mounted, multiple 35- to 50-pound devices on an airframe is not a trivial task, par ticularly on small transport category

aircraft. This effort would require significant design and analysis support.

The modification includes extensiv e analysis to determine local drop trajectory effects and additional e ngineering to perfo rm aero-elastic stability analysis. Requiring the mult iple ranges of PMS would require multiple hard mounting points. This extensive aircraft m odification effort would be a significant expense.

This quantity of instrumentation repres ents state of the art as applied to quantifying large drop conditions. Ho wever, 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 ai rplanes are derivative in

nature and have extensive service hist ory, this cannot be assumed while proposing new certification rulemaking. With the low-altitude nature of SLD icing, increased risk would be pr esent 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 te st personnel. Most manufacturers will require some method of risk reduction. Wind tunnel testing is commonly used, as is dry air flight testing wit h simulated ice shapes. Dry air flight testing with “built-up” shapes allows for testing with varying degrees of

contamination to assess and reduce ri sk before flights wit h the full-span, representative ice shapes.

These simulated ice shapes will likely be the result of analysis and icing tunnel tests. This analysis can consi der a wide variety of conditions to determine critical ice shapes. The si mulated 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 mitigat ion 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 testi ng in natural SLD conditions.

### IPHWG Task 2 WG Re port - 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 te sting with simulated ice shapes. Each form of simulation has particular str engths 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 ar eas (potential hinge moment and stall handling effects)

• Ice shapes on unprotected areas (C L max , C M , and C D )

• Verification of exit c ues (visual or device based)

• Performance effects (drag fr om extended accretion areas) 13

Each of these areas of concern can be addressed through simulation methods. The following discussion prov ides examples of the simulation techniques and the relationships to the areas of interest.

Analytical Simulation

Analytical simulation methods incl ude impingement and accretion models based on computational fluid dynamics. There are existing Appendix C tools which are in the pr ocess of being modified to account for large drop

effects. Part of the development process includes validation of the large drop physics. These methods hav e the potential of simulating impingement and accretion aft of the pr otected areas, as well as resulting shapes on unprotected areas. Analytic al simulation appears to provide the best method of accounting for the va riability in drop distributions that are present in nature. Once t he SLD ice accretions are defined, aerodynamic results could be obtained thro ugh 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. Th is will become more accurate when the

13 The issue of performance (power margins relativ e to drag rise) in SLD conditions is a lesser concern for many transport category aircraft. Larg e-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 Re port - 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 ar eas over the tota l airframe. For aircraft that have potentially limit ed 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 prov ide 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. Ho wever, the distribution e ffects could conceivably be addressed through superposition of large and small drop effects (time/LWC scaled) or for accreti ons 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 det ermine aerodynamic results through wind tunnel or dry-air flight testing.

Icing tunnels could conceivably be us ed to examine impingement limits relative to fuselages or windscree ns with scale models. While scaling SLD icing shapes is still an emerging ar ea, 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 limit ed 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 relati ve to large drop impingement. This technique could potentially be used in a manner similar to icing tunnel

### IPHWG Task 2 WG Re port - 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 exami ne any potential hinge

moment or C L max effects. Alternately, the testing could be used to define

simulated ice shapes to evaluate aerodynam ic effects. This method also has the advantage of being able to comb ine the effects of thermal systems (such as runback) with direct accret ion 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 f light testing has long been used as a cornerstone of ice protection system dev elopment and certification. The primary differences betw een Appendix C and SLD co nditions 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 met hods of compliance. Simulation of SLD conditions will likely be similar as it may take a combination of a couple of methods to adequately repr esent the conditions and determine the degradation effects.

The realities of a new aircraft devel opment and certific ation program are such that early evaluation of ici ng effects via simulation is not only

desirable but essential. While program practices vary among manufacturers, the follo wing illustrates a general timeline for a large aircraft manufacturer prio r to first flight of a new aircraft model.

### IPHWG Task 2 WG Re port - 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 fli ght of the aircraft until the ti me that it is certified, data for the systems, onboar d computers, and simu lator are finalized. Flight-testing for icing certificati on (either in natur al icing or with manufactured ice shapes) has primarily become a “declare and verify” effort. It is expected t hat there will be no surprises during flight test which will require substantial re-analyses of the pre-f light data. Should a manufacturer be required to wait until AFTER the aircraft is flying to assess the icing effects, clearly ther e 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 t he aircraft flies to investigate ice accretion is highly impractical and unrealis tic. 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 relati ve to testing in natural icing conditions. This requirement wa s imposed by Amendment 25-23 in 1970. In the preamble materials, the stat ed reason for this change is: “The

current state of the art and current practice indicate that the flight tests in measure natural atmospheric icing c onditions specified in paragraph (c)(4) are necessary in all ice protection syst em certification programs . . . .”

### IPHWG Task 2 WG Re port - 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 requi re natural icing tests

should be based on the confidence level of the simulation techniques that could alternatively be used. The si mulation tools and capabilities that NASA, et al., are developing for SLD ar e planned to be validated tools. In fact, many of the large drop effe cts (such as splashing) are being developed through physical testing which also serves as validation points for that particular aspect of the lar ge drop physics. Another factor to be

considered is that t he proposed simulation tools are not completely new techniques; they are a continued pr ogression of current Appendix C simulation techniques.

In addition, there is a precedent to not r equire testing in specific conditions throughout the Appendix C envelopes . In AC 25.1419-1 it states:

Past experience indicates that fli ght testing in natural Intermittent Maximum icing conditions may be accompanied by severe turbulence and possible hail encount ers 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 t he Appendix C Intermittent Maximum conditions without requiring flight tests in such conditions.

Conclusion

There are multiple factors that contribut e to the impracticality of testing in natural SLD conditions. As presented, t he 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 Re port - Appendix D Acceptable Compliance Methods For Appendix X Conditions (Natural Ice Flight Testing Necessity)

12/19/2005 Page D-18 • Requiring natural SLD flight-tes ting with current state-of-the-art particle measurement technol ogy 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 allowanc e of validation

methods other than natural ici ng 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 leve l 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 reli able 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 te sts in natural SLD conditions not be required in the proposed IP HWG Task 2 rulemaking.

### IPHWG Task 2 WG Re port - Appendix D Acceptable Compliance Methods For Appendix X Conditions (Natural Ice Flight Testing Necessity)

12/19/2005 Page D-19 A UTHORITIES ’ R ESPONSE

The authorities’ response to the iss ues raised by the manufacturers was

as follows:

Probability of Encounteri ng Appendix X Conditions

The IPHWG has moved away from t he 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 Appendi x C, Continuous Maxi mum, 40 MVD Langmuir E distribution, see Figure D- 2. With SLD conditions defined with a Dmax of greater than or equal to 100 μm, SL D conditions were encountered 4 percent of the time during SLD res earch flights conducted from 1995 to 2000. Encountering SLD icing conditions 4 percent of the time is less probable than encountering icing 24 perc ent of the time. However, 4 percent of the time is not improbable. Applicants may have to relocate to geographical areas wher e 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, μmLWC, 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 Re port - Appendix D Acceptable Compliance Methods For Appendix X Conditions (Natural Ice Flight Testing Necessity)

12/19/2005 Page D-20 The authorities concede that fi nding SLD conditions that will be meaningful may take a longer ti me than Appendix C conditions.

However, the authorities have comment s on several of the statements:

“A typical certification program w ould likely not have this level of meteorological support.” This statement was m ade in reference to a research program that, “had access to significant meteorological resources, such as frequent fore casts 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 c onditions every time t hat 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. Th is tool represents a big improvement over what was available only a few y ears 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 “airplan e effects” code that will include “severe.” Although “severe” does not differentiate between SLD and conventional icing encounter s, it does represent conditions that exceed the capabilities of the airplane’s ic e protection system, which could be an

indication of SLD conditions.

An acceptable Appendix X natural ic ing flight test program has not been defined. However, it may not be t oo different from current Appendix C natural icing flight testing. Specific Appendix C LWC/drop size/temperature points cannot be ta rgeted due to the improbability of finding specific conditions. AC 25 .1419-1 states: “At least one exposure to icing conditions within the par t 25, Appendix C, continuous maximum icing envelope should be obtaine d.” Many times now it is difficult to judge the acceptability of an Appendix C encount er. For this reason the draft AC 20-73A will have appendices that provi de tools for such determination.

The authorities concede t hat Appendix X may be different since specific drop spectra are defined.

### IPHWG Task 2 WG Re port - 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 inst rumentation required to measure Appendix X conditions is more extensiv e than that currently required for Appendix C. Efforts ar e underway to develop Appen dix X instrumentation that is appropriate for certification. Commercial resources are currently available to measure Appendix X ic ing conditions. Equipment for measuring Appendix X ic ing conditions may also be rented. Some manufacturers may contract this serv ice for Appendix C certifications. Analysis is required to mount Appendi x 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 concent rated at lower altitudes, it is anticipated that flight te sting 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 alti tude after desired ice accretions are obtained. One of the objec tives 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 natur al icing flight tests.

Simulation Tools

The authorities agree that simula tion 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 aut horities believe that Appendix C natural icing flight tests are required regardless of whether natur al Appendix X flight testing is required. To reiterate the comment under “Simulation Tools,”

validation of the simulation tools is critic al if they are to be used in lieu of natural Appendix X icing f light test. For an airplane certificated to the

### IPHWG Task 2 WG Re port - 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 agr ee that flight testing in Appendix X is analogous to f light testing in in termittent maximum icing conditions. Flight testing in in termittent 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 anal ogous 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 diffi culties that the m anufacturers 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 ri sk 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.

M AJORITY W ORKING G ROUP P OSITION

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 us ing 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 natura l icing conditions. Acknowledging the difficulties in flight testing in natural Appendix X conditions, the majority of the IPHWG agree that flight testi ng in natural Appendix X conditions would not be required as in §25.1419 for Appendix C icing conditions. Flight testing in natur al Appendix X icing conditions was then

considered as one of the availabl e methods to support the required analysis.

Flight testing in natural Appendix X conditions should not be necessary if:

### IPHWG Task 2 WG Re port - 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 t hat the critical ice protection design points (i.e., heat loads, crit ical 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 fo r 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 airp lane performance with propeller and airframe ice accretion, asymmetric ice accreti ons due to propeller wash, engine performance with inlet (including cooling) ice accretions, or stall warning and characteristics with ice accr etion that affects air data used by stall protection systems; and

4. If the airplane is a derivati ve and is being certificated for flight throughout Appendix X icing c onditions, the service record of ancestor airplanes does not include a pattern of accidents or incidents due to in-flight encounters with Appe ndix 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 requirem ents 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 met hods of predicting Appendix X ice accretions should be shown to provide similar results (ice accretion thickness, location). One method s hould 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 Re port - Appendix D Acceptable Compliance Methods For Appendix X Conditions (Natural Ice Flight Testing Necessity)

12/19/2005 Page D-24 M INORITY P OSITION OF A IRBUS
, B OEING
, E MBRAER AND S AAB

Airbus and Boeing remain the minority IP HWG 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 nece ssary if . . . .” Even Airbus and Boeing agree that if the numerous criteria are sati sfied, then natural icing flight testing should not be necessary. However, that is not the point. The primary reasons for sustained disagreem ent with the majority are that the criteria remain overly burdensome in terms of cost and resources to be expended by the manufactur ers, and they believe that a flight test program in natural Appendix X conditions is mo re likely to result in an extensive research project than an airp lane 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 pr udent 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 re search 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 Ap pendix C ice-shape generation CFD codes, e.g., Lewice 2.0: it has no t 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 in ches but the wing chord of the 737- 300, for example, is approximatel y 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 thos e of all large modern aircraft are swept. Given these ex ceedances of the validation data, whether LEWICE 2.0 generates ice shapes that correlate with those accreted in natural Appendix C condi tions is unknown, and thus Boeing

does not utilize the LEWICE 2.0 code fo r certification purposes. There is

no reason to believe that the situat ion will be improved for Appendix X.

Validation of NASA’S LEWI CE 2.0 code utilized an extensive icing tunnel ice-shape database including modern airf oils; however, no such extensive database exists or is planned for NA SA’s Appendix X code validation. Unfortunately, based upon historical experience with Appendix C certifications, Airbus and Boeing bel ieve that it will be many years and numerous certification attempts before the authorities will accept even a

### IPHWG Task 2 WG Re port - 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 dete rmining 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 te sting in Appendix X conditions to be “found necessary” at the discretion of the authorities is an unacceptable

business situation. The Airbus and B oeing view is that the manufacturers should have the liberty and responsibilit y of determining which means of compliance are appropriate for their si tuation. They must not be burdened with the uncertainty that at the final hour of a certificat ion program, the

authorities will declare t hat natural icing flight testing in Appendix X conditions is necessary and therefore r equired. The rationale presented in

the “Initial Manufacturers’ Position ” remains valid. Airbus and Boeing stand by that position.

Notwithstanding the majority’s cont ention that the phrase “as found necessary” is not intended to force t he applicant to perform natural icing flight testing in Appendix X conditi ons, the fact is that the language remains in the proposed rule. If the ma jority sincerely accepts that natural icing flight testing in Appendix X conditions is merely one of the acceptable additional means of co mpliance and is not intended to be required, then they should be amenable to deleting the phrase “as found necessary” rather than stating in guidanc e material that they don’t actually intend for it to be necessary. Anot her good-faith alternative would be to incorporate into the rule itself l anguage similar to t he “should not be necessary if . . . “ provision from the Majority Working Group Position and proposed AC (of course, changing “shoul d 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 consens us on this issue stems from a fundamental difference of opinion that has not changed since the beginning of this effort: the authoritie s (or the majority) believe that the proposed new rule for Appendix X condi tions is necessary to improve safety for all Part 25 airplanes; Airbus and Boeing do not believe that safety will be improved for large jet tr ansports. 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 Re port - Appendix D Acceptable Compliance Methods For Appendix X Conditions (Natural Ice Flight Testing Necessity)

12/19/2005 Page D-26 M AJORITY R ESPONSE TO M INORITY P OSITION (ALPA, CAA/UK, C ESSNA
, FAA/FAA T ECH C ENTER
, M ETEOROLOGICAL S ERVICES C ANADA
, NASA,

T RANSPORT C ANADA /T RANSPORT D EVELOPMENT C ENTER )

Criteria Overly Burdensome

The minority state, “the criteria re main 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 c onsidered 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 ackn owledges the difficu lties 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 a ccept 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 A ppendix X conditions should not be necessary has been included in the pr oposed AC. The authorities concur with the proposed AC, which provides opt ions other than icing flight tests to verify Appendix X ice shapes. T he development of the methods for determining the ice shapes is expected to be incremental relative to existing Appendix C capabilit ies. 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 Re port - 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 degr ee that it is acceptable to the authorities and thus not require the use of two methods.”

The majority acknowledges that gover nment entities are actively pursuing the development of engine ering tools for use in determining Appendix X ice shapes. However, it is ultima tely the applicant’s responsibility to substantiate that the engi neering tools employed dur ing 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 pr udent 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 w ould 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 re quirement 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 us e of similarity analyses that allows an applicant to take advantage of past certifications by applying the previous substantiation to another program. The us e of similarity analyses has been successfully used on many programs. There have been cases

where a manufacturer has substant iated 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 certificati on programs increases. There is no need to revise the rule as suggested by the minority.

“As Found Necessary”

1. The minority position st ates: ‘The potential for natur al icing flight testing in Appendix X conditions to be “found necessary” at the discretion of the authorities is an unacceptabl e business situation.’

This appears to imply that the authoritie s will arbitrarily decide that natural icing flight tests in Appendix X condi tions 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 Re port - Appendix D Acceptable Compliance Methods For Appendix X Conditions (Natural Ice Flight Testing Necessity)

12/19/2005 Page D-28 ambiguity regarding the m eaning 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 r eport 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, t he 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 comp liance with the regulation. The applicant will choose to us e 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) whic h 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 m ean that with regard to flight testing in natural icing cond itions, the majority “don’t actually intend for it to be necessary.”

The majority finds this to be an incorre ct 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. Includi ng 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 iss ue of the applicability of the rule. This issue is addressed in Appendix F of this Working Group Report.

### IPHWG Task 2 WG Re port - Appendix E Necessity to Require a Means to De termine 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 Re port - Appendix E Necessity to Require a Means to De termine Exceedance of Appendix X

12/19/2005 Page E-2 N ECESSITY O F A M EANS T O D ETERMINE E XCEEDANCE O F A PPENDIX X

The tasking statement requires: "In additi on, consider the need for a regulation that requires installation of a means to discriminate between conditions within and outside the certification envelope.” Fo r 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 cert ification envelope is requi red by the rule language and clarified in the advisory material. Th is 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 env ironment in terms of an engineering design standard. The database used fo r developing Appendix X consists of

measurements made in winte rtime stratiform clouds. No measurements were

made in cumulus clouds. Convective fl ows within cumulus clouds are known to produce higher liquid water (LWC) cont ents than stratiform clouds. Appendix H of this report provides more detail on why it was reasonabl e to develop Appendix X using wintertime stra tiform cloud data only.

The IPHWG proposed Advisory Circula r AC 121-XX contains the following

statements: “The A ppendix C conditions were designe d to include 99% of icing conditions. Evaluation of icing data has indicated that the probability of encountering icing outside of Appendix C droplet conditi ons is on the order of

10–2”.

The IPHWG has proposed to use the 99t h percentile of li quid water content (LWC) for Appendix X. Therefore, fo r Appendix X encounters the probability of LWC values greater than Appendix X is on the order of 10–2.

There are other mitigating considerations t hat are conservative aspects of an ice protection system certificati on program. For example, the probability of being in icing is assumed to be one for certificat ion purposes. Ice shapes described in Part II of Appendix X for use in dem onstrating compliance with the airplane handling and performance requirements of 14 CFR § 25.21(g)(2) & (g)(4) are defined to provide a conservative assessm ent of the icing e ffects to ensure an acceptable level of safety. Holding operat ions 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 ca tch point is typically a very specific set of conditions and conservative assump tions such as aircraft weight, holding airspeeds, and ambient temperatures . The probability of an aircraft simultaneously achieving or exceedi ng these design assumptions is not quantifiable but is conservative. Many certification programs have further

conservative factors such as assumi ng minimum system tolerances such as minimum pressure regulators or temperature controller set points.

### IPHWG Task 2 WG Re port - Appendix E Necessity to Require a Means to De termine Exceedance of Appendix X

12/19/2005 Page E-3 C ONCLUSIONS

No definitive quantitative analysis can be performed to show that the hazard of flight in conditions beyond certificat ion limits meets commonly accepted probabilities for cata strophic events. However, gi ven the conservative factors described above, the IPHWG believes that th e probability of exceed ing the limits for aircraft certification is sufficiently low to prevent a catastroph ic event, and a means to alert flight crews when 14 CFR part 25 Appendix X icing cond itions 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 APPLI CABILITY 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 P OSITION STATEMENT SUPPORTING EXCLUSION FROM §25.1420 FOR A IRPLANES WITH C ERTAIN D ESIGN F EATURES

(Airbus, Boeing, Embraer, Cessna 14 )

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 Cessna 14 (“the major airplane manufacturers”) is strongly supported by the safety record of the cla ss of airplanes proposed for exclusion, which today routinely fly in icing conditions bot h within and outside of Appendix C. Compliance with the proposed new certification require ments of §25.1420 to address icing environments including s upercooled large droplet conditions (“SLD”) would be extremely costly, and no safety improvement can be expected. Consequently, the major airplane manufacturers c ontend that there is no justification for making eit her the certification require ments or the certification process more burdensome for these types of airplanes.

Introduction

The Ice Protection Harmonization Work ing Group (“IPHWG”) assembled accident

and incident databases of events in SLD c onditions 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 pr evented the event). Review of the databases revealed that airplanes of certai n types or with certain design features have experienced no accidents and very few in cidents due to in-flight icing, either inside or outside of Appen dix C environments. It is therefore reasonable to conclude that no increase in safety with re gard to in-flight icing need be achieved for these types of airplanes. The majo r airplane manufacturers thus propose the concept of “exclusionary design features,” with the intent that future airplane designs which incorporate these feat ures be exempt from demonstrating compliance with proposed §25.1420.

The major airplane manufacturers propos e 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 ha ve all three of these ma in features. However, there are other design featur es that warrant considerat ion 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-i n-Icing Certification

The icing certification rules were establis hed in the 1940’s a fter World War II. The certification process wa s subsequently defined by gradual development of the advisory material. The 14 CFR/JAR require ment is that an airplane be able to operate safely when flying in icing co nditions. Compliance is shown by evaluating the ice protection system (“IPS”) design and the airplane’s performance when subjected to an env ironment defined by design icing envelopes known as 14 CFR/JAR Part 25, Appendix C. Advisory material provides guidance on the conservative cr itical ice shapes to be considered when showing compliance. Ice accretion a ssociated 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 ic ing parameters for a given size cloud: mean effective diameter (“ME D,” 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 accumu lation. In-flight assessment of the actual shape and thickness is very diffi cult and results are approximate. These difficulties are compounded by a nother 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 measur ements. 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; f light 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 transiti on away from reliance upon natural icing flight testing as the primary m eans of certification compliance.

An airplane certified for flight in ic ing today typically has no environmental limitations on its in-flight operation in icing condition s. Since Ap pendix 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 condi tions of Appendix X do not pose a greater or addition al 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 hor izontal extent of the condi tions after takeoff. No

accidents nor incidents have occurred due to takeoff and subsequent flight in these conditions. Therefore, for airp lanes possessing the exclusionary design features, certification for flight in ici ng using Appendix C cond itions for design of

the IPS and determination of the critical ice shapes should be considered an equivalent method of compliance provid ing global protecti on (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 te st of use, and since the total of experience with existing aircraft is more comprehensive than any data-collecting program, it is sugges ted that the future changes in criteria be based primarily on oper ating experience rather than on meteorological data.

This opinion is very much in unison wit h the assertion of the major airplane manufacturers – that the current large transport category airplanes, a fleet of nearly 20,000, already demonstrat e safe operation in icing.

Exclusionary Design Features

The major airplane manufacturers propose that airplanes possessing a combination of all three specific desi gn features be exempt from demonstrating compliance with the proposed §25.1420. T he 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 de fining the limit between “larger” and “smaller” airplanes, review of the ici ng safety record reveals the facts:

• Zero in-flight accidents with larger airplanes 15

• Numerous fatalities from accidents of smaller airplanes.

The advantage of size with regard to in-f light icing considerations is well known based upon decades of testi ng and experience and does not have to be re-proven.

Based upon review of the accident/inc ident database, it is proposed that size be an exclusionary design feature fo r airplanes with gross weight in excess of 60,000 pounds (27,000 kilogr ams). 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 w ould 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 forc e/trim effects is not applicable to such systems. Since the loads gener ated at the control surfaces are reacted against the actuator and its mounting, changes in these loads cannot be transmitted through the cockpit c ontrols. The use of irreversible powered flight controls is implic itly related to reduced airplane susceptibility. The choice of irreversib le flight controls is related to the aerodynamic loads on the control surfac es. 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 ici ng environment. The scale issue is discussed in

item 1 above.

3. Wing Leading-Edge High-Lift Devices . For wings without ice contamination, wing leadi ng-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 ai rplane 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 hav e smaller relative CLmax losses

due to ice accretion than are suffered by wings with fixed leading edges. This reduced sensitivity to the adver se effects of ice accretion is applicable to the critical takeoff, initial climb, approach, and landing phases of flight.

Significant facts associat ed 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 fe wer 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 l eading edges of the

horizontal and vertical tails. For ex ample, 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 comme rcial 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. T he 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 featur es which may have a positive effect on mitigating performance degradation due to ice accumu lation. Such features should be considered as potentially pr oviding a level of safety equivalent to the three primary exclusionar y 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 exam ple, some tails are designed to provide full functionality wit h critical ice accretion. Some airfoil designs are less susceptible to icing effe cts 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 roo t, 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 susc eptible 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 dev ice 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 uncontam inated 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 perfo rmance 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 excl usionary design features enjoy an exemplary safety record with regard to in-f light icing. Tribute for this must be given to the current certification standar ds and methods, which are successful for these types of airplanes. This succe ss 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 ic ing certification standards adequately protect many airplanes against any type of ici ng 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 mo re stringent certification requirements penalizes new airplanes relative to previous ones, which have proven to be safe as certified to the previous requirem ents. 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 c onsistent with the recommendations of the Commercial Aviation Safety Team, which has determined t hat the operational data support application of an expanded icing envelope onl y to airplanes without thermal anti-icing systems. 16 Burdening all airplanes with certification efforts that result in no significant increase in safe ty does not benefit the flying public and is contrary to the purposes of ARAC rulemaking.

Cessna Position Statement

As a manufacturer of small-scale Tr ansport and Normal Category airplanes, a

scale-based discriminator will not include mo st Cessna products. However, it is recognized that there is technical justif ication for the reduc ed susceptibility of certain airplanes based on scale and other design features. Cessna is supportive of the recognition that airpl anes with certain desi gn features are less susceptible and will have a reduced safety benefit as a result of large drop rulemaking. It is also recognized that improved operat ing procedures and training is a major factor in increasing safety in icing conditions, whether in Appendix C or large dr oplet 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 certific ation 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 Harmonizatio n 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 M AJORITY P OSITION

- R ESPONSE TO E XCLUSION FROM §25.1420 FOR A IRCRAFT WITH C ERTAIN D ESIGN F EATURES

(ALPA, CAA/UK, FAA/FAA Tech Center , Meterological Services of Canada, NASA, SAAB, Transport Canada/Trans port Development Center)

Background

The existence of SLD icing conditions, consisting of freezing drizzle and freezing rain, is unquestioned. Transport airplanes, regardless of airp lane size, weight, high-lift configuration design, or flight c ontrol system design, r outinely operate or may be inadvertently exposed to these icing conditions. Yet, during the approval of the airplane for flight in icing conditi ons, transport airplanes are not required to show compliance with minimum airworth iness requirements that would ensure safe operation in these SLD icing conditi ons. The safety margins when operating

in these conditions are unknown.

While some transport airplane manufacture rs remain silent on the operation of their aircraft in SLD icing conditions, others recommend that their airplanes not be operated in SLD icing conditions. Fo r example, Gulfstream states the following in the operations manual fo r their Gl-GV airplanes: "Under no circumstances will flights be planned thr ough 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 operatio nal experiences in SLD indicate that the 14 CFR part 25 Appendix C icing c onditions 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 assessm ents 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 info rmation used to establish Appendix C:

Since these criteria have stood the te st of use, and since the total of experience with existing aircraft is more comprehensive than any data-collecting program, it is sugges ted that the future changes in criteria be based primarily on oper ating 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 primar ily 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 SL D icing conditions is further mired in related confused industry practices and unclear info rmation provided to pilots relative to deciding when operating their airplane in SL D icing conditions is safe. Annually, the FAA issues a Flight Standards Info rmation 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 explicit ly state that the fl uids do not provide protection during flight, some flight crews interpret the inclusion of light freezing rain fluid protection guidelines in t he FSAT as an FAA approval for flight operations in SLD icing c onditions. Action by the FAA to expand the 14 CFR part 25 icing condition standards to incl ude SLD will address operational issues associated with flightcrews lacking clear guidelines fo r 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 irreversib le 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 res ponse 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 tr ansport category airplanes. In general, specific airworthiness standards are defined with the intent of ensuring safety of flight. These standards state airworthi ness objectives or absolute requirements, and have been formatted to be generally applicable to future airplane designs. Typically, a limit is included in th e airworthiness requirements when the requirement applies to a specific configur ation 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 airworth iness requirement is only appropriate when there is high confidence that the boundary of the app licability can be appropriately chosen. To evaluate t he confidence in the proposed boundary one

must consider if there ar e unknown factors involved. For example, the safety record of large aircraft may be due in par t to the prevalence of safety equipment that are not typically installed in sma ller 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 airp lanes. Another example would be the unknown effect of future new, modifi ed, or innovative airplane designs and modifications on past operational experience. These two examples illustrate that we cannot predict with conf idence that the past servic e experience of airplanes with specific design features will be applicable to future designs.

Historically the FAA has not limited t he 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 specia lists of Europe, North America, and Sout h 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 ru lemaking drafted by the FTHWG, commonly referred to as §25.21(g), et al. These proposed airworthiness requirements are applicable to all trans port 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 devis ing 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 gener ally extend the proposed requirements of §25.21(g), et al., to Appendix X SLD icing conditions.

Operations in Appendix X SL D icing conditions are expect ed to result in airplane performance degradations and adverse changes in airplane stability and control different than, and potentially gr eater than, those resulting from operations in part 25, Appendix C. Limiting the applicabi lity 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. Theref ore, limiting the applicability of the proposed §25.1420 would re sult in the unacceptable situation of requiring higher levels of safety for ce rtain airplanes and lower levels of safety for other airplanes with regard to perfo rmance, 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 des ign feature that reflects the alleviating effect of airplane size on the adverse aerodynamic effects of ice accretion. However, the ratio of wing and control su rface sizes to airplane weight varies between airplane designs. Also, the chord of wings and control surfaces vary with the aspect ratio of these surfaces. Ther efore, airplane takeoff weight is not a consistent indicator of lifting and c ontrol surface size or chord.

The proposed 14 CFR §25.21(g), et al., recognizes the benefic ial effects of airplane size, relative to the adverse effects of ice accretion, by the est ablishment of airplane performance thresholds at which airplane performance degradatio ns resulting from

ice accretion must be included in the ai rplane 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 per formance 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 ai rplanes relative to the ice contaminated stability and control requirements pr oposed by §25.21(g), et al. Actual substantiation of the effects of airpl ane size on the aerodynamic effects of ice accretion, as is required by the propos ed §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 requi res analysis based on 14 CFR Appendix C icing envelopes. The larger drops in SL D conditions will yield impingements, and ice accretions, further aft than Appendix C conditions, along with potentially different accretions in the leading edge area. T hese SLD shapes and locations have not been quantified in terms of performance degradation and handlin g 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 provi de protection against uncommanded motion of a flight control surface due to a ridge of ice af t of the protected ar ea 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. Moreov er, 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 ic e accretion. Thus, especially SLD ice accretion might change the stall characteristi cs 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 perfo rmance margins defined by subpart B of 14 CFR part 25 are predi cated on stall speed. However, any aerodynamic

benefits attributed to the devic es in icing conditions would not be present when

the devices are stowed for the cruise configuration.

Arguments have been made that airplanes wi th leading edge high-lift devices are less sensitive to leading edge surface r oughness. However, maintenance and repair requirements of 14 CFR part 41 and the requirements of 14 CFR §121.629, and similar other operat ing rules, ensure that airplane surface finishes

are maintained to manufacturers’ standar ds and ensure that transport airplane surfaces are free of adhering ice prio r to takeoff. Also, means used for demonstrating compliance with 14 CFR par t 25, subpart B, provide for using the most adverse of allowable tolerances, su ch as equipment tolerances and surface finishes, for those demonstrations. The issues of surface smoothness and the adverse aerodynamic effects of n on-smooth surfaces are addressed by compliance with existing regulations and c onformance with existing certification practices.

The benefits of various leading edge high-li ft device designs relative to leading edge surface roughness may vary. A signifi cant portion of these benefits are due to boundary layer control resulting fr om a well-designed slot between the leading edge high-lift device and the bas ic 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 s ensitivity to leading edge surface roughness as “hard wing” airplanes. The takeoff position of slats may be automated, with the slat ex tended and sealed at takeoff operational attitudes and further extended and deflected, opening a slot at the slat tra iling-edge, as stall angles of attack are approached. Such sl at designs may provide tolerance to wing leading surface roughness. The le vel 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 in clude: slats that may be slotted or sealed to the basic wing leading edge, ov er or under deflected, deflection and slotting of the slat that may be automa ted as a function of stall warning or airplane angle of attack; Krueger flaps t hat may be slotted or sealed to the wing leading edge, flexed to opt imum curvature or confo rmed to the wing’s leading edge lower surface; vortilons or some ot her 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-li ft devices varies from 100 percent for some early turbo-jet airplane slats to t he span of two slats for later airplane designs and to none for Krueger flaps. T he 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 Khodadoust 17 states that there is very little applicable data available addre ssing 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 li ft 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 aircra ft this kind of rough SLD ice may result in adverse aerodynamic effects.

Use of wing leading edge high-lift devices to limit the applicabilit y of the draft 14 CFR §25.1420 is considered inappropria te since: the adverse aerodynamic effects of icing must be addressed fo r all phases of flight and airplane configurations; the variabi lity of wing leading edge hi gh-lift device and airplane ice protection designs (and associat ed benefits that may be attributed to utilization of the devices); and, the lack of applicable da ta available that address the aerodynamic benefits resu lting from the devices du ring flight in icing.

Issue No. 2
– Exclude airplanes with other desi gn features (configuration of airfoil design, stall protection dev ices, thermal ice protection system)

Response No. 2

Airplane design features may exist that mitigate the adverse effects of ice accretion on airplane performance and stabi lity and control, such as leading edge sweep, airfoil camber and thickne ss 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 accr etion on aircraft aerodynamics,” Progress in Aerospace Sciences, Vol. 37, p. 730, 2001.

18 Reinmann, John J., “Icing: Accret ion, 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 spec ific design features the minority has proposed to be used to limit the app licability of the proposed rule.

Configuration or Airfoil Design

The minority states that la rger drops split into thousands of small ones, which

never hit the airframe. Although understood for other ap plications (e.g., ink jet printing and fuel atomizati on), the bursting of large wa ter drops because of shear forces as the drops approach surfaces and the bouncing and splashing of large drops is an emerging area of icing ph ysics 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 bu rsting 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 system s varies with respect to coverage and capability. Running wet syst ems are becoming more prev alent 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 t he 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 t hermal ice protection systems may vary and

offer little confidence as a basis for lim iting 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 accomp lished. Therefore the majority does not concur that it is possible to c onclude that the cost s would outweigh the benefits for the airplanes t hat are proposed to be excl uded 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 ic ing because there is a trend toward minimizing the areas protected from ice accretions and increasing the

areas which run wet rather than minimi zing the protected surface wetness by evaporation of the impinging cloud drops.

2. Zero accidents or incidents do not m ean that sufficient safety margins are always maintained.

The majority acknowledges that the manuf acturer 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 A ppendix X. The general trend for new

aircraft designs is to further improve airplane efficiency by reducing the protected areas, reducing the amount of heat, changi ng 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 pr actices ensure future airplanes will have an acceptable safety record in Appendix X icing conditions.

The safety margin for Appendix X c onditions afforded by compliance with Appendix C would be dependent upon the specific airplane and the means of compliance. Therefore, without dete rmining 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 suffici ent to ensure that Appendix X does not pose a greater or addi tional 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 re commendations of the Commercial Aviation Safety Team (CAST), whic h determined that th e 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 inc onsistency. This issue references the CAST Loss of Control (LOC) Joint Sa fety 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 accident s, all of which involved airplanes equipped wit h pneumatic deicing boots. With such a limited evaluation, no data was examined to just ify a decision regarding the adequacies or inadequacies of a thermal anti-icing system or aircraft size . Under issue No. 2, the majority expl ains why this proposed rule should not be limited to airplanes without thermal ice pr otection systems.

Issue No. 6 – Cessna Position Statement

a. There is technical justificat ion for the reduced susceptibility of

certain aircraft based on scale and other design features.

b. Supportive of recognition t hat aircraft with certain design features are less susceptible and will have a reduced safety benefit as a result of superc ooled large drop rulemaking.

c. Improved operating pr ocedures 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 pr opose 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 feat ures help reduce the effect of icing. However, these design feat ures 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. Ho wever, the majority does concur with the general statement that operating procedures and training are important factors that improve the sa fety 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 M INORITY P OSITION R EGARDING C ERTIFICATION TO A P ORTION OF A PPENDIX X

(ALPA, Metrological Se rvices of Canada)

The Appendix X rule and AC provide for “certification with a range of techniques for example, limiting the range of liquid water content s, 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 c ontent . . . .” There is no guidance for either an applicant or an appr oving authority for how to apply the four spectra and their individual LWC’s in the Append ix X rule to “acc ount 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 environm ents. There are no “envelopes” in Appendix X to determi ne what spectrum or LWC should be used to represent the chosen “portion of t he envelope” for a par tial spectrum or

limited LWC certification. Appendix X contains four “average spectra” which were not designed or demonstrated to repr esent actual SLD environmental data for other drop sizes or other LWC’s than those containe d 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 proce ss would be necessary to insure valid ice shapes and provide consistent safety le vels 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 ma terial, since Appendix X provides no way to relate such a proposal to act ual SLD icing environments. Accurate representation of the environm ent 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 M AJORITY R ESPONSE (Airbus, Boeing, CAA/UK, Cessna, Em braer, 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 particu lar, the AC acknowledges that initial certifications to a portion of Appendix X will likely include a ll of freezing drizzle or all of freezing rain, or will be restricted by phase of fligh t. The AC material also discusses some potential concepts that could be used to a ccomplish 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 ex it” 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 ne ed to be related to the proposed method of identifying the icing conditions for t he portion that requires exiting. Language was added to the AC to clarify that cert ifications to a portion of Appendix X

should consider the methods used for dev eloping Appendix X, with a reference to a detailed report to be released by the F AA Technical Center. Further, proposed advisory material discusses that the AF M Limitations section should contain appropriate limits that consider the certification assumptions with respect to approved portions of Appendix X. As su ch, if an aircraft is only approved for a portion of the freezing drizzle envir onment 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 certif y to a portion of Appendix X.

Eliminating the alternative for certificat ion to a portion of Appendix X creates a more prescriptive rule that has the pot ential 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 unres tricted in the full SLD environment. Removing certification to a portion remo ves any benefit for manufacturers to certify for a part of the A ppendix X envelopes if the ai rcraft can safely operate therein. The ability to certify to a portion of the env ironment allows manufacturers to certify to as large an ic ing envelope as safely possible. This provides a balance of economic pressures (aircraft operators desire to dispatch with a minimum of restrictions), yet provi des a consistent level of safety (with established margins) via the ty pe certification process.

In addition, the ability to certify to a por tion of Appendix X provides incentive for technology development relative to both det ection 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 des ire expressed in the Minority Position to provide a validated method for certific ation to a portion of Appendix X in the

AC materials is not consistent with a llowing 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 cert ify with this option. As with many new types of certification efforts, extens ive 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 dr afted to be performance-based and not prescriptive. The majo rity of the IPHWG reco mmend 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 t hat the performance-based rule language and the added draft advisory material pr ovide sufficient guidance to future applicants and regulatory personnel regardi ng 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 M INORITY P OSITION A GAINST C ERTIFYING TO F REEZING D RIZZLE OR F REEZING R AIN U SING A S UBSET OF THE A PPENDIX X C URVES

(Meteorological Service of Canada, ALPA)

The four spectra with associated LWC limits were developed to describe the entire Appendix X environm ent as it could be determined by all of the measurements available, rather than producing a continuous envelope that covered all SLD conditions. A managea ble number (four) of curves were selected to allow tests using codes, ici ng tanker, and wind tunnel simulations. During development of these sp ectra, partial certificati on to a portion of Appendix

X was not considered. Later it was proposed that a m anufacturer 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 spec tra that might be

observed below cloud at the surface. In addition, airport observers do not identify freezing drizzle or fr eezing precipitation using the precise criteria used to develop these curves, so this could pos e a problem. For ex ample, freezing drizzle is defined by the surface observe r 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 observe r 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 prec ipitation rate. It also shows the freezing drizzle and freezing rain precipitati on rates, measured every minute using a heated gauge with the occurr ence of freezing drizzle or rain as being reported by the ground observer. T he surface data were obtained from an analysis performed by Transport Canada for several stations in Quebec, and contain 3,692 minutes of fr eezing drizzle and 10,833 minutes of freezing rain. Obviously, the gauge data show s 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 dr izzle. This is not very surprising. The surface observer needs to quickly estimate the drizzl e/rain distinction using visual cues which are difficult to use, and precipitat ion rates are naturally highly variable, especially on a time scale of minutes. T he extreme drizzle rates as measured by the gauge were probably incorrectly classi fied 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 maxi mum criteria of A ppendix X for freezing

drizzle because the observer had misidentifi ed 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 o ff 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 m ade 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 defin itions of freezing drizzle and rain are different than those used for Appendix X, and the fact that extreme values of

surface observations of freez ing drizzle fall well within the freezing rain envelope, it would not be prudent to allow certificat ion to freezing drizzl e 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-f light versus Ground Observations

Note: Shows the 50, 75, 95, 99, 99.9% precipitation rate values of freezing drizzle and freezing rain as determined fr om 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.

M AJORITY R ESPONSE (Airbus, Boeing, CAA/UK, Cessna, Em braer, FAA/FAA Tech Center, NASA, SAAB, Transport Canada/TDC)

The majority of the IPHWG support the co ncept of certification to a portion of Appendix X. Discussion of the motivation fo r retaining the abilit y to certify to a portion is contained in the above IPHWG ma jority position relative to the ALPA position statement against certification to a portion. This position statement will focus on the concerns raised by the Minorit y Position relative to certifying to a portion of Appendix X and to the difficult y in differentiating between freezing drizzle and freezing rain at the terminal areas.

Certification to a portion of Appendix X

The minority position states that dur ing the development of Appendix X, certification to a portion was not consider ed. The majority of the IPHWG do not agree with this statement, as use of a por tion 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 ex tent as determined by the applicant." As such, certification to a portion has been intended since concept approval. During much of the development of t he draft SLD rule language and supporting documentation, some of the IPHWG fo cused 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 a llowing 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 t he freezing precipitation were stratified into four categories, based on the maximum drop size contained in the spectra of each

measurement. Measurement s containing maximum drop sizes of less than 500

μ m, but greater than the maximum drop sized commonly considered as a cloud drop size (100 μ m), were defined as freezing dri zzle. The maximum drop size of

100 μ m is consistent with the use of the Langmuir D distribution for the Appendix C stratiform cloud maximum average dr op size. Measurements containing maximum drop sizes greater than 500 μ m were defined as freezing rain. These definitions of freezing dri zzle and freezing rain were selected to be consistent with accepted meteorological definiti ons 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 μ m. 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 strati fy the continuum of freez ing precipitation into commonly accepted regimes of freezing dr izzle and freezing rain. The database used for characterizing the freezing prec ipitation environment can be thoughtfully and carefully stratified using other definit ions. 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 pr ecipitation contained in the Appendix X data base. The propos ed 14 CFR § 25.1420 addresses safe inflight airplane operations in Appendix X and does not address terminal area freezing precipitation diagn osis and reporting. Gr ound-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 freez ing precipitation, and ground-based measurements of precipitat ion rates were used to ens ure that the measured inflight precipitation rates were compatible . This comparison of the precipitation intensities provided confidenc e that airplane deicing and anti-icing practices did not permit dispatch of airplanes into freez ing precipitation alo ft rates greater than those addressed by Appendix X. No te that ground-based observations of precipitation rates (discussed in the mi nority position), espec ially 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-ph ase precipitation. Theref ore, the majority of the IPHWG has concerns about the signifi cance of the ground and in flight comparison presented in the minority opinion.

The need for accurate diagnosis of freezi ng precipitation in the terminal area currently exists for dispatching airplanes for takeoff in freezing precipitation. Reported freezing precipitation is used to re strict takeoff in freezing precipitation that exceed the capability of anti-icing fluids to protect th e airplane from ice accumulation during ground operations and t he 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 cat egory (drizzle or rain) and intensity of freezing precipitation hav e 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 def inition in the Amer ican Meteorological Society's "Glossary of Meteorology" states that the fall rate is "usually" not greater than 1 mm/hr. The mi nority position also states that "it is very difficult for a surface observer to identify freezing dr izzle visually and distinguish it from freezing rain." However, an expert consul ted by the majority states that impact behavior is generally accepted as a reliabl e 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 substant ially 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 mi nority then states the high rates are "probably" due to rain being "incorrectly cla ssified" as drizzle. The use of the word "probably" acknowledges that ot her 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 sm aller 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 misdi agnosis can occur in discriminating between freezing drizzle and rain, and s upports 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 leve l 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 an d the precipitation levels below the cloud. This results in the use of 99 perc ent LWC over the full 12,000 ft. Previous

IPHWG deliberations determi ned that the mean vertical extents are closer to 3,500 ft and 2,900 ft for freezi ng 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 effe ct of decreasing the aircraft’s vertical

climb rates and increases the durati on 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 A ppendix X will have a means to determine

exceedance conditions. The result of th is situation would be that exceedance cues would be present and would trigger an exit from the rest ricted portion of the

SLD environment. The results of an airc raft taking off into a freezing rain

encounter that was misidentified as freez ing drizzle would not be significantly

different than those associated with an in advertent encounter with freezing rain aloft. This type of operation would not resu lt 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 resu lt will likely cause an increase in the number of diversions, or ot her operational consideratio ns, but will not result in an unsafe condition. Data has been prov ided regarding the economic impact of diversions for use in the rulemaking pr ocess (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 reli able indicator). Furthe rmore, the majority

of the IPHWG also believe that the cons ervatism 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 Append ix X & Part 33 Appendix D

12/19/2005 Page H-1 

### APPENDIX H - DEVELO PMENT OF 14 CFR PART 25 APPENDIX X

& 14 CFR PART 33 APPENDIX D

### IPHWG Task 2 WG Report - Appendix H Development of Part 25 Append ix X & Part 33 Appendix D

12/19/2005 Page H-2 This appendix provides discussion of t he 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, A PPENDIX X

Appendix X was derived from a database containing in-situ measurements of supercooled large drop (SLD) icing conditi ons by suitably instrumented research aircraft. Data from many research ca mpaigns, 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 dete rmine the spatial aspects of SLD icing conditions, such as altitude and temperat ure ranges. Concerns about significant amounts ice crystals in traditionally meas ured liquid water content (LWC) data, and the desire to more precisely def ine SLD drop spectra and LWC values, resulted in a subset of more precise, multiple sensor data from recent Meteorological Service of Canada (MS C) and the National Aeronautics and Space Administration (NASA) research flights being used for these purposes.

This insured both the wides t applicability and the great est confidence in the critical details for the Appendix X envir onment. See following references for

more information.

G ENERAL

The relationship between Appendix C and A ppendix X icing conditions is limited. The data measured to produce Appendix X wa s taken in wintertime stratus cloud formations. As such, the Appendix X cond itions are most closely associated with

the stratus conditions used to define the existing Appendix C, Continuous Maximum conditions. Nearly all of t he data contained in the IPHWG SLD icing

conditions database are related to wintertim e, stratiform cloud conditions. It is known, however, that supercooled la rge drop icing conditions are generally plentiful in warm season, vigorously growing convective clouds (Cumulus Congestus, Towering Cumulus, Cumulo nimbus) 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 dr izzle and raindrops.” This means that airplanes penetrating the cores of these cl ouds 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 characteri zation 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 Append ix 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 characterizati on of summer SLD icing conditions are:

• The relative ease of visually reco gnizing 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 expos ures (at least in Cumulus Congestus and Towering Cumulus clouds).

The in situ data obtained for determini ng 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 A ppendix 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 cl assified 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 Append ix X, including freezing drizzle and freezing rain. The two types of freezing precipitation are further divided into conditions in which the drop median volu me diameters are either less than or greater than the 40 μm, the largest m ean 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 En vironment Including Large Droplet Icing Conditions." Dividing the freezing dri zzle and freezing rain into these two categories approximates freezi ng precipitation icing condit ions 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 Append ix X & Part 33 Appendix D

12/19/2005 Page H-4 Appendix X consists of m easured data that was divide d 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 dr op diameter. By definition, this acknowledges the dist ributed 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 ici ng conditions. As referenced in

DOT/FAA/CT-88/8-1 “Aircraft Ici ng 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 ic ing 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 ty pe of distributions for determining the drop size intervals do not adequately def ine the bi-modal nat ure of freezing precipitation. Consequently, the distributions of drop sizes are defined as part of Appendix X. The need to include the distri butions 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 sa fe flight of the airplane.

The variation among freezing precipitation drop diameter distributions is very large. Subdividing freezing drizzle and freez ing rain allowed selection of the four average drop diameter distributions that are representative of the total range of drop distributions. Appendix X drop diam eter distributions are shown as

cumulative mass distributions.

Also, the maximum values of freezing drizzle and freezing rain liquid water content decrease with decreasing tem perature as defined in 14 CFR 25, Appendix X, figures 1 and 4, respective ly. Following the same standard as 14 CFR 25, Appendix C, the values of tota l 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 f eet 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 driz zle has been reported at temperatures as cold as -13° F (-25°C).

### IPHWG Task 2 WG Report - Appendix H Development of Part 25 Append ix X & Part 33 Appendix D

12/19/2005 Page H-5 The maximum vertical extent of freez ing rain is 7,000 fe et. 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 horizont al extents. Hori zontal extents of exposures in FZDZ and FZRA can be anywh ere 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.

T OTAL W ATER C ONTENT VERSUS L IQUID W ATER C ONTENT

For approximately 40 to 50 percent of the in-situ measur ements in icing conditions between -5° C and –30° C, some ic e 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 temperat ure 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 cl oud 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 func tion of temperature are very similar. The 90 to 95 percent val ues are almost the same. Since Appendix

X includes extreme water content values , it does not appear that there would be a significant difference betw een using TWC or LWC.

G EOGRAPHICAL R ESTRICTIONS

The current archive used to produce r epresentative spectra and extremes for LWC for Appendix X consists of data co llected 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 tem perature 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 fo r 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 lo wer drop concentrations. The frequency

of occurrence of SLD conditions also va ries 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 Amer ica and in Europe. However, there are very few in-situ measurements from t hese areas. The micro-physics of large

### IPHWG Task 2 WG Report - Appendix H Development of Part 25 Append ix X & Part 33 Appendix D

12/19/2005 Page H-6 droplet formation are the sa me and it is not expected that the drop diameter distributions would be greatly affected. Therefore, the existing data is considered sufficient for rulemaking.

P ROBABILITY OF O CCURRENCE

The MSC in-situ icing database shows t hat 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 t he probability of occurrence of SLD icing conditions because these flights we re directed into areas with icing that were expected to contain SLD. Cons equently, the true probabi lity of occurrence of SLD icing conditions would be lowe r. For example, the frequency of occurrence of freezing prec ipitation 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 Gr eat Lakes area (2 percent) and in Newfoundland (5 percent).

Climatology studies performed by the Nati onal Center for Atmospheric Research (NCAR) for North America show t hat the annual average frequency of occurrence of SLD ranges from 0.4 to 5 per cent of the time in a spatial column over a measuring station. This study indi cates 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 Gr eat Lakes area where

most of the Canadian data were obtai ned, 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 t hat 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, r epresenting 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 accura tely identify the differences between freezing drizzle and freezing rain. The prec ipitation conditions at the ground are primarily out of cloud. Appendix X is an engineerin g design standard that considers freezing drizzle and freezing ra in aloft, and the four spectra selected contain cloud droplets. This can lead to differences between dispatch standards and conditions tested during t he 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 Append ix X & Part 33 Appendix D

12/19/2005 Page H-7 standard due to the limited exposure dur ing transition between ground freezing precipitation env ironment and aloft.

A PPENDIX X R EFERENCES

1. Ashenden, R. and Mar witz, J. D. (1996); "S upercooled 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 (1 995); " Aircraft Icing Measurem ents in East Coast Winter Storms," Journal of Applied Meteorol ogy, Vol. 34, pages 88-100, dated January

1995.

3. Cober, S.G., G.A. Isaac, A.V. Ko rolev, and J.W. Strapp, 2001: Assessing cloud-phase conditions. J. A ppl. Meteor., 40, 1967-1983.

4. Cober, S. G. et al (2001); "Characteriza tions 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), "Defini ng Characteristic Cloud Drop Spectra From In-Situ Measurements," AI AA-2003-0561, 41st Aerospace Sciences Meeting & Exhibit, 6-10 January 2003, Reno, NV

6. Cober, S.G., T.P. Ratv asky, and G.A. Isaac, 2002: Assessment of aircraft icing conditions observed during AIRS. AIAA 40t h Aerospace Sci. Meeting and Exhibit, Reno Nevada, 14-17 Januar y 2002, AIAA 2002-0674.

7. Cober, S.G., and G.A. Isaac, 2002: Ai rcraft 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); “Canadi an 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: Prelimi nary results from the Alliance Icing Research Study (AIRS). AIAA 39th Aero space Sci. Meeting and Exhibit, Reno Nevada, 8-11 January 2001, AIAA 2001-0393.

### IPHWG Task 2 WG Report - Appendix H Development of Part 25 Append ix X & Part 33 Appendix D

12/19/2005 Page H-8 11. Jeck, R. K. (1996); “Repr esentative Values of Icing- Related Variables Aloft in Freezing Rain and Freezing Drizzle" Te chnical note DOT/FAA/AR-TN95/119, 44 pp., dated March 1996.

12. Jeck, R. K. editor; "Data and Analyse s Contributing to the Development of Engineering Specifications for Freezing Rain and Freezing Drizzle Conditions for Aviation Purposes," FAA Technical Repor t 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. dat ed January 12-15,1998.

14. Shah, A. D. et al (2000); "Engineer ing Analysis of the Atmospheric Icing Environment Including Large Droplet Icing Conditions", SAE 2000-01-2115, dated April 2000.

14 CFR PART 33, A PPENDIX D

A data base of commercial service icing ev ents (see Appendix I of this report) in the mixed phase / glaciated environment es tablished that the current FAR Part 25 Appendix C icing envelope with super cooled liquid water droplets does not cover the range of altitude and ambient tem perature 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 Da y temperature limits. Hence, the range of the new FAR Part 33 Appendix D was increased beyond FAR Part 25 Appendix C envelope to t he Mil Std 210A temperat ure limits. A common

characteristic in most of these events wa s the proximity to convective / tropical storm cloud formations, and thus Appendix D is bei ng 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 sear ch was perfomed which revealed limited data for deep convective atmospheres. For example, a series of airborne

measurements of cloud total water c ontent (TWC) were conducted between

December 1956 and March 1958 in three tr opical locations, resulting in an

extensive set of 101 cloud measurement f lights with 44.5 hours of in cloud data (McNaughtan data). Most of the more re cent data on the properties of mixed phase clouds has been collected by the at mospheric research community, but

these studies were not focused on meas uring maximum total water content areas of deep convective storms as in the case of the McNaughtan data.

The accuracy of the TWC measurement s 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 Append ix 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 experim ental 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 ex amined relative to temperature and

altitude and were consistent with the M il-Std hot day boundary. As such, the MilStd was used as limiting tem perature/altitude condition.

The general slope of the TWC reduction with distance scale was derived from the McNaughtan data set. A standard distance sca le 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 recogni tion of a total air temperature (TAT) anomaly, an indicator of the presence of hi gh TWC. The short distance limit of

4.6 nm is the lowest that could be deduced from the Mc Naughtan data set. It was the opinion of the EHWG that this was adequate given the information in the

event database and current technical know ledge 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, A PPENDIX D R EFERENCES

1. THE ANALYSIS OF MEASUREMENTS OF FREE ICE AND ICE/WATER

CONCENTRATIONS IN THE ATMO SPHERE 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 SIZ ES 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 MEAS UREMENTS IN THUNDERSTORM

OUTFLOW REGIONS DURING ALLI ED/BAE 1997 FLIGHT TRIALS, Strapp, J.W., P. Chow, M. Maltby, A.D. Bezer, A. Korolev, I. Stomberg, and J. Hallett, 37 th 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 I NCIDENTS 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 acci dents to identify those which may be considered in the benefit analysi s of the regulatory evaluati on. 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 asso ciated 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 t he 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 cont end that these events are not relevant

because it is not believed that future Pa rt 23 airplanes will have a Part 25 icing

certification basis.

I CING I NCIDENT D ATABASE R EVIEW

The icing incident database for Task 2 in cluded 81 events. These 81 events were obtained following a revi ew 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 ex tracted 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, fo r 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 §25.141 9 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 associat ed with turbo-propeller aircraft, and one event in which the type of power plant c ould not be determined. Of these, there

were 4 references to mi xed conditions (2 each for turbo-propeller aircraft and turbojet aircraft), and six events which occurred out side 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 airc raft and they are included in Table I-1 below. Thus, the final list of 28 inci dents potentially applicable to Task 2 is:

Number of incidents Aircraft Type

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

I CING A CCIDENT D ATABASE R EVIEW

The IPHWG developed an icing accident database in support of Task 2 that

included 182 events. T hese 182 events were obtained a fter a review of several databases, including the National Transporta tion 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 em ployed in the selection of events for

the database. There were limited refer ences 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 (incl uding 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-Appen dix 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 Britanni a 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 Mooreshea d,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 Appendi x 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 Aircra ft Accidents For Aircraft With Appendix C Certific ation Criteria

M INORITY P OSITION ON A PPENDIX I OF THE IPHWG W ORKING G ROUP R EPORT (A CCIDENTS AND I NCIDENTS R ELEVANT TO THE P ROPOSED R ULE )

(Embraer, with support of the R egional 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 larg e drops conditions (includes freezing drizzle and freezing rain). The operatin g rules do not prohibit operations in supercooled large drop conditions. The accident history indicates that flight crews of certain types of aircraft have lo st 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 envir onment requirements are inadequate ”. Embraer agrees with this statement; however, the pur pose 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 reas ons for the requirement improvement are

not associated with the EMB-120 aircraft model.

The IPHWG examined incidents and acci dents to identify those which may be

considered in the benefit analysis of the regulatory eval uation. Descriptors such as severe ice, freezing rain, freezing drizzl e, side window icing, heavy icing, large droplets, SLD, and for mixed ice conditi ons descriptors of sleet, snow, snow grains and ice were employed in the sele ction 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 “Acci dents and Incidents Relevant to the Proposed Rule” includes three events with the EM B-120 aircraft (164, 171 and 179), which are presented in Table I-3 above:

Some reasons were also taken into acco unt to consider t he 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 airc raft considered by the IPHWG as applicable to this benefit analysis should be removed from the Working Group report, as either the IPS wa s not activated, there was improper pilot action, or both. This is very clear in the NTSB r eport associated to each event. Below is a summary of each of the ev ents 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 selectio n 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 perform ance size wise and time wise was not compatible with al l 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 pr ior to the loss of control. FDR data shows that the crew let the airspeed dec rease 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 fo r 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 an alysis performed by Embraer, during the accident investigation, dem onstrated that the loss of performance size wise and time wise was not compatible with all data available for t he 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 (Pal m Beach) accident, the NTSB report concluded that the pr obable cause of the accident was: “The failure of the flight crew to maintain airspeed during an enco unter with severe icing conditions,

which resulted in an inadvert ent stall, loss of control, and structural damage to the airplane”. Although there are indicati ons that the crew had activated the deicing boots, the “light” m ode 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 ai rcraft speed and let it reach values

much lower than the 160 KIAS thre shold specified in the AFM for operation in icing conditions

o According to Embraer analysis of t he accident that shows that there was a linear increase in performanc e degradation for approximately 3 minutes, it is apparent that the de-ic ing system was activated in “light” mode (three minutes cycle), conflicting with instructions presented in the AFM to select “heavy” (one mi nute cycle) mode in severe icing

conditions.

In none of the incidents/accidents abov e it was demonstrated that the icing conditions prior to the upsets were out side 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 impr ovements 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, instru cting 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 Al arm, 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 lik ely 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, Embr aer believes that the only way FAA can

construct a defendable benefit estimate is to factor the total benefit from accidents prevented by the percentage cont ributed by each corrective action that has been or will be implemented. This woul d 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 cl ear that the vast am ount of benefit would be accrued solely through that system. Th is would leave only accidents in SLD

conditions so severe that the 160 KIAS th reshold would be insu fficient to prevent

stall and loss of control as an accident t hat would be credibly prevented by the proposed regulation. There is nothing indi cating that the events involving EMB120 would fall in this category, what re inforces 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.

R EGIONAL A IRLINE A SSOCIATION S TATEMENT OF S UPPORT OF E MBRAER M INORITY
P OSITION

RAA concurs with the Embraer position that with all three of the Emb-120 accidents/incidents, there is nothing to in dicate that the ic ing conditions were outside the current Appendix C envelope and that th e Emb-120 events should not be cited as justificati on for the proposed new rule.

M AJORITY R ESPONSE

(ALPA, Boeing, CAA/UK, FAA/FAA Tech Center, MSC, NASA, SAAB, Transport Canada/TDC)

Considering the descriptions of the a ccidents in the NTSB reports, the three EMB-120 events included in Table 5 were more closely reviewed.

Determining the actual icing conditions afte r an accident is very difficult. In all 3 events, the NTSB reports did not quantif y 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 wa s “potential SLD cond itions”, since the actual icing conditions for eac h 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 Bl uff accident that t he flight crew had activated the deicing boots, there was no ev idence that they failed to activate the deicing boots. In the Orl ando, FL event, the flightcrew did activate the “light” mode of the deicing boots, but the AFM di d not specify the mode to be used for severe icing, and in fact stated that ic e 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 coul d 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 exclusio n 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, t he 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 i dentified as contributing factors affecting the crew’s performance and abilit y to safely operate the airplane in these icing conditions. The above improv ements (such as improvements to the ice detection systems and AFM proc edures) are considered appropriate responses by applicants to Appendix C as well as Appendix X icing conditions. Failure to monitor airspeed in icing condition s is a systemic factor in icing related accidents of airplanes without an auto-throttl e, 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 improvem ents listed by Embraer for the EMB-120 were not a requirement for icing certif ication when the EMB-120 was certificated, and is not a requirement for Appendix C ic ing certification. This is direct indication that improvements to the cert ification requirement s 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 require ment is even lower, 5%. In addition, the Low Speed Alarm referenced by Embr aer was designed to the accident FDR data, not part 25 regulations. One woul d 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 warn ing designed to CFR Section 25.207. This analysis supports the need for impr ovements 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 show ed that the lift lo ss and drag increase measured in EMB-120 flight te sting with critical Appendix C ice shapes could not duplicate the performance losses experienc ed in the three EMB-120 accident events included in Table 5, and an EMB-120 in cident included in Table 3, derived from the flight data recor ders. For example, Figure 1 of the NTSB performance group report for the Orl ando, FL accident, dated March 13, 2002, showed the following lift losses at the current st all warning shaker angle of attack:

Flight test with critical Append ix C intercycle ice: 20%

Pine Bluff accident: 28%

Monroe accident: 30%

Westair incident: 37%

Orlando accident: 41%

Based upon the above, improv ements to the design of t he airplane to meet the proposed SLD icing environment rule c ould 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 R EVIEW OF A CCIDENTS AND I NCIDENTS

An assessment of worldwide commerc ial engine service history was conducted for small and large engines covering t he period from 1989 through 2003. No

accidents were attributed to the effect s 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 list ed 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) displa ys incidents in Mixed Phase/Glaciated conditions that resulted in reportable rollb acks, 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 G ENERAL A SSUMPTIONS

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 certif ied with the “detect and exit” option ((§25.1420(a)(1))

There is some indication that the dev elopment and certification costs will be roughly the same for all certification opti ons in Appendix X (full, portion of, detect and exit), but the additional operational costs (training, dispatch, diversions, etc.) will be strongly dependant upon t he type of certification.

There were no efforts made to identif y 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 wit h certification in Appendi x C was discussed, without

clear conclusions. The costs were provided assuming no benefit from an Appendix C certificat ion applied to an Appendix X cert ification. There may be some reduction in the cost when considering a combined Appendix C and Appendix X certificati on program, but this is an unknown factor.

E XAMPLE C OSTS

Table J-1 provides example costs t hat 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

Aircraft 3
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 $12,300,000 2 $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 $10,000 5 $10,000 5 $10,000 5
Total $12,135,600 1 $20,300,000 1 $6,805,000 1

Table J-1 - Example Costs

Note 1: Compliance will likely requ ire a combination of the approaches, but

individual manufactures will have to det ermine which exact combination will be required to meet the cert ification 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 requir ed. In light of this, a conservative economic analysis would consider the to tal costs. For derivative aircraft programs that have an ancestor aircraft that has already been certified to Appendix X, it is expect ed that the program cost s 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 per cent of airplane cert ification programs will require natural icing flight testing.

Note 2: Quote assumes cons iderably 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 det ector for activation of the ice protection system and/or for knowing when to exit icing conditions. The large transport manufacturers, whose airplanes will like ly be certified in accordance with § 25.1420(a)(3), anticipate installation of an ic e 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 benef its 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 approximat ely equally divided between unrestricted operation, operation in a porti on of X, and detect and exit.

Note 7: For unrestricted ai rcraft, 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 aircra ft to be equipped with such a system.

E NGINE C ERTIFICATION C OSTS

• 14 CFR Part 33.68 – Three additional ic ing test conditions required, cold point plus ground operation in SL D and snow, with allowance for certification in the latter two by anal ysis and/or in the case of snow by equivalent liquid water). Five additional test conditi ons (climb, cruise, idle descent, holding, approach) in Mix ed 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 limit ed test facilities for te sting 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 fr om spray nozzles provide velocities lower than flight speed. If facilit ies for producing the ice crystals with adequate velocity to simulate flight s peeds are required, they may need to include ice processing and hi gh air pressure equipment.

Industry Relief

• 14 CFR Part 33.77 – Rule changes allo w 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 sl ab and natural icing tests within the nominal ability to measur e 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 glac iated icing conditions are hazardous to turbine engine operation, severe incidents involving this type of meteorological condition are not common. Many cu rrently 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 compli ance by similarity analysis.

Additional Requirements

• Validated analysis of engine icing in gl aciated environment is required for assessment of this icing threat.

• Manufacturers will have to demonstrate to FAA that they have thoroughly addressed Critical Point Analysis require ments 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 requ ire additional analysis of the engine inle t capability in descent and holding conditions

• The addition of Appendix X Ground Te st 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 com ponent 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 $50,000 a TBD*

Ice crystal facility $1,000,000 b Engine Analysis SLD TBD* $10,000* $20,000 c TBD* Engine Icing Tests TBD** TBD** $600,000 d $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 met hod, however this may not meet requirements and result in more expensive facilities.

c If additional test facilities to provi de 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 m anufacturers facility cannot be used and altitude facility required costs wil l be considerably higher

* Rule allowance for ice crystal compliance by similarity reduces cost significantly and promotes designing proven ice crystal-t olerant features into new engines. Comparative analysis done by one engi ne 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 ic ing test costs for new engine certification program. Estimate from one testing fac ility 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 te st partially offset by additional soft body tolerance analysis

NOTE: Additional SLD costs for airpl ane 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 N ON RECURRING COSTS ( DEVELOPMENT
, CERTIFICATION )

Flight tests in natural conditions were i dentified as being a ma jor contributor to

the overall cost. There is no clear visi bility on the amount of flight hours that could be needed for certification (low probability for SLD encounters as compared to Appendix C conditions; no r equirements defined in terms of MVD, LWC, or exposure times). A large trans port aircraft manufacturer evaluated the additional test time at about 100 flight te st hours. It is recognized that the accretions obtained during the test cam paign may not be sufficient to allow the validation of the icing codes used to gener ate the critical ic e shapes for a full Appendix X certification. The analysis should therefore pr ovide 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 lik ely be significantly higher than subsequent applicat ions) or certification of derivatives (no general rule can be proposed on the nature of the modifications being applied).

For each airplane category assumed (l arge 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 artifi cial icing behind a tanker -Flight test with si mulated ice shapes -Documentation (Certificati on 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 cert ification in Appendi x C will obviously depend upon the performance penalties which could be required for operation in Appendix X. Costs provided by the m anufacturers are based on a scenario in which a specific set of minimum s peeds /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 R ECURRING COSTS

On the manufacturers’ side, introduction of the new rule may lead to major modifications in the ice protection system s (e.g., increased size of protected surfaces requiring additional power; addition of new detection or warning means).

An evaluation of these recurri ng costs is not available.

On the operators’ side for the non-recurri ng costs, a breakdown equally divided into three categories (large, regional , business/commuter) can be envisaged. This is based on the assumption that lar ge transport airplanes will probably apply for certification in all of Appendix X, r egional airplanes for full or a portion of Appendix X, and business/commuters fo r the detect and exit option. The economic analysis should take into account the following consequences:

- Impact on dispatch (performance lim ited 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 ava ilable figures (probability to encounter Appendix X conditions, number of airplanes certified to the new rule, estimated proportions in each category) provided an ex ample calculation of the operational costs. The impact on dis patch numbers are based on ai rcraft certified without

restriction or certified for a portion of Appendix X (i.e., WAT limited takeoffs). The

diversion and cancelled-f light numbers are based on airc raft 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 available 2
Weather/ATC training not available 2

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 RECOMM ENDED 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 exis ting §25.1419 requirements as well as §25.1420. As part of the revision, Sub- part B related requirements were moved to the FTHWG proposed advisor y 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 provi de guidance to supplement the engineering judgment that must form t he 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 regulat ory authorities, and Federal Aviation Administration airplane type certif ication 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 th is particular method of compliance when the acceptable method of compliance described in this document is used. While these guidelines are not m andatory, they are derived from extensive Federal Aviation Administration and industry ex perience in determining compliance with the pertinent regulations.

d. This advisory circular does no t 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 applie s to certification of part 25 transport category airplanes for flight in icing condition s. 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 conditi ons defined by 14 CFR 25, Appendix X. The certification options ava ilable are: 1) to safely operate throughout Appendix X, 2) To operate safely throughout a porti on of Appendix X, 3) Limited to safely exiting Appendix X conditions. Sect ions 25.1419 and 25.1420 set forth the specific airframe requirements for dem onstrating compliance with the icing conditions defined in Appendices C and X. Additionally, for other parts of the airplane (i.e., engine, engine inlet, pr opeller) there are more specific icing requirements and associated guidance defin ed in the referenced regulations and advisory circulars listed in paragraph 3 below. Some of the icing-related regulations must be complied with, even if t he 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 informati on on flight tests for ice protection certification; additional information on flight tests for showing compliance with the airplane performance and hand ling 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 (T C's), Supplemental Type Certificates (STC's), and amendments to existing TC's fo r airplanes certified under Part 4b of the Civil Aviation Regulations (CAR) and Pa rt 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 Cent er, 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 C onducive 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 Ai rplane Mechanical Systems

AC 25.629-1 Flutter Substantiation of Tr ansport 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 deriv ation 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 fo r Aeronautics (RTCA), Inc., 1140 Connecticut Ave., NW, Suite 1020, Washington, DC 20036. The following SAE

Documents can be ordered from the Societ y 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 Ic e 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 Protec ted Areas of Lifting Surfaces (2) Ice Shapes on Unprotec ted 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 a nd 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 S ystem 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 wi nd 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 Abnorma l 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 Secondar y 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 Ce rtification 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 ce rtificated under Part 04 of the CAR. Section 04.5814 required that if deicer boot s were installed, then positive means must be provided for the deflation of all wing boots. There were no other references to an airplane ice prot ection system (IPS) in Part 04.

(2) Part 4b of the CAR was codi fied on December 31, 1953. The requirement for positive means of def lating deicer boots was incorporated, without change, in § 4b.640. Section 4b. 640 stated that, “W hen an ice protection system is installed, it shall be of an appr oved type. If pneumatic boots are used, at least two independent s ources of power and a positiv e means for the deflation of the boots shall be provided.” Secti on 4b.351(b)(ii) provided requirements for pilot compartment vision in icing condi tions. Section 4b. 406 provided propellerdeicing requirements. Se ction 4b.461 provided induc tion system deicing and anti-icing requirements. Section 4b.612(a)(5) required that the airspeed indicating system be provided with a heat ed 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 Append ix 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 t he current Appendix C envelopes with a few exceptions. These envelopes described the liquid water content, the mean effective diameter of droplets, the tem perature, and horizontal and vertical extent of the supercooled icing cloud environm ent. 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 bet ween the Amendment 4b- 2 figures and the current Appendix C envelopes, if the co rrection for the difference in value

between nautical and statut e miles is made. Ther e 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 Ici ng 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 A ugust 12, 1957, revised the icing envelopes to the current requirement s 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 chang es which extend the currently effective provisions governi ng intermittent maximum icing

conditions so as to cover condition s which might be critical insofar as the turbine engine induction system is concerned. In this regard, the data are being extended in a ccordance 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 regulatio ns are applicable in the main to the airframe. The changes being made in section 4b.461 require the turbine powerplant to be subjec ted to the same icing conditions and require that the induction syst em be protected to prevent serious engine power loss. A sim ilar requirement is incorporated with respect to certification of turbine engines by an amendment to Part 13 which is being made conc urrently 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, becam e § 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 des ired was changed in part 25 to reference the icing conditions specified in § 25.1419. Section 4b.461, Induction system deicing and anti-icing provisio ns, became § 25.1093.

(2) Amendment 25-5, effective July 29, 1965, revised § 25.1325(b) to require that the correlation between air pr essure in the static pressure system and true ambient atmospheric static pressu re 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 Ma y 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 co mpliance but were not mandatory. Amendment 25-23 also revised § 25.1309 to include additional requirements for certificating equipment, system s, 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 failu re analysis, supported by appropriate test, to ensure that the safe ty objectives of the probabili ty 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 otherwis e 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 t he operating limitations prohibit operations at night in icing conditions.

(6) Amendment 25-43, effectiv e 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 De cember 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 improv ement. In addition, the contents of § 25.1416(c), which required a means to i ndicate to the flightcrew that the pneumatic deicing boot system is receivi ng 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 perfo rmance 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 f light in icing is desired. Prior that revision, compliance with § 25.1419 was only required if the airplane had ice protection provisions. Editorial revisi ons 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 dete rmination of when to operate ice protection systems to ensure safe operat ion in Appendix C icing conditions.

(11) Amendment 25-XX, effectiv e 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 dropl et icing conditions that must be considered when an airplane is cert ificated 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 te rms 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 ic e accretion. The flightcrew is responsible for monitoring t he icing conditions or ice accretion as defined in the AFM, typically using total air tem perature and visible moisture criteria, visible ice accretion, or specific airframe ice accretion thickness, and activation by the flightcrew of the anti-icing or de-ic ing system(s) is necessary. The advisory system provides informati on to advise the f lightcrew of the presence of ice accretion or icing condi tions, but it can only be used in conjunction with other means to determine the need for, or timing of, activating the anti-icin g or de-icing system.

b. AIRFRAME ICING: Ice accretions on portions of the airplane, with the exception of the propulsion system in cluding the air induction system, on which supercooled liquid droplets may impinge.

c. ANTI-ICING: The prevention of ice fo rmation or accumulation on a protected surface, either:

• by evaporating the impinging water or

• by allowing it to run back and off t he 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 arti ficial 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 cycl es 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 remo val 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 superc ooled liquid precipitation that freezes upon impact with the ground or exposed obje cts, 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 temperat ure 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 cont rolled by signals from the flight deck controls. Loads generated at the contro l 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 dr op diameter that divides the total liquid water content pres ent 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 calcul ated, 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 t he 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 prot ected surfaces to areas t hat cool sufficiently for the migrating ice to adhere to the surface.

u. MONITO RED SURFACE: The surface of c oncern 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 t he IPS must be activated. The system annunciates the presence of ice a ccretion or icing conditions and may also provide information to other ai rcraft systems. A primary automatic system automatically activates the anti- icing or de-icing systems. With a primary manual system, the flightcrew activa tes 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 su rface must occur prior to – or coincidentally with – the presence of ic e on the monitored surface. Examples

of reference surfaces include windshie ld wiper blades or bolts, windshield posts, ice evidence probes, propeller spinner, and the surface of ice

detectors. The reference surface ma y also be the monitored surface.

y. REVERSIBLE FLIGHT CONTROLS: The flight deck controls are connected to the pitch, roll, or yaw c ontrol 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 transmitt ed back to flight deck controls.

(1) Aerodynamically boosted flight controls: Reversible flight control systems that employ a movabl e tab on the trailing edge of the main control surface linked to the pilot’s contro ls or to the structure in such a way as to produce aerodynamic forces that mo ve, 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 provi ded, usually a hydraulic actuator, to apply force to a control surface in additi on 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 cool ed for freezing to ta ke place. This ice type is frequently associated as an unwanted product of thermal deicing systems.

aa. SIMULATED ICE: Ice shapes that are fabr icated 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 outsi de temperature,” or “amb ient 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 distributi on that has an MVD greater

than 40 μm and includes freezing dri zzle and drops with diameters less than 100μm (defined by 14 CFR 25, Appendix X).

ag. FZDZ/L: A drop diameter distributi on that has an MVD less than 40 μm and includes freezing drizzl e and drops with diameters less than 100μm (defined by 14 CFR 25, Appendix X).

ah. FZRA/G: A drop diameter distributi on that has an MVD greater

than 40 μm and includes freezing rain, freezing drizzle and drops with diameters less than 100μm (defi ned by 14 CFR 25, Appendix X).

ai. FZRA/L: A drop diameter distributi on that has an MVD less than 40 μm and includes freezing rain, freezing drizzle and drops with diameters less than 100μm (defi ned 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 env elopes 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 ic ing environment for superc ooled 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 detec t that the airplane is operating in Appendix X; and following detection, the ai rplane 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 fli ght. Following detecti on of conditions that exceed the selected portion of Appendix X, the airplane must be capable of operating safely while exiting all icing cond itions. 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 aler t flight crews when those portions of Appendix X are exceeded (§ 25.1420(a )(2)) or Appendix X conditions are encountered (§ 25.1420(a)(1)). Certific ation to Appendix X (§25.1420(a)(3)) requires that the airplane operat e safely throughout Appendix X.

The four spectra with associated LWC limits were developed to describe the entire Appendix X environm ent as it could be determined by all the

measurements available. Init ial certifications to a portion of Appendix X will likely include all of freezing drizzl e 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 al lows latitude for certification with a range of techniques; for example, limiting the range of liquid water contents. In doing these type of limited certificati ons, ice shapes will be developed for the portion of the envelope for which the aircra ft is approved as well as for detecting and exiting icing conditions beyond the selected portion.

The ice shapes developed for the approv ed portion of the envelope should account for the icing conditions in terms of drop distribution and water content and consider the proposed method for identif ying the icing conditions that require exiting. The definition of the certificated portion of Appendix X should be based

on measured characteristics of the sele cted icing environment and consistent with the methods used for developing Appen dix X. A detailed report on the development of Appendix X is availabl e from the FAA Te chnical 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 Append ix X, including freezing drizzle and freezing rain. The two types of freezi ng precipitation are further divided into conditions in which the drop median volu me diameters are either less than or greater than the 40 μm, the largest m ean effective diameter of Appendix C continuous maximum icing conditions. Appendix X consists of measured data that was divided into droplet distributi ons within these four icing conditions. These distributions were averaged to produc e the representative distributions for

each condition.

The icing conditions data were divided as follows: 1. Measured icing condit ions 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 cond itions with an MVD gr eater than 40 μm and a maximum drop size from 100 μm to 500 μm were classified as FZDZ/G 3. Measured icing condit ions with an MVD less than 40 μm and a maximum drop size greater than 500 μm were classified as FZRA/L 4. Measured icing cond itions with an MVD gr eater than 40 μm and a maximum drop size greater than 500 μm were classified as FZRA/G

The data measured to produce Appendix X wa s taken in wintertime stratus cloud formations. As such, the Appendix X cond itions are most closely associated with

the stratus conditions used to define the existing Appendix C, Continuous

Maximum conditions. Based on the me thods used for Appendix X development, the boundary between Appendix C and Appendi x X is Continuous Maximum icing conditions with maximum drop sizes bel ow approximately 100μm. Use of only

stratiform cloud data is considered acceptable since stratiform clouds predominate during prolonged freezing pr ecipitation conditions. See FAA Technical Center report DO T/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 dist ributed 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 si zes and associated fractional water contents in a specific A ppendix C icing condition.

The distributions assumed for Appendix C ic ing conditions are inappropriate for freezing precipitation icing c onditions 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, t he 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 la rger drops affects the water catch on airplane components, the drop impingem ent, 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 precipitati on drop diameters varies greatly. Subdividing freezing drizzle and freezing rain allowed se lection of four average drop diameter distributions that are repr esentative of the total range of measured drop distributions. These four distribut ions are shown as cumulative mass distributions in Appendix X.

Also, the maximum values of freezing drizzle and freezing rain liquid water

content decrease with decreas ing 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 standar d 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. T he maximum altitude of freezing drizzle is the same as that of an Append ix C continuous maximum icing cloud, 22,000 feet (Appendix X, figure 3). The temper atures are as cold as -13° F (-25°C).

The maximum vertical extent of freez ing rain is 7,000 fe et. 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 laps e rate with altitude is not appropriate for freezing drizzle and freezi ng 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 dri zzle 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 horizont al extents. Hori zontal extents of exposures in FZDZ and FZRA can be anywh ere 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 us e of Appendix X rela tive 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 ici ng conditions rather than two when determining critical icing conditi ons 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 cumula tive distributions in Appendix X. The mass proportions for the bins were selected to provide reasonable resolution of the upper range of the di stributions. 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 ma y be appropriate. Different methods should consider the impingement characte ristics of the geometry relative to drop size.

Table 1. FZDZ distributions represented using 10 bins.

FZDZ/L (MVD < 40 μ m) FZDZ/G (MVD > 40 μ m) (a) (b) (b)

Bin Proportio

n of Mass Left Boundary

Point ( μ m) Massweighted Midpoint ( μ m) Right Boundary

Point ( μ m) Left Boundary

Point ( μ m) Massweighted Midpoint ( μ m) Right Boundary

Point ( μ m)

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 re solution 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 μ m) FZRA/G (MVD > 40 μ m) (a) (b) (b)

Bin Proportio

n of Mass Left Boundary

Point ( μ m) Massweighted Midpoint ( μ m) Right Boundary

Point ( μ m) Left Boundary

Point ( μ m) Massweighted Midpoint ( μ m) Right Boundary

Point ( μ m)

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 re solution 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 di stributions associated with Appendix C cloud drop sizes, the Appendix X dr op size distributions have a larger percentage of the total water mass in la rge drop sizes. The mass of the larger drop sizes has sufficient inertia to over come the local airflow effect and tend towards straight-line trajectories. T he collection efficiency and the impingement limits are both influenced by the inerti a 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, behav ior of the water drops are influenced by the scale of the object being consider ed. 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 infl uence the trajectory of the supercooled water droplets (Both Appendix C and Append ix X). This results in decreasing collection efficiency with increases in scale.

The Appendix X icing conditions contai n larger drop sizes, have a greater percentage of total liquid wa ter 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 impingem ent limits that are

further aft than those determined for A ppendix 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 si ze and trajectory influence, see FAA Technical Report DOT/FAA/ CT-88/8-1, and AC 20-73A.

An additional effect of the supercooled la rge drop environment is the variability in the distribution of drop sizes. Ice shapes developed from Appendix C icing

conditions sometimes assume uniform dr op 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 va riations of drop sizes and the larger percentage of total water c ontained in the larger dr ops within Appendix X tends to magnify the effects of di stributed drop sizes. The di stribution of water drops, or more directly, the percentage of wate r 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 protect ed areas of lifting surfaces. Appendix X icing conditions may result in a ridge of ic e or ice roughness aft of the protected

areas.. On aircraft with reve rsible flight controls, a ridge of ice may result in

uncommanded deflections of t he 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 maximu m lift at lower angles of attack. Without adjustments to the stall prot ection system activation schedules, the Appendix X ice buildups can also resu lt in an aircraft upset or stall.

(2) Ice shapes on unprotec ted areas of lifting surfaces . Ice shapes resulting from Appendix X icing conditi ons can be more extensive than those from Appendix C icing conditi ons. When portions of t he airfoil leading edge are unprotected, there is potential to affect the perfo rmance and handling qualities.

(3) Ice shapes on unpro tected 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 vi sibility, air data sensor locations, and potential airflow disturbance (for example on airpl ane nose) from Appendix X accretions that could influence the perfo rmance of the air data sensors should be

evaluated.

(4) Drag and Power effects. Airc raft 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 lowe r 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 in formation regarding the evaluation of potential effects on airplane performance.

(5) Engine Considerations. Airframe and air induction system components ice accumulations resulting from Appendix X ici ng 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 certificat ion plan to the cognizant FAA Aircraft Certification Office (ACO) for approval. The certification pl an should include the following basic information:

a. A general airplane description that includes dimensions, airworthiness limitations, and other data t hat 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 t he certification methods fo r each applicable section of part 25, including a description of analyses and tests, or re ferences to similarity of designs, that the applicant intends for certification of the IPS. These methods of showing compliance should be agr eed upon between the applicant and the FAA early in the certification program pr ior to conducting certification tests.

e. A failure hazard assessment to de termine the criticality of the system.

f. If the ice protection or detecti on 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 co mpleted, that will require special operating procedures.

i. If the IPS or detection systems co ntain complex electronic hardware (such as programmable logic devices (P LD) or application specific integrated circuits (ASIC)), plans for providing a le vel of design assurance of these devices commensurate with their potential contri bution to aircraft hazards and system failures, which could result from elec tronic 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 anal ysis used. All analyses should be validated by tests or should have been va lidated by the applicant on a previous program. To utilize a prev iously 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 ar eas to determine the pot ential ice accretions. Analytical simulation provides a method of accounting for the vari ability in droplet distributions. In addition, analytical simu lation provides the capability to examine impingement relative to visual icing c ues and analyze the location of detection

devices for detrimental local flow effects. Thermal characteristics of IPSs relative to icing conditions may also be eval uated using computational simulation.

During the substantiation process of ici ng simulation tools, cross comparisons

should be made with natural icin g flight test data, tanker test data, icing tunnel data and analytical codes to show that simulating supercooled large droplets results in equivalent or conservative resu lts. 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 ar e required in § 25.1419. Engineering judgment is required in ev aluating the results of t hese comparisons, as most simulation tools are steady-state simu lations 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 def ined 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, t he applicant should include supporting data and rationale in the analyses for allowi ng those areas or components to be unprotected. The applicant should show that the lack of protection does not adversely affect the handling characterist ics or performance of the airplane, as required by § 25.21(g), nor the operation and functioni ng of affected systems and equipment (e.g. pitot probes).

(1) Leading edges of wings, winglet s, and wing struts; horizontal and vertical stabilizer; canards; and other lifting surfaces.

(2) Leading edges of control surf ace 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 accret ion. Inlets (inc luding 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 r equired for compliance with § 25.1403.

(14) Vortex generator s installed on lifting surfaces and the fuselage, and stall improvement devices such as strips, vortilons, and fences.

(15) Any structure that extends in to the free stream such as cameras, camera mounts, and video equipment.

Note 1 : Ice protection of fuel tank vent s, 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 Airp lanes,” provides guidance for showing compliance with § 25.629. The flutter analyses should reflect any mass accumulations on unprotected and protec ted surfaces from exposure to Appendices C and, as appropriate, X icing conditions, including any accretions that could develop on control surfac es. 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 pow er source to determine that it is adequate to supply the IPS plus all other e ssential 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 ai rplanes must comply with the engine isolation requirement s of § 25.903(b).

d. Failure Analyses.

(1) Failure Analysis fo r § 25.1419 (Appendix C Conditions) . Advisory Circular (AC) 25.1309-1A, “System Design Analysis,” provides guidance for demonstrating compliance with the requirem ents 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 el ectronic hardware faults and malfunctions to system failure conditions, and classify the har dware assurance level appropriately, see RTCA/DO-254, "Design Assurance Guidance for Airborne Electronic Hardware." The probability of encountering Appendi x C icing is considered one for quantitative analysis.

(2) Failure analysis for § 25.1420 (Appendix X Conditions) . The application of the system safe ty 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. Asse ssing 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 classificati on 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 hist ory of similarly designed aircraft, as well as the icing event history of all convent ional design aircraft. When assessing the similarity to previous designs, the spec ific 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 suppor t of hazard classifications. If a new or derivative aircraft has similar design features to a previously certificated airplane, this service history may be appr opriate to support the determination of

the hazard classification.

While definitive statistics ar e not available, a historic al 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 cl oud) 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 pr evious design, an asse ssment 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 test ing. The aerodynamic effects of such shapes could be evaluated with wind tunnel testing or potentially, computational fluid dynamics. See AC 20-73A fo r additional guidance on assessing the aerodynamic effects of ice buildups. T he hazard classification typically takes place early in a certificat ion program. Therefore, a conservative assessment of the hazard may be required until sufficient s upporting data is available to reduce

the hazard classification.

(b) Probability of Encountering Appendix X

. The appendix C conditions were determined on the 99 percentile pr obability of exceeding the icing conditions (Ref: NACA TN 2738, “A Pr obability Analysis of the Meteorological Factors Conducive to Aircraft Icing in the United States”). In other words, the probability of encountering ic ing outside of A ppendix 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 fr om 0.4% to 5% of the time in a

100 km radius column over various stati ons. Determining the actual occurrence of SLD in the atmosphere is difficult us ing the data sets available. It is recognized that there are large geographical differences and changes with seasons that influence the probability of SL D occurrence. However, to a first approximation, the probability of occurrence of SLD for any particular location in North America, representing the altit ude 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 duri ng the winter is less frequent. These numbers do not account for the full pr obability of an aircraft encountering supercooled large drops within the 100 km radius of the column. Based on this information, on an annual basis th e 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 pr obability of equipment failure with the probability, defined above, of encountering appendix X icing conditions. Therefore, if the applicant can suppor t a hazard level of “hazardous” and considering the above probability of encount ering 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 safe ty 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 cons idered feasible. When visual cues are the method of detecting Appendix X c onditions to determi ne when to exit, an assessment of the appropriateness and r easonableness 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 anticipat ed 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 perfo rmed when required. In part, this is

coincident with the substantia tion of the specific cues as described in paragraph

13.f. The workload of the visual det ection method should be considered in combination with the operati onal 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 i dentifying Appendix X accumula tions that require exiting from icing conditions.

e. Similarity Analyses

.

(1) In the case of certificati on based on similarities to other type certificated airplanes previously approv ed 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 conduct ed 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 evaluati on of both the system and installation

differences. The differences should be evaluated for their effect on the functionality of the IPS and on sa fe 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) r equires flight testing in measured natural icing conditions. Ho wever, flight test data fr om previous certification programs may be used to show complianc e with 14 CFR 25.1419(b) if it can be shown that the data is appl icable to the airplane in question. If there is uncertainty about the similarity analysis, the manufacturer should conduct flight tests in measured natural icing condi tions for compliance with § 25.1419(b).

(4) On derivative airplane designs, sim ilarity to previous type designs that have shown compliance to § 25.1420 can be used as a method of compliance if

the effects of differences ar e substantiated. Natural ic e 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, includi ng 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 consi der the various airplane operational configurations, phases of flight, and t he associated angles of attack. The analysis should establish the upper and lowe r aft droplet impinge ment limits that

can then be used to establish the aft ice fo rmation 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 compre ssor blades, impeller vanes, inlet screens and ducts, as well as propelle rs, are examples of powerplant components subject to damage from s hedding 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 s hedding from the forwar d 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, aile rons, flaps, and spoilers are also subject to damage, especially those construct ed of thin metallic, nonmetallic, or composite materials. Currently ava ilable 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 pr otected leading edge on propeller efficiency and airplane performance.

i. Pitot Probe Ice Protection

. Compliance to the TS O qualification standard for electrically heated pitot probes (TSO-C 16) is not sufficient by itself in demonstrating compliance to the inst allation requirements of §§ 25.1309(a), 25.1323, 25.1326, 25.1419, and 25.1420. Sect ion 25.1309(a) requires that the system must perform its intended f unction under any foreseeable operating

condition. Sections 25.1419 and 25.1420 r equire that an airplane certificated for flight in icing must be able to safely operate in part 25 A ppendix C and Appendix X icing conditions, respectively. It is unlikely that the condi tions of Appendices C

or X that are critical to the air data system equipment will be encountered during flight tests. Consequent ly, certification programs should supplement the icing flight tests with icing tunnel test and/or tanker test dat a, as necessary. In-service experience during severe atmospheric c onditions has shown that pitot tubes qualified to the older standards have re sulted 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 maximu m 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. Li quid water content is not specified but it influences heat requirements. Alt hough functioning of pitot probes are evaluated in natural icing conditions during certification test pr ograms, 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 App endix 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, rec ent cloud characterization research has indicated that approximately 40 percent of icing conditi on events consist of liquid water drops and ice crystals (mixed-phase ic ing 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 evaporat e the ice crystals. Recently, some aircraft manufacturers and foreign certific ation authorities have required pitot and pitot-static probes to be tested in ice crystal and mixed-phase icing conditions along with supercooled liquid water conditi ons. As a resul t, some pitot tube manufacturers now use the icing environm ent of British S pecification (BS) 2G.135, “Specification for Electrically-H eated Pitot and Pitot-Static Pressure Heads,” in addition to the require ments 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 cr ystal 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 capabilit y, but only temperature and airspeed are specified. The precip itation 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 warn ing 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-freezi ng 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 characte ristics (rivulets or thin layers) of

the runback water or the resultant ic e shapes; however, some codes may be

able to estimate the mass of the runback ice. Therefore, runback ice should be determined experimentally. Runba ck ice should be considered when determining critical ice shapes. Si mulated runback ice shapes may be used when evaluating the effects of critical ice shapes. Consideration should be given to potential hazards resulti ng from the shedding of runback ice.

6. SIMULATED ICE SHAPES and ROUGHNESS AC 25.21-1 contains guidance on the icing exposure during va rious phases of flight that should be considered when determining simulated ic e 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 impingem ent limits and ice shape predictions (see SAE ARP5903), flight in meas ured natural icing conditi ons, 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. Ther efore, 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 critic al 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 oper ate in the continuous maximum and intermittent maximum icing conditi ons 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 certif ication requirements when all IPSs are installed and functioning. This can no rmally 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 crit ical to basic airplane aerodynamics, IPS design, and powerplant functions. All IPS equipment should perform its intended function throughout the ent ire operating envelope.

The primary purposes of flight testing are to determine t hat the IPS is acceptably effective and performs its int ended functions during flight as predicted by analysis or ground testing and to evaluate any degradation in performance and flying qualities. Performance and handling qualities requiremen ts are identified in § 25.21(g). Flight tests to show co mpliance with these requirements are addressed in AC 25.21-1. Flight test ing also verifies the adequacy of the flightcrew procedures and limitations for t he use of the IPS in normal, abnormal, and emergency conditions.

Flight testing is also used to confirm t hat the powerplant installation as a whole (engine, inlet, anti-ice system, etc.) perfo rms satisfactorily while in icing conditions. In addition, the APU shoul d be evaluated for performance in icing conditions if it performs essential functi ons. 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 specia l airplane/engine operating procedures.

Both 14 CFR §§ 25.1419 and 25.1420 require t he 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 effect s 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 equi pment may be warranted to verify the

basic function of each item. Quantitative data may be ob tained, as necessary, to verify the system designs. These data in clude operating temperatures of thermal devices, deicing fluid flow rates and flow patterns for deicing liquid devices, and operating pressures and rates of in flation of pneumatic components.

b. Dry Air Flight Tests with Ic e Protection Equipment Operating . Initial dry air flight tests are usually the first fli ght 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 t he 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 ty pical dry air flight test practices. Other types of equipment should be evaluat ed 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 des ign parameters and thermal performance analyses.

(a) Normally, the system comp onents 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 te sts 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 t hen can be demonstrated through a comparison of the measured data to t he theoretical analysi s 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 t he 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 defi ned 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 env elope 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 hav e no significant effect on airplane performance and handli ng 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 dr y air, the followin g system parameters should be monitored to confirm proper f unction. It is suggested that system current, brush block voltage (i.e., bet ween each input brush and the ground brush) and system duty-cycles be monitor ed to ensure that adequate power is applied to the deicers. Surface tem perature measurement s should be made during dry air tests. These surface te mperature measurement s are useful for correlating analytically predicted dry air te mperatures with act ual 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 expe cted during flight in icing. Any significant vibrations should be investigated.

(c) Consideration should be giv en to the maximum temperatures to which a composite propeller blade ma y be subjected when the deicers are energized. It may be useful to monito r deicer bond-side temperatures. When performing this evaluation, the most crit ical 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 ma y be needed to verify the thermal analysis. Thermal analysis shoul d substantiate that the windshield surface temperature is sufficient to main tain anti-icing capabi lity without causing structural damage to the windshield. An evaluation of the vi sibility, including distortion effects through the protect ed area, should be made for both day and night operations. In addition, the size and location of the pr otected area should be reviewed for adequate visibility, espec ially 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 equival ent 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 t he 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 indicati on on an electronic display for each pitot source. Additional guidance on an a cceptable means of compliance with § 25.1326 is provided in AC 25-11. Su rface 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 bl eed air extraction on engine and airplane performance should be examined and included in the AFM performance data. The surface heat dist ribution analysis should be verified for varying flight conditions in cluding 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 e ffect 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 Ap pendix X icing conditions. Scale effects and scaled roughness effects should be subst antiated 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-st atic sensors, angle of attack sensors, and static pressure ports should be assessed.

For failure conditions of the IPS that are not extremely im probable, validation testing may be required (e.g. flight test s, flight simulato r, wind tunnel test, validated CFD) to demonstrate that the effe ct on safety of flig ht (as measured by degradation in flight characteristics) is commensurate with the failure probability. Dry air flight tests with pr edicted 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 t ankers, are normally employed to demonstrate that the IPS performs during f light 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 t he case of natural icing, testing should be accomplished within the icing condi tions 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 charac teristics 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 vari ous means, e.g., a rod mounted on the airfoil and bl ack 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 fo llowing parameters s hould 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 necessa ry 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 wa ter drop size distribution.

(g) Median volume drop diameter and drop diameter spectra. When

measurement of the ic ing environment drop diameter is necessary, instrumentation used for measuring dr op sizes should be appropriate for the Appendix C and App endix X icing conditions. For aircraft to be certified to a portion or all of Appendix X, measurement and record ing of drop diameter spectra should be accomplished.

Microscopic measurement of drop impact cr aters on a gelatin oil or soot slide exposed to an icing cloud may be an impractical method for measuring a median volume drop diameter becaus e of the splashing and bouncing characteristics of large drops. Also, use of gelatin oil or s oot slides is impractical for recording the drop diameter spectra of Appendix X type clouds. Medi an 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 e quivalent. Depending on t he drop size measurement capability of the instrum ents, more than one instru ment may be required to measure the expected range of drop diamet ers. Large drop in strumentation on the test aircraft may not be required if te sting 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 contai n mixed-phase icing (liquid plus ice). The instrumentation should provide t he capability to be able to distinguish between liquid drops and ice crystals. The equipment should be calibrated and properly maintained to obt ain quality data.

(2) Artificial Icing . Flight testing in artifici al icing environments, i.e., behind icing tankers, repr esents one way to predict ic e protection capabilities of

individual elements of t he ice protection equipment and determine local ice shapes. Due to limited cross-sectional ar ea of the plume, testing is usually limited to components such as, heated pito t tubes, antennas, air inlets including engine induction air inlets, empennage, airfoil sections, and windshields. Calibration and verification of the icing cl oud 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 hi gh confidence in local icing effects. However, it may be difficult to obtain sm all drop sizes with some spray nozzles. Therefore, these methods could produce larger ice bu ild-ups and different ice shapes than those observed in natural ic ing conditions within the icing envelope of part 25, Appendix C. Th is 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 C Lmax 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 simula te the resulting ice accumulations.

Tanker simulation could also be used to estimate accretion areas that could be used in drag estimates from extended accr etion 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 temper ature) should be considered in this type of testing.

(3) Appendix C Natural Icing Flight Testing

.

(a) Section 25.1419(b) requires m easured natural icing flight tests. Flight tests in measured natural icing conditions are intended to verify the ice protection analysis, to check for icing anom alies, 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 anal ysis. 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 accom panied by lightning strikes, severe turbulence and possible hail encounters t hat could extensively damage the test airplane. When design analyses show that the critical ice protection design points (i.e., heat loads, critical ice s hapes, accumulation, and accumulation rates, etc.) are adequate under these conditions, and sufficient gr ound or flight test data exist to verify the analysis, the flight te st in intermittent ma ximum icing conditions may not be necessary.

(c) Flight testing in natural icing c onditions should also be used to verify AFM procedures for activation of the I PS, including recognition and delay times associated with IPS activation and verify the analytically pr edicted location and general physical characteristics of the ice accretions. Critical ice accumulations should be observed, where possible, and su fficient data taken to allow correlation with dry air testing. Remotely located ca meras either on the te st airplane or on a chase airplane have been used to documen t ice accumulations on areas that cannot be seen from the test airpl ane’s flightdeck or cabin.

(d) For an airplane with a therma l 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 ce rtification program. The tests usually encompass the measurem ents of the surface temperature time history. This time history includes the ti me the system is activated, the time the surface reaches an effective temperature, an d the time the majori ty of ice is shed from the leading edge. The dat a should be recorded in t he flight test report.

(4) Appendix X Natural Icing Flight Testing . Section 25.1420(a) requires that the airplane operate safely in the SLD ici ng 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 nat ural Appendix X icing conditions may be necessary to:

1 . Verify the general physical charac teristics and location of the simulated ice shapes utilized for dry-air test ing, 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 Appen dix X icing conditio ns should not be necessary if:

1 . The design analyses show that the critical ice protection design points (i.e., heat loads, critical ice s hapes for performance and handling qualities, accumulation, and accumulation rates, et c.) 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 Appendi x X ice accretions should be shown to provide similar results (ice accretion th ickness, 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 desi gns were validated as in (i); and

3. Adequate analyses and/or tests ar e accomplished if there is a need to evaluate more than one airplane component simultaneously. As examples, the evaluation of airplane per formance with propeller and airframe ice accretion, asymmetric ice accretions due to propeller wash, engine performance with inlet (including cooling) ice accreti ons, 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 A ppendix X icing conditions should be accomplished for airplane derivatives whose ancestor airplanes have a service record that includes a pa ttern of accidents or incid ents 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 protect ed 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 remain ing, etc., should be

evaluated to determine that the indicators are plainly visible to the pilot and that the indications provided can be effectiv ely 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 fl uid anti-ice systems should be tested to demonstrate that it does not become opaqu e at low temperatures. The AFM should include information so the flightcr ew will know how long it will take to deplete the amount of fluid re maining 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 Appendi x C and Appendix X freezing drizzle. This type testing has been used to evaluate ice s hapes on unprotected areas as well as the performance of ice protection systems, such as pneumatic deicing or thermal systems, and any resulting protected area accr etions, such as intercycle, residual and thermal runback. Aerodynamic effect s (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, incl uding 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, wit h appropriate scaling corrections, to examine impingement limits relative to fuselages or windscreens with scale models relative to Appendix C and Appen dix 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, va lidate location of detection devices, and

determine total accretion areas that coul d 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 require ments. Ice shapes defined by computation, icing tunnels or icing tankers can be used. Scale aerodynamic and roughness effects should be substantiat ed using methods found acceptable to

the authorities.

8. CERTIFICATION TO § 25.1420 . Section 25.1420 requires that the airplane operate safely in the supercool ed 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 Pa rt 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 judgmen t, and flight testing in na tural icing conditions to demonstrate compliance with icing requirem ents. The interim SLD tools and test techniques are an extension of curr ent 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 dr op impingement and ice accr etion computer codes. However, extension of the progress made in freezing dri zzle 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 t he 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 co nditions 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 defi nes Appendix X and c ould include phase of flight limits, such as takeoff or holding, in Appendix X or a por tion of Appendix X. For example, an airplane ma y be certificated to takeof f 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 conditi ons within Appendix X during recognition and subsequent exit from icing conditions. Methods of defining the sele cted Appendix X icing conditions boundary should be considered early in the certificat ion process, with concurrence from the appropria te certification authority. See paragraphs 15 of this AC for specific guidance on AF M limitations and operating procedures.

(1). Selection of Appendix X Boundary . As discussed in paragraph 3.c., “Effects of Appendix X Icing Conditions,” the physi cs of ice accretion is complex. However, some generalizations can be made relative to the effects of supercooled water drop impingement and ic e 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 Repor t DOT/FAA/CT-88/8-1, “Aircraft Icing

Handbook” for detailed discussion on the icing physics process.) The modified inertia parameter is a dim ensionless parameter that re lates the major influences on the trajectory of a water drop. These influences are drop size, drop density, local air density, drag forces on a drople t, 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. Examinat ion of the modified inertia parameter is of interest for the inertia of Appendix X large drops in that increas es in the modified inertia parameter typically result in increa sed collection efficiency and further-aft impingement limits. Also, as discussed in paragraph 3.c of this AC, other influences that affect the object's wate r 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 dr ops 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 defi ned in Appendix X. Drop size distributions and the related di stribution of water mass are important considerations for water catch, impingement and ice limits as well as runback ice.

Consideration of these infl uences allows comparison of critical icing conditions

that can be correlated to ice accretions in aft locations, such as behind protected areas. Conditions with equivalent modifi ed inertia parameters result in similar impingement characteristics.

The modified inertia parameter is commonl y used in icing wind tunnel testing to compare high altitude flight conditions to t he available low altitude test conditions. To simulate the impingement effects of high altitudes and true airspeeds, larger dropsizes are used at the lower altitude te st conditions. As an example, an IPS designed to protect against 50μm dr op impingement at 22,000 ft and 200 KCAS (290 KTAS) would be equivalently protec ted 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 di fficult to implement. Consideration of the drop inertia effects throughout the flight envelop e will likely be required. While technology does not currently exist to discriminate icing conditions based on specific drop sizes (exclusive of iner tia 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 exam ple, it may be possible to develop

an exit cue such that unrestricted f light in Appendix X freezing drizzle is approved; however, exit is requir ed 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 r ange from direct observation of ice accretions aft of the airplane’s protected su rfaces 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 sp inners aft of a reference point,

• accretions forming on radomes af t of a reference point, and/or

• accretions forming aft of the protected surfaces

(i) The visual cues should be devel oped 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 visibl e 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 adver se 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 t he difficulty of observi ng 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 validat ed through icing tanker test ing or potentially through icing wind tunnel tests. The cues shoul d consider the drop distributions of Appendix X, to establish t hat these cues would be approp riate in the restricted appendix X icing conditions. If a referenc e 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 ac tivation of the dei cer boot system. However, the accident and incident histor y 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 dete rmine 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 typica lly 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 comp lete cycle of the dei cing boots and allow completion of any subsequent cycle previous ly started. This note is equally applicable to other deicing systems that sim ilarly do not activate all portions of the deicing systems simultaneously.

10. EMERGENCY AND ABNORM AL 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. Thes e demonstrations should be conducted with anticipated ice accumulation on normally pr otected 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 requi rements regarding the activation of the IPS. These requirements are only applic able 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 c onditions from which they must exit, respectively.

a. COMPLIANCE WITH § 25.1419(e)(1) and (e)(2). These sections of the rule provide alte rnatives to the operat ion of the IPS based on icing conditions defined in § 25.1419(e)(3). These alternatives require either a primary ice detection system, or substant iated 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 acti vate the IPS prior

to an unsafe accumulation of ice on t he airframe, engine components, or engine air inlets. The primary ice detection syst em must perform its intended function for the airplane configurations, phases of flight, and within ic ing envelopes of Appendix C and Appendi x X. Design of the primar y ice detector system should account for reasonable time delays in t he 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 t he ice detector to function properly within the entire required icing conditions and airplane oper ating envelope. Approval of the primary ice detector should include measured natural icing flight tests to verify analyses and laboratory test results, as we ll 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 detec t 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 operat ion of the IPS using AFM procedures. An advisory system is not the prime means used to determi ne if the IPS should be activated. When there is an advisory syst em installed on an airplane, the flight crew has primary responsibility fo r determining when the IPS should be activated. Analyses and tests simila r to those performed for a primary ice detector should be performed for an adv isory 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 ens ure 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 airfra me surface where the sensor is adequately exposed to the icing environm ent. Flow field and boundary-layer analyses of candidate installation posit ions should be accomplished to ensure

that the ice detector sensor is not shielded from impingi ng Appendix C and Appendix X water drops. The ice det ection system should be shown to operate in the range of conditions defined by Appendix C and the appl icable portion of Appendix X. Sections 25. 1419 and 25.1420 also require a combination of tests and analysis to demonstrate the perform ance of the ice detector and the system as installed on the airplane. This could in clude icing tunnel an d 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 sa fely operated with the ice accretions formed at the time the ice protecti on system becomes e ffective, following

activation of the ice detector. The detec tor and its installation should minimize nuisance warnings.

(d) Ice Detector System Safety Considerations . The applicant should consult AC 25.1309-1A for guidance on complianc e 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 c haracteristics of the airplane in icing

conditions without activation of the ai rframe ice protection system(s) are

demonstrated to result in a less severe hazard category. If visual cues are primary, failure of an advis ory 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 obs ervation of ice accretions on reference surfaces. Examples of visual means could be:

• accretions forming on the windshiel d 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 wit hin 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 eval uated from the most adverse flightcrew

seat locations in combination with the range of flightcrew heights, within the approved range of eye reference point locati ons, if available. A visual cue is

### IPHWG Task 2 WG Report - Appendix K IPHWG Recommended Advisory Materials§ §25.1419, 25.1420

12/19/2005 Page K-48 required for both the left and right seats. If a single visu al cue is used, it should

be visible from either seat. Considerat ion should be given to the difficulty of observing clear ice. If a reference surf ace 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 te sting in measured natural icing.

b. COMPLIANCE WITH § 25.1419(e)(3). This paragraph of §25.1419 provides an alter native 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 t he airplane is in conditions conducive to airframe icing during all phases of flight.

(1) Temperature Cue. The temperature cue us ed in combination with visible moisture, should be calibr ated and be readily available to the flightcrew. A minimum temperature lim itation 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 to tal 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 substant iate that the IPS need not be operated during certai n phases of flight, the ic e accretions that form during these phases should be considered w hen establishing the airplane is able to safely operate in the continuous maximum and intermittent maximum icing conditions of appendix C and the applic able portions of Appendix X.

d. COMPLIANCE WITH § 25.1419(g). This paragraph of § 25.1419 requi res that after the initia l activation of the IPS:

(a) the IPS must operate continuously, or

(b) the airplane must be equipped wit h a system that automatically cycles the IPS, or (c) an ice detection system must be prov ided to alert the flightcrew each time the IPS must be cycled.

Some examples of systems which automatically cycle the IPS are:

### IPHWG Task 2 WG Report - Appendix K IPHWG Recommended Advisory Materials§ §25.1419, 25.1420

12/19/2005 Page K-49 • A system that senses ice accretion on a detector and correlates it to ice accretion on a protected surface. Th is 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 ic e 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 wh ich 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 deice rs does not meet th e 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 a ccretions 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 f light crew is able to determine which cycle time is appropriate. The ice shedding effectivene ss of the selected means for cycling the ice protection system should be eval uated during testing in natural icing conditions and any inter-cycle and runback ice should be considered when showing compliance with § 25.21(g). S ee paragraphs 11.a.(1) of this AC for guidance on installed ice detector per formance 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 f light 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 detecto r 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 ensur e that the ice det ector 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 Appendi x X when validated considering methods described in SAE ARP 5903. It should be ensured that the system minimizes nuisance warnings w hen operating in icing conditions.

The low probability of finding conditions conducive to ice accumulation in Appendix X may make natural icing fli ght tests difficult as a means of

### IPHWG Task 2 WG Report - Appendix K IPHWG Recommended Advisory Materials§ §25.1419, 25.1420

12/19/2005 Page K-50 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 functi oning of the system and to correlate the signals provided by the detectors and the actual ice accretion on the surface.

(2) Aerodynamic Performance Monitor (AP M). A crew alerting system could be developed using pressure probes and signa l processors to quantify pressure

fluctuations in the flow field from c ontamination 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 incl uding, 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 opera tion (§§ 25.1093 and 25.1301).

b. Engine Alternat e 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 retr action 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 ar e not protected. Therefore, the se nsors’ functions should be evaluated to ensure operation in the icing conditions of Appendices C and, as appropriate, X. The activation point s of artificial stall warning and stall identification systems, if installed, may need to be reset for operations in icing conditions to provide adequate stall warn ing 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 equipm ent 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 subst antiated visual cues for compliance with § 25.1419(e)(2).)

### IPHWG Task 2 WG Report - Appendix K IPHWG Recommended Advisory Materials§ §25.1419, 25.1420

12/19/2005 Page K-51 g. Primary and Secondary Flight Control Surfaces . Primary and secondary flight control surfaces shoul d remain operational after exposure to icing conditions. It should be demonstr ated 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 landi ng configuration.

h. Ram Air Turbine . The ram air turbine shou ld 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 obstructi on 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 limitati on, § 25.1403 requires that a means be provided for illuminating or otherwise det ermining 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 t he 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 illumina tion 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 illumi nation of the component of interest is available without excessive glare, refl ections, or other distractions to the flightcrew. These tests may be conv eniently accomplished both in and out of clouds during the airplane cert ification 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 appropri ate manual should identify the ice characteristics the flightcrew is expect ed to observe and the flightcrew action

associated with the observation.

14. CAUTION INFORMATION . Section 25.1419(c) r equires that caution information be provided to alert the fli ghtcrew when the IPS is not functioning normally. Caution information should be pr ovided if the failure condition of the IPS can result in a hazardous airplane condit ion. 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 numeric al probability of the failure event (i.e., even if a numerical probability anal ysis indicates the system failure is

### IPHWG Task 2 WG Report - Appendix K IPHWG Recommended Advisory Materials§ §25.1419, 25.1420

12/19/2005 Page K-52 improbable or extremely improbable, t he 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) us ed to identify a failure condition should

be evaluated to ensure that the sensors ar e 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 t hat the system is functioning normally. For example, if a pneumatic deicing system (boots) requ ires a specific minimum pressure and pressure rise rate to adequately shed ice, then caution information should be provided if the minimum pressu re and pressure rise rate are not

attained. If this caution information is not provided, the flightcrew may erroneously believe that the boots are oper ating normally when, in fact, the boots might not be inflating with sufficient pre ssure or rate of inflation to adequately

shed the ice. The need for caution informa tion 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 s hould address airframe, engine, and equipment ice protection systems, and other systems that may nee d to be specifically configured for operation in icing conditi ons. 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 flui d anti-icing/deicing systems capacities and flow rates.

(3) Airplane speed limitations (i f any) and minimum temperature for deicing boot operation for ai rplanes equipped with boots.

(4) Environmental limitations fo r equipment operations as applicable (e.g., minimum temperatur e for boot operation or ma ximum altitude for boot operation).

(5) Minimum engine speed if the airf rame IPS does not function properly below this speed.

(6) A list of required placards.

### IPHWG Task 2 WG Report - Appendix K IPHWG Recommended Advisory Materials§ §25.1419, 25.1420

12/19/2005 Page K-53 (7) If compliance is based on §§ 25.1420(a)(1) or 25.1420(a)(2), the Limitations section must include a require ment that the flightcrew exit all (Appendix X and C) icing conditions i mmediately upon recognition that the Appendix X icing conditions exceed the cert ification boundary. Fo r certification of detection and exit of Appendix X in accordance with § 25.1420(a)(1) and §25.1420(a)(2) the Limitations section shoul d contain appropriate limits that consider the certification assumptions wit h respect to portion (phase of flight, freezing drizzle, freezing rain, etc.)

b. Operating Procedures Section

. (1) Section 25.1585 requires that oper ating procedures be provided in the AFM for: (1) normal procedures pecul iar 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 system s, and (3) emergency procedures for

foreseeable but unusual situations in wh ich 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 emer gencies associated with the IPS. The system components should be described with suffici ent clarity and depth that the pilot can understand their function. Unless flightcrew actions are accepted as normal airmanship, the appropriate procedures sh ould be included in the FAA-approved AFM, AFM revision, or AFM supplemen t. 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, in cluding climb, holding, and approach configurations and speeds. The AFM s hould define when the ice protection equipment should be activated consist ent with the system operation during

certification.

(3) Emergency or abnormal procedur es, 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 s upply adequate electrical power for all systems at low engine speeds, load-shedding in structions 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):

### IPHWG Task 2 WG Report - Appendix K IPHWG Recommended Advisory Materials§ §25.1419, 25.1420

12/19/2005 Page K-54 (a) The method used to deter mine 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 certai n phases of flight to Appendix X, information defining these restricted phases should be provided. This information should include any aircraft c onfigurations associated with the restricted phases of flight such as fl ap position, gear extension, or airspeed.

(7) The method for determining when the IPS must be activated must be provided. Information should be prov ided to indicate that a de-icing system should not be de-activated until the completion of an entire deicing cycle after leaving icing conditi ons. An anti-icing system should not be de-activated before leaving icing conditions.

(8) The following statem ent should be included: Convective clouds, above the freezing al titude, that are vigorously growing should be avoided since they may c ontain 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 it s “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 provis ions 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 eval uated 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.

### IPHWG Task 2 WG Report - Appendix K IPHWG Recommended Advisory Materials§ §25.1419, 25.1420

12/19/2005 Page K-55 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 fo llowing hypothetical example is used.

The aircraft is a transport category, tu rbofan aircraft. It is a derivative of an existing type. The existing aircraft has al ready been type certificated for flight in icing including §25.1419 (A ppendix 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 co mpliance with 14 CFR 25, sub-part B requirements and AC 20-147 contains guidance for engine and engine

installations. All relevant certificati on requirements must be met. However for this example only the following aspects will be discussed.

Impingement aft of t he protected areas of lifting surfaces. The aircraft has ice protection over the full span of the wing and horizontal stabilizer with minimal unprotected areas. Si nce this is a derivative aircraft, the system has already been evaluated and cert ificated for Appendix C conditions. The aircraft does have reversible flight controls on the roll and pitch axis. An assessment of hinge moment and lift reduc tion effects is required. See AC 25.21-1 for details on perform ance and handling qualities guidance. Impingement aft of the engine inlet protected surfaces. Ice accumulation aft of the engine inlet protected su rfaces 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 complia nce 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 ar eas 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 ex posure 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 airf rame (which do not accrete ice under Appendix C

### IPHWG Task 2 WG Report - Appendix K IPHWG Recommended Advisory Materials§ §25.1419, 25.1420

12/19/2005 Page K-56 conditions) should be examined relative to the increased impingement and

runback on accretion areas related to A ppendix X. Issues such as windshield visibility, static port location and potentia l airflow disturbance from Appendix X

accretions that could influence sensors (TAT, pitot, AOA) should be examined. Use of computational fluid dynamic analysi s, 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 i ndicates that the wi ng is the critical surface for formation of a potential ridge behi nd 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 horizont al stabilizer would be addressed, but will not be presented in this example.

Determination of Exit Cues The aircraft-operating envelope encom passes 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 in formation. 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 pr imary concern in this example, an analysis of the operational co nditions will be made to determine the critical flight conditions for such accreti ons. The conditions considered as critical were determined to be the following:

Highest holding altitude within the conditi ons of Appendix X that maximizes water catch (typically a function of ram air e ffects and LWC content at defined static temperatures) Highest true airspeed ex pected during holding conditions (typically high weight hold speed; pr oduces maximum water catch aft of protected areas upper surface.)

Note: Other operating conditions s hould be considered as required to

determine the critical conditions.

Through a combination of impingement anal ysis, analytical ice shape prediction and icing wind tunnel testing, it has been det ermined that there is potential for an

### IPHWG Task 2 WG Report - Appendix K IPHWG Recommended Advisory Materials§ §25.1419, 25.1420

12/19/2005 Page K-57 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 exampl e, at a lower altitude and a similar equivalent airspeed (similar AOA) , slightly larger dropsizes are required to produce a similar accretion of ice behi nd the protected area. However, this combination of drop size, liquid water conten t, and relative inertia to the aircraft is sufficient to potentially produce accumula tions behind the protected areas. See

discussion on the modified iner tia 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 s pecific drop size determined by analysis and testing under the operating conditions of a high alti tude, high speed hold. The target

criterion for a visual exit cue is a surfac e on the aircraft that is readily visible to the flight crew that will indicate wate r 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 s pecific portion of the windshield as a

reference surface that can be correlat ed to ice accumulation on the wing. Preliminary impingement analysis indicates t hat this is feasible as there is significant movement of the aft im pingement limits on t he windshield with increasing drop sizes when the specific flight conditions of interest are considered. Based on this analysis, a s pecific location on the aircraft side window is proposed as the visible c ue 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 accreti on of approximately 1/16-in ch is required on the

reference surface for the crew to detect it. This reference-surface accretion is then correlated to the specific ice accret ion 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 prot ected area at the specific critical operating condition. T he difference in ice thickness is due to

differences in the local collection effi ciencies between the side window and the area aft of the leading edge protected su rface. 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.

### IPHWG Task 2 WG Report - Appendix K IPHWG Recommended Advisory Materials§ §25.1419, 25.1420

12/19/2005 Page K-58 Evaluation of critic al ice accretions Wind tunnel testing of this critical a ccretion 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 address ed in the stall warning system. Dry air flight-testing is performed us ing simulated ice shapes. This flighttesting is performed in a built up m anner with limited spanwise exposure to evaluate the effects with minimal safety risk. The stall warning margins are

evaluated and result in a modification of t he 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. S ee 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 th rough flight-testing with an icing tanker. The tanker is equipped to provide a specific MVD with an associated droplet distribution. As the tanker cannot si mulate the full extent of icing conditions within Appendix X, a validat ion of the specific conditions will be made. A fixed nozzle simulating the specif ic drop size to be used as the exit cue

will be selected.

Icing tanker testing indicates that t he side window does begin to accumulate ice at the specific condition considered, wh ich 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 s hape behind the protected areas. The location is such that it is adequately il luminated by existing cockpit lighting without glare or reflection and is r eadily observable by crewmembers.

Exiting from restricted Appendix X icing conditions Flight tests are performed in accordanc e 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 perform ance and handling qualities guidance.

### IPHWG Task 2 WG Report - Appendix K IPHWG Recommended Advisory Materials§ §25.1419, 25.1420

12/19/2005 Page K-59 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 modi fications (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 anal ysis provided in par agraph 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 c onditions,§ 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 Characteristi cs, Stability, and Control If there is a potential for the increased engine power or thrust to affect airplane handling qualities in icin g 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 simila rity is used as a method of compliance, it should be supplemented with analysis or testing, or both. In some cases, an analysis that subst antiates similarity, along with an installation survey conducted by the engine manufacturer, may be sufficient. The analysis should include:

1. Heat requirements for the in let 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 address ed by analysis must be addressed by flight testing. The data to be used fo r similarity, including the data from past certification on which the similarity anal ysis 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 certific ation 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 syst em 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 shoul d be compared between the currently installed engine and the pr oposed 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 s hed 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 te mperatures of th e proposed engine and existing, certified engine should be compar ed. If there is a reduction, the

effectiveness of the IPSs must be substantiated.

c. Propeller changes

Section 25.929 requires a means to pr event 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. T he 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 t han the accretions obtained on the same propeller on an airplane that was flight te sted in icing conditions and shown to have no appreciable loss in thrust.

The typical deicing boot manufac turer report also contains a caveat that it does not address propeller runback ice. Similari ty to another propeller that was flight tested in icing conditions is usually done to address runback. Similarity would include propeller and deicing boot aerodynam ic and thermal similarity, deicing timer, propeller RPM, and flight conditi ons. 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 performanc e is checked during flight tests in icing conditions.

The propeller installation, including spinner and cowl geometry, must be compared to previously tested installati ons in icing conditions. Changes that

could allow moisture to reach t he brush blocks must be avoided.

If the proposed propeller is calculated to hav e higher efficiency than the existing, approved propeller, the guidance under “1. E ffects 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 change s” section above. It should be noted that § 25.1093 applies to engine oil and ac cessory 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, fairi ngs 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 evaporat ion of impinging water. (d) Assume the shape on blade antenna will be similar to airfoils and the shape on low prof ile 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, r un an ice accretion code to determine a less conservative ice shape.

(4) Flight tests in measured nat ural icing or wit h simulated ice shapes should be accomplished to dete rmine 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 fairi ngs. An analysis to show the impact on maximum lift coefficient, in combination wit h flight tests with no ice accretions, have been accepted in the past.

f. Gross weight increases

(1) The impingement analysis shoul d cover a weight range that is applicable to any increase in gross weight.

(2) The impingement analysis shoul d also evaluate unprotected

areas such as fuel vents. (3) If there is a potential for t he increased gross weight to affect airplane handling qualities in icin g 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 us ed for center of gravity envelope increases. The one exception is an increas e of forward center of gravity limit on airplanes may make an airp lane 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 altit ude is applied for, the applicant should

demonstrate:

(1) the IPS operating pressure s (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 an d 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 acc eptable pneumatic operating pressure, which should allow for descent, shoul d be established and published in the

AFM/POH.

j. Modifications to lifting and control surfaces

Critical ice accretions (i ncluding consideration of pr e-activation, intercycle, residual, and runback icing) may have to be re-defined, especially if the changes affect operational wing angle of attack. Stall strips ar e good collectors of ice and are an example where leading edge ice accr etions should be re-defined. If ice accretions are changed or t he 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 depend ing on their location. The applicant should provide data on expected ice accretions . Flight conditions to consider are the hold, descent, and approach. Substant iation on the effects on stall speeds, stall characteristics, and stab ility and control should be provided.

l. Modifications to ice protection systems Critical ice accretions (i ncluding consideration of pr e-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 CLO UD 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 M INORITY P OSITION ON THE N ECESSITY OF AN AFM S TATEMENT C ONCERNING C UMULIFORM C LOUD L ARGE D ROPLET I CING (E XCEEDANCE OF A PPENDIX X)

Supported by: ALPA

The addition of an explicit caution concer ning convective cloud icing in future aircraft flight manuals is necessary fo r 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, wit h Appendix X addressing only stratiform clouds, that is not necessarily the case. Appendix C certificat ion 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 Appendi x X icing. In the moder n ATC environment, pilots routinely face decisions concerni ng penetration of cumuliform clouds and operations near thunderstorms. While pi lots understand that numerous hazards are associated with these conditions, without an AFM warning they will not understand that icing certific ation does not provide the same safe operation in convective cloud icing as it does in other ic ing. 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 ma y result in hazardous degradations of aircraft performance and handling characteristics and should be

avoided.

{Note: A WARNING is defined as an operat ing procedure, technique, etc., that may result in personal injury or loss of life if not carefully followed. A CAUTION is an operating procedure, technique, et c., that may result in damage to equipment if not carefully followed. Ba sed on these definitio ns, 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 t he 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 c onditions that may be encountered in or near convective clouds that are vigorous ly growing above the freezing level

are not evaluated in aircraft certificat ion. Such conditions may result in hazardous degradations of aircraft perfo rmance and handling characteristics,

and should be avoided.

M AJORITY P OSITION ON THE N ECESSITY OF AN AFM S TATEMENT C ONCERNING C UMULIFORM C LOUD L ARGE D ROPLET I CING (E XCEEDANCE OF A PPENDIX X)

Supported by: Airbus, Boeing, Bom bardier, 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 la rge droplet convective cloud ic ing conditions that are not considered during certification. However, the use of a Warning or Caution in the Limitations section is not appropriate. Fo r a Warning or a C aution, 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 ni ght, there is not a good means for the flightcrew to consistently avoid these cl ouds. 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 instru ctions. 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 avoidi ng these clouds when possible.

In addition, the majority do not concur t hat the AFM information is necessary for the airplanes that are ce rtificated 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 al titude, 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 Ae roplane Handling Qualities and Performance in Supercooled Large Drop Icing Conditions

Prepared for the IPHWG

Introduction

The rules that the FTHWG proposes fo r performance and hand ling qualities for flight in the icing conditions of Ap pendix X are based upon those developed for flight in Appendix C ici ng conditions. These pr oposals address the options identified by the IPHWG for CS/FAR 25.142 0; 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 ici ng 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, wit h 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) "operati ng safely while exiting all icing conditions", the FTHWG considers that these are not failure cases and the temptation to link this text with the phr ase "continued safe flight and landing", commonly used in association with system fa ilure 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 cert ification for Appendix C icing conditions (discussed later).

Continued operation in the ic ing conditions of Appendix X

For continued operation, the FTHWG consi ders that little change is required from the requirements for Appendix C. The list of requirements except ed for flight in Appendix C were reviewed and found larg ely 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

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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 practi ce, it is expected that some applicants may prohibit take-off in Appendix X conditions. Otherwise, the ra tionale behind the requirements is exactly that applied fo r Appendix C; that for continued operation in icing conditions, there should e ffectively be no degradation in handling qualities and any degradation in performance should be strictly limited by means of the same thresholds developed for A ppendix C. The proposed requirements are thus, with the one except ion identified above, ident ical to those proposed for Appendix C.

The FTHWG also considers that it is worth clarifying that the op tional certification for continued operation in the supercool ed 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 operat ion to a portion of the Appendix X icing conditions is acceptable, provided that there are not too m any 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 st andpoint but has accepted a further subdivision to each based on particle size.

The concept of limiting continued operatio n in Appendix X to specific flight phases, not explicitly stated in CS/F AR 25.1420 but addressed in the IPHWG draft AC, is more difficul t. In certification ther e is a general understanding on what is intended by reference to the differ ent flight phases in terms of aeroplane configuration, speed etc. Operationally, there is not usually such a clear

distinction. Take-off and fi nal take-off can be considered a distinct flight phase in that operation in this phase can be aut horised or not and a clear go/no go

decision can be made on the ground. Otherwise, the onl y discriminant that can sensibly be recognised by the crew is aer oplane 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 cons ider 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 condi tions 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 not ing that the FTHWG

has followed the advice of the IPHWG in assuming that the aeroplane leaves all icing conditions after encountering A ppendix X icing conditions and does not continue in Appendix C icing conditions.

"Detect and exit" Appendix X certification integral wit h Appendix C ce rtification

The "detect and exit" cases of 25.1420(a)(1 ) are normal flight conditions arising from an inadvertent encounter with Append ix 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 certificat ion is sought, then "detect and exit" for Appendix X

must also be addressed.

Performance and handling qualit ies for "detect and exit"

Regarding specifically handling qualities and performa nce, 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" aeropl ane. Hence, with the exception of stall speeds, landing speeds and distanc es and landing c limb performance, the FTHWG considers that performanc e 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-o ff case, CS/FAR 25/143(c)(1), addressing controllability following engine failure at V 2 . No justification for a relaxed standard of handling qualities could be ident ified, bearing in mind that "detect

and exit" is not a failure case and that the aeroplane, once cl ear of all icing conditions but retaining the ice accreti on, 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 t heir 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 margi nal. It is anticipated that the same philosophy will hold for Appendix X.

• Sandpaper ice , included in the previous A ppendix C proposals, does not feature in the Appendi x 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 envelo pe accretion covering both cases.

• "Prior to detection of Appendix X" – The FTHWG consider s 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 requir ements (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 operating25.121 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 fli ght in the icing conditions of Appendix C (EASA NPA 16/ 2004 and the corresponding FAA NPRM) require revision to accommodate the pro posed 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, fo r 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 limi tations and operating procedures provided in the aeroplane Flight Manual.

(2) Unless otherwise prescribed, comp liance 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 ope rating 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 perf orming 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 re quirements of CS 25.143(k) and 25.207(i) after entry into the supercool ed large drop icing conditions of Appendix X and before the detecti on of those conditions. Compliance must be shown using the ice accreti ons 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 atmos pheric condition) of the supercooled large drop icing conditions of Appe ndix 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 aer oplane and its ice protection system in accordance with the ope rating limitations and operating procedures provided in the aeroplane Flight Manual.

(6) No changes in the load distributi on 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 allo wed for flight in icing conditions or with ice

accretion. "

Proposal 2

Amend CS 25.103 Sta ll 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 V SR

....

(b) V CLMAX is determined with:

(1) Engines idling, or, if that resultant thru st causes an appreciable decrease in stall speed, not more t han zero thrust at the stall speed;

(2) Propeller pitch controls (if app licable) 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 V SR is being used;

(4) The weight used when V SR 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 V SR and not greater than 1.3 V SR

.

(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-o ff 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 am bient temperature within the operational limits selected by the appl icant; and

(2) In the selected configuration for take

- off.

in the selected configuration for t ake-off at each weight, altitude, and ambient temperature within the operati onal limits selected by the applicant

-

(1) In non-icing conditions; and

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-12 (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 great er of 5.6 km/h (3 kt) CAS or 3%

V SR ; or

(ii) The degradation of the gr adient of climb determined in accordance with CS 25.121(b) is greater than one-half of the applicable

actual-to-net take-off f light 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 dat a 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) V 2 , 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) V 2MIN ;

(2) V R 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) V FTO , in terms of calibrated airspeed, mu st 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 -

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-13 (1) 1.18 V SR ; and

(2) A speed that provides the manoeuvring capability specified in CS 25.143( h ).

(h) In determining the take-off speeds V 1 , V R , and V 2 for flight in icing conditions, the values of V MCG , V MC , and V MU 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 determi nation in accordanc e 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 automati c 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

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-14 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 aer oplane 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 configurati on, 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 th rust setting (see AMC 25.119(a) ) - and

(b) A climb speed which is

-

(1) Not less than

-

(i ) 1·08 V SR for aeroplanes with four engines on which the application of power results in a signi ficant reduction in stalling speed; or

(ii) 1·13 V SR for all other aeroplanes;

(2) Not less than V MCL ; and

(3) Not greater than V REF

. (a) In non-icing conditions, with a climb speed of V REF determined in accordance with CS 25.125(b)(2)(i),

(b) In icing c onditions with the applicable “Landing Ice” accretion specified by CS 25.21(g) , and with a climb speed of V REF determined in accordance with CS 25.125(b)(2)(ii). "

Proposal 7

Amend CS 25.121 Climb: One-e ngine-inoperative to read:

"(a) Take-off; landing gear extended

. ....

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-15 (b) Take-off; landing gear retracted

. In the take-off configuration existing at the point of the flight path at which t he 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 thr ee-engined aeroplanes, and 3. 0% for four-engined aeroplanes, at V 2 and with -

(i) The critical engine inoperative, t he 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 t here 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-paragr aph (b)(1) of this paragraph must

be met:

(i) In non-icing conditions; and

(ii) In icing c onditions with the applicable “Take-off Ice” accretion specified by CS 25.21(g) , if in the configurati on 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 th e greater of 5.6 km/h (3 kt) CAS

or 3% V SR ; or

(B) The degradation of the grad ient 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 thr ee-engined aeroplanes, and 1. 7% for four-engined aeroplanes, at V FTO and with -

(i) The critical engine inoperative and the remaining engines at the available maximum continuous power or thrust ; and

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-16 (ii) The weight equal to the weight exis ting at the end of the take-off path,

determined under CS 25.111.

(2) The requirements of sub-paragr aph (c)(1) of this paragraph must

be met:

(i) In non-icing conditions; and

(ii) In icing c onditions 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 “T ake-off Ice” accretion:

(A) The stall speed at maximum take-off weight exceeds that in non-icing conditions by more than th e greater of 5.6 km/h (3 kt) CAS

or 3% V SR ; or

(B) The degradation of the grad ient 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 correspond ing to the normal all-enginesoperating procedure in which V SR for this configuration does not exceed 110% of

the V SR 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 thr ee-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 V SR : and

(iv) Landing gear retracted.

(2) The requirements of sub-paragr aph (d)(1) of this paragraph must

be met:

(i) In non-icing conditions,

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-17 (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 c limb 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 f light paths to read:

"(a) For the en-route c onfiguration, the flight paths prescribed in subparagraphs (b) and (c) of this paragraph must be determined at each weight, altitude, and ambient temper ature, within the operating limits established for the aeroplane. The variation of weight al ong 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 V FTO , with -

(1) The most unfavourable centre of gravity;

(2) The critical engines inoperative;

(3) The remaining engines at the av ailable maximum continuous power or thrust, and

(4) The means for controlling the engi ne-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 gradien t of climb of 1.1% for two-engined aeroplanes, 1.4% for th ree-engined aeroplanes, and 1.6% for four-engined aeroplanes.

-

(1) In non-icing conditions,

(2) In icing c onditions with the applicable “En-route Ice” accretion specified by CS 25.21(g) , if:

(i) 1.18V SR 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% V SR , or

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-18 (ii) The degradation of the gradient of climb is greater than one-half of the applicable actual-to-net f light 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 surfac e 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 c onditions with the applicable “Landing Ice” accretion specified by CS 25.21(g) if V REF in icing conditions is greater than V REF 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 V REF , must be maintained down to the 15m (50 ft) height.

(i) In non-icing conditions , V REF may not be less than:

( A ) 1.23V SR0 ;

( B ) V MCL 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, V REF may not be less than:

(A) The speed determined in s ub-paragraph (b)(2)(i) of this paragraph;

(B) 1.23 V SR0 with the applicable "Landing Ice" accretion specified by CS 25.21(g) if that speed exceeds V REF selected in nonicing conditions by more than 9.3 km/h (5 kt) CAS; and

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-19 ( C ) A speed that provides the manoeuvring capability specified in CS 25.143(h).

(3) Changes in configur ation,....(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 Controll ability 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 aer oplane 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 V 2 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

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-20 (a) and (b) through (c) of this paragraph (See AMC 25.143( d )): ....[Table unchanged]

(e) Approved operating procedures .... that are prescr ibed 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 fo r the particular flight phase. For aeroplanes with unpo wered 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 po wer. It must be possi ble to promptly recover

from the manoeuvre without exceed ing 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)

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-21 (3) Changes in longitudinal contro l force to maintain speed with increasing sideslip angle must be progressive with no reversals or unacceptable discontinuiti es. (Based on FAA/JAA/ TC/ALPA position in original FTHWG Appendix C report)

( 3 ) Advisory material only (Manuf acturers' 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 operati on 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 operati on 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 operati on of any ice protection system isdependent upon means of recognition other than that defined in subparagraph (j)(1) of this pa ragraph, 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 a fter 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 i ce accretion on a reference surface, the requirements of CS 25.143 are app licable with the ice accretions specified by CS 25.21(g).

(2) If detection of Appendix X ici ng conditions is by an approved ice detection system, it must be shown th at 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). "

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-22 Proposal 11

Amend CS 25.207 Stall warning to read:

"(a) Stall warning with ....

(b) The warning must be furnished ei ther through the in herent aerodynamic qualities of the aeroplane or by a device that will give clearly distinguishable indications under expected condi tions of flight. However, a visual stall warning device that requires the attention of the cr ew 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 pr escribed 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 opera tion of the ice protection system prescribed in sub-paragraph (h)(2) of th is 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/s 2 (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 suffi cient 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 airs peed deceleration rate s of at least 1m/s 2
(2 knots per second), with the flaps and la nding gear in any normal position, with the aeroplane trimmed for straight flight at a speed of 1.3 V SR , and with the power or thrust necessary to main tain level flight at 1.3 V SR

. When demonstrating compliance with this s ub-paragraph with ice accretions, the same test

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-23 technique as for the aeroplane without ice accretions must be used for

recovery.

(g) Stall warning must ....

(h) For flight in icing c onditions prior to activation of normal operation of the ice protection syst em, the following apply:

(1) If activation of normal operati on of any ice protection system is dependent upon visual recognition of a specified ice accretion on a reference surface, the requirement s of this paragraph except subparagraphs (c) and (d) are appl icable with the ice accretion s specified by CS 25.21(g) . (Based on position of group l ess ALPA in original FTHWG Appendix C report)

(1) If activation of normal operati on 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 operati on of any ice protection system isdependent 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/s 2 (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 ch aracteristics when recovery is

initiated not less than 1 second after the onset of sta ll warning, with

the ice accretion s 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 sp eed is reduced at rates not exceeding 0.5 m/s 2 (one knot per second), the stall warning in straight and turning flight must be sufficie nt to allow the pilot to prevent stalling, using the same test t echnique 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 accretion s 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/s 2 (3 kt) per

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-24 second airspeed deceleration rates of CS 25.201(c)(2) need not be

demonstrated.

(i) For flight in ici ng 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 excep t sub-paragraphs (c) and (d) are applicable with the ice accreti ons specified by CS 25.21(g).

(2) If detection of Appendix X ic ing 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 sp eed is reduced at rates not exceeding 0.5 m/s 2 (one knot per second), the stall warning margin in straight and turning fli ght 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 in itiated not less than 1 second after the onset of stall warning, with th e 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 sp eed is reduced at rates not exceeding 0.5 m/s 2 (one knot per second), the stall warning in straight and turning flight must be sufficie nt to allow the pilot to prevent stalling, using the same test t echnique 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/s 2 (3 kt) per second airspeed decel eration 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 ve locity, demonstrated to be safe for take-off and landing, must be establishe d for dry runways and must be at least

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-25 37 km/h (20 kt) or 0.2 V SR0 , whichever is greater, except that it need not exceed 46 km/h (25 kt).

(2) The crosswind component fo r 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, V FC

/M FC

. V FC /M FC 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 V MO /M MO and V DF /M DF , except that, for altitudes where Mach number is the limiting factor, M FC need not exceed the Mach number at which effective speed warning occurs.

(c) The maximum speed for stabili ty 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) V FC

, or

(3) A speed at which it is demons trated that the airframe will be

free of ice accretion due to th e effects of increased dynamic

pressure. "

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-26 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 fo rth 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 conditi ons is desired, the aer oplane 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 condit ions 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/m 3 , the mean effecti ve 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.

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-27 Part II - Airframe Ice Accretions for Showing Complianc e with Subpart B

(a) Ice accretions - General . CS 25.21(g) stat es 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 cons ideration 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 gr oup 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 cons ideration the atmospheric conditions of part I of this Appendi x, and all flight conditi ons within the operational limits of the aeroplane (e.g. configurat ion, configuration changes, speed, angle-of-attack, and alti tude). The following i ce 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 operati on, occurring between lift-off and 122m (400 ft) above the take-off surface, assuming accretion starts at liftoff in the take-off maximum icing condi tions 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 operati on, between 122m (400 ft) and 457m (1500 ft) above the take-off surface, assu ming 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 operati on, 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 operati on, during the holding flight phase.

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-28 (5) Landing Ice is normally Holding Ice , unless modified by ice protection system operati on 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 comp liance with the requireme nts 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 th e most adverse effect on handling characteristics may be used for performance tests provided any difference in performance is c onservatively 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 co nditions 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 V EF , 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.

### IPHWG Task 2 WG Report - Appendix M

FTHWG RECOMMENDATIONS FOR 14 CFR 25 SUB PART B

12/19/2005 Page M-29 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).

### IPHWG Task 2 WG Report - Appendix N

FTHWG RECOMMENDATIONS FOR ADVISORY MATERIAL

12/19/2005 Page N-1



### APPENDIX N - FTHWG RECOMM ENDATIONS FOR ADVISORY

MATERIAL

### IPHWG Task 2 WG Report - Appendix N

FTHWG RECOMMENDATIONS FOR ADVISORY MATERIAL

12/19/2005 Page N-2 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 in tended to address Appendix X icing conditions and to consolidate performanc e and handling quality informati on that was previously

contained in AC 25.1419-1A.

### IPHWG Task 2 WG Report - Appendix N

FTHWG RECOMMENDATIONS FOR ADVISORY MATERIAL

12/19/2005 Page N-3 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 R equirements 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 F light-related Systems 4.8 Aeroplane Fli ght Manual (JAR 25.1581)

5 Acceptable M eans 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-i noperative (JAR 25.121) 6.7 En-route Flight Path (JAR 25.123) 6.8 Landing (JAR 25.125)

### IPHWG Task 2 WG Report - Appendix N

FTHWG RECOMMENDATIONS FOR ADVISORY MATERIAL

12/19/2005 Page N-4 6.9 Controllability and Manoeuvr ability - 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 Later al 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 C onfiguration and Ancestry A3.2 Wing A3.3 Empennage A3.4 Aerodynamic Balancing of Flight Control Surfaces A3.5 Ice Protection/Detection System

### IPHWG Task 2 WG Report - Appendix N

FTHWG RECOMMENDATIONS FOR ADVISORY MATERIAL

12/19/2005 Page N-5 1 Purpose

.

1.1 This ACJ describes an acceptable means fo r showing compliance with the requirements related to performance and handl ing characteristics of Large Aeroplanes as affected by flight in the icing conditions that are defined in Appendix es 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 judgem ent 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, Appendix es C and X

.

1.4 Section 5 describes acceptable met hods and procedures that an applicant may use to show that an aeroplane meets these require ments. 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 t hose types or models, some judgement will often be necessary for det ermining that any particular method or procedure is adequate for showing comp liance with a particular requirement.

1.5 Section 6 provides an acceptable flight test programme wh ere flight testing is selected by the applicant and agreed by t he Authority as being the primary means of compliance.

1.6 The three appendices provide additiona l 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)

### IPHWG Task 2 WG Report - Appendix N

FTHWG RECOMMENDATIONS FOR ADVISORY MATERIAL

12/19/2005 Page N-6 • 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 larg e drop icing conditions)

• JAR 25.1581 (Aeropl ane 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 Appendix es C and X to JAR-25.

4.1.1 Operating rules for commerc ial 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 depos it (ice or other contamin ant) that might adversely affect the performance and/or controllabi lity 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 approp riate ice removal and ice protection procedures dur ing flight preparat ion on the ground.

4.1.2 Appendix es C and X to JAR-25 define the ice accretions to be used in showing compliance with JAR 25.21(g). App endix 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.

### IPHWG Task 2 WG Report - Appendix N

FTHWG RECOMMENDATIONS FOR ADVISORY MATERIAL

12/19/2005 Page N-7 4.1.3 Certification experience has shown that it is not necessary to consider ice accumulation on the propeller, inducti on system or engine components of an inoperative engine for handling qualities substant iation. 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 re maining 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 lo wer 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-o f-attack can result from aeropl ane configuration (e.g., increased flap extension increasing the downw ash 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 char acterized 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 mome nt, forcing the elevator trailing edge down. This elevator hinge mo ment reversal can be of sufficient magnitude to

draw the control column forward with a l evel 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 wi th 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 tail plane stall margin with tailplane icing contamination. An acceptabl e flight test procedure fo r determining susceptibility to ICTS is presented in paragraph 6.9.3 of this ACJ.

4.1.6 Supercooled large drop icing conditi ons or runback ice can result in the formation of a ridge of i ce aft of the protected area on the wing upper surface, leading to separated airflow over the a ileron. Ice induced airflow separation upstream of the aileron can have a significan t 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 aile ron 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 f light 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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12/19/2005 Page N-8 ACJ and consist of bank-to-bank roll ma noeuvres, steady heading sideslips, and

stall tests.

4.1.7 JAR 25.21(g) requires that for an aer oplane to be certified for flight in icing conditions, the aeroplane must be ab le to meet certain performance and handling quality requirements of JAR- 25 subpart B while operating in the atmospheric icing environment defi ned in Appendix C to JAR-25.

4.1.8 JAR 25.21(g) also requires compliance with certain subpart B performance and handling qualities requi rements with ice accretions on the aeroplane that are defined in Appendi x X to JAR-25. The only subpart B performance requirements that must be met for an encounter with supercooled large drop atmospheric icing conditions b eyond those for which the airplane is certified for are: (1) stall speed (J AR 25.103), landing c limb (JAR 25.119), and landing (JAR 25.125). Relative to the othe r 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 ai rplane is certified.

4.1.9 For Appendix X atmospheric ici ng 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 Appe ndix X atmospheric icing envelope. Applicants may also choose to certify fo r 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 aeropl ane is not certified for flight, the aeroplane must be capable of encounter ing 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 wh en the aeroplane has encountered icing conditions beyond those for which it has been certifi ed. Procedures should be provided in the Aeroplane Flight Manual to enable a saf e exit from all icing conditions after an encounter with icing conditions beyond tho se for which the aeroplane is certified.

4.1.11 To certify the aeroplane for opera tions in Appendix X icing conditions only for certain flight phase(s) , the flight phase(s) for wh ich operation is approved should be defined such that a flight crew 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 th rust or power settings that may be

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12/19/2005 Page N-9 encountered during that phase of flight ( not just at the conditions specified in Subpart B requirements), and should incl ude an adequate time in the icing conditions to address operational variabilit ies. For the i ce 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 gr ound roll, take-off and final take-off segments), en-route, holding, and approa ch/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. Appendi xes C and X to JAR-25 define the ice accretions, as a function of flight phase, th at must be considered for certification approval for flight in those respective ic ing conditions. In order to reduce the

number of ice accretions used for de monstrating compliance, any of the applicable ice accretions (or a composit e accretion representing a combination of accretions) may be used to show co mpliance with a particular Subpart B requirement if that accretion is eith er the ice accretion identified in the requirement or is shown to be more con servative than the ice accretion identified in the requirement. In addition, the ice accretion that has th e most adverse effect on handling characteristics may be used for compliance with the airplane performance requirements if any differen ce in performance is conservatively taken into account. The ice accretion( s) used to show compliance should consider the speeds, configurations (inc luding configuration changes), angles of attack, power or thrust setti ngs, etc. for the flight ph ases and icing conditions that they are intended to cover. For example, if certification fo r flight in the supercooled large drop icing conditions of Appendix X is desired in addition to the icing conditions of Appendix C, co mpliance with the a pplicable subpart B requirements may be shown using the mo st critical of the Appendix C and Appendix X ice accretions.

4.2 Proof of Compliance (JAR 25.21(g)).

4.2.1 Demonstration of complia nce with certification requirements for flight in icing conditions may be accomplished by any of t he means discussed in paragraph 5.1 of this

ACJ.

4.2.2 Certification experience has shown t hat 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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12/19/2005 Page N-10 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 st all identification system activa tion 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 ava ilable for each flight condition must be appropriate to the aer oplane operating limitations and no rmal procedures for flight in icing conditions. In general, it is acc eptable to determine the propulsive power or thrust available by suitable analysis, substantiated when r equired by appropriate flight tests (e.g. when determining the power or thrust available after 8 seconds for JAR 25.119). The following as pects should be considered:

a. Operation of induction system ice protection.

b. Operation of prope ller ice protection.

c. Operation of engi ne ice protection.

d. Operation of airfra me ice protection system.

4.4.2 The following should be consid ered 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, in cluding engine induction system and/or engine components, and propeller spinner and blades.

b. The incremental airframe drag due to ic e accretion with normal operation of the ice protection system.

c. Changes in operating speeds due to flight in icing conditions.

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12/19/2005 Page N-11 4.4.3 Certification experience has s hown 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 us e of appropriate speed adjus tments to account for operation in icing conditions, any changes in the procedures established for take-off, balked landing, and missed approaches s hould be agreed with the Authority.

4.4.5 Performance associated with flight in icing condition s is applicable after exiting icing conditions until the aeropl ane critical surfaces are fr ee 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 fo r the aeroplane with

ice accretions.

4.6 Failure Conditions (JAR 25.1309)

.

4.6.1 The failure modes of the ice protec tion system and the resulting effects on aeroplane handling and performance shoul d be analysed in accordance with JAR 25.1309. In determining the probability of a failure condition, it should be assumed that the probabi lity 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 wit hin a quantitative analysis. This probability should not be reduced based on phase of flight. The "Failure Ice" configuration accretion is defined in Appe ndix 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 condi tion 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 condi tion 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 procedur e that requires the aeroplane to leave the icing conditions as soon as practicable, it s hould be shown that the aeroplane is capable of continued safe flight and landing with t he “Failure Ice" configuration. The operating procedures and related speeds should provide an adequate operating

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12/19/2005 Page N-12 envelope and acceptable performance and handling characteristics to ensure continued safe flight and landing.

4.6.5 For failure conditions that are im probable but not extremely improbable, the analysis and substantiation of continued sa fe flight and landing, in accordance with JAR 25.1309, should take into consideratio n 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 requirement s in other subparts of JAR-25, and associated guidance material. However, certification experi ence has shown that other flight related systems aspects should be considered when determining compliance with the flight requirements of subpart B. For ex ample, the following aspects may be relevant:

a. The ice protection systems may not ant i-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 affect s 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 af fect the propulsive thrust available, aerodynamic drag, pow erplant control, or flight control.

e. Operation of stall warni ng and stall identification rese t features for flight in icing conditions, including the e ffects of failure to operate.

f. Operation of icing condition sensor s, ice accretion s ensors, 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 conditi ons, including stall and maneuverability demonstrations with the airplane under flight guidance system control

.

h. Installed thrust. This includes op eration of ice prot ection systems when establishing acceptable power or thrust se tting procedures, cont rol, stability, lapse rates, rotor speed margins, temperature margins, Automatic Reserve Power (ARP) operation, and power or thrust lever angle functions.

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12/19/2005 Page N-13 4.8 Aeroplane Flight Manual (JAR 25.1581)

.

4.8.1 Limitations.

4.8.1.1 Where limitati ons are required to ensure safe operation in icing conditions,

these limitations s hould be stated in the Aeroplane Flight Manual ( AFM ).

4.8.1.2 Performance limitations shoul d be presented for flight in icing conditions that reflect any effects on li ft, drag, thrust, and operating speeds due to operating in icing conditions. Th ese limitations may be presented in the

Performance Information Section of th e AFM and included as limitations by specific reference in the Limi tations Section of the AFM.

4.8.1.3 Any airspeed limitations associat ed with flight in icing conditions should be presented, such as the minimum airspeed that should be maintained for each normal airplane confi guration in icing conditions.

4.8.1. 4 The Limitations section of the AFM should include, as applicable, a statement similar to the following: “I n icing conditions the aeroplane must be operated, and its ice protection systems us ed, as described in the operating procedures section of this manual. W here specific operational speeds and performance information have been established fo r 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 simila r to the following, clearly stating the

extent of the certification for flight in icing conditions: “This aircraft has been certified for ope rations under freezing drizzle or freezing rain icing conditions (as appropriate). If these conditions are anticipated at takeoff [for aeroplanes approved to take o ff 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 detect ing when the certified icing conditions

have been exceeded and that intentional fli ght, including take off and landing, into these conditions is prohibited. A requireme nt to exit all icing conditions must be included if the uncertified portion of Appe ndix X is encountered. The statements in the Limitations section should generall y be concise with additional information provided in the AFM and Flightcrew Operating Manual (FCOM) procedure

sections. See ACJ 25.1419/142 0 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 FC OM wording are provided in Appendix 4 of this AC for some of the possible certi fication options. (Note: This example

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12/19/2005 Page N-14 AFM wording only addresses the extent of certification for Appendix X icing conditions. Other limitations and pr ocedural 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 proced ures for flight in icing conditions should include normal operation of the aeroplane includi ng operation of the ic e protection system and operation of the aeroplane following ice protection system failures. Any changes in procedures for other aeroplane system failu res that affect t he capability of the aeroplane to operate in icing cond itions 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 syst em 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 certi fied. Examples of acceptabl e procedures statements are provided in Appendix 4 of this AC for some of the possible certification options.

4.8.2.3 Abnormal operati ng procedures should include the procedures to be

followed in the event of annunciated ice protection system failures and suspected unannunciated failures. Any changes to ot her abnormal procedures contained in the AFM, due to flight in icing cond itions, 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 accept able methods and procedures that an applicant may use to show that an aeropl ane 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) shoul d be shown by one or more of the

methods listed in this section.

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12/19/2005 Page N-15 5.1.3 The compliance process should address all phases of flight, including takeoff, climb, cruise, holding, descent, l anding, and go-around as appropriate to the aeroplane type, considering it s 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 co mpliance 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 a ccretion on the airfra me with an operative ice protection system and wit h ice protection system fa ilures 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 AC J. That appendix also provi des the different types of artificial ice shapes to be considered.

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12/19/2005 Page N-16 5.2 Flight Testing.

5.2.1 General

.

5.2.1.1 The extent of the flight test programme s hould consider the results obtained

with the non-contaminated aer oplane 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 perf ormance and flight characteristics test programme on an aer oplane with ice accret ion. A suitable programme that is sufficient to demonstr ate compliance with the requirements can be established from experienc e 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 charac teristics, and handling characteristics of the non-contaminated aeroplane. In particular, it is not nec essary 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 charac teristics of the aeroplane at high altitude (i.e. above the upper limit specified in Appendix es C and X to JAR-25). An acceptable flight test programme is provided in section 6 of this ACJ.

5.2.1.3 Certification experie nce has shown that tests are usually necessary to evaluate the consequences of ice pr otection 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 ma y be based on flight testing in dry air using artificial ice shapes that have been agreed with t he Authority.

5.2.2.2 Additional limited flight test s should be conducted in natural icing conditions, which are discuss ed 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 c onditions is the primary means of compliance, the conditions should be measured and recorded. The tests should ensure good coverage of Appendix es 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 accr eting ice (including the icing atmosphere, configuration, speed and durat ion of exposure) should be agreed with the Authority.

5.2.3.2 Where flight testi ng with artificial ice shapes is the primary means of compliance, additional limited flight test s should be conducted in natural icing conditions. The objective of these tests is to corroborate the handling characteristics

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12/19/2005 Page N-17 and performance results obtained in flight test ing with artificial ice shapes. As such, it is not necessary to measure the atmo spheric characteristics (i.e. liquid water content (LWC) and median volumetric diamet er (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 addi tional flight test s in natural icing conditions if such tests have been already performed with the ancestor. Depending on the extent of certificati on being sought for flight in the icing conditions of Appendix X and the m eans for showing compliance with the applicable aeroplane performance and handling characteristics requirements, it may also may not be necessary to c onduct 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 t esting in the natural atmospheric icing conditions of Appendix X is necessary.

5.3 Wind Tunnel Testing and Analysis. Analysis of the resu lts of dry air wind tunnel testing of models with artifi cial 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 Te sting 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 ma y be based on results of dry air wind tunnel tests, flight tests, computational analysis, and engi neering 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 handl ing characteristics. The effects of the ice shapes used in this analysis may be determi ned 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 incl udes 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 performanc e and handling characteristics. This analysis should consider the similarity of the configur ation, operating envelope, performance and handling characteristics, and ice protecti on system of the ancestor aeroplane.

5.6.2 The analysis may include flight test dat a, dry air wind tunnel test data, icing tunnel test data, engineering simulator analysis, service history, and engineering judgement.

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12/19/2005 Page N-18 6 Acceptable Means of Complianc e - Flight Test Programme.

6.1 General.

6.1.1 This section provides an acceptabl e 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 co mpliance 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 ph ase are defined in Appendixes C and X to JAR-25. Additional guidance for dete rmining 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" fo r the majority of t he 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 particul ar application. In addition, the ice

accretion that has the most adverse eff ect on handling characteristics may be used for airplane performance tests prov ided 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 st all 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 signific ant configuration changes occur with extension of trailing edge flaps (such as wing leadi ng 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 positi on appropriate to t he configuration.

b. Normal stall test altitude.

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12/19/2005 Page N-19 c. In the configurations listed below, tr im the aeroplane at an initial speed of

1.13 to 1.30 V SR . Decrease speed until an acceptable stall identification is obtained.

i. High lift devices retracted conf iguration, "Final Take-off Ice."

ii. High lift devices retracted c onfiguration, "En-route Ice."

iii. Holding configuration, "Holding Ice."

iv. Lowest lift take-off conf iguration, "Holding Ice."

v. Highest lift take-off conf iguration, "Take-off Ice."

vi. Highest lift landing conf iguration, "Holding Ice."

6.3 Accelerate-stop Distance (JAR 25.109). The effect of any increase in V 1 due to take-off in icing conditions may be determined by a suitable analysis.

6.4 Take-off Path (JAR 25.111). If V SR in the configuration defined by JAR 25.121(b) with the “Takeoff Ice" a ccretion defined in Appendix C to JAR

- 25 exceeds V SR for the same configuration without ice accreti ons 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 ta ke-off speed increase, thrust loss, and drag increase on the take-o ff path may be determined by a suitable analysis.

6.5 Landing Climb: All-engines-operating (J AR 25.119). Acceptable Test Programme . The following represents an acceptable test programme:

a. "Holding Ice."

b. Forward centre of gravity positi on appropriate to t he configuration.

c. Highest lift landing configuration, landing climb speed no greater than V REF

.

d. Stabilise at the specified speed a nd 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 a cceptable test programme:

a. Forward centre of gravity positi on appropriate to t he configuration.

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12/19/2005 Page N-20 b. In the configurations listed below, stabilise the aeroplane at the specified speed with one engine inoperative (or simula ted 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 in crement as agreed wit h the Authority.

i. High lift devices retracted configuration, final take-off climb speed, "Final Takeoff Ice."

ii. Lowest lift take-off configurat ion, landing gear retracted, V 2 climb speed, "Takeoff Ice."

iii. Approach configuration appropriate to the highest lift landing configuration, landing gear retracted, approac h 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 positi on appropriate to t he configuration.

c. En-route configur ation and climb speed.

d. Stabilise at the specified speed wi th 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 asymme tric 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 evaluati on is usually necessary to evaluate the aeroplane's controllabilit y 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 nec essary 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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12/19/2005 Page N-21 6.9.2 General Controllabilit y and Manoeuvrability . The following represents an acceptable test programme fo r general controllability and manoeuvrability, subject to the provisions outlined above:

a. "Holding Ice."

b. Medium to light weight, aft centre of gravity positi on, symmetric fuel loading.

c. In the configurations listed in Tabl e 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 th is may be limited to 1.3g at V REF ), and pushover to 0.5g (except that t he pushover is not required at V MO and V FE ); and

ii. Deploy and retract dec eleration devices.

TABLE 2: Trim Speeds

Configuration Trim Speed High lift devices retracted configuration: • 1.3 V SR , and

• V MO or 250 KIAS, whichever

is less Lowest lift takeoff configuration: • 1.3 V SR , and

• V FE or 250 KIAS, whichever

is less Highest lift landing configuration: • V REF , and

• V FE or 250 KIAS, whichever

is less.

d. Lowest lift take-off configurat ion: At the greater of 1.13 V SR or V 2 MIN , with one engine inoperative (simul ated), conduct 30° banked turns left and right with normal turn reversals and, in wings-lev el flight, a 5 knot speed decrease and

increase.

e. Holding configuration, maximu m landing weight , minimum holding speed (highest expected holding angle- of-attack) and highest lift landing configuration, maximum landi ng weight, forward c.g, V REF (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 th e other direction. When the airplane reaches approximately 20° bank in th e opposite direction, apply the same lateral control input in the opposite direct ion. Release input and recover as the

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12/19/2005 Page N-22 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 read ily 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-ar ound with one engine inoperative (simulated) using the recommended procedure.

h. Conduct an approach and landing using the recommended procedure. In addition satisfactory controllability s hould be demonstrated during a landing at V REF 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 co mpliance 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 wit hout exceeding 50 pounds pull control force.

Industry Position
6.9.3.1 For pushover manoeuvres, it sh ould be shown that the aeroplane is controllable down to zero g or the lowest l oad factor obtainable if limited by elevator power. It should be shown that a push forc e is required down to 0.5 g load factor, and that it is possible to promptly recove r from the manoeuvre wit hout exceeding 50 pounds pull control force.

TC Position
6.9.3.1 For pushover manoeuvres, it sh ould 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 recove r from the manoeuvre wit hout exceeding 50 pounds pull control force.

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12/19/2005 Page N-23 6.9.3.2 For sideslips, changes in longitudinal control force to maintain speed with increasing sideslip should be progressi ve, with no reversals or unacceptable discontinuities (see paragr aph 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 followin g 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 li sted below, with the aeroplane in trim, or as nearly as possible in trim, at the specified trim speed, perf orm a continuous manoeuvre (without changing trim) to reach zero g norma l load factor or, if limited by elevator control authority, the lowe st 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 V SR , target speed not more than 1.23 V SR , or

- Trim speed V FE , target speed not less than V FE - 20 knots.

ii. Highest lift landing configuration at go-around power or thrust, and the more

critical of:

- Trim speed 1.23 V SR , target speed not more than 1.23 V SR , or

- Trim speed V FE , target speed not less than V FE - 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 V SR , and power or thrust for -3° flight path angle.

6.9.4 Controllability prior to Normal O peration of the Ice Protection System and prior to Detection of Appendix X icing conditions. The following repr esents an acceptable test programme fo r compliance with the applicable controllability requirements with the ice accretion prior to normal oper ation of the ice protection system

.

6.9.4.1 Where the ice prot ection system is activated as described in paragraph A1.2.3.3.a of Appendix 1 of th is 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, pa ragraphs 6.9.1, 6.9. 2 and 6.9.3 of this

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12/19/2005 Page N-24 ACJ are also applicable using the ice accr etion prior to detection of Appendix X icing conditions.

6.9.4.2 Where the ice prot ection system is activated as described in paragraphs A1.2.3.3.b,c,d or e of Appen dix 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, tr im 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 configurat ion if different), holding speed, power or thrust for level flight.

ii. Landing configuration, V REF for non-icing conditions, power or thrust for landing approach (limit pu ll up to stall warning).

6.9.4.3 When the primary means of de tecting Appendix X icing conditions is an approved ice detection system, cont rollability 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 dete ction of the Appendix X icing conditions.

6.10 Longitudinal Control (JAR 25.145).

6.10.1 No specific quantitative evaluat ions are required for demonstrating compliance with JAR 25.145(b) and (c). Qua litative evaluations should be combined with the other testing. The results from the non-co ntaminated aeroplane tests should

be reviewed to determine whether there ar e any cases where there was marginal compliance. If so, these ca ses 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 positi on, symmetric fuel loading.

c. In the configurations listed below, trim the aeroplane at 1.3 V SR . Reduce speed using elevator control to sta ll warning plus one second and demonstrate prompt recovery to the trim speed using elevator control.

i. High lift devices retracted configur ation, maximum continuous power or

thrust.

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12/19/2005 Page N-25 ii. Maximum lift landing configuration, maximum continuous power or thrust.

6.11 Directional and Lateral C ontrol (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 whethe r there are any cases where there was marginal compliance. If so, thes e cases should be repeated with ice.

6.12 Trim (JAR 25.161). Qualitative evaluations shoul d be combined with the other testing. The results from the non-contam inated aeroplane tests should be reviewed to determine whether there are any cases where there was marginal co mpliance. 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 sp ecifically investigated.

6.14 Demonstration of Static Longit udinal Stability (JAR 25.175).

6.14.1 Each of the following cases shoul d 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. Alt hough the maximum speed for substantiation of stability characteristics is the lower of 300 knots CAS or V FC (JAR 25.253c), the maximum speed for demonstration can be limit ed to 280 knots CAS, provided that the stick force gradient can be satisfacto rily extrapolated to 300 knots CAS or V FC (e.g. there is no gradient dec rease with increasing speed).

6.14.2 Acceptable Test Programme . The following represents an acceptable test programme for demonstration of static longitudi nal stability:

a. "Holding Ice."

b. High landing weight, aft centre of gr avity position, symmetric fuel loading.

c. In the configurations listed below, tr im 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 re tracted, trim at 1.3 V SR

.

ii. Cruise : With high lift devices retracted, trim at V MO or 250 knots CAS,

whichever is lower.

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12/19/2005 Page N-26 iii. Approach : With the high lift devices in the approach position appropriate to the highest lift landing confi guration, trim at 1.3 V SR

.

iv. Landing : With the highest lift landing c onfiguration, trim at 1.3V SR

.

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 latera l 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 substant ially 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 positi on, symmetric fuel loading.

c. In the configurations listed below, tr im 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 configurati on: Trim at best rate-of-climb speed,

but need not be less than 1.3 V SR

.

ii. Lowest lift take-off configuration: Tr im at the all-engi nes-operating initial climb speed.

iii. Highest lift landing conf iguration: Trim at V REF

.

6.16 Dynamic Stability (JAR 25.181). Provided that there are no marginal compliance aspects with the non-contaminat ed aeroplane, it is not necessary to demonstrate dynamic stability in specific tests. Qualit ative evaluations should be combined with the other testing. Any tendency to sustain osc illations in turbulence or difficulty in achieving precise atti tude control should be investigated.

6.17 Stall Demonstration (JAR 25.201).

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12/19/2005 Page N-27 6.17.1 Sufficient stall testing should be c onducted to demonstrat e that the stall characteristics comply with the requirement s. In general, it is not necessary to conduct a stall programme which encompasse s all weights, c entre of gravity positions (including lateral asymmetry), alti tudes, high lift configurations, deceleration device configurations, straight and turning fli ght stalls, power off and power on stalls.

Based on a review of the stall characteri stics of the non-cont aminated aeroplane, a

reduced test matrix can be established. However, additional testing may be necessary if:

• the stall characteristics with ice accret ion show a significant difference from the non-contaminated aeroplane,

• testing indicates marg inal 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 out lined 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 positi on, 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 determinati on. 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 configurati on: Straight/Power Off, Straight/Power On, Turning/Power Of f, Turning/Power On.

ii. Lowest lift take-off configuration: St raight/Power On, Tu rning/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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12/19/2005 Page N-28 e. For the configurations listed in pa ragraph 6.17.2(d)(i) and (iv) (and any other configuration if deemed more cr itical), 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 shoul d 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 progra mme 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 positi on, symmetric fuel loading.

c. Normal stall test altitude.

d. In the configurations listed below, trim the aeroplane at 1.3 V SR with the power or thrust necessary to ma intain 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 deceleratio n of at least 2 knot/sec. Decrease speed until 1 sec after stall warning and recover us ing 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 airplan es 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 prot ection 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 pr ior 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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12/19/2005 Page N-29 Appendix X icing conditions. For assessing compliance with the ice accretion prior to detection of Appendix X icing c onditions, 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 prot ection system is activated as described in paragraphs A1.2.3.3.b,c,d or e of A ppendix 1 of this ACJ, it is acceptable to demonstrate adequate stall warning with the ice accreti on prior to normal system operation, as

follows:

a. In the configurations listed below, trim the aeroplane at 1.3 V SR

.

i. High lift devices retracted conf iguration: Straight/Power Off.

i. Landing configuration: Straight/Power Off.

b. At decelerations of up to 1 knot pe r second, reduce the speed to stall warning plus 1 second, and demonstrat e that stalling can be prev ented using the same test technique as for the non-contaminated aer oplane, 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 de tecting Appendix X icing conditions is an approved ice detection system, the stall warning ma rgin prior to detection of Appendix X icing conditions shoul d be assessed using the criteria of paragraph 6.18.3.2 with the ice accretion prior to det ection of the Appendix X icing conditions. For assessing complia nce with the ice accretion prior to detection of Appendix X icing conditions, it is assu med 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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12/19/2005 Page N-30 6.19.2 The results of the steady heading sides lip tests with “Landing Ice” may be used to establish the safe cross wind componen t. If the flight test data show that the maximum sideslip angle demonstrated is si milar to that demonstrated with the noncontaminated aeroplane, and t he flight characteristics (e.g. control forces and deflections) are similar, then the non-contaminated aer oplane crosswind component

is considered valid.

6.19.3 If the results of t he comparison discussed in paragraph 6.19.2, above, are not clearly similar, and in the absence of a more rational analys is, a conservative analysis based on the results of the st eady heading sideslip tests may be used to establish the safe crosswind component. The crosswind value may be estimated

from:

V CW = V REF x sin (sideslip angle) / 1.5

where:

V CW is the crosswind component,

V

REF is the landing reference s peed appropriate to a minimum landing weight, and sideslip angle is that demonstrated at V REF (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 st ability tests (see paragraph 6.14 of this ACJ).

6.20.2 It is also necessary to demonstrat e 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

V DF /M DF with the de-icing boots operat ing 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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12/19/2005 Page N-31 6.21.1.1 Whether the flight testing has been performed with artificial ice shapes

accretions or in natural icing conditions, add itional 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 descr ibed in this section is to corroborate the handling characteristics and performance resu lts obtained in flight testing with artificial ice shapes accretions. At least a qua litative assessment should be

made that the artificial ice accretions are conservative relative to the ice accretions obtained in natural atmosphe ric icing conditions, and to confirm that ice does not accrete in unexpected places.

6.21.1.2 ACJ 25.1419/1420 prov ides guidance on when flight testing in the natural atmospheric icing condit ions 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 atte mpt should be made to accomplish the test manoeuvres as soon as possible after exit ing the icing cloud to minimise the atmospheric influences on ice shedding.

6.21.1. 4 During any of the manoeuvres specif ied in paragraph 6.21.2, below, the behaviour of the aeroplane should be consist ent with that obtained with artificial ice shapes. There should be no unusual control responses or uncommanded aeroplane motions. Additionally, during t he 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 operati on 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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12/19/2005 Page N-32 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 V SR Deceleration to stall warning.

Landing flaps, gear

down Optional (aft range) V REF • 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 a ccretions in different configurations on unprotected surfaces. Each test conditi on should be accomplished with the ice accretion that exists at that point. T he 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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12/19/2005 Page N-33 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 V SR • 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) V REF • 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 contro ls, in the absence of an agreed substantiation of the crit icality of the artificial ice shape used to demonstrate compliance with the controllability require ment, the pushover test of paragraph 6.9.3 should be repeated with a thin accretion of natural ice.

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12/19/2005 Page N-34 6.21.4 Existing propeller speed limits or, if r equired, revised propeller speed limits for flight in icing, should be verified by flight tests in the natural icing conditions of Appendix C, and, if a pplicable, Appendix X.

6.22 Failure Conditions (JAR 25.1309).

6.22.1 For failure conditions which are annunc iated 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 carri ed out for the most critical probable failure condition where the associated proc edure 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 t hat are no longer protec ted - the “Failure ice” accretion described in parag raph A1.3.2 of this AC; and

iii. On the normally protected surfaces that are still functi oning 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 follo wing the critical failure condition.

b. Medium to light weight, aft centre of gravity positi on, symmetric fuel loading.

c. In the configurations listed below, tr im 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 configurat ion if different): Holding speed, power or thru st 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 V SR

. Decrease speed to stall warning plus 1 second, and demonstrate prompt recovery

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12/19/2005 Page N-35 using the same test technique as for t he non-contaminated aeropla ne. Natural stall warning is acceptable for the failure case.

i. High lift devices retracted conf iguration: 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 re sults 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 meas ured by degradation in flight characteristics) is commensur ate 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 agr eed with the Authority for the specific failure condition.

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12/19/2005 Page N-36 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 shoul d be determined. The parameters to be

considered include:

• the flight conditions (e.g. aeroplane configur ation, 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 Appe ndix X and subsequently exiting all icing conditions. Therefore, certification for flight in icing conditions must consider at least the following ice accretions: (a) op erations in Appendix C icing conditions, and (b) detecting and exiting Appendix X icing conditions. J AR 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 atmosphe ric 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 atmos pheric 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 conditi ons, (b) operations in the Appendix X icing conditions for which approval is s ought, 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 a pply for each of these ice accretions.

A1.1.3 In accordance with JAR 25.1420(a)(3), in addition to certi fication for flight in Appendix C conditions, an aeroplane may be certified for operation throughout the atmospheric icing conditions of Appendix X to JAR-25. Certif ication 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 condi tions. JAR 25.21(g) specifies which subpart B airplane performance and hand ling 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 i ce accretions (or a composite accretion representing a combination of accretions) may be used to show compliance with

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12/19/2005 Page N-37 a particular Subpart B requirement if th at accretion is either the ice accretion identified in the requirement or is show n 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 characteris tics may be used for compliance with the airplane performance requirements if any difference in performance is conservatively taken into account. The ic e accretion(s) used to show compliance should consider the speeds, configurations (includi ng 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 ac cretion 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 t he unprotected aerofoil leading edges and all unprotected airframe parts on which ice may a ccrete. The effect of ice accretion on protuberances such as antennae or fl ap hinge fairings need not normally be investigated. However aeroplanes that are characterised by unusual unprotected airframe protuberances, e.g. fixed landi ng 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 t he most critical unprotected main aerofoil surface (e.g. wing, horizontal or vertical stabi lisers) 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 unpr otected main surfaces an anal ysis may be performed to

determine the maximum ice accretion associ ated with this maximum pinnacle height. In the absence of such an acceptable analy sis 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 tr uncate the pinnacle height of 3 inches (75 mm) if sufficient service history exists on simila r ice protection system designs. The shape and texture of the ice are important and should be agreed with the Authority.

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12/19/2005 Page N-38 Discussion: This issue has been debated extensively in both the FTHWG and IPHWG without resolution. The FTHWG provid ed language is essentially unchanged from the draft 25.21(g) materials relati ve to Appendix C. The FTHWG could not

achieve consensus in the time availabl e 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 th e primary means of determining ice shapes. Ice shape size varies with geometry, which can l ead 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 consiste nt 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 just ification 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 certifica tion experience to that identified in paragraph A1.2.1.3 for Appendix X ici ng 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 protecti on systems are normally assumed to be operative. However, the applic ant 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 accr etion 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 av erage drag increment determined over the de-icing cycle may be used for performance calculations.

b. Runback ice which occurs on or dow nstream of the pr otected surface.

c. Ice accretion prior to normal operat ion 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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12/19/2005 Page N-39 by calculation, assuming that the Ta keoff Maximum icing conditi ons defined in Appendix C exist, and:

• aerofoils, control surfaces and, if applicable, propellers ar e free from frost, snow, or ice at the start of the take-off;

• the ice accretion st arts at lift-off;

• the critical ratio of thrust/power-to-weight;

• failure of the critical engine occurs at V EF ; and

• flight crew activation of the ice pr otection system in accordance with an AFM procedure, except that after commencement of the take-off ro ll 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 Appendix es C and Xto JAR-25 for the takeoff phase are:

• "Take-off ice": The most critical ic e 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 norma lly operated as anti- icing systems (i.e. designed to prevent ice accretion on the pr otected 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-ic ing systems depending on the phas e 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 operat ing limitations and operating procedures established by the applicant. Si nce ice protection systems are normally only operated when icing conditions are encountered or when airframe ice is detected, means of flight crew determination of ic ing 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 syst em is activated. In accordance with

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12/19/2005 Page N-40 § 25.21(g)(3), compliance must be shown with ice accretions corresponding to

both the Continuous Maximum ici ng conditions of Appendix C and the icing conditions of Appendix X (or to reduce th e number of ice accretions tested, the

most critical of the two i ce accretions if the most crit ical ice accretion can be determined). This additional ice accretion is to acc ount for flight crew delay in either identifying the conditions and activating t he ice protection systems (see paragraphs A1.2.3.3(a), (b) and (c) below), or acti vating the ice protection system following indication from an ice detection system (see par agraph 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. fo r a thermal ice protection syst em, the time to heat the surface and remove the ice).

A1.2.3.3 The means of detecting that th e aeroplane is in icing conditions should be the same for all icing conditions for whic h certification is desired, regardless of whether the icing conditions are wit hin 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 norma l operation of any ice prot ection 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 s hould 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 a ccretion 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 conditi ons 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 norma l operation of any ice prot ection system is dependent on visual recognition of the first indication of ice accretion on a reference surface (e.g. ice accretion probe), the ice a ccretion 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 firs t indication on the reference surface, plus

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12/19/2005 Page N-41 ii. the ice accretion equivalent to thirty seconds of operation in icing conditions ( the Continuous Maximum icing conditi ons 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 pr otection 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 A ppendix C, Part I(a), plus

iii. the ice accretion during the system response time.

b. [RESERVED]

c. If activation of norma l operation of any ice prot ection system is dependent upon pilot identification of icing conditions (a s defined by an appropriate static or total air temperature and visible moisture conditi ons), the ice accretion should not be less than that corresponding to the ice accr eted during probable crew delays in recognition of icing conditions and operat ion of the system, determined as follows:

i. the ice accretion equivalent to thirty seconds of operation in icing conditions ( the Continuous Maximum icing condi tions of Appendix C, Part I(a) and the icing conditions of Appendix X), plus

ii. the ice accretion durin g 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 t he 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 syst em, determined as

follows:

i. the ice accretion corresponding to t he time between entry into the icing

conditions and indication from the ice detection system, plus

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12/19/2005 Page N-42 ii. the ice accretion equivalent to ten seconds of operation in icing conditions ( the Continuous Maximum icing condi tions 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 norma l operation of any ice prot ection system is automatic following indication from an ice detection syst em, the ice accretion should not be less than that corresponding to the ice accreted prior to indica tion from the ice protection system and operation of the sys tem, determined as follows:

i. the ice accretion on the protect ed surfaces corresponding to the time between entry into the icing conditions and activation of the system, plus

ii. the ice accretion durin g 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 cond ition is not annunciat ed, 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 co ndition is annunciated and the associated procedure does not require the aeroplane to ex it 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 annuncia ted 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 specif ied 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 accreti on existing at the time the flightcrew becomes awar e that they are in Appendix X icing conditions, they have initiated the appropria te action(s) identified by aeroplane Flight Manual limitations or procedures, and any resulting systems changes (e.g., reset of stall warning or stick pushe r activation points) are functional.

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12/19/2005 Page N-43 A1.4.1.1 Both direct entry into Appendix X icing conditions and en try from flight in Appendix C icing conditions should be cons idered. 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 identifi cation system activation point s, 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 det ect Appendix X icing conditions that exceed those for which the airplane is certi fied should be based on the means of

detection. Guidance for cer tification 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 fo r a detection means that relies on the flightcrew to see and recognize a visual icing cue than fo r an ice detection system that provides an attention-getting alert to the flightcrew.

A1.4.1.3 Visual detection requires time for accumulati on on the reference surface(s) of enough ice to be reliably identi fied 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 id entification is a function of the distinguishing characteristics of the ice (e.g., size, sh ape, contrast from the surface feature that it is pr esent on), the distance from the pilots (e.g., windshield vs. engine vs. wingtip), and the relative vi ewing angle (location with respect to the pilots’ primary fields of view).

b. Pilot scan time of the reference su rface(s) will be influenced by many factors. Some factors include phase of fl ight, 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 associat ed with some phases of flight in instrument conditions (e.g. approach a nd 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 i ce to be reliably identified by either pilot in all atmospheric and lighting condi tions. 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 mount ed engine, or wing tip – 5 minutes.

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12/19/2005 Page N-44 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 fo r operating safely after encountering Appendix X atmospheric icing conditions fo r 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 at mospheric 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 certifi ed 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 accretion 1

Final Take-off No accretion 1

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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12/19/2005 Page N-45 Flight Phase/Condition Appendix X Detect-and-Exit Ice Accretion

Approach Approach Detect-and-Exit Ice

The more critical of holdi ng detect-and-exit ice and the combination of: (1) Ice accreted during a descent in the cruise configuration fr om 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 ma de, (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 Appe ndix C continuous maximum icing conditions. Landing Landing Detect-and-Exit Ice

The more critical of holdi ng 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 nauti cal 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 fr om 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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12/19/2005 Page N-46 Flight Phase/Condition Appendix X Detect-and-Exit Ice Accretion

Pre-Activation Pr e-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 activat ed, and the ice protection system to become fully ef fective in performing its

intended function.

Pre-Detection Pre-Detect ion 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 ti me for the flight crew to refer to and act on proce dures, 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 Appendi x X icing conditions are recognized.

Failure Failure Ice

No accretion 2

Notes:

1 T/O is not permitted wh en Appendix X conditions beyond those for which the aeroplane is certified to operate exist in the vicinity of departure airport.

2 Unnecessary to consider unintenti onal encounter with Appendix X icing conditions beyond those for which the aeropl ane is certified to operate with a failed ice protection system.

A1.4.3 Ice accretions for encounter s 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 fo r operating safely in the Appendix X atmospheric icing conditions for which the aeroplane is approved.

A1.4.3.2 The ice accretions should incl ude consideration of combinations of Appendix C and Appendix X icing conditi ons within the scenarios defined in paragraph A4.1.3.3. For exampl e, flight in Appendix X c onditions may result in ice accreting, and potentially fo rming 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 app licable flight phase scenari o may result in a more

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12/19/2005 Page N-47 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 flig ht 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 Appe ndix X icing environment to 2,000 feet above the landi ng surface, followed by transition to the appro ach configuration and manoeuvring for 15 minutes at 2,000 feet above the landing surface; and (2) Holdin g ice (if the airplane is certified for holding in Appendix X conditions).

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12/19/2005 Page N-48 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 surf ace with a transition to the landing configuration, followed by a go-around manoeuvre beginning with th e 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 Pr e-Activation Ice Ice accreted during the time for the flightcrew to recognize icing conditions and activate the ice protection system, plus the ti me for the ice protection system to perform its intended function.

Pre-Detection Pr e-Detection Ice Ice accreted during the time fo r the flightcrew to detect Appendix X conditions, refer to and initiate procedures, and any time for systems to perform their intended functions (if applicable). Pr e-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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12/19/2005 Page N-49 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 characte ristics. If analytic al data show that other reasonably expected ice shapes c ould be generated which could produce higher performance decrements, then the ic e shape having the most adverse effect on handling characteristics may be used fo r performance tests provided that any difference in performance can be conservatively taken into account.

A2.1.2 The artificial shapes should be repres entative 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 t he 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 arti ficial 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 ic ing cloud (e.g. from an icing tanker).

A2.2.2 In the absence of another agreed def inition of texture the following may be

used:

A2.2.2.1 For small amounts of ice (for exam ple the amount of ice accreted during a deicing system rest time), t he roughness should be typically:

• roughness height: 1 mm

• particle density: 8 to 10/cm 2

A2.2.2.2 For large amounts of ice (for exam ple on an unprotected, exposed surface), the roughness should be typically:

• roughness height: 3 mm

• particle density: 8 to 10/cm 2

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12/19/2005 Page N-50 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 condi tions 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 cover age 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 horizont al stabiliser if this can be shown to be conservative.

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12/19/2005 Page N-51 Appendix 3 - Design Features

A3.1 Aeroplane Configuration and Ancestry . An important design feature of an overall aeroplane configurati on that can affect perform ance, controllability and manoeuvrability is its size. In addition, the safety record of t he 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 ma y be used to evaluate its general arrangement and systems integration.

A3.2 Wing . Design features of a wing that c an affect performance, controllability, and manoeuvrability include aerofoil type, le ading 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 number s make the transition to turbulent flow near the leading edge when non- contaminated, thus reduci ng 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 C LMAX due to ice by increasing the overall

level of C L . Gapping the slat may improve th e 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 aut omatic slat-gapping device may generate a greater C LMAX with ice than the certified C LMAX with the slat sealed and a non-contaminated leading edge. This may provide effective protection against degradation in stall perfo rmance or characteristics.

A3.2.4 Lateral Control. The effectiveness of the late ral control system in icing conditions can be evaluated by comparison wit h closely related ancestor aeroplanes.

A3.3 Empennage . The effects of size and aerofoil type also apply to the horizontal

and vertical tails. Other desi gn features include tailplane sizing philosophy, aerofoil design, trimmable stabiliser, and control surfac e 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. Ho wever, these effects usually result in

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12/19/2005 Page N-52 changes to aeroplane handling a nd/or control characteristics rather than degraded performance.

A3.3.1 Tail Sizing. The effect on aeroplane handlin g characteristics depends on the tailplane design philosophy. The tailp lane may be designed and sized to provide full functionality in icing c onditions without ice protection, or it may be designed with a de-icing or ant i-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-induc ed aerodynamic hinge moment reversal.

A3.3.4 Control Surface Size. For mechanical elevator controls, the size of the surface significantly affects the contro l 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 F light Control Surfaces. The aerodynamic balance of unpowered or boosted reversible flight contro l surfaces is an important design feature to consider. The design should be carefully evaluated to account fo r the effects of ice accretion on flight control system hinge moment characteri stics. Closely balanced controls may be vulnerable to overbalance in ic ing. The effect of ice in front of the control surface, or on the surface, may ups et the balance of hinge moments leading to either increased positive force gr adients or negative force gradients.

A3.4.1 This feature is particularly import ant with respect to lateral flight control systems when large aileron hinge moments are balanced by equally large hinge moments on the opposite aileron. Any asymmetr ic disturbance in flow which affects this

critical balance can lead to a sudden uncomm anded deflection of the c ontrol. 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 f eatures that reduce the ice accretion on the wing and/or tailplane.

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12/19/2005 Page N-53 A3.5.1 Wing Ice Protection/Detection. An ice detection system that activates a wing de-icing system ma y ensure that there is no signi ficant ice accretion on wings that are susceptible to performanc e 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 charac teristics 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 cont rols can reduce the risk of ice-induced aerodynamic hinge moment reversal.

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12/19/2005 Page N-54 Appendix 4 – Examples of Aeroplane Fli ght Manual limitations and operating procedures for operations in supercooled large drop icing conditions.

A4.1 Approved for Appendix C icin g conditions only (no Appendix X)

A4.1.1 AFM Limitations

Intentional flight, including takeoff and land ing, into freezing drizzle or freezing rain conditions is prohibited. If freezi ng drizzle or freezing rain conditions are encountered, or if {insert cue descripti on here}, immediately request priority handling from air traffic contro l to facilitate a route or al titude change to exit all icing conditions. Stay clear of all icing conditions for th e remainder of the flight, including landing, unless it can be dete rmined that ice accretions no longer

remain on the airframe.

A4.1.2 AFM Operating procedures (normal procedures section)

Freezing drizzle and freezing rain conditi ons are severe icing conditions for this aeroplane. Intentional flight, including ta keoff and landing, into freezing drizzle or freezing rain conditions is prohibited. A flight delay or diver sion to an alternate airport is required if these conditions exist at the depart ure or destination airports.

{insert cue description here} is one indicat ion of severe icing for this aeroplane. If severe icing is encountered, immediately requ est 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 re mainder of the flig ht, including landing,

unless it can be determined that ice accret ions no longer remain on the airframe.

A4.1.3 Flightcrew Operatin g Manual operating procedures

Warning: Severe icing may result from environmental conditions outside of those for which the airplane is cer tified. Intentiona l flight into severe icing conditions may result in ice build- up on protected surfaces exceed ing 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 usi ng the ice protection systems, and may seriously degrade the performance and controllability of the aeroplane.

Operations in icing conditions were evalua ted as part of the certification process for this aeroplane. Freezing drizzle and freezing rain conditions were not

evaluated and are considered severe ic ing conditions for this aeroplane.

Intentional flight, including takeoff and land ing, into freezing drizzle or freezing rain conditions is prohibited. A flight de lay or diversion to an alternate airport is required if these conditions exist at the depa rture or destination airports. {insert cue description here}is an indication of severe icing conditions that exceed those

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12/19/2005 Page N-55 for which the airplane is certified. If severe icing is enc ountered, immediately request priority handling from air traffic control to facilitate a route or altitude change to exit all icing conditions. St ay clear of all icing conditions for the remainder of the flight, including landi ng, unless it can be determined that ice accretions no longer remain on the airframe.

A4.2 Approved for Appendix C and freez ing drizzle conditions of Appendix X (i.e., not approved for freezing rain)

A4.2.1 AFM Limitations

Intentional flight, including takeoff and land ing, into freezing rain conditions is prohibited. If freezing rain conditions are encountered, or if {insert cue description here}, immediately request priori ty 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 ici ng conditions for this aeroplane. Intentional flight, including takeoff and land ing, into freezing rain conditions is prohibited. A flight delay or diversion to an alternate airport is required if these conditions exist at the depart ure or destination airports.

{insert cue description here} is one indicati on of severe icing for this aeroplane. If severe icing is encountered, immediately requ est 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 re mainder of the flig ht, including landing,

unless it can be determined that ice accret ions no longer remain on the airframe.

A4.2.3 Flightcrew Operatin g Manual operating procedures

Warning: Severe icing may result from environmental conditions outside of those for which the aeroplane is certified. Inte ntional flight into severe icing conditions may result in ice build- up on protected surfaces exceed ing 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 usi ng the ice protection systems, and may seriously degrade the performance a nd controllability of the aeroplane.

Operations in icing conditions were evalua ted 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 land ing, into freezing rain conditions is prohibited. A flight delay or diversion to an alternate airport is required if these

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12/19/2005 Page N-56 conditions exist at the departure or destin ation 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 contro l to facilitate a route or al titude change to exit all icing conditions. Stay clear of all icing conditions for th e remainder of the flight, including landing, unless it can be dete rmined that ice accretions no longer

remain on the airframe.

A4.3 Approved for Appendix C, and appr oved for Appendix X except for en route and holding

A4.3.1 AFM Limitations

Intentional holding or en r oute flight into freezing drizzle or freezing rain conditions is prohibited. If freezing drizzle or freezing rain conditions are encountered during a hold (i n any aeroplane configurati on) or in the en route phase of flight (climb, cruise, or descent wit h high lift devices and gear retracted), or if {insert cue description here}, immedi ately 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 re mainder of the flight , including landing,

unless it can be determined that ice accret ions no longer remain on the airframe.

A4.3.2 AFM Operating procedures (normal procedures section)

Freezing drizzle and freezing rain condi tions encountered during a hold (in any aeroplane configuration) or in the en route phase of fl ight (climb, cruise, or descent with high lift devices and gear re tracted) 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 indicati on of severe icing for this aeroplane. If severe icing is encountered, immediately requ est 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 re mainder of the flig ht, including landing,

unless it can be determined that ice accret ions no longer remain on the airframe.

A4.3.3 Flightcrew Operatin g Manual operating procedures

Warning: Severe icing may result from environmental conditions outside of those for which the airplane is cer tified. Intentiona l flight into severe icing conditions may result in ice build- up on protected surfaces exceed ing 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 usi ng the ice protection systems, and may seriously degrade the performance a nd controllability of the airplane.

Operations in icing conditions were evalua ted as part of the certification process

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12/19/2005 Page N-57 for this aeroplane. En route (climb, crui se, and descent with high left devices and gear retracted) and holding flight (in any aeroplane c onfiguration) in freezing drizzle and freezing rain conditions were not evaluated and are considered severe icing conditions for this aeroplane.

Intentional holding or en r oute flight into freezing drizzle or freezing rain conditions is prohibited. {insert cue d escription here}is an indication of severe icing conditions that exceed those for which th e airplane is certified. If severe icing is encountered, immediately request pr iority 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 usab le description here} conditions are encountered, or if {in sert cue description here}, immediately request priority handling from air traffic control to facilitate a route or altitude change to exit all icing conditions. St ay clear of all icing conditions for the remainder of the flight, including landi ng, 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 prohibite d. A flight delay or diversion to an alternate airport is required if these c onditions exist at the departure or destination airports.

{insert cue description here} is one indicati on of severe icing for this aeroplane. If severe icing is encountered, immediately requ est 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 re mainder of the flig ht, including landing,

unless it can be determined that ice accret ions no longer remain on the airframe.

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12/19/2005 Page N-58 A4.4.3 Flightcrew Operatin g Manual operating procedures

Warning: Severe icing may result from environmental conditions outside of those for which the airplane is cer tified. Intentiona l flight into severe icing conditions may result in ice build- up on protected surfaces exceed ing 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 usi ng the ice protection systems, and may seriously degrade the performance a nd controllability of the airplane.

Operations in icing conditions were evalua ted 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 diversi on to an alternate airport is required if these conditions exist at the departure or destination airports. {insert cue description here}is an i ndication of severe icing conditions that exceed those for which the airplane is certi fied. If severe icing is encountered, immediately request priority handling from ai r traffic control to f acilitate a route or altitude change to exit all ic ing 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 (R EVISION TO AC 20-147)IPHWG Task 2 WG Report - Appendix O

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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, fo r demonstrating compliance with the applicable engine induction system icing and engine ice ingestion requirements. These requirements are applic able to the Federal Aviati on Regulations, parts 23, 25, and 33 of Title 14 of the Code of Feder al 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 par t 23 or 25 aircraft. The guidance in

this AC is not intended to address turbos haft engine installati ons, or the rotary wing aircraft they are installed on. Due to the complexity that those aircraft and installations pose for icing, AC 20-73, Ai rcraft Icing Protection, is considered the primary AC for those installations. Fu rther, this AC also provides guidance materials on mixed phase (supercooled li quid 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 Administra tion (FAA) and industry experience in determining compliance with the relevant regulations. This AC does take precedence over the engine and engine inst allation 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 Certificat ion 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.IPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-3 2. APPLICABILITY.

a. The guidance provided in this document is directed to engine manufacturers, modifiers, foreign r egulatory authorities, FAA engine type certification engineers and their designees.

b. This material is neither mandatory nor r egulatory 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 demonstrati ng compliance that an applicant may elect to present. Terms such as “should,” “sha ll,” “may,” and “must” are used only in the sense of ensuring applicability of th is 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 t he FAA becomes aware of circumst ances that convince us that following this AC would not result in compliance with the app licable regulations, we will not be bound by the terms of th is 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 exis ting regulatory requirements.

3. RELATED REGULATIONS.

a. Part 23, Ai rworthiness Standards: Normal, Utility, Acrobatic, and Commuter Category Airplanes, §§ 23.901(d)(2), 23.1093, and 23.1419.

b. Part 25, Ai rworthiness Standards: Transport Category Airplanes, §§

25.1093 and 25.1419.

c. Part 33, Airworthiness St andards: 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 Gui de for Certification of Part 23 Airplanes.IPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-4 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 th rust. Propulsion systems certified under these requirem ents and operated in a ccordance 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 engi ne 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 standar d test conditions have been determined, through more than 30 years of certificat ion experience, to provide an adequate and consistent basis for engine icing certif ication with good service experience.

The successful demonstration of the test co nditions outlined in the regulation is intended to address many potential engine power conditions, aircraft flight conditions, and environmental conditions th at 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 occu rrence rate is far more probable than the remote threat posed by the rain and ha il environmental threat in Appendix B of part 33. Therefore, a dire ct comparison of guidance betw een the acceptable test outcomes of the icing certification requi rements and the rain and hail certification requirements is not appropriate. Agai n, it should also be recognized that although Appendix C, Appen dix X and Appendix D ar e the certification standards, it is still possible to have in-ser vice icing conditions that are more severe than the Appendix C, Appendix X and Appendi x D conditions. The Appendix C and X conditions were designe d to include 99 percent of icing conditions. Evaluation of icing data has indicated that the probability of encountering icing LWC levels outside of Appendi x C droplet conditions is on the

order of 10-2 1 . The applicant may assume this probability for encountering the large droplet conditions conducive to ic e accumulation aft of the airframe’s protected areas. It shoul d be considered as an average probability throughout the flight. Often, experience has shown that the actual ic ing environment in nature can be a combination of conditions, such as a continuous maximum cloud followed by a intermittent maximum cl oud followed by a continuous maximum

1 Note that exceedance of Appendix C LWC & MV D and operating below Appendix C temperature simultaneously is on the order of 10-3.IPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-5 cloud, and so on. While technology efforts need to be completed regarding the analysis and testing requirements for co mpliance 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 follo wing 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 fr om the impact of supercooled water droplets on propulsion system surf aces 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 wa ter content and large droplet sizes. Droplets impacting the surface do not fr eeze immediately, but run back along the surface until freezing occurs. Glaze ic e 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 frac tion and lower adhesive properties than

rime ice. Glaze ice is often a concern fo r 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 si zes. Rime ice typically forms in an aerodynamic shape, on both rotating and stat ic engine hardware. The freezing fraction is high for rime ice, typically appr oaching a value of 1.0. Rime ice typically has greater adhesion properties than glaz e ice but often a lower density. Adhesion properties increase with lower te mperature up to a test point where no additional adhesion is gained with additional lower temperature.

(3) Mi xed or intermediate ice . A combination of glaze and rime ice which forms with rime patches slightly aft of t he glaze ice portions. This ice forms at temperatures, liquid water content, and dr oplet 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 (f or example, high-speed camera which should view fan and booster or low pressure compressor (LPC) inlet guide vaneIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-6 components), and engine instrumentati on (such as, vibration pickups, temperature probes, speed pickups, and so on).

e. Icing condition . A meteorological conditi on defined by the following parameters:

(1) Li quid 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 ME D 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 ma ss is in larger particles and half the ice mass is in smaller particles.

(4) Temperature . The total temperature asso ciated with the icing cloud environment. Appendix C, Appendix X and Appen dix D temperatur es are static ambient temperatures. When a critical test point analysis is conducted, the local total temperatures at the engine inle t should be considered, based on applying the Appendix C, Appendix X and Appendix D static te mperatures and assumed flight Mach number.

f. Powe r loss instabilities . Engine operating anomalies such as nonrecoverable or repeating sur ge, stall, rollback, or fl ameout, can result in engine power or thrust cycling.

g. Scoop factor (concentration factor) . The ratio of nacell e inlet highlight area (A H ) to the area of the c aptured air stream tube (A C ) [scoop factor = A H /A C ]. The highlight area is defined as the area bounde d by the leading edge of the nacelle inlet. Scoop factor potentially concent rates 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 si mulated 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 sur ge, 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 Airp lane Directorate, and the Small Airplane Directorate) expects there will not be a notic eable power or thrust loss. This isIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-7 especially important when considering t hat icing encounter s 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 in strumentation, or flight deck instrumentation (such as, N 1 , N 2 , 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 cons idered stabilized when either ice is not forming on any engine parts, or the accret ing 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 s hed cycle has been demonstrated through the recording of measured engine param eters. The applicant should determine what parameters need to be monitored to determine steady operation of the engine during the icing test. Variations in measured paramet ers 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 (f or example, fan rotor speed, engine pressure ratio). A sustained measurabl e power loss is considered a “severe power loss” in the context of the icing r equirement. Power or thrust losses that are not sustained are temporary in natur e and may be related to the effects of ingesting super-cooled water or ice parti cles, or possibly the effects of ice accumulation or ice shedding. The engine’s momentary response during shedding may be from the t hermodynamic 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 genera lly in effect from the top of descent (TOD) to the point at which the approach pha se of flight begins (typically defined when flaps are deployed) and idle RPM incr eases to reflect the flight control requirements during approach.

m. Span. The length of an engine co mpressor airfoil (rotor or stator) measured from the flowpath ID to the flowpath ODIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-8 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 ic e 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 certif icate data sheet, intended as the maximum power for continuous operation.

q. Takeoff power. Engine rating requir ed by 14 CFR Part 33.7 (c)(1)(iii) and documented in the engine type certificate data sheet.

7. DISCUSSION

. The induction system ici ng requirements of §§ 23.1093, 25.1093, and 33.68 are intende d 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 prol onged encounter, should not be of consequence to the crew, and should not in validate the engine's compliance with other part 33 requirements (for exampl e, §§ 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 o perational or maintenance in terventions. Operational procedures to assist ice shedding, such as throttle manipulation, should not be relied on, or be required to comply wit h parts 23, 25, and 33 in-flight icing requirements. It is acceptable to pr ovide engine throttle manipulation (for example, power run-ups to shed ice) inst ructions to shed accumulated ice during ground operations. These instructions wi ll be used as a recommendation for inservice ground operation, alt hough they would be mandatory if they were utilized during the ground icing compliance demonstr ation 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 te st results under §§ 33.68, 23.1093, and 25.1093. The applicant should determine the parameters, which may include

both visual and instrumented indica tions that need to be monitored. The

demonstration should include stable bui ld 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 re sponses during the ice accumulation process (for example, isochronous cont rol response to accreting ice) are considered acceptable as long as there ar e no power losses. At the conclusion of the test point, during the acceleration to takeoff power, the measured parameters should demonstrat e a smooth and steady acceleration characteristic, unless the applicant can provide an accept able justification for a performance change while on test point (for exampl e, thermodynamic engine response to shed ice ingestion is acceptable). Close c oordination is necessary by all parties to ensure that test plans are in reasonab le bounds for the anticipated use of theIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-9 airplane. The body of this AC is arr anged 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 droplet s and ice particles coexist in a cloud, often around the outskirts of a deep convecti ve cloud formation. Ice crystal icing condition exists when all of the liquid wa ter 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 mi nimal with the possi ble exception of two design features. They are (1) pr onounced inlet bends (such as particleseparator inlets), or inlet flow reve rsals, where inlet flow can stagnate and accumulate ice, and (2) high solidity, hi gh 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 engi ne core stream due to mixed phase/ice crystal conditions. Adverse effects of th is type of icing condition on the engine are uncommanded rollback of power or fl ameout 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 tem perature warmer than the ISA standard atmosphere and outside of the FAR Part 25 Appendix C and Appendix X icing envelopes. FAR Part 33 A ppendix D has been established to include this icing envelope. Malfunctioning of the Total Air Temperat ure (TAT) probe has been noted during many of these engine icing events and is a known indicator of ice crystals. Ice accumulati on 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 certificati on requirements for mixed phase or ice crystal ic ing are discussed in this AC.

b. Auto-recovery systems . The use of auto-recove ry systems is acceptable for certain engine certification testi ng. The FAA supports the use of autorecovery systems, or other protecti ve engine systems or devices, while in service, and allows the use of auto-re covery systems during ice slab ingestion certification testing as def ined 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 eviden t. 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 encount ers (§ 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 norma l ice sheds or accumulations while operating in typical icing conditions. Details will be provided later in this ACIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-10 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 ex tent is the distance vertic ally (vertical extent) or horizontally (horizontal ext ent) that a cloud extends. Vertical extent is normally

measured in feet while horizont al 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 re lationships 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 anal yses. These factors are applicable to airframe flight profiles where the straight-line flight portion of the evaluations may use the cloud extent factor include d in Appendix C in Figures 3 and 6, Appendix X in Figure 7 and Appendix D in Fi gure D-3, as applicable. However, engines and induction systems being eval uated under § 33.68 have historically not been limited to or evaluated against a s pecific aircraft flight profile when considering icing environments. Instead, under § 33.68, they are evaluated for unlimited operation in icing. It is emphasized t hat the criteria in Appendix C represent those icing conditions that could result from encounters with supercooled clouds. Other conditions t hat may contribute to aircraft engine icing conditions include freezing precipitation (t hat is, rain and drizzle) covered by Appendix X. Ice crystals, and mix ed conditions (meaning, mixture of supercooled water droplets and ice crystal s) are defined in Appendix D. To

account for the differences described abov e, multiple clouds should be assumed, with an extent factor equal to 1.0, as act ual cloud extent is no t a consideration for engine operations, particularly in an ai rcraft hold pattern. This approach will assure unlimited engine and induction syst em operation within the atmospheric conditions defined by Appendix C, Appendix X and Appendix D, and as experience indicates, in actual ici ng environments that include Appendix C, Appendix X and Appendix D in its entirety. The cloud ho rizontal extent factor

was not intended to be used to limit the seve rity of exposure to icing conditions,

where it is reasonable to assume the ai rcraft will be required to operate in that condition (for example, the holding pa ttern which may require repeated passes through a severe icing environment, or c ontinuously remain in that severe environment). As a general rule, engi nes and induction systems should be shown to operate continuously in icing withou t regard to time in icing conditions. The cloud extent factor defined in Append ix D relies primarily on limited data

derived from Reference 1. The data covered several geographical regions including eastern Asia where ice crystal-re lated service events are prevalent.IPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-11 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 env elope 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 t he engine manufacturer, and prolonged operation in icing (for exampl e, in-flight hold pattern), or repeat icing encounters. The CPA should be validated by empirica l test data. This analysis should

consider both critical ice accumulation c onditions (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 hav e included within their CPA, a best practice of including conditions that ma y be outside of Appendix C, Appendix X and Appendix D, that have been identifie d through service difficulty of other engine models. Often the CPA is suppl emented with developmen t test data (for example, wet and dry testing with the rmocouple components). The methodology

used to calculate ice accretions should a ccount for freezing fraction and pertinent aerodynamic effects. For example, wate r ingestion into fan inlet and core inlet, water impingement rates for critical surf aces, forward aircraft air speed effects, engine configuration effects such as in ter-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 shoul d be determined from energy balance calculations of required heat load s 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, prov ides additional gu idance 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 certif ication test points (§ 33.68C) . The FAA maintains this view of the CPA complementing the standard table test points provides an adequate me thodology to cover all icing conditions. The FAA recognizes an improvement in the fidelity of analysis tools that are available today. However, based on expe rience 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

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12/19/2005 Page O-12 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 point s for future certific ation projects. The content of an icing certific ation 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 c an be expensive and often impractical to overcome. To directly address this r epeated shortfall, the FAA has identified

several national and international ici ng resources for accomplishing § 33.68 compliance testing. These resources incl ude 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 su rfaces (for example, blades, vanes, sensors) may eventually shed. The shed ice can subsequently cause engine damage if it impacts an engi ne 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 senso r(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 s peed and the adhesive strength of the ice to the surface. The adhesive strength of ice generally increas es 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 temperatur e, exposure time, and rotor speed are important considerations in addition to more typica l parameters, such as icing condition and scoop factor In particular, extended operatio n 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 sc enario 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 se t in the high-pressure compressor (intermediate pressure comp ressor for three spool engi nes). Establishing the critical conditions for these glaze ice a ccretions requires careful consideration as they occur at specific limit ed conditions of low freezing fractions over a range ofIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-13 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 ic e testing should be evaluated against the possibility of multiple occurrences, since icing is a common environmental

condition.

(4) Engine op erability 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 fr om 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 acc eptable, should be documented in the engine’s installation manual. The applican t should consider as part of the CPA both engine accelerations and decelerations relative to operability challenges. Critical test point testing should dem onstrate 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 accreti ons 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 measurem ents are used by the engine control systems control law to est ablish power or thrust ratings, or to schedule other systems requir ed to operate the engine (for example, variable stator vanes). Critical sensors s hould be designed to operate without accreting

ice sufficient to cause an erroneous m easurement 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 ic e crystal icing environment must be representative of the meteor ological conditions defin ed in FAR Part 33 Appendix D. Typically, the supercooled droplet te st points include those described in the rule and any additional test points identifie d as part of the CPA. The applicantIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-14 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 constructi ng an icing test matrix:

a. Section 33.68 Acceptable Means of Compliance . The engine should be capable of operating a cceptably under the meteorologi cal conditions of Appendix C and Appendix X over t he 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 compli ance, if used in conjunction with the

critical conditions determined in the CPA. § 33.68(C) does allow for the elimination of specif ic standard certification test poi nts if the applicant chooses to

run a similar CPA test conditi on that can be shown to be more severe in terms of ice accretion mass at critical engine locati ons or to produce an equivalent level of safety. Similar is defined as being at e ssentially the same level of engine low rotor RPM, the same type of ice accreti on (glaze or rime), and for the same or longer period of icing exposure.

Specifically for the icing conditions defi ned 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 equivalent ly within the inlet duct of a direct connect test facility. Cons ideration of local enrichm ent 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 glaz e icing conditions for at least 10 minutes each, typically during separ ate 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 buildin g up at the end of 10 minutes at the three higher power settings, c ontinue running until the engine demonstrates stabilized operatio n (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 comp liance with §33.68. This test matrix includes power settings from idle to takeoff during exposure to conditions typical of high altitude wher e 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 le vel flight, or longer if the natural engine ice shed cycle is not established. Special consideration and testsIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-15 should be conducted to adequately subs tantiate 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 limi ted 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 sa fely in an in-flight holding phase without a time limit. The test program for turbofan and turboprop applications should include test points (for example, ic ing 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 tabl e 33.68-1 represents a mixed rime and glaze icing condition that was orig inally derived from the JAR-E ice requirements of LWC, tem perature 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 expos ure 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 conti nue 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 s peed and above, for engines with icing protection systems (including systems fo r probes), stabilize the engine for at least 2 minutes in the icing at mosphere with these protection systems off, prior to turning on the icing pr otection system to simulate the delay expected for the pilot to recognize t he icing condition. Fully automatic systems may use an appropriate delay. Systems that are automatic and controlled by the full authority digita l electronic/engine control (FADEC) do not require the 2-minute delay in ic e protection system demonstration. Where the engine’s anti-icing system re lies on an ice detector to indicate the presence of icing conditions, del ay in anti-ice selection is likelyIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-16 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 def initions section and is intended to

address both stabilized ice accret ion 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. E ngine 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 in let screen: The inlet screen compliance demonstration for icing in cluding any associated shed cycle may be addressed separately from the part 33.68 engine ice compliance testing, but then should be noted in t he engine installation manual. If not addressed under part 33.68, complia nce could then be demonstrated under the aircraft inlet requireme nts of 23.1093, 25.1093, 27.1093, or 29.1093. Special attent ion should be given to these inlet features, including specialized tests or analysis.

• At the conclusion of each steady st ate icing test point, the engine should be accelerated to takeoff power (throttl e movement of 1 second or less) or shedding should be induced by other meth ods, depending on the critical shed methodology. The throttle moti on should be the most critical when considering the ice-shed effects on engi ne operability. In some cases, a quick deceleration before accelerati ng to takeoff pow er 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 fr eezing fog, a supercooled large droplet (SLD) test point that repr esents 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 te mperature range and LWC levels, but the larger droplets can notably penetrate fu rther back on the surface of the engine spinner with the potential for ice shedding in to fan blades. Allowance for analysis as means of compliance offers the opt ion of comparing a new engine design for spinner icing/de-icing characteristics and fan blade robustne ss to prior engines with good service experience. The guidance contained in AC 33.2B, with re spect to snow ingestion testing, is

considered outdated. The ground-fog icing, SLD and falling or blowing snowIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-17 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 operat ion cannot be demonstrated then periodic engine speed run-ups should be demonstr ated. These run-ups would then become mandatory in the AFM in icing cond itions 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. Ba sed on review of service events, airports have continued to operate with falling-snow concent rations 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 resu lt. The maximum precipitation rate for moderate snow is 2.5 mm/hr water equiva lent. 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 dat a 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 extrem e 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/h r (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). Wi thin 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 c onditions 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 pow er range, within the limitations established for the airplane for such operation.

c. Ground Idle Demonstrations: Operate steadily at gr ound idle setting for at least 30 minutes under icing conditions of §33.68(D) followed by acceleration to takeoff setting. Since a broad temper ature range is provided, the applicant should identify the most critical tem perature, as determi ned by the CPA, and target that range. Turboprop engine inlet screen icing compliance demonstrations may be addressed separat ely 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 sp eed from static surf aces is not likely at 30 minutes and beyond. For these engines , this test generally establishes the maximum allowable time in the engine operating manual bet ween engine run-ups to shed ice. The AFM should specify the maximum demonstr ated time between run-ups. Demonstration that ice sheds fr om compressor static surfaces during the run-up establishes that the engine can operate c ontinuously (including runups, if necessary) at the ground icing condit ion, as might well occur at an airport where aircraft can be held on the ground se veral hours in bad weather awaitingIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-18 takeoff. Alternate run-up procedures t hat can be shown to provide acceptable engine acceleration to takeoff power are also acceptable.

d. Mixed phase or ice crystal ici ng: It has been established that mixed phase or ice crystal icing conditions that ar e hazardous to turbine engines exist around the outskirts (anvil) of a deep convective cl oud formation as well as near tropical storms such as monsoons, hurricanes, and typhoons. A compilation of mixed phase/ice crystal icing condition currently ava ilable is in Referenc e 7. This report recommends that ice water content up to 1.2 g/m3 could exist in thunderstorm anvil cloud above 25,000 f eet altitude, but also incl udes information from ADS-4 citing total water concentrations (TWC) in t he 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 requirement s, it was necessary to define an extension to the icing envelopes defined in FAR Part 25 Appendix C and Appendi x X. FAR Part 33 Appendix D has been created for this pur pose. TWC levels in Appendix D are based upon predicted “adiabatic” condensat ion rates in convective storm updrafts. This has been shown to be a reasonable assumption based on limited field trials. Until a more comprehens ive mixed phase and ice crystal icing envelope is defined, Appendix D should be tr eated as the critical mixed phase/ice crystal icing condition in demonstrating comp liance to certificati on requirements.

10. TEST SETUP CONSIDERATIONS . The Liquid Water Content (LWC) levels defined in Appendix C and Ap pendix X of Part 25 ar e intended as supercooled droplet ambient icing conditions. Test s 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 duc ting may be more or less than the engine inlet LWC concentration if the engine we re 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 exampl e, operation at idle descent power with simulated forward airspeed that is le ss than flight speed might require a compensating increase in test level LW C concentration above Appendix C and Appendix X conditions. This increase w ould be greater for larger supercooled droplet diameters. Engine size is also a variable that affects the difference in

LWC inlet concentration between f light conditions and the engine test environment, with the great est potential compensation requirements being for small engines.

Icing tests that provide a simulated ici ng cloud by direct connection of facility piping to the front flange of the engine, where no inlet ai r spillage is allowed, may cause test facility effects that could al ter the test parameter s (for example, LWC and MVD). The applicant must provide t he FAA with substant iation that the required simulated test conditions adequatel y simulate an installed engine flyingIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-19 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 set up. 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 cryst al icing conditions defined in part 33 Appendix D : Flight-testing is the most accu rate method of demonstrating engine operation in this icing condition. Two import ant considerations in flight testing in mixed phase or ice crystal icing condi tions are the measurement of ambient meteorological data and the ability to pr oject the measured engine performance to the critical icing point cited in this AC. To address the latter, a fully instrumented engine with tem perature sensors strategica lly 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 measur ed data would be required to substantiate satisfactory engine operation at the critical point. To support the scaling calculation, an accurate measur ement of the ambient meteorological condition is essential. Past engine flight tests in this environment had

demonstrated that a combination of li quid water content probe, total water content probe, particle si zing and imaging probe, ice detector and TAT probe is necessary to fully characterize the ambient environment. Onboard real time meteorological data displa y with GPS positioning capab ility were also very helpful tools in locating the area where hi gh ice crystal concent ration exists. This allows the pilot to position the aircra ft 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 indi cation if the test objective is met and

minimizes the number of sorties into this hazardous environment.

Simulation of the critical mixed phase/ice crystal ici ng 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 actual ly simulate the natural environment. A testing standard for this icing condition is not established. Therefore, using ground simulation as a means of demons trating compliance to the mixed phase/ice crystal icing certification r equirement 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 ic e crystal icing), the engine should operate without the accumulati on of ice, which would adversely affect engine operation (for example, flam eout, surge, stall, run-down, high vibrations, slow acceleration or lack of throttle respons e) or cause a sustained loss of power or thrust. Additionally, the applicant should accurately monitor icing test point conditions either through video surveillanc e or instrumentation and provide theIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-20 means to identify the source of ice dam age, especially in those instances where test apparatus may shed ice (for exam ple, 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 pow er losses below the engine power and thrust ratings selected in accordance wit h § 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 t he fan and compressor during the ice shed ingestion process are generally accept able. If temporary power loss or temporary high vibrations are deemed acceptable to t he 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 ci rcumstances, some limited damage may be accepted. The acceptance of any damage mu st fully account for the cumulative damage from repeat encounter s, 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 leve l considered to be within measurement capability (1.5%);

(3) Temporary power loss . Although not general ly acceptable, any resultant temporary steady st ate power loss, surge or high vibrations, if found acceptable by the Administrator, shoul d 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 test s, rig tests, service experi ence) 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 cu mulative damage for repeated encounters

should be evaluated.

(6) Communication of results . The Installation and Operating Manuals required by § 33.5 should provide info rmation describing any resultant engine condition observed during engine certification icing tests. The engineIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-21 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 th is run-up must be documented. Documentation must contain a descripti on of the run-up requirements and the required run-up intervals and it must be contained in the operating manual and

airworthiness limitations se ction 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 o peration should be documented in the Installation Manual. Both the engine certifying ACO and the installing ACO should carefully consider any high vibrat ions 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 cr ew action. Examples of engine characteristics that may not be trans parent to the flight crew are exhaust gas temp fluctuations, or audible surging. Any engine control system function wh ich may be acceptable and required for engine ice protection certification in part 33 should not create an adverse interaction with other aircraft sys tems, aircraft handling qualities and performance, and human factor s considerations. Any unacceptable interactions with the airplane may result in the engi ne 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 fact ors are critical issues

which must be addressed. Additionally, any engine system required to show compliance with § 33.68 should

meet the followi ng 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 conf igured in its most critical dispatch state for certification icing tests;

(3) Electronic faults . If the system uses electroni cs, substantiate that the function is not lost due to any single or probable multiple electronic faults; (4) Ot her environmental testing . The function should not be affected when the system and any associated electroni c systems are exposed to requiredIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-22 operating environments, including high in tensity radiated fields (HIRF) and lightning; and

(5) Power requirements . For those systems that are powered solely with a dedicated engine alternator (either direct ly or using another engine system such as full authority digital electronic/e ngine control (FADEC)), it should be demonstrated that over the operating envelop e, that the function (that is, sensing and performance) is provided at the mini mum certified rotor speeds. Minimum certified engine speed is the minimum id le 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 signi ficantly outside Appendi x C, Appendix X and Appendix D, and should be communicated to the installing ACO if activation is expected or experienced in these rare o ccasions. Auto-recovery systems are not the primary protection fo r continued safe engine operation during normal ice sheds, or accretion while operating in ic ing conditions described in Appendix C, Appendix X and Appendix D. Therefore, it is acc eptable to perform § 33.68 compliance testing with auto-recovery systems enabled, but they should not activate throughout the § 33.68 test sequenc e. Additionally, continuous ignition should not be selected during § 33.68 co mpliance testing. To assure nonactivation of an enabled auto-recovery system , it may be necessary to display an instrumented signal that monitors auto-reco very system activati on. If activation monitoring cannot be accomplished, then di sabling of the auto-recovery system may be necessary.

e. Operating instructions . Any operating procedure (for example, ground runup procedures) required to ensure c ontinued operational compliance with ground icing conditions or falling and blowi ng snow evaluated under § 33.68(d), and §§23.1093(b)(ii), or 25.1093(b)(2), should be co mmunicated to the installer in the Operating Instructions as a requiremen t, 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 t hat they can be effectively implemented

in-service.IPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-23 f. Special Considerations for Mixed Phase / Ice Crystals

14-CFR part 33 requirement s for the type certificat ion approval of engines operating in an ice crystal environment are located in section 33.68 and Appendix D of Part 33. These requirem ents have been developed in response to

service events. The root cause of t hese 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 th reat to turbine engines in recent years. In response to this recent rec ognition, the FAA has worked within a crossfunctional industry/governmental group to develop standards to address this threat. During this rule and guidance devel opment process, it is noted that ice crystal tools and test techniques hav e 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 ty pe certification program.

In the near term, new engines must be s hown 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 anal ysis to specific field events. This analysis approach should show that t he new engine cycle and/or design features will result in accept able engine operation. Acceptable operation would in clude the absence of rollback, rundown, stall, flameout, and unacceptable compresso r blade damage as described under paragraph (b) of this section. Additionally, the manufacturer should show progress towards incorporating available technology and be driving to wards a thorough substantiation for operation in ice crystals.

Long-term, an acceptable demon stration 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 s hould be demonstrated to the FAA through a combination of testing and validated anal ysis using the latest tools and technology when proposing the complianc e methodology. Computational tools used in this analysis should be calibrated by either rig calibration test data or engine test measurements.IPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-24 Comparative Analysis Guidance

As stated earlier, until ice crystal tool s and test techniques have been developed and validated, the engine manufacturer shou ld use a comparative analysis to specific field events. This analysis mu st 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 su spected susceptible design features as well as mitigating design features.

Susceptible design features coul d include such features as:

• Stagnation points which could provi de 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 desig n features. Mitigating desi gn 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 spacin g of stators set to enhance tolerance to ice blockage (generally denoted by the hy draulic diameter of the stator passage)

• Core compressor airfoil soft body damage tolerance

(i). Alternate method of compliance by similarity to engi nes proven safe to operate in mixed phase or ice crystal icing conditions: Although it has been established that mixed phase or ice cryst al icing conditions are hazardous to turbine engine operation, seve re incident involving this type of meteorological condition is not common. Many curr ently certified engi ne 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.IPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-25 Firstly, the baseline (certified) engine s hould 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 evidenc e can be field service experience or certification test report. Next, the icing pertinent engine features that may influence mixed phase or glaciated ice accr etion 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 tar get engine does not make it more susceptible to mixed phase or ice crystal icing than the basel ine engine. This analysis should consider both environment al and engine operational effects on accumulation, accretion locations, as we ll as the most crit ical engine operating conditions for ice shedding.

If the new engine cycle and/or design feat ures contain innovative design ideas such that a comparative analysis with current engines and specific engine events is not possible, then the applicant should de monstrate 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 crystal s. 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 dev eloped 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 ic e accretion (with allowance for use of liquid water in the near term if test ing with ice crystals is not feasible)

• Evaluate tolerance of new design f eatures over a r ange of test points

( ii ). Compliance Considerations

Engine icing events in atmospheric conditi ons associated with convective clouds and a mixed phase/ice crystal environment appear to be associated with accretion of ice in regions of the compre ssor which are further aft than typical for supercooled droplet environments. Under standing of this process is limited, however. It is expected that, as the ‘sta te-of-the-art’ progresses in these areas,IPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-26 the applicants can utilize advanced met hods 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 a llows impinging cryst als to melt and

accrete on the surface. This is a comb ination of conductive & convective heat transfer due to melting, evaporation and su rface contact. The form of ice can be rime or glaze, this is dictated by loca l temperature/pressure conditions and thus a function of power setting and ambient at mospheric 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 ici ng envelope, to include cruise, hold and descent power settings in mixed phase / ice crystal conditions defined in Appendix D. There is cu rrently 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 ambi ent 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, ev en 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 approx imately 50 deg (120 deg F) to promote ice crystal melting. Power le vel adjusts the accretion site forward or aft within the engine.

• High altitude (low air density) to a llow 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 A ppendix D in the followi ng two figures. They cover the various flight phases in cluding cruise, idle descent and holding. The water contents shown in Figure 2 r epresent the level for the standard exposure distance of 17.4 nautical mile s and must be adjusted to reflect the expected icing exposure period using the dist ance 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 site s more rearward and at higher local air temperatures than woul d exist with only supercooled liquid. These sites also will have higher air loads which can limit ice accretion due toIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-27 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 - ftAmbient Temperature - deg C

Appendix D Cruise

Idle Descent Hold

Figure 1. Appendix D Icing EnvelopeIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-28 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 ftTWC - 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 ContentIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-29 12. APPENDIX X WATER IMPINGEMENT

. The water impingement region becomes potentially greater for the lar ge water droplets of Appendix X. This occurs because larger droplets have great er 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 know n as the modified inertia parameter. This parameter is a relationship bas ed upon the momentum of droplet, air viscosity and size scale of object (with co rrection for Reynolds No) and is used to

correlate the streamline effect on deflect ing 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 

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12/19/2005 Page O-30 The reduction in impingement efficiency below unity from the correlation indicates the degree to which droplets are deflected fr om the object due to streamlines moving around the object. For most shapes a modifi ed inertial parameter of 10 or greater implies a ballistic path for the droplets. Th is 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 t he Appendix C droplet size range is seen to behave ballistically for small “target” object di mensions typical of engine airfoils, airfoil cascades and probes. Consequently, Appendi x 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 A ppendix 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

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12/19/2005 Page O-31 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 ingesti on resulting from nacelle surfaces and to

establish limits for ice releas ed from other aircraft surfaces in FAR 25 certification. It is intended that the engine manufacturer will c onservatively 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 wit hin this AC). The induction system manufacturer or installe r should assess these accumulations in accordance with §§ 23.901(d)(2), 23.1093, or 25.1093 and provide pertinent test variables to the engine manuf acturer for incorporation in to a test demonstration in accordance with § 33.77. In the case wher e an application or product inlet has not been selected at the time of engine certificati on, the engine manufacturer should provide all pertinent inlet assumptions and test data and re sults in the engine installation manual for use by the future installer. The dimensio ns of this slab are related to engine size (defined by inlet highlight area) based on earlier certif ication 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 demons tration should consider ice slab sizes and trajectories aimed at critical engine locati ons 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 applican t 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 comp liance demonstration should be assessed for appropriateness against parts 23 and 25 co mpliance requirements. Minimum ice slab density equivalent to a 0.9 specific grav ity 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 fo r the engine installation, then the part 33 test results can often be used by the airfra me manufacturer to co mply with the natural icing flight test requirements. Execut ion of the ice slab ingestion test typically involves targeting the slab to the air stream ahead of t he fan at the outer diameter of the inlet

duct. This is intended to mimic the ice rel ease from the inlet and results in impact on

the outer diameter of the fan.

a. Validated Analysis with Equi valent Soft Body Tolerance Testing The alternate compliance method allows use of a validated analysis process in conjunction with appropriate so ft body damage testing. If the applicant satisfies theIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-32 requirements of this alternate compliance app roach, the engine shal l 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 dam age testing may in some circumstances include objects that are larger than t he standard ice slab based upon inlet area. Certification for larger than the standard ice slab by this me thod is currently not allowed. However, it is expected that, as the ex perience 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 tec hniques such as finite element analysis – Blade material properties for yiel d and/or failure as appropriate – Dynamic / time variant capability – Thrust variance prediction if requi red to account for blade damage – Appropriate engine and/or component testi ng with impact in outer 1/3 span location

to anchor results This model may be used alone, or in conjuncti on 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 (dis tortion, cracking, tearing) must be assessed against the criteria of paragraph 15(a) below

The following figures help to describe t he contribution of t hese parameters in establishing the threat to the fan blade.IPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-33 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 roto r speed and the velocity of the ice slab as influenced by the air stream velocity and dens ity 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

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12/19/2005 Page O-34 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 ic e slab orientation is still conservative relative to ice slab testing.

Ice Slab Break Up The analysis shall utilize the largest sl ab size consistent with a conservative assessment of slab “break up” that can o ccur within the air str eam ahead of the fan. Historically, the ice slab breaks up into smalle r pieces during the ice slab testing. Data

derived from a number of tests are included in the next figur e. 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 orig inal 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 - inchIce 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:IPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-35 a. Sustained power losses . Any fan blade bending or damage should be assessed against potential sustained power loss. Comp liance 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 f an diameter, the 33.77 requirement is met. In the event that pow er loss exceeds 1.5%, the manufacturer must provide a validated analysis that shows cons istency with the bird test results and verifies that the ice slab would produce less than the 1.5-% power lo ss. 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 t he engine installation

manual.

b. Engine operability . Damage should not adversely affect engine operability (that

is

, should not cause surge, flameou t, 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 fo r a conservative flight cycle scenario (for example, within fly back limits or greater if appropriate te sting 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 t he damage is not readily evident to the crew or visible on preflight inspection (f or example, tail mounted positions).

d. Other anomalies . Damage should not result in any other anomaly (for example, vibration) that may cause the engine to ex ceed operating or stru ctural limitations.

e. Auto-recovery systems. If during § 33. 77 ice slab ingestion testing, an engine does incur a momentary flameout and auto-religh t, 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 additiona l dispatch criteria would be required, where the ignition system must be functional (that is, fully operable) prior to each dispatch. The reason for t he additional dispatch criteria is to ensure the ignition system’s critical relight function is reliabl y available during the subsequent flight. The reason for the allowance of auto-recovery syst ems 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 prov ide information on the size, thickness, and density of the ice slab ingested, any anomal ous behavior such as high vibrations and any affect on the engines ability to operate at the commanded power setting or rating.IPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-36 The icing certification report should incl ude information regarding ice slab orientation and trajectories, slab breakup, impact locati ons, description of any resultant damage, and any other pertinent data defin ing the engine's capability or response to the ice ingestion event. Additionally, if the auto-re covery system is required to comply with § 33.77, then the functional state of the re covery system (for example, one igniter inoperative) becomes a limitat ion that needs to be communicated to the installer to ensure compliance with the delayed activati on requirements of §§ 23.1093 or 25.1093.

SECTION 3. INDUCTION SYSTEM ICIN G PROTECTION (§§ 23.1093 and 25.1093)

Discussion In recent years, applicants have been using t he results of FAR33.77 for compliance with

25.1093 in lieu of a test demonstr ation 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 require s 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 installe d 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 cons idering icing environm ents, 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 requir ed 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 icin g conditions. The only exce ption would be for low engine power conditions where sustained level flight is not possible. Even then, a conservative approach must be used where multiple horizont al and vertical cloud extents in series

are assumed.

The applicant must adequately analyze the per formance 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 runba ck or lip ice. It must be shown by analysis, and verified by test, that the engi ne inlet anti-ice system provides adequate protection under all flight operations. The following conditions mu st be considered. Additional critical points may be identif ied, 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 ant i-ice system performance and the fan blade capability are equivalent to prev ious certification experienc e, then certification may be shown by similarity to previous designs.IPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-37 Inlet Design Point Selection If the inlet is evaporative under the critical points in continuous maximum icing conditions, and is running wet under intermi ttent maximum icing conditions, then the design is satisfactory. Engine inlet anti-ice systems have histor ically 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 promot e 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 Tabl e 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 c onditions defined in Table A:

Descent: If the engine inlet anti-ice system is not fu lly evaporative during des cent 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 calc ulated 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 def ined below (size or kinetic energy), than the amount of ice the engine has been satisfactorily demons trated 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 airp lane operations. The extended airplaneIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-38 holding condition is defined as 45 minutes at t he 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 t he 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 intermi ttent maximum. If the inlet is running wet in a maximum continuous atmospheric condition, then the applicant should

calculate the amount of ice that would a ccumulate during the holding conditions defined in Table A. Ingestion of these calculated qu antities 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 ener gy), 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 Appendi x C for extended airplane operations. The straight line flight evaluat ion must be investigat ed including the use of

the extent factor. If the in let is evaporative under contin uous maximum icing conditions, and is running wet under intermittent maxi mum 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 expos ure). 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 In let Anti-ice Accretion Analysis Inlet lip ice can form as the result of a 2-mi nute delayed activation of the engine inlet anti-ice system.

The latest FAR33.77 testing criteria were dev eloped to account for historical means of compliance for the 2-minute delay ed 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 (amendm ent 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 inle t perimeter is ingested as one piece, consistent with the historical approach taken by the engine m anufacturers. Since ma ximum damage to fan would occur at high fan speed, and this critical condition is also when the inlet has theIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-39 most heat, therefore 2 minutes is a reasonable time to include both pilot reaction to conditions and time to shedding a fter 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 airpla ne into a region that is susceptible to ice build-up, then ingestion of th e calculated quantities of ice mu st 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 ma nufacturer will conservative ly consider the potential installation effects of the engine induction system. Also, there should be close

coordination with the installer to ensure that potential airf rame 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 wit hin this AC). The induction system manufacturer or installe r should assess these accumulations in accordance with §§ 23.901(d)(2), 23.1093, or 25.1093 and provide pertinent test variables to the engine manuf acturer for incorporation in to a test demonstration in accordance with § 33.77. In the case wher e an application or product inlet has not been selected at the time of engine certificati on, the engine manufacturer should provide all pertinent inlet assumptions and test data and re sults 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 el ect to compare the ice on the basis of the kinetic energy of the ice slab.

Kinetic energy may be used as an acceptabl e 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 c ontact with the fuel tanks’ upper surfaces, and the airplane is exposed to conditions of at mospheric 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 fu selage tanks has been consumed, clear ice mayIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-40 form on cold wing surfaces during some operat ions. Moreover, some past analysis has shown the potential for ice formation on t he 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-mount ed engines, equipped with wing mounted ice detection systems, the applicant must demonstr ate 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 engi ne 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. Consi der that a wing mounted ice detection system advises the presence of cl ear ice build-up on the upper surface of the wings when the clear ice thickness reac hes 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 testi ng on an airfoil, Ref (Q). The results of

these tests indicate that for unheated airc raft surfaces, a mix ed phase environment

results in the same or less ice accreti on 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 operat ion, the local power density was much higher around the stagnation ar ea in the mixed-phase co nditions 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 t hat either fully or partially stick to the

surface.

This may explain why engines with ‘pitot’ styl e inlets have not proved to be susceptible to mixed phase icing and the Appendix C compliance methods adequately address these installations for operation in Append ix D of Part 33. Engines designed with reverse flow intakes, or with intakes involvi ng 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 qua litative analysis of the design and supported by similarity to pr evious 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 impi ngement limits, differi ng catch efficiency, distribution effects, and water contents fo r Appendix X should be accomplished.

Ground Taxi Exposure to Appendix X of Pa rt 25 Conditions: Service experience considered for this advisory material indicates that t here are engine fan damage eventsIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-41 as a result of exposure to SLD during gr ound 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. Considerat ion 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 cu rrent 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 cap able of operating throughout the flight power range of the engine (including idli ng), without the accumulation of ice on the engine, inlet system components, or airfra me 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 oper ability events experienced on nat ural 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. Thes e evaluations include: (1) The adequacy of the flight crew procedures fo r operation in icing conditions , (2) acceptability of tactile inputs to the flight crew as the airpl ane responds to engine fan blade ice shedding during a variety of airplane oper ating conditions, (3) perform ance of the engine vibration indication system as well as other engine indi cation 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. Identificati on of ice source . A means should be provided to aid in identifying the source of any ice that may be ingest ed by the engine during the natural icing certification testing. S pecial attention should be giv en to non-representative ice accretions on flight test instrumentation pr obes 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 diamet er, temperature) against the

time to ensure that the ic ing encounter is represent ative 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, typi cally an adequate test sequence includes threeIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-42 natural fan ice shed cycles at each of the following conditions (with inlet anti-ice turned "on"); descent (flight-idle), holding (power nec essary to maintain level flight for a range of anticipated airplane gross weight condi tions), and maximum climb, unless a more critical engine power setting exists. These encounters should be conducted at a steady state engine thrust level and al though not preferred, sometime s have involved flying through the same icing cloud multiple times (l apping) 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 ai rplane to exit the icing conditions between each fan shed cycle for the purpose of cleari ng any other unprotected airplane surfaces from ice. To avoid masking any adverse engine operating conditions during the natural icing encounter, the test engi ne’s ignition system should be se lected off during the icing conditions. This may require pulling several ai rplane circuit breakers to disable the test engine’s auto-ignition or reco very 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 co ncurrence with the FAA for eng ine 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 r ange, and ground idle. The effect of ingesting snow during ground operations can and should be evaluat ed. The applicant must identify and evaluate the critical temperat ure for the configuration pr oposed. A temperature range between 25 and 34 degrees Fahrenheit has been found conducive to the heavy snow environment and to providing the “wet sti cky snow” which may accumulate on unheated surfaces (airframe and engine) subject to impingement. Co lder temperatures may be critical to some configurations. In thes e cases, colder exteri or 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 s now 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/m 3 , 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 ple num 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 s hould 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:IPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-43 a. Acceptable damage . The extent of damage is equiva lent to or less than that incurred and accepted during eng ine certification testing.

(1) All systems operating normally . The extent of damage is equivalent or less than that incurred and accept ed during the § 33.68 tests.

(2) Delayed acti vation 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 (f or example, inboar d 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 aut o-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 ant i-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 pr oduce 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 in to 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 e ffects of an inadequate particle-separating

inlet that is not in full co mpliance with §§ 23.1093 or 25.1093.

b. Damage from testing in non-representative icing conditions . Damage resulting from icing test conditions which fall sign ificantly outside Appendi x 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 cr iteria established in paragraph 11b of this AC.IPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-44 23. CONCLUDING REMARKS

. Although applicants may c onduct representative tests under §§ 33.68, 33.77, 23.1093, and 25.1093, flight test events during airplane certification may still occur which appear incons istent. In all likelihood, those results would not be inconsistent when judged in light of the scope, intent, and limitations of the certification testing. On ly through reliable instrumentation and photographic evidence can the icing test disparities be fully under stood. Because of the relatively frequent encounters with icing conditions in conjuncti on with the potential impact on safety, the FAA takes a conservative approach when accepting icing compliance standards.

/s/ David W. Hempe Manager, Aircraft Engineering DivisionIPHWG Task 2 WG Report - Appendix O

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12/19/2005 Page O-45 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. Pf eiffer and G. P. Maier

3. THE ICING OF AN UNHEATED NO N-ROTATING CYLINDER IN LIQUID WATER DROPLET-ICE CRYSTAL CLOUDS, E. P. Lozowski, J. R. Stallabrass & P. P. Hearty, National Research C ouncil Canada Report LTR-LT-96, February,

1979.

4. SNOW AND ICE PARTICLE SIZ ES 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, DO T/FAA/AR-03/48, Ka mel 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: "M IXED PHASE ICING CONDITION, A REVIEW", James T Riley.

8. SNOW CONCENTRATION MEASURE MENTS AND CORRELATION WITH VISIBILITY, J. R. Stallabrass, AGARD Conference Pr oceedings No. 236 Icing Testing for Aircraft Engines.

9. THE ESTIMATION OF SNOWFALL RATE USING VISIBILITY , Rasmussen, R.M., J. Vivekanandan, J. Cole, B. Myer s, and C. Masters, 1999 : Journal of Applied Meteorol ogy, 38, 1542-1563.