Subject: COMPLIANCE OF 

TRANSPORT CATEGORY 

AIRPLANES WITH 

CERTIFICATION 

REQUIREMENTS FOR FLIGHT 

IN ICING CONDITIONS

Date: Draft 

Initiated By: 

ANM-110 

AC No: 25-XX

Change: 

1. PURPOSE. This advisory circular (AC) describes an acceptable means of 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. Part 25 

contains the certification requirements for transport category airplanes. The compliance means 

described in this document are intended as guidance. They are meant to supplement the 

engineering judgment that must form the basis of any compliance findings for §§ 25.1419 and 

25.1420. 

2. APPLICABILITY. 

a. This AC provides guidance directed to airplane manufacturers, modifiers, foreign 

regulatory authorities, and Federal Aviation Administration (FAA) airplane type certification 

engineers and their designees. This guidance applies to certification of part 25 transport 

category airplanes for flight in icing conditions. 

b. This material is neither mandatory nor regulatory in nature and does not constitute a 

regulation. It describes acceptable means, but not the only means, for demonstrating 

compliance with the applicable regulations. The FAA will consider other methods of 

demonstrating compliance that an applicant may elect to present. While these guidelines are 

not mandatory, they are derived from extensive FAA and industry experience. On the other 

hand, if we become aware of circumstances that convince us that following this AC would 

not result in compliance with the applicable regulations, we will not be bound by the terms of 

this AC, and we may require additional substantiation or design changes as a basis for 

finding compliance. 

c. This material does not change, create any additional, authorize changes in, or permit 

deviations from, regulatory requirements. 

d. The guidance provided in this AC applies to certification of part 25 transport category 

airplanes for flight in icing conditions. If certification for flight in icing conditions is desired, 
AC 25-XX

ii

the airplane must be able to safely operate throughout the icing envelope defined in 14 CFR 

25, Appendix C. Additional icing requirements in part 25 apply to airplanes for which at 

least one of the following applies: 

 

• The airplane’s gross weight is less than 60,000 lbs, or 

• The airplane is equipped with reversible flight controls. 

Such an airplane, besides being able to operate safely in Appendix C icing conditions, must 

also be able to safely operate in or exit the icing conditions defined by 14 CFR part 25 

Appendix O. The applicant, however, has several certification options available for 

Appendix O icing conditions. The airplane can be certified for— 

 

• The ability to detect Appendix O conditions and safely exit all icing conditions, 

• The ability to operate safely throughout a portion of Appendix O icing conditions and 

safely exit all icing conditions when that portion of Appendix O is exceeded, or 

• The ability to operate safely throughout all Appendix O icing conditions. 

Sections 25.1419 and 25.1420 provide specific airframe requirements for certification for 

flight in the icing conditions defined in Appendices C and O. Additionally, for other parts of 

the airplane (i.e., engine, engine inlet, propeller) there are more specific icing requirements 

and associated guidance. These are contained in the regulations and advisory circulars listed 

in Appendix 1 of this AC. 

Some icing-related regulations must be complied with even if the airplane is not certificated 

for flight in icing. Examples of such rules are— 

• § 25.629(d)(3), 

• § 25.903, 

• § 25.975, 

• § 25.1093, 

• § 25.1323(i), 

• § 25.1324, and 

• § 25.1325(b). 

This AC contains information on flight tests for ice protection certification. Additional 

information on flight tests for showing compliance with the airplane performance and 

handling qualities requirements for icing certification may be found in AC 25-25X. AC 20-
AC 25-XX

iii

73A also contains flight-in-icing approval guidance. The guidance in this AC is applicable to 

new type certificates (TCs), supplemental type certificates (STCs), and amendments to 

existing TCs for airplanes certified under part 4b of the Civil Aviation Regulations (CAR) 

and under part 25, for which approval under the provisions of § 25.1419 is desired. 

3. Cancellation. Advisory Circular 25-1419-1A, Certification of Transport Category 

Airplanes for Flight in Icing Conditions, dated 5/7/04, and Advisory Circular 25-1419-2, 

Compliance with the Ice Protection Requirements of § 25.1419(e), (f), (g), and (h), dated 

10/27/2009, are cancelled.

Ali Bahrami

Manager, Transport Airplane Directorate

Aircraft Certification Service

AC 25-XX

iv

Contents

1. BACKGROUND.

2. DEFINITION OF TERMS.

3. ICING ENVELOPES.

a. The Appendix O Environment. 

b. Use of Icing Envelopes.

4. CERTIFICATION PLAN.

5. ANALYSES.

a. Analytical Simulation Methods.

b. Substantiating Icing Simulation Analytical Tools.

c. Analysis of areas and components to be protected. 

d. Flutter analysis. 

e. Power Sources. 

f. Failure Analyses. 

(1) Failure analysis for § 25.1419 (Appendix C Conditions).

(2) Failure Analysis for § 25.1420 (Appendix O Conditions).

g. Similarity Analyses.

h. Impingement Limit Analyses.

i. Ice Shedding Analyses.

j. Propeller Deicing Analysis.

k. Pitot Probe Ice Protection.

l. Stall Warning System Ice Protection.
AC 25-XX

v 

m. Runback Ice.

6. SIMULATED ICE SHAPES AND ROUGHNESS.

7. COMPLIANCE TESTS (Flight/Simulation). 

8. CERTIFICATION TO § 25.1420. 

9. PNEUMATIC DEICER BOOTS. 

10. EMERGENCY AND ABNORMAL OPERATING CONDITIONS. 

11. ICE AND ICING CONDITIONS DETECTION SYSTEMS. 

a. Compliance with § 25.1419(e)(1) and (e)(2).

b. Compliance with § 25.1419(e)(3).

c. Compliance with § 25.1419(f).

d. Compliance with § 25.1419(g).

e. Compliance with § 25.1420(a)(1) or (a)(2).

 

12. PERFORMING INTENDED FUNCTIONS IN ICING.

a. Engines and Equipment.

b. Engine Alternate Induction Air Sources.

c. Fuel System Venting.

d. Landing Gear.

e. Primary and Secondary Flight Control Surfaces. 

f. Ram Air Turbine.

13. ICE INSPECTION LIGHT(S). 

14. CAUTION INFORMATION.

15. AIRPLANE FLIGHT MANUAL (AFM). 

a. Operating Limitations Section.
AC 25-XX

vi 

b. Operating Procedures Section.

16. GUIDANCE FOR AMENDED TCs AND STCs. 

17. INSTRUCTIONS FOR CONTINUED AIRWORTHINESS.

APPENDICES

Appendix 1 RELATED REGULATIONS AND DOCUMENTS

Appendix 2 HISTORY OF ICING CERTIFICATION RULES

Appendix 3 EXAMPLE OF CERTIFICATION TO § 25.1420(A)(1) USING VISUAL 

CUES 

Appendix 4 GUIDANCE FOR AMENDED TCs AND STCs

Appendix 5 CAPABILITIES OF ENGINEERING TOOLS FOR COMPLIANCE WITH 

 APPENDIX O REQUIREMENTS OF PART 25
1 
AC 25-XX

1 

1. BACKGROUND. Before 1953, airplanes were certificated under part 04 of the CAR. The 

first icing rule, section 04.5814, required that if deicer boots were installed on an airplane, 

positive means must be provided for deflation of all wing boots. There were no other references 

to an airplane ice protection system (IPS) in part 04. Part 4b of the CAR was codified on 

December 31, 1953, and it was subsequently recodified into part 25 effective February 1, 1965. 

Over the years, as our knowledge has increased about icing conditions and their effects on safe 

operation of airplanes, we have made changes to the regulations. A detailed history of icing 

certification requirements can be found here in Appendix 2. 

2. DEFINITION OF TERMS. For purposes of this AC, the following definitions should be 

used. 

Advisory Ice Detection System—A system that advises the flightcrew of the presence 

of ice accretion or icing conditions. An advisory ice detection system can only be used in 

conjunction with other means (most commonly, visual observation by the flightcrew) to 

determine the need for, or timing of, activating the anti-icing or deicing system. When using 

an advisory ice detection system, the flightcrew is responsible for monitoring icing 

conditions or ice accretion as defined in the airplane flight manual (AFM), typically using 

total air temperature and visible moisture criteria or visible ice accretion. The flightcrew is 

responsible for activating the anti-icing or deicing system(s).

Airframe Icing—Ice accretion on the airplane, except for on the propulsion system. 

Anti-Icing—Prevention of ice accretions on a protected surface, either by : 

• Evaporating the impinging water, or 

• Allowing the impinging water to run back and off the protected surface or freeze 

on non-critical areas.

Appendix C Icing Conditions—The environmental conditions defined in Appendix C of 

14 CFR part 25. 

Appendix O Icing Conditions—The environmental conditions defined in Appendix O of 

14 CFR part 25. 

Note: These definitions of terms are intended for use 

only with respect to § 25.1419 and § 25.1420. 
AC 25-XX

2 

Artificial Ice—Real ice formed by artificial means, such as by a spray rig in a tunnel or by a 

tanker.

Automatic Cycling Mode—A mode of operation of the airframe deicing system that 

provides repetitive cycles of the system without the need for the pilot to select each cycle. 

This is generally done with a timer, and there may be more than one timing mode. 

Deicing—The removal or the process of removal of an ice accretion after it has formed on a 

surface.

Drizzle Drop—A drop of water measuring 100 to 500 micrometers, or microns (μm) (0.1 – 

0.5 mm) in diameter.

Freezing Drizzle (FZDZ)—Supercooled drizzle drops that remain in liquid form and 

freeze upon contact with objects colder than 0°C. 

Freezing Precipitation—Any form of supercooled liquid precipitation that freezes upon 

impact with the ground or exposed objects colder than 0°C —freezing rain or freezing 

drizzle.

Freezing Rain (FZRA)—Supercooled rain drops that remain in liquid form and freeze 

upon contact with objects colder than 0°C. 

Glaciated Icing Conditions—Icing conditions in which only ice crystals are present. 

Ice Protection System (IPS)—A system that protects certain critical aircraft parts from 

ice accretion. 

Icing Conditions— The presence of atmospheric moisture and temperature conducive to 

airplane icing. 

Icing Conditions Detector—A device that detects the presence of atmospheric moisture 

and temperature conducive to airplane icing.

Irreversible Flight Controls—Force or motion originating at the airplane’s control 

surface (through aerodynamic loads, static imbalance, or trim tab inputs, for example) that is 

transmitted to the actuator and its mounting. The force or motion cannot be transmitted 

directly back to the flight deck controls. This term refers to flight controls in which all of the 

force necessary to move the pitch, roll, or yaw control surfaces is provided by hydraulic or 

electric actuators, the motion of which is controlled by signals from the flight deck controls. 

Liquid Water Content (LWC)—The total mass of water contained in liquid drops within a 

unit volume of air, usually given in units of grams of water per cubic meter (g/m3). 
AC 25-XX

3 

Mean Effective Diameter (MED)—The calculated drop diameter that divides the total 

liquid water content present in the drop size distribution in half. Half the water volume will 

be in larger drops and half the volume in smaller drops. This value is calculated, as opposed 

to being arrived at by measuring actual drop size. The MED is based on an assumed 

Langmuir drop size distribution. The fact that it is a calculated measurement is how it differs 

from median volume diameter, which is based on actual drop size. 

Median Mass Dimension (MMD)—The particle size (sphere or equivalent mass) which 

divides the total ice mass present in an ice particle distribution in half. Half the ice mass is in 

larger particles and half the ice mass is in smaller particles.

Median-Volume Diameter (MVD)—The drop diameter that divides the total liquid water 

content present in the drop distribution in half. Half the water volume will be in larger drops 

and half the volume in smaller drops. This value is obtained by actual drop size 

measurements.

Migrating Ice—Ice resulting from the down-stream migration and buildup of ice from wet 

protected surfaces to areas that cool sufficiently for the migrating ice to adhere to the surface. 

Mixed-Phase Icing Conditions—Icing conditions in which both ice crystals and 

supercooled liquid water drops are present. 

Monitored Surface—The surface of concern regarding the ice hazard (for example, 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.

Primary Ice Detection System—A detection system used to determine when the IPS 

must be activated. This system announces the presence of ice accretion or icing conditions, 

and it may also provide information to other aircraft systems. A primary automatic system 

automatically activates the anti-icing or deicing IPS. A primary manual system requires the 

flightcrew to activate the anti-icing or deicing IPS upon indication from the primary ice 

detection system. 

Rain Drop— A drop of water greater than 500 μm (0.5 mm) in diameter.

 

Reference Surface—The observed surface used as a reference for the presence of ice on 

the monitored surface. The reference surface may be observed directly or indirectly. Ice 

must occur on the reference surface before – or at the same time as – it appears on the 

monitored surface. Examples of reference surfaces include windshield wiper blades or bolts, 

windshield posts, ice evidence probes, the propeller spinner, and the surface of ice detectors. 

The reference surface may also be the monitored surface.

Reversible Flight Controls—Force or motion originating at the airplane’s control surface 

(through aerodynamic loads, static imbalance, or trim tab inputs, for example) that is 

transmitted back to flight deck controls. This term refers to flight deck controls connected to 
AC 25-XX

4 

the pitch, roll, or yaw control surfaces by direct mechanical linkages, cables, or push-pull 

rods in such a way that pilot effort produces motion or force about the hinge line. 

(1) Aerodynamically boosted flight controls—Reversible flight control systems 

that employ a movable tab on the trailing edge of the main control surface linked to the 

pilot’s controls or to the structure in such a way as to produce aerodynamic forces that 

move, or help to move, the surface. Among the various forms are flying tabs, geared or 

servo tabs, and spring tabs. 

(2) Power-assisted flight controls—Reversible flight control systems in which 

some means is provided, usually a hydraulic actuator, to apply force to a control surface 

in addition to that supplied by the pilot to enable large surface deflections to be obtained 

at high speeds. 

Runback Ice—Ice formed from the freezing or refreezing of water leaving one area on an 

aircraft surface that is above freezing temperature and flowing downwind to another area that 

is sufficiently cooled for freezing (or refreezing) to take place. This ice type is frequently an 

unwanted product of thermal deicing systems. 

Simulated Ice—Ice shapes fabricated from wood, epoxy, or other materials by any 

construction technique. 

Static Air Temperature—The air temperature that would be measured by a temperature 

sensor that is not in motion in relation to that air. This temperature is also referred to in other 

documents as “outside air temperature,” “true outside temperature,” or “ambient 

temperature.” 

Substantiated Visual Cue—Ice accretion on the airplane’s protected surface or on a 

reference surface that has been demonstrated by testing to correlate with ice accretion on a 

monitored surface. 

Supercooled Large Drops (SLD)— Supercooled liquid water that includes freezing rain 

or freezing drizzle.

Supercooled Water—Liquid water at a temperature below the freezing point of 0°C. 

Water Catch—The amount of water that strikes an aircraft surface. 

3. ICING ENVELOPES. 

a. The Appendix O Environment. The conditions included in the part 25 Appendix C 

icing envelopes are unchanged by the addition of Appendix O. See AC20-73A and 

DOT/FAA/CT-88/8-1 "Aircraft Icing Handbook" for details on the Appendix C icing 

envelopes. Appendix O was developed to provide the definition for a representative icing 
AC 25-XX

5 

environment for supercooled large drops, which are not included in Appendix C and which 

include freezing drizzle and freezing rain conditions. 

(1) Section 25.1420 requires that the airplane operate safely in the supercooled large 

drop icing conditions defined in part 25, Appendix O. This section is applicable to 

airplanes for which either of the following applies— 

(a) The airplane’s maximum gross weight is less than 60,000 lbs, or 

(b) The airplane is equipped with reversible flight controls. 

(2) Section 25.1420 requires compliance with § 25.1420(a)(1), (a)(2), or (a)(3). When 

complying with § 25.1420(a)(1), the applicant must provide a method for detecting that 

the airplane is operating in Appendix O icing conditions. Following detection, the 

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

applicant seeks certification for safe operation in portions of Appendix O conditions, 

such as freezing drizzle only, or during specific phases of flight, § 25.1420(a)(2) applies. 

If this option is chosen, following detection of conditions that exceed the selected portion 

of Appendix O, the airplane must be capable of operating safely while exiting all icing 

conditions. Certification for a portion of Appendix O (§ 25.1420(a)(2)) or none of 

Appendix O (§ 25.1420(a)(1)) requires substantiated methods of alerting flightcrews 

when those portions of Appendix O are exceeded (§ 25.1420(a)(2)) or when Appendix O 

conditions are encountered (§ 25.1420(a)(1)). Section § 25.1420(a)(3) applies when the 

applicant seeks certification for all of the icing conditions described in Appendix O. An

airplane certified to § 25.1420(a)(3) must be capable of safely operating throughout the 

conditions described in Appendix O and does not need a means to detect Appendix O 

conditions. In effect, when § 25.1420(a)(3) is chosen, the airplane is certificated for 

flight in icing without any specific airplane flight manual procedures or limitations to exit 

icing conditions. 

Note: While this AC provides guidance on compliance with § 25.1420(a)(2), 

further advancements in engineering tools may be necessary before an applicant 

is able to substantiate compliance with this option. A 2009 review of 

engineering tool capabilities found that current technology does not support 

distinguishing between freezing drizzle and freezing rain in flight. Therefore 

certification for a portion of Appendix O will be challenging and will require 

close coordination with certifying authorities. Consult Appendix 5 of this AC 

for additional details on engineering tool capabilities.
AC 25-XX

6 

(3) Appendix O defines freezing drizzle and freezing rain environments by using four spectra of 

drop sizes with associated liquid water content (LWC) limits. An FAA detailed report on the 

development of Appendix O is available from the FAA William J. Hughes Technical Center 

(reference report DOT/FAA/AR-09/10, dated March 2009). Following are the four drop size 

spectra: 

(a) Freezing drizzle environment with a median volume diameter 

(MVD) less than 40 microns (μm). In addition to drizzle drops, which are 

defined as measuring 100 μm to 500 μm in diameter, this environment contains 

drops less than 100 μm, with a sufficient number of drops less than 40 μm so the 

MVD is less than 40 μm. 

(b) Freezing drizzle environment with an MVD greater than 40 μm. In 

addition to freezing drizzle drops, this environment contains smaller drops, with 

diameters less than 100 μm. 

(c) Freezing rain environment with an MVD less than 40 μm. In addition 

to freezing rain drops, which are defined as measuring more than 500 μm in 

diameter, this environment also contains smaller drops of less than 500 μm with a 

sufficient number of drops less than 40 μm so the MVD is less than 40 μm. 

(d) Freezing rain environment with an MVD greater than 40 μm. In 

addition to freezing rain drops, this environment also contains smaller drops of less 

than 100 μm. 

(4) Initial certification for flight in a portion of Appendix O conditions will likely 

include all of freezing drizzle or all of freezing rain. Such certification could be restricted 

to operation in Appendix O conditions by phase of flight. Certification for flight in a 

portion of Appendix O conditions depends upon the applicant substantiating an 

acceptable way for the flightcrew to distinguish the portion of Appendix O conditions for 

which the airplane is certified from the portion of Appendix O conditions for which the 

aircraft is not approved. Certification for a portion of Appendix O allows latitude for 

certification with a range of techniques. Ice shapes will need to be developed to test for

the portion of the envelope for which the aircraft is approved, as well as for detecting and 

exiting icing conditions beyond the selected portion. 

(5) Ice shapes developed using the approved portion of the icing envelope should 

account for the range of drop distribution and water content and consider the proposed 

method for identifying icing conditions that must be exited. The definition of the 

certificated portion of Appendix O for a particular airplane should be based on measured 

characteristics of the selected icing environment and be consistent with methods used for 

developing Appendix O. 
AC 25-XX

7 

(6) Water content versus drop size relationships defined in Appendix C, Figures 1 and 4, 

are defined in terms of mean effective drop diameter. Part 25 does not require 

consideration of specific distributions for Appendix C icing conditions. As referenced in 

DOT/FAA/CT-88/8-1 "Aircraft Icing Handbook" and AC 20-73A, “Aircraft Ice 

Protection,” single mode distributions such as Langmuir distributions have been used to 

represent the range of drop sizes and water content associated with a specific Appendix C 

icing condition. 

(7) The distributions assumed for Appendix C icing conditions are not appropriate for 

freezing precipitation icing conditions. This is because 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, distributions of 

drop sizes are defined as part of Appendix O. The need to include distributions comes 

from the fact that a larger percentage of the cloud’s total water mass is contained in the

larger diameter drops of Appendix O. The water mass of larger drops affects the water 

catch on airplane components. It also affects drop impingement, icing limits, and the ice 

buildup shape. Ice accretions resulting from the water catch must be considered for their 

effects on safe flight of the airplane.

(8) Liquid water content maximum values for freezing drizzle and freezing rain decrease 

as temperature decreases. This is defined in Appendix O, figures 1 and 4, respectively. 

Appendix O values of total liquid water follow the same standard as those in Appendix C. 

They represent liquid water content for a horizontal extent distance of 17.4 nautical miles 

(20 statute miles).

(9) The maximum vertical extent distance of 12,000 feet for freezing drizzle includes the 

cloud layer and all drizzle precipitation below the cloud that is within 12,000 feet of the 

cloud top. The maximum altitude of freezing drizzle is the same as that for an Appendix 

C continuous maximum icing cloud—22,000 feet (Appendix O, figure 3). Temperatures 

range to as low as -13° F (-25°C). 

(10) The maximum vertical extent distance for freezing rain is 7,000 feet. The 

maximum altitude of freezing rain is 12,000 ft. Temperatures range to as low as 8.6°F 

(-13°C). 

(11) It is not appropriate to use the United States Standard Atmosphere Temperature 

Lapse Rate with Altitude for freezing drizzle and freezing rain. Typically, freezing rain 

is associated with temperature inversions. Freezing drizzle is associated with other 

deviations from the standard lapse rate. When generating ice shapes associated with 

vertical transits through freezing drizzle and freezing rain, the critical temperature should 

be determined and used for the duration of the vertical exposure. 

(12) Liquid water content of continuous freezing drizzle and freezing rain decreases as 

the length of the precipitation area, or its horizontal extent, increases. The horizontal 

extent of an airplane’s exposure to freezing drizzle and freezing rain can be anywhere 
AC 25-XX

8 

from a brief encounter to an extended one. An “f-factor” is used in Appendix O to 

calculate liquid water content per cubic meter based on the length of the cloud (its 

horizontal extent). The f-factor used in Appendix O is the same as is used in Appendix C 

for these calculations. This f-factor is discussed in Appendix N of AC 20-73A. 

b. Use of Icing Envelopes. 

(1) AC 20-73A addresses use of Appendix C. Use of Appendix C is not changed by the 

addition of Appendix O. Appendix O is designed to be similar to Appendix C and to be 

used in much the same manner. The principal differences between using Appendix O 

and using Appendix C are that for Appendix O, the applicant must now——

• Consider four icing conditions rather than two when determining critical icing 

conditions, and 

• Address drop size distributions (DSD). 

(2) Applications of drop size distributions typically require a bin tabulation of the 

proportion of mass (liquid water content) to drop diameter. Tables 1 and 2, which follow, 

represent 10-bin tabulations for the cumulative distributions in Appendix O. Mass 

proportions for the bins were selected to provide a reasonable resolution of the upper 

range of the distributions. The shaded columns (a) and (b) in the tables contain values 

typically used as input to ice accretion computer codes. For some simulation techniques, 

different methods of segregating the bins may be appropriate. 
AC 25-XX

9 

Table 1 

Freezing Drizzle Drop Size Distributions Represented Using 10 Bins. 

Freezing Drizzle Environment 

with MVD < 40 μm

Freezing Drizzle Environment with 

MVD > 40 μm

(a) (b) (b)

Bin

Proportion 

of Mass

Left 

Boundary 

Point 

(μm) 

Massweighted 

Midpoint 

(μm)

Right 

Boundary 

Point

(μm)

Left 

Boundary 

Point

(μm)

Massweighted 

Midpoint

(μm)

Right 

Boundary 

Point

(μm)

1 0.100 1 9 10 1 11 15

2 0.200 10 13 16 15 22 36

3 0.200 16 18 20 36 64 106

4 0.200 20 23 27 106 150 197

5 0.100 27 29 33 197 221 247

6 0.050 33 36 40 247 261 276

7 0.050 40 48 66 276 292 309

8 0.050 66 97 137 309 330 354

9 0.025 137 163 202 354 370 390

10 0.025 202 252 388 390 417 473

Note: Drop size distributions (DSD) in one-micron-drop bin resolution are available in 

electronic files from the William J. Hughes Technical Center.
AC 25-XX

10 

Table 2 

Freezing Rain Drop Size Distributions Represented Using 10 Bins.

Freezing Rain Environment 

with MVD < 40 μm

Freezing Rain Environment with 

MVD > 40 μm

(a) (b) (b)

Bin

Proportion 

of Mass

Left 

Boundary 

Point 

(μm) 

Massweighted 

Midpoint 

(μm)

Right 

Boundary 

Point

(μm)

Left 

Boundary 

Point

(μm)

Massweighted 

Midpoint

(μm)

Right 

Boundary 

Point

(μm)

1 0.100 1 7 8 1 9 14

2 0.200 8 10 12 14 35 236

3 0.200 12 15 19 236 399 526

4 0.200 19 25 53 526 645 765

5 0.100 53 255 468 765 833 912

6 0.050 468 534 595 912 957 1008

7 0.050 595 655 718 1008 1066 1129

8 0.050 718 792 883 1129 1197 1288

9 0.025 883 942 1034 1288 1345 1420

10 0.025 1034 1191 1553 1420 1545 2228

Note: Drop size distributions (DSD) in one-micron-drop bin resolution are available in 

electronic files from the William J. Hughes Technical Center.

(3) When compared with the Langmuir distributions of drop sizes commonly associated 

with Appendix C clouds, Appendix O drop size distributions have much larger drop 

sizes and a larger percentage of the total water mass in large drop sizes. The liquid 

water contents associated with icing conditions in Appendix O are different from those 

associated with Appendix C. These differences may affect water collection efficiency. 

They may result in impingement limits that are further aft for Appendix O icing 

conditions than they are for Appendix C icing conditions. For more discussion of the 
AC 25-XX

11 

effects of drop size and trajectory influence, see FAA Technical Report DOT/FAA/CT88/8-1, and AC 20-73A. 

(4) An additional effect of the supercooled large drop environment is the variability in 

distribution of drop sizes. Ice shapes developed to simulate Appendix C icing 

conditions are sometimes calculated by using a monodispersed drop distribution. This is 

because predicted main ice shapes are not highly sensitive to commonly assumed 

distributions for Appendix C (such as the Langmuir distributions). In a monodispersed 

drop distribution, all drop diameters are assumed to be equal to the median-volume 

diameter (MVD). The large variations of drop size and the larger percentages of water 

in the larger drops defined by Appendix O tend to magnify effects of distributed drop 

sizes. This may be reflected in the main ice shape and in runback ice characteristics.

4. CERTIFICATION PLAN. At the start of the design and development of an airplane, the 

applicant should submit to the cognizant FAA Aircraft Certification Office (ACO) for approval 

a certification plan that addresses the ice protection system. This certification plan should 

include the following basic information: 

a. A general description of the airplane that includes dimensions, airworthiness limitations, 

and other data that may be relevant to certification of the airplane’s ice protection system 

(IPS). 

b. Description of the IPS.

c. A compliance checklist addressing each section of part 25 applicable to the IPS. 

d. Identification of the certification methods for each applicable section of part 25, 

including a description of analyses and tests or references to similarity of designs, that the 

applicant intends to use for certifying the IPS. The applicant and the FAA should agree 

upon these compliance means early in the certification program and before certification 

testing begins. 

e. A failure hazard assessment to determine the criticality of the system.

f. If the ice protection or detection systems contain software, software plans, as described in 

Radio Technical Commission for Aeronautics (RTCA) DO-178B, “Software Considerations 

in Airborne Systems and Equipment Certification” (or another acceptable means of 

compliance for software).

g. Projected schedules of design, analyses, testing, and reporting. 

h. A list of any anomalous results from completed icing certification tests (relevant to 

requirements of §§ 33.68 and 33.77) that will require special operating procedures. 
AC 25-XX

12 

i. If the IPS or icing detection systems contain complex electronic hardware (such as 

programmable logic devices (PLD) or application-specific integrated circuits (ASIC)), plans 

for providing a level of design assurance for these devices commensurate with their 

potential contribution to aircraft hazards and system failures. Such hazards or failures could 

result from electronic hardware faults or malfunctions. The applicant should consult 

RTCA/DO-254, “Design Assurance Guidance for Airborne Electronic Hardware,” and 

RTCA/DO-160E, “Environmental Conditions and Test Procedures for Airborne Equipment” 

for how to provide design assurance for these devices. 

 

5. ANALYSES. The applicant should prepare analyses to substantiate the choice of ice 

protection equipment for the airplane. Such analyses should clearly state the basic protection 

required and the assumptions made, and delineate methods of analysis used. All analyses 

should be validated by tests or should have been validated by the applicant on a previous 

certification program. The applicant who uses a previously validated method should 

substantiate why that method is applicable to the new program.

a. Analytical Simulation Methods. Analytical simulation methods for icing include 

impingement and accretion models based on computational fluid dynamics. The applicant 

will typically use these methods to evaluate protected as well as unprotected areas for 

potential ice accretions. Analytical simulation provides a way to account for the variability 

in drop distributions. It also makes it possible to examine impingement in relation to visual 

icing cues and to analyze the location of detection devices for detrimental local flow effects. 

b. Substantiating Icing Simulation Analytical Tools. During the substantiation 

process for icing simulation analytical tools, such as icing codes, cross comparisons should 

be made with data derived from using other types of tools to show that the analytical 

simulation results are equivalent or conservative. Other types of test data that the icing 

simulation analytical tool data could be compared with are— 

• Natural icing flight test data, 

• Tanker test data, and

• Icing tunnel data. 

Results of analysis with icing codes, as well as results of icing tunnel tests and tanker tests, 

should be compared to results of natural icing flight tests in Appendix C conditions that are 

required by § 25.1419. Engineering judgment is required in evaluating these comparisons, 

because most simulation tools are steady-state simulations and they’ll be compared with 

tests made in natural icing conditions, which are variable. See AC 20-73A for additional 

guidance about substantiating icing simulation tools. The guidance in this paragraph 

applies to airframe, engine, and propeller icing tools for Appendix C and the applicable 

portion of Appendix O. 
AC 25-XX

13 

c. Analysis of areas and components to be protected. In evaluating the 

airplane’s ability to operate safely in Appendix C icing conditions and relevant icing 

conditions of Appendix O, and in determining which components will be protected, the 

applicant should examine those areas listed in AC 20-73A to determine the degree of 

protection required. An applicant may determine that protection is not required for one or 

more of these areas or components. If so, the applicant’s analysis should include the 

supporting data and rationale for allowing those areas or components to remain unprotected. 

The applicant should show that the lack of protection does not adversely affect handling 

characteristics or performance of the airplane, as required by § 25.21(g). The applicant 

must also show that the lack of protection does not affect the operation and functioning of 

affected systems and equipment (e.g., pitot probes). Accessory cooling air intakes that face 

the airstream could become restricted with ice accretion. Inlets (including National 

Advisory Committee for Aeronautics (NACA) inlets) that do not accrete ice in Appendix C 

conditions may accrete ice in the Appendix O conditions. 

d. Flutter Analysis. Advisory Circular (AC) 25.629-1A, “Aeroelastic Stability

Substantiation of Transport Category Airplanes,” provides guidance for showing 

compliance with § 25.629. For airplanes approved for operation in icing conditions, that 

AC addresses nominal configurations with ice accumulations on unprotected surfaces as 

well as ice accumulations resulting from any single failure in the deicing system or any 

combination of failures not shown to be extremely improbable. Advisory 

Circular 25.629-1A also provides guidance for consideration of inadvertent encounters with 

icing for airplanes not certificated for flight in icing conditions, as required by 

§ 25.629(d)(3). Flutter analyses should reflect all mass accumulations of ice on protected 

and unprotected surfaces from exposure to Appendix C and, as appropriate, Appendix O 

conditions. This includes any accretions that could develop on control surfaces. 

e. Power Sources. The applicant should evaluate the power sources in the IPS design 

(e.g., electrical, bleed air, and pneumatic sources). An electrical load analysis or test should 

be conducted on each power source to determine that it is adequate to operate the IPS as 

well as to supply all other essential electrical loads for the airplane throughout the airplane 

flight envelope. The effect of an IPS component failure on availability of power to other 

essential loads should be evaluated in accordance with § 25.1309. All power sources 

affecting engines or engine IPSs for multiengine airplanes must comply with the engine 

isolation requirements of § 25.903(b). 

f. Failure Analyses. 

(1) Failure analysis for § 25.1419 (Appendix C conditions). Advisory Circular 

25.1309-1A, “System Design Analysis,” provides guidance for demonstrating 

compliance with § 25.1309. The hazard classifications of IPS failure conditions are

typically substantiated with analyses and/or testing. A failure analysis should include 

the potential contribution of complex electronic hardware faults and malfunctions to 

system failure conditions. It should also classify the hardware assurance level 

appropriately, as outlined in RTCA/DO-254, "Design Assurance Guidance for Airborne 
AC 25-XX

14 

Electronic Hardware." For purposes of a quantitative analysis, the probability of 

encountering Appendix C icing conditions is considered “1.”

(2) Failure analysis for § 25.1420 (Appendix O conditions). Applying the 

system safety principles of § 25.1309 is helpful in determining the need for system 

requirements to address potential hazards from an Appendix O icing environment. The 

following addresses application of the § 25.1309 principles to Appendix O conditions 

and may be used for showing compliance with § 25.1309. 

(a) Hazard classification. Assessing a hazard classification for compliance with

§ 25.1309 is typically a process combining quantitative and qualitative factors. If 

the design is new and novel and has little similarity to previous designs, a hazard 

classification based on past experience may not be appropriate. If the new design is 

derivative in nature, the assessment can consider the icing event history of similarly 

designed aircraft, and, if applicable, the icing event history of all conventional 

design aircraft. The applicant should consider specific effects of supercooled large 

drop icing when assessing similarity to previous designs (see paragraph 8.b.(4), 

“Some areas of interest for Appendix O”). 

(b) Qualitative Analysis. The following qualitative analysis may be used to

determine the hazard classification for an unannunciated encounter with Appendix O 

icing conditions. The analysis can be applied to aircraft shown to be similar to 

previous designs with respect to Appendix O icing effects, and to which the icing 

event history of all conventional design aircraft is applicable. 

1 Assumptions:

• The airplane is certificated to either— 

 Detect Appendix O icing conditions and safely exit all icing 

after detection of Appendix O icing conditions, or 

 Safely operate in a selected portion of Appendix O icing 

conditions and safely exit all icing after detection of Appendix 

O icing conditions beyond those for which it is certificated. 

• The unannunciated encounter is with Appendix O icing conditions 

in which the aircraft has not been shown to operate safely. 

• The airframe and propulsion ice protection systems have been 

activated prior to the unannunciated encounter. 

2 Service history. The applicant may use service history, design, and 

installation appraisals to support hazard classifications for § 25.1309. Service 

history may be appropriate to support a hazard classification if a new or 
AC 25-XX

15 

derivative aircraft has similar design features to a previously certificated 

airplane. 

3 While definitive statistics are not available, a historical perspective can 

provide some guidance. Many aircraft flying through icing have been exposed to 

supercooled large drop conditions without the pilot being aware of it. The 

interval of exposure to the supercooled large drop conditions may have varied 

from a brief amount of time (such as could occur during a vertical transition 

through a cloud) to a more sustained exposure (such as during a hold). Severity 

of the exposure conditions in terms of water content may have varied

significantly. Therefore, not all encounters with supercooled large drop 

conditions will result in a catastrophic event. 

4 Icing event history of conventionally designed airplanes 

certificated before the introduction of § 25.1420. Given the volume of 

aircraft operations and the number of reported incidents that did not result in a 

catastrophe, a factor of around 1 in 100 is a reasonable assumption of probability 

for a catastrophic event if an airplane encounters Appendix O conditions in 

which it has not been shown capable of safely operating. An applicant may 

assume that the hazard classification for an unannunciated encounter with 

Appendix O conditions while the ice protection system is activated is severe 

major, in accordance with AC 25.1309-1A, provided that the following are true.

• The airplane is similar to previous designs with respect to Appendix O 

icing effects, and

• The applicant can show that the icing event history of all 

conventionally designed airplanes is relevant to the airplane being 

considered for certification.

5 If an aircraft is not similar to a previous design, an assessment of the hazard 

classification may require more analysis or testing. One method of hazard 

assessment would be to consider effects of ice accumulations similar to those 

expected for aircraft being certified under § 25.1420. Such ice shapes may be 

defined from a combination of analysis and icing tanker or icing wind tunnel 

testing. Aerodynamic effects of such shapes could be evaluated with wind 

tunnel testing or, potentially, computational fluid dynamics. See AC 20-73A for 

additional guidance on assessing aerodynamic effects of ice buildups. Hazard 

classification typically takes place early in a certification program. Therefore, a 

conservative assessment may be required until sufficient supporting data is 

available to reduce the hazard classification.

(c) Probability of encountering Appendix O conditions. Appendix C was 

designed to include 99 percent of icing conditions. (Ref: NACA TN 2738, “A 

Probability Analysis of the Meteorological Factors Conducive to Aircraft Icing in 
AC 25-XX

16 

the United States.”) Therefore, the probability of encountering icing outside of 

Appendix C drop conditions is on the order of 10-2. The applicant may assume that 

the average probability for encountering Appendix O icing conditions is 1 x 10-2per 

flight hour. This probability should not be reduced based on phase of flight. 

(d) Numerical safety analysis. For the purposes of a numerical safety analysis, 

the applicant may combine the probability of equipment failure with the probability, 

defined above, of encountering Appendix O icing conditions. If the applicant can 

support a hazard level of “hazardous,” using the above probability (10-2) of 

encountering the specified supercooled large drop conditions, the probability of an 

unannunciated failure of the equipment that alerts the flightcrew to exit icing 

conditions should be less than 1 x 10-5. 

(e) Assessment of visual cues. Typical system safety analyses do not address 

the probability of crew actions, such as observing a visual cue before performing a 

specified action. As advised in AC 25.1309-1A, quantitative assessments of crew 

errors are not considered feasible. When visual cues are to be the method for 

detecting Appendix O conditions and determining when to exit them, the applicant 

should assess the appropriateness and reasonableness of the specific cues (see 

paragraph 8b(3)(b) of this AC for additional guidance). Reasonable tasks are those 

for which the applicant can take full credit because the tasks can realistically be 

anticipated to be performed correctly when required. The applicant should assess 

the task of visually detecting Appendix O conditions to determine if it could be 

performed when required. The workload for visually detecting icing conditions 

should be considered in combination with the operational workload during 

applicable phases of flight. The applicant may assume that the flightcrew is already 

aware that the aircraft has encountered icing. The assessment of whether the task is 

appropriate and reasonable is limited to assessing the task of identifying Appendix O 

accumulations that require exiting from the icing conditions. 

g. Similarity Analyses.

(1) For certification based on similarity to other type-certificated airplanes previously 

approved for flight in icing conditions, the applicant should specify the airplane model 

and the component to which the reference of similarity applies. The applicant should 

show specific similarities in the areas of physical, functional, thermodynamic, 

pneumatic, and aerodynamic characteristics as well as in environmental exposure. The 

applicant should conduct analyses to show that component installation, operation, and 

effect on the airplane’s performance and handling are equivalent to that of the same or 

similar component in the previously approved configuration. 

(2) A demonstration of similarity requires an evaluation of both system and installation 

differences. Differences should be evaluated for their effect on IPS functionality and on 

safe flight in icing. If there is uncertainty about the effects of the differences, the 

applicant should conduct additional tests and/or analyses as necessary and appropriate to 
AC 25-XX

17 

resolve the open issues. 

(3) Section 25.1419(b) requires flight testing in measured natural icing conditions. 

Flight test data from previous certification programs may be used to show compliance 

with § 25.1419(b) if the applicant can show that the data is applicable to the airplane in 

question. If there is uncertainty about the similarity analysis, the applicant should 

conduct flight tests in measured natural icing conditions for compliance with 

§ 25.1419(b). 

(4) On derivative airplane designs, the applicant can use similarity to previous type 

designs shown to comply with § 25.1420 if the effects of differences are substantiated. 

Natural ice flight testing may not be necessary for a design shown to be similar. At a 

minimum, the following differences should be addressed: 

• Airfoil size, shape, and angle of attack. 

• IPS design. 

• Operating altitude and airspeed. 

• Center of gravity. 

• Flight control system. 

• Engine and propeller operation. 

h. Impingement Limit Analyses. The applicant should prepare a drop trajectory and 

impingement analysis of— 

• Wings, 

• Horizontal and vertical stabilizers, 

• Propellers, and 

Note: The applicant must possess all the data to 

substantiate compliance with applicable regulations, 

including data from past certifications upon which the 

similarity analysis is based.
AC 25-XX

18 

• All other critical surfaces upon which ice may accrete. 

This analysis should consider the various airplane operational configurations, phases of 

flight, and associated angles of attack. The analysis should establish upper and lower aft 

drop impingement limits that can then be used to establish the aft ice formation limit and its

relationship to the IPS coverage. 

i. Ice Shedding Analyses. 

(1) Airframe ice shedding may damage or erode engine or powerplant components as 

well as lifting, stabilizing, and flight control surface leading edges. Fan and compressor 

blades, impeller vanes, inlet screens and ducts, and propellers are examples of 

powerplant components subject to damage from shedding ice. For fuselage-mounted 

turbojet engines (and pusher propellers that are very close to the fuselage and well aft of 

the airplane's nose), ice shedding from the forward fuselage and from the wings may 

cause significant damage. Ice shedding from components of the airplane, including 

antennas, should not cause damage to engines and propellers that would adversely affect 

engine operation or cause serious loss of power or thrust (compliance with § 25.1093). 

The applicant should also consider airplane damage that can be caused by ice shedding 

from the propellers. 

(2) Control surfaces such as elevators, ailerons, flaps, and spoilers, especially those 

constructed of thin metallic, nonmetallic, or composite materials, are also subject to 

damage. Currently available trajectory and impingement analyses may not adequately 

predict such damage. Unpredictable ice shedding paths from forward areas such as 

radomes and forward wings (canards) have been found to negate the results of these 

analyses. For this reason, a damage analysis should consider that the most critical ice 

shapes will shed and impact the areas of concern.

j. Propeller Deicing Analysis. The applicant should perform a propeller deicing 

analysis that— 

(1) Substantiates ice protection coverage in relation to chord length and wingspan. 

(2) Substantiates the ice protection system power density. 

(3) Calculates intercycle ice accretions and resulting propeller efficiency losses. 

k. Pitot Probe Ice Protection. Compliance with the technical standard order (TSO) 

qualification standard for electrically heated pitot probes (TSO-C16a) is not sufficient by 

itself to demonstrate compliance with § 25.1323. Section 25.1323 requires a heated pitot 

probe or an equivalent means of preventing malfunction due to icing conditions specified in 

Appendix C, certain Appendix O icing conditions, and in specific mixed-phase icing and ice 

crystal conditions. It is unlikely that the icing conditions critical to the equipment will be 
AC 25-XX

19 

encountered during flight tests. Consequently, certification programs should supplement the 

icing flight tests with icing tunnel test and/or tanker test data, as necessary. In-service 

experience during severe atmospheric conditions has shown that use of pitot probes 

qualified to the older standards has resulted in airspeed fluctuations and even loss of 

indicated airspeed. Section 25.1326 requires a system to advise the flightcrew when a pitot 

heating system is not operating. An acceptable indication system may consist of separate 

lights or a crew alert indication on an electronic display for each pitot source. Additional 

guidance on an acceptable means of compliance with § 25.1326 is provided in AC 25-11. 

(1) TSO C16a, “Electrically heated pitot and pitot-static tubes,” references Society of 

Automotive Engineers (SAE) Aeronautical Standard AS8006 and supplements the icing 

requirements with specific part 25, Appendix C icing conditions and specific liquid 

water content tests from British Specification (BS) 2G.135 “Specification for 

Electrically-Heated pitot and Pitot-Static Pressure Heads.” However it does not contain 

tests for Appendix O icing conditions, mixed-phase or ice crystal conditions. The 

applicant is responsible for showing that the pitot heat is adequate for the Appendix C 

and applicable Appendix O icing conditions. If this proof of compliance is not obtained 

in flight tests, analysis or icing tunnel test data should be submitted. 

(2) Although Appendices C and O of part 25 only consider the liquid water content of 

icing conditions, recent cloud characterization research has indicated that approximately 

40 percent of icing condition events consist of liquid water drops and ice crystals 

(mixed-phase icing conditions). Also, glaciated atmospheric conditions, which consist 

solely of ice crystals, are encountered during aircraft operations. The ice crystal 

environment may be more critical than liquid water for thermal systems, because 

evaporating ice crystals requires more energy. The mixed-phase and ice crystal 

requirements in § 25.1323(i) provide an environmental definition of conditions that may 

be encountered. If not enough heat is provided to melt the ice crystals in mixed-phase or 

ice crystal conditions, blocked pitot probes can result. Blocked pitot probes can prevent 

accurate airspeed indications. Icing tunnel tests with simulated mixed-phase and 

glaciated icing combined with analysis is an acceptable way to demonstrate compliance 

with § 25.1323. 

l. Stall Warning System and Angle of Attack Ice Protection. Compliance with the 

TSO qualification standard for stall warning instruments (TSO-C54) is not sufficient by 

itself to show compliance with § 25.1324. Section 25.1324 requires a heated angle-ofattack system sensor or equivalent means to prevent malfunction due to icing conditions 

specified in Appendix C, certain Appendix O icing conditions, and specific mixed-phase 

icing and ice crystal conditions. AC 25-25X contains additional guidance on stall warning 

requirements.

(1) TSO-C54, “Stall Warning Instruments,” requires compliance, with some exceptions 

and additions, with performance specifications of SAE Aerospace Standard AS403A. 
AC 25-XX

20 

As in AS393A, these requirements include a test to demonstrate deicing and anti-icing 

capability, but only temperature and airspeed are specified. The precipitation test 

conditions of AS-403A include moderate icing conditions for Type II instruments. 

"Moderate," however, is not defined. Although functioning of angle-of-attack sensors is 

evaluated in natural icing conditions during certification test programs, there is no 

requirement to flight test at the Appendix C or O icing limits because the low probability 

of finding those conditions imposes a burden. The applicant is responsible for showing 

that stall warning heat is adequate for the Appendix C and applicable Appendix O icing 

conditions. If proof of this is not obtained in flight tests, analysis or icing tunnel test 

data should be submitted. 

(2) Section 25.1324 also requires consideration of mixed-phase and glaciated icing 

conditions for angle-of-attack sensors. Conventional trailing vane designs are such that 

ice crystals are not likely to be captured. A qualitative analysis based on the service 

history on trailing vane angle-of-attack sensors is an acceptable means of compliance. 

m. Runback Ice. Water not evaporated by thermal ice protection systems and unfrozen 

water in near-freezing conditions (or in conditions when the freezing fraction is less than 

one) may run aft and form runback ice. This runback ice can then accumulate additional 

mass from direct impingement. Computer codes may be unable to estimate the 

characteristics of the runback water or resultant ice shapes (rivulets or thin layers), but some 

codes may be able to estimate the mass of the runback ice. Thus runback ice should be 

determined experimentally. It should be considered when determining critical ice shapes. 

Simulated runback ice shapes may be used when evaluating effects of critical ice shapes. 

The applicant should consider potential hazards resulting from the shedding of runback ice. 

6. SIMULATED ICE SHAPES AND ROUGHNESS AC 25-25X contains guidance on 

icing exposure during various phases of flight that should be considered when determining 

simulated ice shapes and surface roughness. The shape and surface roughness of the ice should 

be developed and substantiated with acceptable methods. Common practices include— 

• Use of computer codes for modeling drop impingement limits and ice shape 

predictions (see SAE ARP5903), 

• Flight in measured natural icing conditions, 

• Icing wind tunnel tests (see SAE ARP5905), and 

• Flight in a measured controlled simulated icing cloud (e.g., behind an icing 

tanker, see SAE ARP5904). 

When developing critical ice shapes, the applicant should consider ice accretions that will form 

during all phases of flight and those that will occur before activation of the ice protection 
AC 25-XX

21 

system. If applicable, runback, residual, and inter-cycle ice accretions should also be 

considered. 

 

a. The applicant should assume that, during holding conditions, the airplane will remain in 

a rectangular “race track” pattern, with all turns being made within the 17.4 nautical mile 

icing cloud. Therefore, no horizontal extent correction should be used for this analysis. 

b. The applicant should substantiate the drop diameter distribution (mean effective, median 

volume, spectra), liquid water content, and temperature that will cause formation of an ice 

shape critical to the airplane’s performance and handling qualities.

c. Ice roughness used 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). 

a. The following paragraphs address methods normally used to show compliance with 

§§ 25.1419 and 25.1420. These requirements state that the airplane must be able to operate 

safely in the continuous maximum and intermittent maximum icing conditions described in 

Appendix C and in the applicable icing conditions described in Appendix O of part 25. The 

airplane should be shown to comply with certification requirements when all IPSs are 

installed and functioning. This can normally be accomplished by performing tests in those 

conditions found to be most critical to basic airplane aerodynamics, IPS design, and 

powerplant functions. All IPS equipment should perform its intended function throughout 

the entire operating envelope. 

b. The primary purposes of flight testing are to— 

• Determine that the IPS is acceptably effective and performs its intended functions 

during flight as predicted by analysis or ground testing, and 

• Evaluate any degradation in performance and flying qualities. 

Performance and handling qualities requirements are identified in § 25.21(g). Flight tests to 

show compliance with these requirements are addressed in AC 25-25X. Flight testing also 

verifies the adequacy of flightcrew procedures as well as limitations for the use of the IPS in 

normal, abnormal, and emergency conditions. 

c. Flight testing is also used to confirm that the powerplant installation as a whole (engine, 

inlet, anti-ice system, etc.) performs satisfactorily in icing conditions. If the auxiliary power 

unit (APU) performs essential functions, it should be evaluated for performance in icing 

conditions. In accordance with part 33, engine icing certification results should also be 

carefully reviewed before any part 25 engine or airframe icing flight tests to determine if 

there were any anomalous results requiring special airplane/engine operating procedures.
AC 25-XX

22 

d. Both §§ 25.1419 and 25.1420 require two or more means of compliance for flight in 

icing approval. It is common to use a combination of methods in order to adequately 

represent the conditions and determine resulting degradation effects with sufficient 

confidence to show compliance. 

(1) Dry air ground tests. Depending upon details of the IPS design, some 

preliminary ground tests of the equipment may be warranted to verify the basic function 

of each item. The applicant should collect quantitative data, as necessary, to verify the 

system designs. These data should include operating temperatures of thermal devices, 

deicing fluid flow rates, and operating pressures and rates of inflation of pneumatic 

components. 

(2) Dry air flight tests with ice protection equipment operating. The first 

flight tests conducted to evaluate the airplane with the IPS operating are usually dry air 

flight tests. The initial dry air tests are conducted to— 

• Verify that the IPS does not affect flying qualities of the airplane in clear air, and 

• Obtain a thermal profile of an operating thermal IPS. 

Several commonly used IPSs and components are discussed below to illustrate typical 

dry air flight test practices. Other types of equipment should be evaluated as their 

specific design dictates.

(a) Thermal ice protection leading edge systems. Dry air flight tests are 

conducted to verify the system design parameters and thermal performance analyses. 

1 Normally, instruments are installed on system components to measure the 

anti-icing mass flow rate or energy input (for electrical systems), supply air 

temperature, and surface temperatures. The dry air test plan generally includes 

operating conditions such as the climb, holding, and descent phases of a normal 

flight profile. Since the presence of moisture can affect surface temperatures, 

tests should be conducted where no visible moisture is present. 

2 Measurements of supply air mass flow rate, energy input, and air temperature 

allow determination of how much heat is available to the system. The adequacy 

of the IPS can then be demonstrated by comparing the measured data to the 

theoretical analysis for both Appendix C and either Appendix O for compliance 

with § 25.1420(a)(3) or the portion of Appendix O proposed for compliance with 

§ 25.1420(a)(2). Surface temperatures measured in the dry air, for example, can 

be useful in extrapolating the maximum possible leading edge surface 

temperature inflight, the heat transfer characteristics of the system, and the 

thermal energy available for the IPS. Supply air temperatures or energy input 

may also be used to verify that the IPS materials were appropriately chosen for 
AC 25-XX

23 

the thermal environment. 

(b) Pneumatic leading edge boots. Tests should demonstrate a rise and 

decrease in operating pressures, which results in the effective removal of ice. This 

pressure rise time, as well as the maximum operating pressure for each boot, should 

be evaluated throughout the altitude range defined in Appendices C and O. Boots 

should be operated in flight at the minimum envelope temperature (-22 oF o) of 

Appendix C continuous maximum icing conditions and Appendix O. This is to 

demonstrate adequate performance throughout the entire flight envelope and to 

demonstrate that no damage occurs during inflation and deflation. The appropriate 

speed and temperature limitation (if any) on boot activation should be included in 

the AFM. Boot inflation should have no significant effect on airplane performance 

and handling qualities. 

(c) Electrically heated propeller boots. For compliance with provisions of 

§§ 25.901(c), 25.903(b), 25.929, and 25.1419(c)— 

1 When flight testing in dry air, surface temperature measurements should be 

made and monitored to confirm proper function. These measurements are useful 

for correlating analytically predicted dry air temperatures with actual 

temperatures, and as a general indicator that the system is functioning and that 

each deicer is heating. It is suggested that system current, brush block voltage 

(i.e., between each input brush and the ground brush) and system duty cycles be 

monitored to ensure that adequate power is applied to the deicers. 

2 System operation should be checked throughout the full r.p.m. and propeller 

cyclic pitch range expected during flight in icing. All significant vibrations 

should be investigated. 

3 The applicant should consider the maximum temperatures a composite 

propeller blade may be subjected to when deicers are energized. It may be useful 

to monitor deicer bond-side temperatures. When performing this evaluation, the 

most critical conditions should be investigated (e.g., airplane on the ground; 

propellers not rotating) on a hot day with the system inadvertently energized. 

(d) Windshield anti-ice. Section 25.773(b)(1)(ii) requires that the airplane have 

a means of maintaining a clear portion of windshield in the icing conditions defined 

in Appendix C and in certain Appendix O icing conditions. The applicant should 

conduct dry air flight tests to verify the thermal analysis. Measurements of both the 

inner and outer surface temperature of the protected windshield area may be needed 

to verify the thermal analysis. Thermal analysis should show that the windshield 

surface temperature is sufficient to maintain anti-icing capability without causing 

structural damage to the windshield. An evaluation of visibility, including distortion 

effects through the protected area, should be made for both day and night operations. 

In addition, the size and location of the protected area should be reviewed to confirm 
AC 25-XX

24 

that it provides adequate visibility for the flightcrew, especially during the approach 

and landing phases of flight. 

(e) Bleed air systems. Effects of bleed air extraction on engine and airplane 

performance, if any, should be examined and included in the AFM performance 

data. The surface heat distribution analysis should be verified for varying flight 

conditions including climb, cruise, hold, and descent. Temperature measurements 

may be necessary to verify the thermal analyses. In accordance with provisions of 

§ 25.939(a), the maximum bleed air for ice protection should have no detrimental 

effect on engine operation throughout the engine’s power range. 

(3) 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 an accumulation of critical ice shapes 

based on exposure to Appendix C icing conditions and applicable Appendix O icing 

conditions. The specific flight tests used to evaluate airplane performance and handling 

qualities are addressed in AC 25-25X. In addition, effects of ice accretion on locations 

forward of pitot-static sensors, angle-of-attack sensors, and static pressure ports should 

be assessed. For failure conditions of the IPS that are not extremely improbable, 

validation testing may be required to demonstrate that the effect on safety of flight (as 

measured by degradation in flight characteristics) is commensurate with the failure 

probability. Such validation testing may consist of flight tests, flight simulator tests, 

wind tunnel tests, or validated computational fluid dynamics analyses. The applicant 

may use dry air flight tests with predicted critical failed IPS ice shapes, which may 

include asymmetric ice shapes, to demonstrate acceptable operational safety.

(4) Icing flight tests. Flight tests in measured natural icing and tests performed with 

artificial icing tools, such as icing tankers, are normally used to demonstrate that the IPS 

performs during flight as predicted by analysis or other testing. Such tests are also used 

to confirm analyses used in developing the various components, such as ice detectors, 

and ice shapes. Section 25.1419 requires measured natural icing flight tests within the 

icing conditions of part 25, Appendix C. If necessary, measured natural icing flight tests 

should be accomplished in Appendix O conditions. The natural icing flight tests are 

accomplished to corroborate the general nature of the effects on airplane handling 

characteristics and performance determined with simulated ice shapes (see AC 25-25X), 

as well as to qualitatively assess the analytically predicted location and general physical 

characteristics of the ice accretions. If necessary, there should be a means to measure 

and record ice accumulations and impingement limits. These can be approximated by 

various means, such as a rod mounted on the airfoil and black paint on the airfoil to 

increase the contrast between the ice accretion and the airfoil.

(a) Instrumentation. The applicant should plan sufficient instrumentation to 

allow documentation of important airplane, system, and component parameters, as 

well as icing conditions encountered. The following parameters should be 

considered. 
AC 25-XX

25 

1 Altitude. 

2 Airspeed. 

3 Engine power level or speed. 

4 Propeller speed and pitch, if applicable. 

5 Temperatures that could be affected by ice protection equipment or ice 

accumulation or that are necessary for validation of analyses, such as the 

temperatures of— 

• Static air.

• Engine components.

• Electrical generation equipment.

• Surfaces.

• Structural components. 

6 Liquid water content. This should be measured over the complete water drop 

size distribution.

7 Median volume drop diameter and drop diameter spectra. When measurement 

of the icing environment drop diameter is necessary, instrumentation used for 

measuring drop sizes should be appropriate for the Appendix C and Appendix O 

icing conditions. For aircraft to be certified to a portion or all of Appendix O, 

measurement and recording of drop diameter spectra should be accomplished. 

8 Microscopic measurement of drop impact craters on a gelatin oil or soot slide 

exposed to an icing cloud may be an impractical way of measuring a median 

volume drop diameter because of the splashing and bouncing characteristics of 

large drops. Also, gelatin oil or soot slides are impractical for recording the drop 

diameter spectra of Appendix O type clouds. Median volume diameters and 

drop diameter distributions are more practically obtained with more sophisticated 

equipment, such as laser-based drop measuring and recording instruments, or 

their equivalent. Depending on the drop size measurement capability of the 

instruments used, more than one instrument may be required to measure the 

expected range of drop diameters. If testing is performed with a calibrated icing 

tanker, large drop instrumentation on the test aircraft may not be necessary. 

Some icing conditions may contain mixed-phase icing (liquid plus ice). The 

instrumentation should provide the ability to distinguish between liquid drops 
AC 25-XX

26 

and ice crystals. Equipment should be calibrated and properly maintained to 

obtain quality data. 

(b) Artificial icing. 

1 Flight testing in artificial icing environments, such as behind icing tankers, is

one way to predict capabilities of individual elements of the ice protection 

equipment and to determine local ice shapes. Since the ice plume has a limited 

cross-section, testing is usually limited to components, such as heated pitot 

tubes, antennas, air inlets including engine induction air inlets, empennage, 

airfoil sections, and windshields. Calibration and verification of the icing cloud 

produced by the tanker should be accomplished as necessary for meeting test 

objectives. SAE ARP5904 provides recommended practices for using tanker 

icing simulations.

2 Use of an icing tanker can provide high confidence in local icing effects. But

obtaining small drop sizes may be difficult with some spray nozzles. As a result, 

these methods could produce larger ice build-ups and different ice shapes than 

those observed in natural Appendix C icing conditions. Icing tanker techniques 

can be used in a manner similar to icing tunnel testing with respect to ice shape 

development. The plume may be of sufficient size that it could be applied to 

sections of the airframe to examine any potential hinge moment or CLmax effects 

from ice accretions behind protected areas in Appendix C or Appendix O 

conditions. This method also has the advantage of being able to combine the 

effects of thermal systems (such as runback) with direct accretion to simulate 

resulting ice accumulations. Tanker simulation could also be used to estimate 

accretion areas for use in estimates of drag from extended accretion regions 

(such as ice accretions extending beyond a radome) during exposures to 

Appendix O conditions. Atmospheric effects such as humidity and drop 

residence time (time required to bring the drop to static temperature) should be 

considered in this type of testing. 

(c) Appendix C natural icing flight testing. 

1 Section 25.1419(b) requires measured natural icing flight tests. Flight tests in 

measured natural icing conditions are intended to verify the ice protection 

analysis, to check for icing anomalies, and to demonstrate that the IPS and its 

components function as intended. Advisory Circular 20-73A provides additional 

information useful for planning a natural icing flight test program. The airplane 

should be given sufficient exposure to icing conditions to allow extrapolation to 

the envelope critical conditions by analysis. Test data obtained during these 

exposures may be used to validate the analytical methods used and the results of 

any preceding artificial icing tests.
AC 25-XX

27 

2 Past experience indicates that an airplane performing flight testing in natural 

intermittent maximum icing conditions may be exposed to lightning strikes, 

severe turbulence, and hail encounters that could extensively damage it. When 

design analyses show that the critical ice protection design points (i.e., heat 

loads, critical ice shapes, accumulation, accumulation rates, etc.) are adequate 

under these conditions, and sufficient ground or flight test data exist to verify the 

analysis, a flight test in intermittent maximum icing conditions may not be 

necessary.

3 Flight testing in natural icing conditions should also be used to verify AFM 

procedures for activation of the IPS, including recognition and delay times 

associated with IPS activation. Such testing should verify the analytically 

predicted location and general physical characteristics of the ice accretions. 

Critical ice accumulations should be observed, where possible, and sufficient 

data taken to allow correlation with dry air testing. Remotely located cameras 

either on the test airplane or on a chase airplane have been used to document ice 

accumulations on areas that cannot be seen from the test airplane’s flightdeck or 

cabin. 

4 For an airplane with a thermal deicing system, the applicant should 

demonstrate the effectiveness of the deicing operation either in artificial icing 

conditions or during a natural icing flight test certification program. The tests 

usually encompass measurements of the surface temperature time history. This 

time history includes the time at which the system is activated, the time at which 

the surface reaches an effective temperature, and the time at which the majority 

of ice is shed from the leading edge. The data should be recorded in the flight 

test report. 

5 The applicant should perform a test to determine if moisture can be trapped and 

prevent normal operation of deicing boots or surface mounted sensors if the 

moisture freezes during flight. To verify proper operation of the deicing boots 

and surface mounted sensors, the airplane should operate on the ground in 

moderate rain, take off and fly through moderate rain, then climb and maintain 

an altitude above the freezing level. During this test all ice protection systems 

should be operated in accordance with the AFM procedures. 

(d) Appendix O natural icing flight testing. Section 25.1420(a) requires that 

the airplane operate safely in the supercooled large drop icing conditions defined in 

part 25, Appendix O, if the airplane has at least one of the following two attributes: 

• The airplane’s gross weight is less than 60,000 lbs, or 

• The airplane is equipped with reversible flight controls. 

 
AC 25-XX

28 

 

1 Unless shown by other means, such as discussed in the following paragraph 

(7d(4)(d)2), flight testing in measured natural Appendix O icing conditions may 

be necessary to— 

(aa) Verify the general physical characteristics and location of the 

simulated ice shapes used for dry air testing, and in particular, their effects 

on airplane handling characteristics. 

(bb) Determine if ice accretes on areas where ice accretion was not 

predicted. 

(cc) Verify adequate performance of ice detectors or visual cues. 

(dd) Conduct performance and handling quality tests as outlined in AC 

25.21(g). 

(ee) Evaluate effects of ice accretion not normally evaluated with simulated 

ice shapes (on propeller, antennas, spinners, etc.) and evaluate operation of 

each critical aircraft system or component after exposure to Appendix O 

conditions. 

2 Flight testing in natural Appendix O icing conditions should not be necessary 

if— 

(aa) The design analyses show that the critical ice protection design points 

(i.e., heat loads, critical ice shapes for performance and handling qualities, 

accumulation, accumulation rates, etc.) are adequate under Appendix O 

conditions and various airplane operational configurations, and

(bb) Except where Appendix 5 of this AC allows the use of one method to 

perform the analysis in (aa), at least two different methods of predicting 

Appendix O ice accretions must be shown to provide similar results (ice 

accretion thickness, location). One method should be either an icing wind 

tunnel or icing tanker test. 

Note: Icing tunnels and analysis with computational fluid dynamics (CFD) 

codes are considered two different methods, but two different CFD codes are 

not.

(cc) Determination of the appropriate analyses and/or tests should include 

consideration of the need to evaluate more than one airplane component 

simultaneously. Examples would be the evaluation of— 
AC 25-XX

29 

• Airplane performance with propeller and airframe ice accretion or with 

asymmetric ice accretions from propeller wash, 

• Engine performance with inlet (including cooling ducts for engine 

accessories) ice accretions, and 

• Stall warning and characteristics with ice accretion that affects air data 

used by stall protection systems.

3 The need for flight testing in natural Appendix O icing is reduced for aircraft 

limited to exiting from Appendix O conditions per § 25.1420(a)(1) as a result of

reduced exposure to such conditions. 

4 Flight testing in natural Appendix O icing conditions should be accomplished 

for airplane derivatives whose ancestor airplanes have a service record that 

includes a pattern of accidents or incidents due to inflight encounters with 

Appendix O conditions. 

(5) Fluid anti-icing/deicing systems. Flight testing should include evaluation of 

fluid flow paths to confirm that adequate and uniform fluid distribution over the 

protected surfaces is achieved. Fluid flow paths should be determined when the fluid is 

mixed with impinging water during system operation. A means of indicating fluid flow 

rates, fluid quantity remaining, etc., should be evaluated to determine that the indicators 

are plainly visible to the pilot and that the indications provided can be effectively read. 

A shutoff valve should be provided in systems using flammable fluids. Fluid antiicing/deicing systems may be used to protect propellers and windshields as well as 

leading edges of airfoils. The fluid for windshield fluid anti-ice systems should be 

tested to demonstrate that it does not become opaque at low temperatures. The AFM 

should include information advising the flightcrew how long it will take to deplete the 

amount of fluid remaining in the reservoir. 

(6) Icing wind tunnel tests. Icing wind tunnels provide the ability to simulate 

natural icing conditions in a controlled environment. A variety of Appendix C icing 

conditions and Appendix O freezing drizzle conditions can be simulated, depending on 

the tunnel. Icing wind tunnels have been used to evaluate ice shapes on unprotected 

areas and on or aft of protected areas, such as inter-cycle, residual, and runback ice. 

They have also been used to evaluate performance of ice protection systems, such as 

pneumatic and thermal systems. Scale models may be used in icing wind tunnels, with 

appropriate scaling corrections, if the scale testing on the component has been validated 

with full-scale testing or analysis. Hybrid models, with the full-scale leading edge 

extending beyond the impingement limits, may also be used. The applicant may use 

these models to estimate impingement limits, examine visual icing cues, and evaluate 

ice detection devices. 

Surface temperature measurements are typically made for air-data instruments, such as 
AC 25-XX

30 

pitot tubes, pitot-static pressure probes, and angle-of-attack probes, 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.

 

(7) Dry air wind tunnel tests. Dry air wind tunnel testing using scaled models and 

ice shapes has been used to determine ice protection requirements. The scaling, 

including the effect of the roughness of the ice, should be substantiated using methods 

found acceptable to the authorities.

8. CERTIFICATION TO § 25.1420. Section 25.1420 requires that the airplane operate 

safely in the supercooled large drop icing conditions defined in Part 25, Appendix O. For 

guidance on engineering tool capabilities see Appendix 5 of this AC. 

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 O) and 

no special normal or abnormal procedures are required in Appendix O conditions, then a 

means to indicate when the aircraft has encountered Appendix O icing conditions is not 

required. 

b. Certification to §§ 25.1420(a)(1) and 25.1420(a)(2). As an alternative to 

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 for a limited range of 

conditions, from certification to Appendix C only to certification to Appendix C and a 

portion of Appendix O. The icing conditions the airplane may be certified to fly through 

may be defined in terms of any parameters that define Appendix O conditions and could 

include phase of flight limits, such as takeoff or holding, in Appendix O or a portion of 

Appendix O. For example, an airplane may be certificated to take off in portions of 

Appendix O conditions, but not be certificated for holding in those same conditions. 

Substantiated means must be provided to inform flightcrews when the selected icing 

conditions boundary is exceeded. The applicant must show compliance with § 25.21(g) for 

exiting the restricted Appendix O icing conditions. Ice shapes to be tested are those 

representing the critical Appendix O icing conditions during recognition and subsequent exit 

from those icing conditions. Methods of defining the selected Appendix O icing conditions 

boundary should be considered early in the certification process, with concurrence from the 

appropriate aircraft certification office. See section 15 of this AC for specific guidance on 

AFM limitations and operating procedures. 
AC 25-XX

31 

(1) Selection of Appendix O boundary. The physics of ice accretion is complex. 

(See FAA Technical Report DOT/FAA/CT-88/8-1, “Aircraft Icing Handbook” for 

detailed discussion on the icing physics process.) Some generalizations, however, can 

be made about effects of supercooled water drop impingement and ice limits. The 

“modified inertia parameter” is a similarity parameter relating a water drop's inertia to 

the drag from the airflow. The inertia of smaller drops tends to be dominated by the 

airflow effect, so their trajectories tend to follow the local streamlines of air. 

Sufficiently small drops, depending on the size of the drop and the airplane component 

they are approaching, may simply flow around the airframe. (A larger airplane 

component increases the flow disturbance ahead of the component, deflecting airflow 

and the trajectories of smaller drops.) On the other hand, the inertia of larger drops 

tends to dominate the airflow effect and so the trajectories tend toward straight lines. 

Thus large drops are much less likely to miss the airframe. The modified inertia 

parameter depends very strongly on drop size. It depends strongly on aircraft (or model) 

size and aircraft (or tunnel) velocity, and it also depends on drop density and air 

viscosity and density. The modified inertia parameter assumes a spherical drop and 

does not reflect drop distortion, drop breakup, or drop splashing. Standard impingement 

parameters, such as impingement limits, overall impingement, and leading edge 

impingement, all increase as a function of the modified inertia parameter. Other 

influences affecting the object's water collection efficiency, drop impingement, and 

icing limits, ice shape, and runback ice include splashing of the large water drops and 

break-up of the large water drops when forces on the distorted drop exceed the surface 

tension required to hold the drop together. These latter influences become more evident 

for the larger drops defined in Appendix O. Drop size distributions and the related 

distribution of water mass are important considerations for water catch, impingement

limits, and ice limits, as well as for runback ice. Consideration of these influences 

allows comparison of critical icing conditions that can be correlated to ice accretions in 

Note: While this AC provides guidance on compliance 

with § 25.1420(a)(2), further advancements in engineering 

tools may be necessary before an applicant is able to 

substantiate compliance with this option. A 2009 review of 

engineering tool capabilities found that current technology 

does not support distinguishing between freezing drizzle 

and freezing rain in flight. Therefore certification for a 

portion of Appendix O will be challenging and will require 

close coordination with certifying authorities. Consult 

Appendix 5 of this AC for additional details on engineering 

tool capabilities. 
AC 25-XX

32 

aft locations, such as behind protected areas. 

(2) The modified inertia parameter is commonly used in icing wind tunnel testing to 

compare high altitude flight conditions to the available low altitude test conditions. To 

simulate effects of high altitudes and true airspeeds on impingement, larger drops are 

used at the lower altitude test conditions. As an example, an IPS designed to protect 

against 50μm drop impingement at 22,000 ft and 200 knots calibrated air speed (KCAS)

(290 knots true air speed (KTAS) ) would be equivalently protected against 76 μm drops 

at sea level and 200 KCAS (190 KTAS) because of an equivalent modified inertia 

parameter (assuming a 6-foot chord). 

(a) Based on these physical processes, it is difficult to select an Appendix O icing 

conditions boundary based on drop size. Consideration of the drop inertia effects 

throughout the flight envelope will likely be required. While technology does not 

currently exist to discriminate icing conditions based on specific drop sizes 

(exclusive of inertia effects), methods may be developed in the future to certify 

airplanes for flight in icing in such a manner. 

(b) Alternative methods of defining a selected Appendix O icing conditions 

boundary may be considered. For example, it may be possible to develop an exit cue 

so that unrestricted flight in Appendix O freezing drizzle is approved but exit is 

required from Appendix O freezing rain.

(c) See Appendix 3 of this AC for an example of a plan for certificating to 

Appendix C only (§ 25.1420(a)(1)) based on drop size discrimination.

(3) Means for detecting Appendix O boundaries. Determining whether the 

selected Appendix O icing conditions boundary has been exceeded can potentially be 

accomplished using visual cues, an ice detection system, or an aerodynamic performance 

monitor. 

(a) Ice detection systems and aerodynamic performance monitors are discussed in 

paragraph 11a(1) 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.

(b) Substantiated visual cues can range from direct observation of ice accretions aft 

of the airplane’s protected surfaces to observation of ice accretions on reference 

surfaces. Methods used to substantiate visual cues should be agreed upon with 

appropriate certification authorities. Responding to a cue should not require the 

flightcrew to judge the ice to be a specific thickness or size. Examples of potential 

visual cues are— 

• Accretions forming on side windshields, 

• Accretions forming on sides of nacelles, 
AC 25-XX

33 

• Accretions forming on propeller spinners aft of a reference point, 

• Accretions forming on radomes aft of a reference point, and/or 

• Accretions forming aft of protected surfaces. 

1 Visual cues should be developed with the following considerations. 

(aa) Visual cues should be within the flightcrew's primary field of view if 

possible. If outside the primary field of view, the cues should be visible from the 

design eye point and easily incorporated into the flightcrew's visual scan with a 

minimum of head movement while seated and performing their normal duties.

(bb) Visual cues should be visible during all modes of operation (day, night) 

without use of a handheld flashlight.

2 During the certification process, the applicant should verify the ability of the crew 

to observe visual cues or reference surfaces. Visibility of the visual cues should be 

evaluated from the most adverse flightcrew seat locations in combination with the 

range of flightcrew heights, within the approved range of eye reference point 

locations, if available. A visual cue is required for both the left and right seats. If a 

single visual cue is used, it should be visible from each seat. Consideration should 

be given to the difficulty of observing clear ice. The adequacy of the detection 

method should be evaluated in all expected flight conditions. The applicant can 

carry out night evaluations with simulated accretions to assess visibility in and out of 

cloud. 

3 Visual cues should be substantiated by tests and analysis, including tests in 

measured natural icing, or icing tanker tests, or potentially through icing wind tunnel 

tests. The applicant should consider the drop distributions of Appendix O when 

developing the cue, and the applicant should substantiate that these cues would be 

present in all the restricted Appendix O icing conditions. If a reference surface is 

used, the applicant should substantiate that it accumulates ice at the same time as or 

prior to ice accumulation on the critical surfaces. 

4 AC 25-25X should be reviewed for guidance on the time flightcrews need to 

visually detect Appendix O icing conditions. 

(4) Some areas of interest for Appendix O. The following are some areas of 

interest specific to supercooled large drop conditions: 

(a) Ice shapes aft of protected areas of lifting surfaces. Appendix O icing 

conditions may result in a ridge of ice or ice roughness aft of the protected areas. On 

aircraft with reversible flight controls, a ridge of ice may cause uncommanded 

deflections of the control surfaces, which may result in an aircraft upset. Aircraft 
AC 25-XX

34 

may also incur reduced maximum lift and stall at lower angles of attack, independent 

of the flight control system. Appendix O ice buildups can result in an aircraft upset 

or stall if the stall protection system activation schedules are not adjusted. 

(b) Ice shapes on unprotected areas of lifting surfaces. Ice shapes 

resulting from Appendix O icing conditions can extend further aft on the airfoil than 

those from Appendix C icing conditions. When portions of the airfoil leading edge 

are unprotected, performance and handling qualities can potentially be affected. 

(c) Ice shapes on unprotected airframe regions. Areas of the airframe that

do not accrete ice under Appendix C conditions should be examined for 

impingement and accretion in Appendix O conditions. The applicant should 

evaluate such issues as visibility through the windshield, locations of air-data 

sensors, and potential airflow disturbance (for example on the airplane nose) from 

Appendix O accretions that could influence performance of the air-data sensors.

(d) Drag and power effects. Aircraft may exhibit reduced performance in 

Appendix C conditions because ice accretions cause increased drag and reduced lift. 

Such impacts can be exacerbated in Appendix O conditions, since the accretions 

may extend further aft on protected areas, extend beyond protected areas, or occur in 

areas not typically protected (extended accretions on the fuselage and the lower wing 

surface, for example). Ice accretions may be rough and cause large local drag 

increases. Thrust effects of bleed penalties from system operations or degradation of 

thrust from powerplant losses (e.g., propeller icing) should be addressed. See AC 

25-25X for more information about evaluating potential effects on airplane 

performance.

(e) Engine considerations. Ice accumulation on the airframe and on air 

induction system components resulting from Appendix O icing conditions should be 

analyzed for potential ingestion of ice by the engine (relative to requirements of 

§ 25.1093) and potential blockage effects. 

9. PNEUMATIC DEICER BOOTS. Some existing AFMs specify a minimum ice 

accumulation thickness before activation of the deicer boot system. Such AFMs are associated 

with airplanes certified prior to § 25.1419, Amendment 25-129. This amendment added 

§ 25.1419(e), (f), (g), and (h), which do not allow activation of airframe ice protection systems 

based on flightcrew visual observation of a specific thickness of ice. See paragraph 11 of this 

AC for additional guidance on compliance with § 25.1419(e), (f), (g), and (h). 

10. EMERGENCY AND ABNORMAL OPERATING CONDITIONS. The applicant 

should conduct flight tests to demonstrate continued safe flight and landing following failures of 

the IPS using appropriate AFM abnormal and/or emergency operating procedures. Such

demonstrations should be conducted with anticipated ice accumulations on normally protected 
AC 25-XX

35 

surfaces. See Appendix C, part 2, of part 25, and AC 25-25X for subpart B testing with failure 

ice shapes.

11. ICE AND ICING CONDITIONS DETECTION SYSTEMS. Sections 25.1419(e), (f), 

(g), and (h) specify requirements for activation of the IPS. These requirements are only 

applicable to Appendix C icing conditions. Section 25.1420(c) requires compliance with 

§ 25.1419(e), (f), (g), and (h) for the selected portion or all of Appendix O as applicable. 

Sections 25.1420(a)(1) and (a)(2), respectively, require a means to alert the flightcrew that they 

are in Appendix O icing conditions or have reached Appendix O icing conditions from which 

they must exit. 

a. Compliance with § 25.1419(e)(1) and (e)(2). These sections of the rule provide 

alternatives to operation of the IPS based on icing conditions defined in § 25.1419(e)(3). 

These alternatives require either a primary ice detection system or substantiated visual cues 

and an advisory ice detection system. Section 25.1419(e)(2) requires defined visual cues for 

recognition of the first sign of ice accretion on a specified surface combined with an 

advisory ice detection system that alerts the flightcrew to activate the airframe ice protection 

system. An acceptable means of compliance is described in the preamble of that rule, which 

states that the acceptability of visual cues combined with an advisory ice detection system is 

contingent upon the following conditions: 

• The advisory ice detection system annunciates when icing conditions exist or 

when the substantiated visual cues are present.

• The defined visual cues rely on the flightcrew’s observation of the first sign of 

ice accretion on the airplane and do not depend on the pilot determining the 

thickness of the accretion. 

• The flightcrew activates the ice protection system when they observe ice 

accretion or when the ice detector annunciates ice, whichever occurs first. 

(1) Ice detectors. 

(a) Primary ice detector. A primary ice detector must either alert the flightcrew 

to operate the IPS using AFM procedures or automatically activate the IPS before an 

unsafe accumulation of ice on the airframe, engine components, or engine air inlets

occurs. The primary ice detection system must perform its intended function for the 

airplane configurations, phases of flight, and within icing envelopes of Appendix C 

and the applicable portion of Appendix O. Laboratory tests should demonstrate the 

ice detector’s ability to function properly within all of the required icing conditions 

and airplane operating envelope. If the ice detector cannot detect ice at low freezing 

fractions and ice accretes on the airplane in such icing conditions, the applicant 

should show that the airplane can be operated safely with the ice accretions. If safe 

operation cannot be substantiated, installation of an icing conditions detector may be 

required. Approval of the primary ice detector should include flight tests in 
AC 25-XX

36 

measured natural icing conditions to verify analyses and laboratory test results, as 

well as to verify that the ice detector system performs its intended function. The 

primary automatic ice detection system should be designed to prevent frequent 

cycling up and down of engine thrust in intermittent icing conditions. Small thrust 

changes will occur with the opening and closing of the engine bleed valves however, 

resulting in a possible nuisance to the flightcrew. 

(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 flightcrew to initiate operation of the IPS using AFM procedures. 

An advisory system is not the prime means used to determine if the IPS should be 

activated. When there is an advisory system installed on an airplane, the flightcrew 

has primary responsibility for determining when the IPS should be activated. 

Although the flightcrew has primary responsibility for determining when the IPS 

must be activated, if the airplane is certificated in accordance with § 25.1419(e)(2), 

the advisory ice detector is required (i.e. not optional) and must perform its intended 

function for the airplane configurations, for its phases of flight, and within the icing 

envelopes of Appendix C and the applicable portion of Appendix O. Analyses and 

tests similar to those performed for a primary ice detector should be performed for 

an advisory ice detector to understand its characteristics, limitations, and installation. 

If the ice detector cannot detect ice at low freezing fractions and ice accretes on the 

airplane in such icing conditions, installation of an icing conditions detector, i.e. one 

that detects both moisture and temperature, may be required. 

(c) Certification plan. The applicant should present an icing certification plan, as 

suggested by section 4 of this AC, to the cognizant aircraft certification office. This

plan should include the method selected for demonstrating the ice detector system’s 

compliance with §§ 25.1301, 25.1309, 25.1419, and all other applicable sections. 

Advisory Circular 20-73 A, Appendix K, provides guidance on certification of ice 

detector systems. That appendix provides guidance on ice detection response times 

as the freezing fraction drops below 1. That guidance is equally applicable to 

advisory and primary ice detection systems. 

(d) Performance of the ice detector system when installed. The applicant 

should accomplish a drop impingement analysis and/or tests to ensure that the ice 

detector(s) are properly located. The applicant should show that under the various 

airplane operational configurations, phases of flight, airspeeds, and associated angles 

of attack, the ice detector is exposed to free-stream water drops. The ice detector 

should be located on the airframe surface where the sensor is adequately exposed to 

the icing environment. The applicant should conduct flow field and boundary-layer 

analyses of candidate installation positions to ensure that the ice detector sensor is 

not shielded from impinging Appendix C and Appendix O water drops. The ice 

detection system should be shown to operate in the range of conditions defined by 

Appendix C and the applicable portion of Appendix O. Sections 25.1419 and 

25.1420 also require a combination of tests and analysis to demonstrate performance 
AC 25-XX

37 

of the ice detector as installed on the airplane. This could include icing tunnel and 

icing tanker tests to evaluate ice detector performance. The applicant may use drop 

impingement analysis to determine that the ice detector functions properly over the 

drop range of Appendix C and the applicable portions of Appendix O when 

validated through natural or artificial icing tests (e.g. tanker, icing tunnel). The 

applicant should demonstrate that the airplane can be safely operated with the ice 

accretions formed up to the time the ice protection system becomes effective, 

following activation by the ice detector. The detector and its installation should 

minimize nuisance warnings. 

(e) Ice detector system safety considerations. The applicant should consult 

AC 25.1309-1A for guidance on complying with § 25.1309. In accordance with that 

AC, the applicant should accomplish a functional hazard assessment to determine 

the hazard level associated with failure of the ice detection system. The probability 

of encountering the icing conditions defined in Appendix C to part 25 should be 

considered to be 1. The unannunciated failure of a primary ice detection system is 

assumed to be a catastrophic failure condition, unless characteristics of the airplane 

in icing conditions without activation of the airframe ice protection system(s) are 

demonstrated to result in a less severe hazard category. If visual cues are the 

primary means of ice detection, the pilots retain responsibility to monitor and detect 

ice accretions when an advisory ice detection system is installed. However, the 

natural tendency of flightcrews to become accustomed to using the advisory ice 

detection system elevates the importance of the detector and increases the need to 

make flightcrews aware of an advisory ice detection system failure. Therefore, an 

undetected failure of the advisory ice detector should be considered as at least a

major hazard unless substantiated as meriting a lower failure condition classification.

(2) Visual cues. Visual cues can be either direct observation of ice accretions on the 

airplane’s protected surfaces or observation of ice accretions on reference surfaces. The 

first indications of any of the following are examples of what could potentially be used 

as visual cues: 

• Accretions forming on the windshield wiper posts (bolt or blade). 

• Accretions forming on propeller spinners. 

• Accretions forming on radomes. 

• Accretions on the protected surfaces. 

If accretions on protected surfaces cannot be observed, a reference system would be 

necessary if compliance with § 25.1419(e)(2) is desired. The applicant should consider 

providing a reference surface that can be periodically deiced to allow the flightcrew to 

determine if the airframe is continuing to accumulate ice. Without a means to deice the 
AC 25-XX

38 

reference surface, compliance with § 25.1419(e)(2) would require operation of the IPS 

as long are there is ice on the reference surface, even when additional ice is not 

accumulating. As the freezing fraction drops below 1, although some reference surfaces 

may not build up ice, ice may begin to accumulate on protected surfaces of the airplane. 

The applicant should substantiate, for all the icing conditions defined in Appendix C to 

part 25 and the applicable portions of Appendix O, that the reference surface 

accumulates ice at the same time as or prior to ice accumulating on the protected 

surfaces. 

 

(a) Field of view. Visual cues should be developed with the following 

considerations. 

1 Visual cues should be within the flightcrew’s primary field of view, if 

possible. If cues are outside the primary field of view, they should be visible 

from the design eye point and easily incorporated into the flightcrew’s vision 

scan with a minimum of head movement while seated and performing their 

normal duties. 

2 Visual cues should be visible during all modes of operation (day, night, and in 

cloud). 

(b) Verification. During the certification process, the applicant should verify the 

ability of the crew to observe the visual cues. Visibility of the visual cues should be 

evaluated from the most adverse flightcrew seat locations in combination with the 

range of flightcrew heights, within the approved range of eye reference point 

locations, if available. A visual cue is required for both the left and right seats. If a 

single visual cue is used, it should be visible from each seat. The difficulty of 

observing clear ice should be considered, and the adequacy of the visual cue should 

be evaluated in all expected flight conditions. The applicant can carry out night 

evaluations with simulated accretions to assess visibility in and out of cloud. Visual 

cues should be substantiated by tests and analysis, including tests in measured 

natural icing.

b. Compliance With § 25.1419(e)(3). This paragraph of § 25.1419 provides an 

alternative to the primary ice detection system and visual cues, plus the advisory ice 

detection system as defined in § 25.1419(e)(1) and (e)(2). This alternative requires 

operation of the IPS when the airplane is in conditions conducive to airframe icing during 

all phases of flight. 

(1) Temperature cue. The temperature cue used in combination with visible 

moisture should consider static temperature variations due to local pressure variations on 

the airframe. If the engine and airframe IPS are both activated based on visible moisture 

and temperature, a common conservative temperature for operation of both systems 

should be used. For example, if the engine IPS is activated at +5oC static air 

temperature or less, the airframe IPS should be activated at the same temperature, even 
AC 25-XX

39 

if it is substantiated that the airframe will not accrete ice above +2oC static air 

temperature. This would ease the flightcrew workload and increase the probability of 

procedural compliance. 

(2) If this provision is used, the flightcrew should be able to easily determine the static 

air temperature. A display of static air temperature should be provided. As an 

alternative, a placard showing corrections for the available temperature, to the nearest 

degree Centigrade, can be used, so the flightcrew can determine the static air 

temperature in the region of interest (that is, around 0oC). 

(3) Airplane Flight Manual (AFM). The Limitations section of the AFM should 

identify the specific static or total air temperature and visible moisture conditions that

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 

requirements of § 25.1419(e)(1), § 25.1419(e)(2), or § 25.1419(e)(3) are applicable to all 

phases of flight unless it can be shown that the IPS need not be operated. To substantiate 

that the IPS need not be operated during certain phases of flight, the applicant should 

consider ice accretions that form during these phases, without the IPS operating, and

establish that the airplane can safely operate in the continuous maximum and intermittent 

maximum icing conditions of Appendix C and the applicable portions of Appendix O. 

d. Compliance With § 25.1419(g).

(1) This paragraph of § 25.1419 requires that after the initial activation of the IPS— 

• The IPS must operate continuously, or 

• The airplane must be equipped with a system that automatically cycles the IPS, 

or 

• An ice detection system must be provided to alert the flightcrew each time the 

IPS must be cycled.

ome examples of systems that automatically cycle the IPS are— 

• A system that senses ice accretion on a detector and correlates it to ice 

accretion on a protected surface. This system then cycles the IPS at a 

predetermined rate. 

• A system that uses a timer to cycle the IPS. The applicant should substantiate 

that the airplane can safely operate with the ice accretions that form between 

the time one deicing cycle is completed and the time the next cycle is initiated. 

If more than one cycling time (a choice of 1- or 3-minute intervals for 

S
AC 25-XX

example) is provided, it should be substantiated that the flightcrew can

determine which cycle time is appropriate.

• A system that directly senses the ice thickness on a protected surface and 

cycles the IPS.

A common attribute of the above systems is that the pilot is not required to manually 

cycle the IPS after initial activation.

(2) Some types of ice detection systems that alert the flightcrew each time the IPS must 

be cycled could operate in a manner similar to the automatic systems discussed above, 

40 

except that the crew would need to manually cycle the system. Flightcrew workload 

associated with such a system should be evaluated. Because of flightcrew workload and 

human factors considerations, a timed system without an ice sensing capability should 

not be used to meet this requirement. The ice shedding effectiveness of the selected 

means for cycling the ice protection system should be evaluated during testing in natural 

icing conditions. All inter-cycle and runback ice should be considered when showing 

compliance with § 25.21(g). See paragraph 11a(1) of this AC for guidance on installed 

ice detector performance and safety considerations. 

e. Compliance with § 25.1420(a)(1) or (a)(2). These paragraphs of § 25.1420 require 

that a means be provided to allow the flightcrew to detect when Appendix O conditions are 

encountered or when the selected portion of Appendix O conditions is exceeded. Means for 

determining when the selected portion of Appendix O icing conditions is exceeded may 

include visual means, ice detectors, or an airplane performance monitor. The substantiation 

of visual means is discussed in section 8 of this AC. 

(1) Ice detectors. An ice detector system installed for compliance with 

§ 25.1420(a)(1) or (a)(2) is meant to determine when conditions have reached the 

boundary of the Appendix O icing conditions in which the airplane has been 

demonstrated to operate safely. 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. 

The applicant should accomplish a drop impingement analysis and/or tests to ensure that 

the ice detector is properly located to function during the aircraft operational conditions 

and in Appendix O icing conditions. The applicant may use analysis to determine that 

the ice detector is located properly for functioning throughout the drop range of 

Appendix O conditions when validated with methods described in SAE ARP 5903. The 

applicant should ensure that the system minimizes nuisance warnings when operating in 

icing conditions. 

(2) The low probability of finding conditions conducive to Appendix O ice 

accumulation may make natural icing flight tests a difficult way to demonstrate that the 

system functions in conditions exceeding Appendix C. The applicant may use flight 

tests of the airplane under simulated icing conditions (icing tanker). The applicant may 

also use icing wind tunnel tests of a representative airfoil section and an ice detector to 
AC 25-XX

41 

demonstrate proper functioning of the system and to correlate signals provided by the 

detectors with the actual ice accretion on the surface.

(3) Aerodynamic performance monitor (APM). A crew-alerting system using 

pressure probes and signal processors could be developed for quantifying pressure 

fluctuations in the flow field from contamination over the wing surface. This

technology does exist, but full development is necessary before incorporating it into the 

crew warning system.

12. PERFORMING INTENDED FUNCTIONS IN ICING. All systems and components of 

the basic airplane should continue to function as intended when operating in an icing 

environment. This includes the following: 

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 found acceptable for this operation (§§ 25.1093 and 

25.1301). 

b. Engine Alternate Induction Air Sources. Engine alternate induction air sources 

should remain functional in an icing environment. 

c. Fuel System Venting. Fuel system venting should not be adversely affected by ice 

accumulation

d. Landing Gear. A retractable landing gear should operate as intended after an icing 

encounter. 

e. Primary and Secondary Flight Control Surfaces. Primary and secondary flight 

control surfaces should remain operational after exposure to icing conditions. The applicant 

should demonstrate either that aerodynamic balance surfaces are not subject to icing 

throughout the airplane's operating envelope (weight, center of gravity, and speed) or that 

no ice accumulation on these surfaces interferes with or limits actuation of the control for 

these surfaces, including retraction of flaps for a safe go-around from the landing 

configuration. 

f. Ram Air Turbine. The ram air turbine should remain operational. It does not need to 

be tested in natural icing conditions if tested in the icing wind tunnel.

 

13. ICE INSPECTION LIGHT(S). Unless operations at night in icing conditions are 

prohibited by an operating limitation, § 25.1403 requires that a means be provided, during flight 

at night, to illuminate or otherwise determine ice formation on parts of the wings that are critical 

from the standpoint of ice accumulations. 

a. If the flightcrew cannot see the wings, one acceptable means of compliance with this 
AC 25-XX

42 

regulation would be to install an ice evidence probe in a position where the flightcrew can 

observe ice accumulation. The applicant should substantiate that formation of ice on this

device precedes formation of ice on the wings or occurs simultaneously with it. 

Consideration should be given to the need for illuminating the ice evidence probe. 

b. Ice inspection lights should be evaluated both in and out of clouds during night flight to 

determine that the component of interest is adequately illuminated without excessive glare, 

reflections, or other distractions to the flightcrew. These tests may be accomplished both in 

and out of clouds during the airplane certification flight tests. Typically, airplane-mounted 

illumination has been used to comply with this regulation. Use of a hand-held flashlight has 

not been considered acceptable because of the associated workload. The appropriate 

manual should identify the ice characteristics the flightcrew is expected to observe as well 

as the action the flightcrew must perform if such ice is observed. 

 

14. CAUTION INFORMATION. Section 25.1419(c) requires that caution information be 

provided to alert the flightcrew when the IPS is not functioning normally. Caution information 

should be provided if the failure condition of the IPS can result in a hazardous airplane 

condition. It should be assumed that icing conditions exist during the failure event. The 

decision to provide caution information should not be based on the numerical probability of the 

failure event. Even if a numerical probability analysis indicates that system failure is 

improbable or extremely improbable, if the failure could result in a hazardous condition, the 

caution information should be provided. As discussed in the preamble to the notice of proposed 

rulemaking (NPRM), § 25.1420 does not contain a requirement complementary to § 25.1419(c). 

Instead, it relies on compliance with § 25.1309(c) to ensure that adequate warning is provided to 

the flightcrew of unsafe system operating conditions. Warning information required by 

§ 25.1309(c), to alert the flightcrew of unsafe system operating conditions, is applicable to 

§ 25.1420. 

a. Sensor(s) used to identify a failure condition should be evaluated to ensure that they are 

properly located to obtain accurate data on the failure of the IPS.

b. The indication system should not be designed so that it could give the flightcrew a false 

indication that the system is functioning normally because of a lack of an alert. The 

applicant should submit data to substantiate that this could not happen. For example, if a 

pneumatic deicing system (boots) requires a specific minimum pressure and pressure rise 

rate to adequately shed ice, an alert should be provided if that minimum pressure and 

pressure rise rate are not attained. Without an alert, the flightcrew may erroneously believe 

that the boots are operating normally when, in fact, they might not be inflating with 

sufficient pressure or with a sufficient inflation rate to adequately shed ice. The applicant 

should also consider the need for an alert about 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: 
AC 25-XX

a. Operating Limitations Section.

(1) Establishment of all systems limitations when operating in icing conditions. These 

limitations should address airframe, engine, and equipment ice protection systems, and 

other systems that may need to be specifically configured for operation in icing 

conditions. Appropriate icing conditions definitions may need to be provided to ensure 

proper operation of the related IPS. 

(2) Limitations on operating time for ice protection equipment, if these limitations are 

based on fluid anti-icing/deicing systems capacities and flow rates.

(3) Airplane speed limitations (if any).

(4) Environmental limitations for equipment operations as applicable (minimum 

temperature or maximum altitude for boot operation, for example). The AFM should 

include appropriate limitations to be observed in the event that icing conditions are 

encountered when the ice protection system cannot be activated.

(5) Engine limitations (power or speed) required for proper operation of the IPS.

(6) A list of required placards. 

(7) If compliance is based on § 25.1420(a)(1) or § 25.1420(a)(2), the Limitations 

section must include a requirement that the flightcrew exit all (Appendix O and C) icing 

conditions immediately upon recognition that the Appendix O icing conditions exceed 

the certification boundary. The Limitations section must reflect the certification 

assumptions such as phase of flight, freezing drizzle, freezing rain, etc. 

b. Operating Procedures Section.

(1) Section 25.1585 requires that operating procedures be provided in the AFM for— 

• Normal procedures peculiar to the particular airplane type or model that are to 

be carried out in connection with routine operations, 

• Non-normal procedures for cases of malfunction and failure conditions 

involving use of special systems or the alternative use of regular systems, and 

• Emergency procedures for foreseeable but unusual situations in which 

immediate and precise action by the crew may be expected to substantially 

reduce the risk of catastrophe. 

These procedures should include all preflight actions necessary to minimize the potential 

of enroute emergencies associated with the IPS. System components should be 

described with sufficient clarity and depth that the pilot can understand their function. 

43 
AC 25-XX

Unless flightcrew actions are accepted as normal airmanship, the appropriate procedures 

should be included in the FAA-approved AFM, AFM revision, or AFM supplement. 

These procedures should include proper pilot response to cockpit warnings, a means to 

identify system failures, and safe use of the system(s).

(2) Procedures should be provided to optimize airplane operation while in icing 

conditions, including climb, holding, and approach configurations and speeds. 

(3) Emergency or abnormal procedures, including procedures to be followed when IPSs 

fail and/or warning or monitor alerts occur, should be provided. 

(4) For fluid anti-icing/deicing systems, information and method(s) for determining the 

remaining flight operation time should be provided. 

(5) For airplanes that cannot supply adequate electrical power for all systems at low 

engine speeds, load-shedding instructions should be provided to the pilot for approach 

and landing in icing conditions if pilot action is necessary. 

(6) For compliance with § 25.1420(a)(1) or § 25.1420(a)(2)— 

(a) The method used to determine when icing conditions must be exited must be 

provided, as well as appropriate failure indications and crew procedures. 

(b) The operating procedures section should contain guidance for exiting the icing 

environment. 

(c) For an airplane certified to Appendix O for certain phases of flight, information 

defining the restricted phases should be provided. This information should include 

all aircraft configurations associated with the restricted phases of flight, such as flap 

position, gear extension, or airspeed. 

(7) Section 25.1419(h) requires that procedures for operation of the IPS, including 

activation and deactivation, must be established. Procedures for IPS deactivation must 

be consistent with the § 25.1419(e) requirements for activation of the IPS. The exact 

timing of deactivation should consider the type of ice protection system (e.g., deicing, 

anti-icing, or running wet) and all delays in deactivation necessary to ensure that 

residual ice is minimized. Pneumatic boots should be operated for three complete cycles 

following the absence of the cues used for activation. However, if the airplane’s stall 

protection system reverts from an icing schedule to a non-icing schedule when the 

airframe IPS is deactivated, AFM procedures should state that the airframe IPS should 

not be deactivated until the flightcrew are certain that the critical wing surfaces are free 

of ice. For example— 

(a) If the airplane is certificated in accordance with § 25.1419(e)(2), pneumatic 

deicing boots may be deactivated following completion of three boot cycles after 

44 
AC 25-XX

both the substantiated visual cues and the advisory ice detection system no longer 

indicate ice accretion, or, 

(b) If the airplane is certificated in accordance with § 25.1419(e)(3), pneumatic 

deicing boots may be deactivated following completion of three boot cycles after 

leaving icing conditions, even if residual ice is observed on the boots. The boots 

must be activated upon reentry into icing conditions. 

(c) If the airplane’s stall protection system reverts from an icing schedule to a nonicing schedule when the airframe IPS is deactivated, AFM procedures should state 

that the airframe IPS should not be deactivated until the flightcrew are certain that 

the critical wing surfaces are free of ice.

(8) The following statement should be included in the operating procedures section of 

the AFM— 

Convective clouds above the freezing altitude that are vigorously 

increasing in size should be avoided, because they may contain 

icing conditions exceeding those for which this aircraft has been 

certified.

16. GUIDANCE FOR AMENDED TCs AND STCs. General guidance for amended TCs 

and STCs can be found in AC 21.101-1. As stated in its “APPLICABILITY” section, the 

guidance in that AC applies to any STC or amended TC on an airplane for which the applicant 

seeks approval under provisions of §§ 25.1419 and 25.1420. Increases in gross weight, 

increases in engine power, and propeller changes could affect approval for flight in icing and 

these areas should be evaluated using AC 21.101-1 as the method of compliance. An applicant 

wishing to use an alternate means of compliance (AMOC) must consult the cognizant aircraft 

certification office.

For additional guidance, refer to Appendix 4 of this AC. 

17. Instructions for Continued Airworthiness. Instructions for Continued 

Airworthiness (ICA) are required in accordance with 14 CFR 21.50(b) and 25.1729. ICA should 

be prepared in accordance with part 25, Appendix H. At a minimum, the following should be 

addressed: 

a. Basic description of the ice protection system operation, components, and 

installation. 

 

b. Servicing information, such as fluid type and quantities. 

 

c. Location of panels used for inspection, and/or instructions to access components. 

 

45 
AC 25-XX

d. Scheduling information for each applicable component including cleaning, 

inspecting, adjusting, testing and lubrication. 

 

e. Overhaul or replacement periods for components, if any. 

f. Instructions for removing and replacing components. 

 

g. Precautions, such as toxicity of freezing point depressant fluid or flammability of 

some cements. 

h. Cleaning instructions, such as only soap and warm water for pneumatic deicing 

boots. 

 

i. Limitations on the materials that can be applied, such as ice adhesion inhibitors, 

rubber preservatives, cosmetic coatings. 

j. Recommendations on the frequency of use of ice adhesion inhibitors. 

 

k. Special equipment or materials, such solvents, cements, edge filler, conductive edge 

sealer, rollers for pneumatic boot installation. 

46 
AC 25-XX

Appendix 1 

A1-1 

Appendix 1 

RELATED REGULATIONS AND DOCUMENTS

If any regulation or document listed here is revised, the latest version should always be 

used.

REGULATIONS. You can download an electronic copy of 14 CFR from the Internet at 

http://www.gpoaccess.gov/cfr/. A paper copy can be ordered by sending a request to 

the U.S. Superintendent of Documents, U.S. Government Printing Office, Washington, 

D.C. 20402-0001, or by calling telephone number (202) 512-1800; or by sending a 

request by facsimile to (202) 512-2250. 

Section Title

§ 25.21(g) Proof of compliance

§ 25.103(b)(3) Stall speed

§ 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 

§ 25.143(c),(i), & (j) General

§ 25.207(b), (e), (f), (h), 

& (i)

Stall warning

§ 25.237(a)(3)(ii) Wind velocities

§ 25.251 Vibration and buffeting
AC 25-XX

Appendix 1 

A1-2 

§ 25.253(c) High-speed characteristics

§ 25.629 Aeroelastic stability requirements

§ 25.773(b)(1) Pilot compartment view

§ 25.875 Reinforcement near propellers

§ 25.929 Propeller deicing

§ 25.975(a)(1) Fuel tank vents and carburetor vapor vents

§ 25.1091(b) Air Induction

§ 25.1093 Induction system icing protection

§ 25.1305(c)(5) Powerplant instruments

§ 25.1323 Airspeed indicating system

§ 25.1325 

§ 25.13xx

Static pressure systems

Angle of Attack Systems

§ 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

§ 25.1455 Draining of fluids subject to freezing

§ 25.1585 Operating procedures

Appendix C to part 25

Appendix K25.1.3(a) to 

part 25

Appendix O to part 25
AC 25-XX

Appendix 1 

A1-3 

ADVISORY CIRCULARS. You can download an electronic copy of the latest version 

of the following ACs and other, related, FAA documents, from the FAA Internet at 

http://rgl.faa.gov. 

Number Title and Date

AC 20-73A Aircraft Ice Protection, dated August 16, 2006 

AC 20-115B RTCA, Inc. Document RTCA/DO-178B [Radio Technical 

Commission for Aeronautics] dated January 11, 1993 

AC 20-117 Hazards Following Ground Deicing and Ground Operations 

in Conditions Conducive to Aircraft Icing, dated 

December 17, 1982 

AC 20-147 Turbojet, Turboprop, and Turbofan Engine Induction System 

Icing and Ice Ingestion, dated February 2, 2004 

AC 21-16E RTCA, Inc. Document RTCA/DO-160E, Environmental 

Conditions and Test Procedures for Airborne Equipment, 

dated December 20, 2005 

AC 21-40A Application Guide for Obtaining a Supplemental Type 

Certification

AC 25-11A Electronic Flight Deck Displays, dated June 21, 2007 

AC 25-22 Certification of Transport Airplane Mechanical Systems, 

dated March 14, 2000 

AC 25-25X Performance and Handling Characteristics in the Icing 

Conditions Specified in Part 25, Appendix C, dated XX 

AC 25.629-1A Aeroelastic Stability Substantiation of Transport Category 

Airplanes, dated July 23, 1998 

AC 25.1309-1A System Design Analysis, dated June 21, 1988 

 
AC 25-XX

Appendix 1 

A1-4 

Other FAA Documents:

Number Title

DOT/FAA/CT-88/8-

1 

“Aircraft Icing Handbook,” issued March 1991, updated

September 1993, available through the National 

Technical Information Service, Springfield, Virginia 

22161 

DOT/FAA/AR-09/10 Data and Analysis for the Development of an 

Engineering Standard for Supercooled Large Drop 

Conditions, dated March 2009 

FAA Policy No. 

ANE-2003-35-1-RO

Policy for Propeller Ice Protection Equipment, dated 

March 2, 2005

 

Industry Documents. The RTCA documents in the list below can be obtained from 

Radio Technical Commission for Aeronautics (RTCA), Inc., 1140 Connecticut Ave., 

NW, Suite 1020, Washington, DC 20036. The SAE (Society of Automotive Engineers) 

documents can be ordered from the Society of Automotive Engineers, Inc., 400 

Commonwealth Drive, Warrendale, PA 15096. The SAE documents to date have not 

been updated to reflect supercooled large drop analyses and testing but provide good 

reference material.

Number Title

RTCA/DO-178B Software Considerations in Airborne Systems and Equipment 

Certification

RTCA/DO-160E Environmental Conditions and Test Procedures for Airborne 

Equipment

RTCA/DO-254 Design Assurance Guidance for Airborne Electronic Hardware

SAE/ARP5624 Aircraft Inflight Icing Terminology, March 2008 

SAE/ARP5903 Drop Impingement and Ice Accretion Computer Codes, dated, 

October 2003 

SAE/ARP5904 Airborne Icing Tankers, dated 10/2002 
AC 25-XX

Appendix 1 

A1-5 

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 ED103) 

Minimum Operational Performance Specification For Inflight Icing 

Detection Systems

AC 25-XX

 Ap

Appendix 2

HISTORY OF ICING CERTIFICATION RULES

a. Civil Air Regulations (CAR).

Before 1953, airplanes were certificated under part 04 of the CAR. Sectio

pendix 2

A2-1 

(1) n 04.5814 

required that if deicer boots were installed on an airplane, positive means must be 

provided for deflation of all wing boots. There were no other references to an airplane 

ice protection system (IPS) in Part 04.

(2) Part 4b of the CAR was codified on December 31, 1953. The requirement for 

positive means of deflating deicer boots was incorporated, without change, into

§ 4b.640. Section 4b.640 stated that, “When an ice protection system is installed, it 

shall be of an approved type. If pneumatic boots are used, at least two independent 

sources of power and a positive means for the deflation of the boots shall be provided.” 

Section 4b.351(b)(ii) provided requirements for pilot compartment vision in icing 

conditions. Section 4b.406 provided propeller-deicing requirements. Section 4b.461 

gave induction system deicing and anti-icing requirements. Section 4b.612(a)(5) 

required that the airspeed indicating system be provided with a heated pitot tube or 

equivalent means of preventing malfunctioning due to icing.

(3) Amendment 4b-2, effective August 25, 1955, introduced icing envelopes similar to 

those in the current Appendix C to part 25. The graphs added by Amendment 4b-21

were identical in substance and format to those describing the current Appendix C icing 

envelopes with a few exceptions. Those envelopes were defined by descriptions of the 

liquid water content (LWC), the mean effective diameter of drops, the temperature, and 

the horizontal and vertical extent of the supercooled icing cloud environment. In the 

figures introduced by Amendment 4b-2, the units of distances shown on the graphs were 

expressed in statute miles instead of nautical miles. The LWC in the Amendment 4b-2 

figures and the current Appendix C envelopes is identical, however, if the correction for 

the difference between nautical and statute miles is made. There are two significant 

differences between the Amendment 4b-2 icing envelopes and those described in the 

current Appendix C. 

(a) The minimum mean effective diameter in the intermittent maximum conditions 

was 20 μm. The current diameter is 15 μm. 

(b) The minimum cloud horizontal extent in the chart titled Intermittent Maximum 

Atmospheric Icing Conditions Variation of LWC Factor with Cloud Horizontal 

 

1 4b-24a, 4b-24b, 4b-24c, 4b-25a, 4b-25b, and 4b-25c
AC 25-XX

 Appendix 2

Extent was 1.5 statute miles. The current horizontal extent is 2.6 nautical miles 

(3.0 statute miles).

(4) Amendment 4b-6, effective August 12, 1957, revised the icing envelopes to those 

that exist in the current requirements. Amendment 4b-6 revised § 4b.461, Induction 

system deicing and anti-icing provisions. The preamble to that amendment states:

There are included herein changes which extend the currently 

effective provisions governing intermittent maximum icing 

conditions so as to cover conditions which might be critical 

insofar as the turbine engine induction system is concerned. In 

this regard, the data are being extended in accordance with 

NACA Technical Note 2738 and involve a revision of figure 4b25a to cover drop diameters as small as 15 μm and a revision of 

figure 4b-25c to cover distances down to 3.0 mile. The icing 

conditions prescribed in the currently effective regulations are 

applicable in the main to the airframe. The changes being made 

in section 4b.461 require the turbine powerplant to be subjected 

to the same icing conditions and require that the induction 

system be protected to prevent serious engine power loss. A 

similar requirement is incorporated with respect to certification 

of turbine engines by an amendment to Part 13 which is being 

made concurrently with this amendment.

b. Part 25.

(1) Part 4b of the CAR was codified into part 25, effective February 1, 1965. The 

recodification, with minor editorial changes, resulted in the following: 

• The content of § 4b.640, Ice protection, became § 25.1419. 

• Section 4b.406, Propeller deicing provisions, became § 25.929. 

• Section 4b.612(a)(5), Airspeed indicating systems, became § 25.1323(e). 

• Section 4b.351(b)(1)(ii), Pilot compartment view, became 

§ 25.773(b)(1)(ii). 

 

• The § 4b.351 reference to the most severe icing conditions for which 

approval of the airplane is desired was changed to reference the icing 

conditions specified in § 25.1419. 

 

• Section 4b.461, Induction system deicing and anti-icing provisions, 

became § 25.1093. 

A2-2 
AC 25-XX

 Appendix 2

A2-3 

(2) Amendment 25-5, effective July 29, 1965, revised § 25.1325(b) to require that the 

correlation between air pressure in the static pressure system and true ambient 

atmospheric static pressure is not changed when the airplane is exposed to the 

continuous and intermittent maximum icing conditions defined in Appendix C. 

(3) Amendment 25-11, effective June 4, 1967, revised § 25.1585 to require that the 

airplane flight manual include information on use of ice protection equipment. 

(4) Amendment 25-23, effective May 8, 1970, revised § 25.1419 to require that the 

effectiveness of the IPS and its components be shown by flight tests of the airplane or its 

components in measured natural atmospheric icing conditions. Before this amendment, 

flight tests in natural icing conditions were considered one means of compliance but 

were not mandatory. Amendment 25-23 also revised § 25.1309 to include additional 

requirements for certifying equipment, systems, and installations. The regulation was 

revised to require a comprehensive systematic failure analysis, supported by appropriate 

test, to ensure that, as the hazard of a failure increases, the probability of its occurrence 

decreases. 

(5) Amendment 25-38, effective February 1, 1977, added § 25.1403. That rule requires 

means for illuminating or otherwise determining the formation of ice on parts of the 

wings for which ice accumulation is critical. This requirement does not apply if 

operating limitations prohibit operations at night in icing conditions.

(6) Amendment 25-43, effective April 12, 1978, added § 25.1326 to require installation 

of a pitot heat indication system on airplanes equipped with flight instrument pitot 

heating systems.

(7) Amendment 25-46, effective December 1, 1978, added § 25.1416 to require 

specific standards for pneumatic deicer boots. 

(8) Amendment 25-72, effective July 20, 1990, transferred the contents of § 25.1416 to 

§ 25.1419 for clarification and editorial improvement. In addition, the content of 

§ 25.1416(c) was revised. This rule required a means to indicate to the flightcrew that 

the pneumatic deicing boot system is receiving adequate pressure and is functioning 

normally. It was revised to allow use of the “dark cockpit” concept—a warning when 

failure occurs rather than continual pilot monitoring of a healthy system.

(9) Amendment 25-121, effective Oct

• § 25.21. 

• § 25.103. 

• § 25.105. 

• § 25.107. 

ober 9, 2007, revised the following rules: 

 
AC 25-XX

• § 25.111. 

• § 25.119. 

• § 25.121. 

• § 25.123. 

 

• § 25.125. 

• § 25.143. 

• § 25.207. 

• § 25.237. 

• § 25.253. 

Appendix 2

A2-4

These rules provide performance and handling qualities requirements for safe operation 

in Appendix C icing conditions. Section 25.1419 was revised to require compliance 

with § 25.1419 if certification for flight in icing is desired. Prior to that revision, 

compliance with § 25.1419 was only required if the airplane had ice protection 

provisions. Appendix C was revised to identify airframe ice accretion that must be 

considered for showing compliance with Subpart B. Editorial revisions were made to 

§§ 25.773 and 25.941 to retain consistency with the changed § 25.1419 and to revise 

references to changed paragraphs. 

(10) Amendment 25-129, effective September 2, 2009, added § 25.1419(e), (f), (g), and 

(h) to provide requirements for determining when to operate ice protection systems to 

ensure safe operation in Appendix C icing conditions. 

 

(11) Amendment 25-XX, effective xx xx, introduced Appendix O and §§ 25.1420 and 

25.13xx and revised the rules in the following table. 
AC 25-XX

 Appendix 2

A2-5 

Rules Revised by Amendment 25-XX

§ 25.21 § 25.123 § 25.253 § 25.1323

§ 25.105 § 25.125 § 25.773 § 25.1325

§ 25.111 § 25.143 § 25.903 § 25.1521

§ 25.119 § 25.207 § 25.929 § 25.1533

§ 25.121 § 25.237 § 25.1093

Amendment 25-XX expanded the certification icing environment to include freezing 

rain and freezing drizzle. The amendment specified airplane performance and handling 

qualities requirements for operating in these conditions. The amendment expanded the 

engine installation certification and some airplane component certification regulations to 

include ice crystal and mixed-phase icing conditions. The amendment added a new 

icing regulation for angle-of-attack systems. 

AC 25-XX

Appendix 3

A3-1 

Appendix 3

EXAMPLE OF CERTIFICATION TO § 25.1420(A)(1)

USING VISUAL CUES

The following is a hypothetical example of a certification program using the icing conditions of 

Appendix C of part 25 as the boundary for flight in icing. This example offers guidance on 

developing substantiated visual cues to identify that boundary, so that the flightcrew will know 

if that boundary has been exceeded and Appendix O conditions encountered and will be able to 

exit those conditions safely. 

Hypothetical Airplane. 

The aircraft is a transport category, turbofan airplane. It is a derivative of an existing type

which has already been type certificated for flight in icing, including compliance with 

§ 25.1419 (Appendix C). The changes in the derivative airplane did not alter the original 

§ 25.1419 compliance. The derivative airplane design is characterized by the following: 

• The cruise altitude is above the Appendix C envelope. 

• The wing design does not include leading edge devices. 

• The airplane has ice protection over the full span of the wing and tailplane.

• The airplane has reversible flight controls on the roll and pitch axes.

AC 25-25X contains guidance on compliance with 14 CFR 25, sub-part B requirements. 

AC 20-147 contains guidance for engine and engine installations. All applicable certification 

requirements must be met. However for this example, only some aspects will be discussed.

Impingement Aft of the Protected Areas of Lifting Surfaces.

The airplane has ice protection over the full span of the wing and horizontal stabilizer, with 

minimal unprotected areas. Since this is a derivative airplane, the system has already been 

evaluated and certificated for Appendix C conditions. This airplane does have reversible flight 

controls on the roll and pitch axis. An assessment of hinge moment and lift reduction effects is 

required. See AC 25-25X for detailed guidance on performance and handling qualities. 
AC 25-XX

Appendix 3

A3-2 

Impingement Aft of the Engine Inlet Protected Surfaces. 

The applicant should evaluate ice accumulation aft of the engine inlet protected surfaces for the 

possibility of ice ingestion by the engine. For this example, the cumulative mass of this ice is 

less than that used for demonstrating compliance with § 33.77. Therefore, this airplane will be 

found compliant with § 25.1093 based on this known ingestion standard. Refer to AC 20-147 

for further information on engine and engine installation certification.

Ice Shapes on Unprotected Areas of Lifting Surfaces. 

The airplane is designed with full span ice protection on the wing and horizontal stabilizer. The 

unprotected areas (wing tip, stabilizer tip) are a small percentage of the effective airfoil areas. 

Aerodynamic effects of Appendix O accretions on the minimal exposure areas are unlikely to 

be critical, but should be investigated. See AC 25-25X for details on performance and handling 

qualities guidance. 

Ice Shapes on Unprotected Airframe Regions. 

 

The applicant should examine other areas of the airframe that do not accrete ice under Appendix 

C conditions because of the increased impingement limits and potentially additonal runback in 

Appendix O conditions. Issues such as windshield visibility, static port location, and potential 

airflow disturbance from Appendix O accretions that could influence sensors (total air 

temperature, pitot, angle of attack) should be examined. A computational fluid dynamic 

analysis (icing impingement analysis) combined with a satisfactory field service history on 

derivative models may be sufficient to alleviate any potential concerns.

Air Data Sensors. 

The applicant should examine ice protection on air-data sensors to ensure adequate protection 

from exposure to Appendix O icing conditions. 

Design Substantiation. 

A review of the impingement analysis and runback ice data versus the protected area for the 

wing and horizontal stabilizer indicates that the wing is the critical surface for formation of a 

potential ridge behind the protected areas in front of the ailerons. The horizontal stabilizer has 

farther aft protection limits and has sufficient margin for contamination aft of the protected 

areas. Again, it is assumed that the horizontal stabilizer would be addressed, but that will not be 

presented in this example.
AC 25-XX

Appendix 3

A3-3 

Note: Other operating 

conditions should be 

considered as required to 

determine the critical 

conditions. 

Determination of Exit Cues. 

Although this airplane will be certified only for flight in Appendix C conditions, its operating 

envelope could result in the airplane’s exposure to any of the Appendix O icing conditions. 

Climbs and descents should be examined, but because of the transient nature of these 

encounters, they may not produce the critical accretions for Appendix O. 

The recommended procedure for holding in icing conditions should be considered (e.g., whether 

flaps or landing gear should be extended). Airplane-flight-manual-recommended procedures 

would include this operating information. The case to be evaluated will consider flaps up, gear 

up configuration. Since ice accretions aft of the protected areas on the upper wing surface are 

the primary concern in this example, operational conditions are analyzed to determine the 

critical flight conditions for such accretions. The following conditions are determined to be 

critical: 

• Highest holding altitude within the 

conditions of Appendix O that 

maximizes water catch (typically a 

function of ram air effects (effects of air 

exerting pressure) and LWC at defined 

static temperatures). 

• Highest true airspeed expected during 

holding conditions (typically high 

weight hold speed, which produces maximum water catch aft of protected areas 

upper surface.)

Through a combination of impingement analysis, analytical ice shape prediction, and icing 

wind-tunnel testing, the applicant has determined that there is potential for an ice ridge to begin 

accumulating 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. This 

specific drop size and liquid water content will be proposed as a condition for determining when 

icing conditions must be exited, and a visual exit cue will have to be set (see following 

paragraph). Note that such a drop size criterion is not a fixed boundary. At a lower altitude, 

and at a similar equivalent airspeed (similar angle of attack), slightly larger drop sizes are 

required to produce a similar accretion of ice behind the protected area. But this combination of 

drop size, liquid water content, and inertia in relation to the airplane is sufficient to potentially 

produce accumulations behind the protected areas. See discussion on the modified inertia 

parameter in paragraph 8b(1) of this AC and DOT/FAA/CT-88/8-1 Aircraft Icing Handbook, 

Chap. I, section 2 and Chapter IV, section 2.2.2. 

Determination of Visual Exit Cue. 

The target boundary will be icing conditions that exceed the specific drop size and liquid water 
AC 25-XX

Appendix 3

A3-4 

content determined in the preceding paragraph (for high altitude, high speed hold). The target 

criterion for a visual exit cue is a surface on the aircraft readily visible to the flightcrew that will 

indicate water collection in such drop sizes. A computational fluid dynamics (CFD) analysis of 

the cockpit/fuselage area has determined that there are perceptible changes in impingement 

limits with increasing drop sizes along the side windows. The desired visual cue uses a specific 

portion of the windshield as a reference surface that can be correlated to ice accumulation on 

the wing. Preliminary impingement analysis indicates that this is feasible because as drop sizes 

increase, there is significant movement of the aft impingement limits on the windshield when 

the specific flight conditions of interest are considered. Based on this analysis, a specific 

location on the airplane side window is proposed as the place to assess a visible cue to 

determine when the Appendix O conditions that require exiting icing conditions have been 

encountered. 

Definition of Critical Ice Accretion. 

The applicant proposes that the exit cue be based on the first sign of accretion. It has been 

determined that an accretion of approximately 1/16-inch is required on the reference surface in 

order for the crew to detect it. This reference-surface accretion is then correlated to the specific 

ice accretion on the monitored surface (behind the leading edge). The applicant should consider 

the ice accretion corresponding to the first indication on the reference surface and factor in 

recognition time and time to initiate escape as described in AC 25-25X, Appendix 1. This 

correlation is based on a combination of analysis and icing wind tunnel testing. 

The time it takes to observe the visual cue results in accretion of a 1/8-inch ice ridge behind the 

protected area at the specific critical operating condition. The difference in ice thickness 

between the ice on the reference surface and that on the wing is due to differences in the local

collection efficiencies between the side window and the area aft of the leading edge protected 

surface. Consideration of additional accretion during the exit from the icing conditions is 

required to determine the full ice shape to be evaluated for aerodynamic degradation effects.

Evaluation of Critical Ice Accretions. 

Wind tunnel testing of this critical accretion has determined that there are no hinge moment 

situations that result in changes in roll forces. However, there is a potential increase in stall 

speed that must be addressed in the stall warning system. Dry air flight testing is performed 

using simulated ice shapes. This flight testing is performed in a manner in which the ice shapes 

are gradually built up with limited spanwise exposure to evaluate the effects with minimal 

safety risk. The stall warning margins are evaluated. This evaluation results in a modification 

of the stall warning system to ensure that adequate stall margin exists both for the period before 

detection and during the exit from all icing conditions. See AC 25-25X for details on 14 CFR

25, sub-part B compliance.

Validation of the Visual Cue. 
AC 25-XX

Appendix 3

A3-5 

The applicant will evaluate and validate use of the visual cue by flight testing with an icing 

tanker. The tanker is equipped to provide a specific median volume diameter icing condition 

with an associated drop distribution. Since the tanker cannot simulate the full array of

Appendix O icing conditions, validation of specific conditions will be made. A fixed nozzle 

simulating the specific exit cue drop size will be used. 

Icing tanker testing indicates that the side window begins to accumulate ice in the specific 

circumstances considered, which validates the analysis. At slower speeds, the drops do not 

have sufficient inertia to reach the side windows. This validates the fact that the visual cue is 

not overly sensitive and will not result in false indications. Tanker testing also shows good 

correlation with the predicted ice shapes, thus validating the expectation of a specific ice shape 

forming behind the protected areas. This location is adequately illuminated by existing cockpit 

lighting without glare or reflection, and is readily observable by crewmembers.

Exiting from Restricted Appendix O Icing Conditions. 

Flight tests are performed in accordance with 14 CFR 25.21(g)(2) and (4) to show that the 

airplane can safely exit Appendix O icing conditions. See AC 25-25X for details on 

performance and handling qualities guidance. 

AC 25-XX

Appendix 4

A4-1 

Appendix 4 

GUIDANCE FOR AMENDED TCs AND STCs

The guidance in this AC applies to any supplemental type certificate (STC) or amended type 

certificate (TC) on an airplane for which the applicant seeks approval under 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) should consult the cognizant aircraft certification office.

a. These are some examples of modifications for which compliance with icing regulations 

should be revisited. This list is for example only and is not meant to be all inclusive.

• Engine changes. 

• Propeller changes. 

• Engine inlet or accessory inlet changes. 

• Antennae installations or other external modifications. 

• Gross weight increase. 

• Center of gravity envelope increase. 

• Flight envelope increase. 

• Turboprop conversion. 

• Modifications to lifting surfaces. 

• Installation of vortex generators. 

• Modifications to ice protections systems. 
AC 25-XX

Appendix 4

A4-2 

In addition to the guidance on similarity analysis provided in paragraph 5g of this AC, the 

following guidance on specific modifications should be followed:

b. Engine Changes. The applicant should address effects of increased engine power or 

thrust on airplane handling qualities in icing conditions, as well as on compliance with

engine induction icing regulations, § 25.1093. 

(1) Effects of increased engine power or thrust. 

(a) Stability and control. If increased engine power or thrust has potential for 

affecting airplane handling qualities in icing conditions, the applicant should conduct 

flight tests with ice accretions as described in AC 25-25X. 

(b) Ice-contaminated tailplane stall (ICTS). ICTS should be addressed. See 

AC 25-25X for compliance materials on tailplane stall.

(2) Engine induction icing. 

(a) Similarity.

If similarity to a previous design is used as a method of compliance, such 

documentation should be supplemented with analysis or testing, or both. In some 

cases, an analysis substantiating similarity, along with an installation survey 

conducted by the engine manufacturer, may be sufficient. The analysis should 

include the following: 

• Heat requirements for the inlet lip ice protection, if installed.

• Location of the lip deicer thermostat, if installed.

• Inlet material to which the deicer is attached (heat sink may be different).

• Inlet geometry.

• Engine ignition system.

• Engine sensors. 

Any differences that cannot be addressed by analysis should be addressed by flight 

testing. The data to be used for similarity, including data from a past certification on 

which the similarity analysis is based, must be available to the applicant and must be 

provided to the FAA at their request. 
AC 25-XX

Appendix 4

A4-3 

(b) Falling/blowing snow and ground ice fog for turbine engines. 

If similarity to a previously certified airplane will be used as a method of 

compliance, the certification plan should state to which attributes of the ancestor 

airplane similarity will be shown. This is important, because similarity may be 

based on a certified installation that does not have falling/blowing snow or ground 

ice fog in its certification basis. Service experience by itself cannot be used to show 

compliance. For turboprop installations, compliance is normally shown by testing. 

With regard to falling and blowing snow, inlets such as the oil cooler, as well as the 

induction system, are of concern. The certification plan should identify how 

compliance with these sections will be shown. 

(c) Ice shedding. 

Data showing engine compliance with § 33.77 should be compared between the 

currently installed engine and the proposed engine. If the ice slab used to show 

compliance is smaller for the proposed engine, ice shedding from the airframe 

should be re-addressed. Engine inlet lip ice shedding should be addressed. The 

applicant should compare the amount of ice mass that could be shed by the proposed 

installation to the amount that could be shed by a similar, approved installation. 

Alternately, the amount of ice that could be shed should be compared to Part 33 

engine compliance data. 

(d) Ice protection system (IPS) operation.

The applicant should compare the proposed engine to the existing, certified engine 

for— 

• Bleed air mass flows, 

• Pressures, and 

• Temperatures. 

If there is a reduction in any of these, the effectiveness of the IPS should be 

substantiated. 

c. Propeller Changes. 

Section 25.929 requires a means to prevent or remove hazardous ice accumulation on 

propellers or accessories where ice accumulation would jeopardize engine performance.

The typical analysis report from the deicing boot manufacturer is not sufficient by itself to 

show compliance with § 25.929. The typical report calculates inter-cycle 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 STCs, it would be acceptable 
AC 25-XX

Appendix 4

A4-4 

to show that inter-cycle ice is equal to or less than accretions obtained on the same propeller 

on an airplane flight tested in icing conditions and shown to have no appreciable loss in 

thrust. 

The typical deicing boot manufacturer report also contains a caveat that it does not address 

propeller runback ice. A comparison of similarity to another propeller that was flight tested 

in icing conditions is usually done to address runback. Points of similarity would include 

propeller and deicing boot aerodynamic and thermal similarity, and similarity of the deicing 

timer, propeller RPM, and flight conditions. Note that metal and composite propellers have 

different thermal masses.

As a final qualitative check for both inter-cycle and runback ice on new airplane 

certification programs, airplane performance should be checked during flight tests in icing 

conditions. 

The propeller installation, including spinner and cowl geometry, should be compared to 

previously tested installations in icing conditions. Changes that could allow moisture to 

reach the brush blocks must be avoided. 

If the proposed propeller is calculated to have higher efficiency than the existing, approved 

propeller, the guidance in this appendix titled (1) Effects of increased engine power 

or thrust, under Engine Changes, should be followed. 

d. Engine Inlet or Accessory Inlet Changes. 

Guidance for engine or accessory inlet changes is provided in the Engine Changes

section above. Section 25.1093 applies to engine oil and accessory cooling inlets as well as 

to induction inlets. 

e. Antennas, Installations or Other External Modifications. 

(1) When antennas, cameras, fairings for such installations, or other external installations 

such as drain masts are installed on aircraft, the applicant should show the following: 

 

(a) The predicted ice accretion does not contribute significantly to drag. 

(b) There is no ice-shedding hazard due to impact or ingestion on downstream 

structure, engines or propellers. 

(c) There is no ice-related performance reduction of lifting surfaces. 

 

(d) There is no ice-related effect on downstream air-data sensors or ice detectors. 

(2) The applicant may conduct a very conservative, simple analysis first for (1)(a) and 

(1)(b). If the initial analysis fails, it can be refined to determine if it was overly conservative. 

The conservative analysis can assume the following: 
AC 25-XX

Appendix 4

A4-5 

(a) The water catch area is the full frontal area of the installation.

 

(b) Collection efficiency is one. 

(c) No runback or evaporation of impinging water will occur. 

(d) The ice shapes on blade antennae will be similar to airfoils, and the shapes on 

low profile antennae will be single horn shapes. 

(3) The applicant should determine the critical icing condition, which should include the 

airplane holding in continuous maximum conditions. If the analysis shows a problem, then 

the applicant can accomplish one or more of following: 

(a) Determine realistic collection efficiency either with an ice accretion code or with the 

“FAA Icing Handbook.” 

 

(b) Determine the real impingement limits by using an icing code, which may 

reduce the collection area. 

 

(c) Run the full configuration in an icing code to determine if the installation is in a 

shadow zone. 

(d) If drag is a problem, run an ice accretion code to determine a more realistic ice 

shape. 

(4) The applicant should conduct flight tests in measured natural icing or with simulated ice 

shapes to determine if the ice accretions will have any detrimental effects if any of the 

following conditions exist: 

(a) The installation is upstream of air-data sensors or an ice detector. 

 

(b) The installation is on a lifting surface. 

(c) The installation could create a wake on a lifting surface. As an example, if an 

external modification is large enough (e.g., dish antenna), it may interfere with the 

flow field around the tail and the susceptibility to ice contaminated tailplane stall 

may need to be addressed. 

(5) The one exception to (4)(b) is fairings. For fairings, an analysis to show the impact on 

maximum lift coefficient, in combination with flight tests with no ice accretions, may be 

acceptable. 

f. Gross Weight Increases. 

If increased gross weight causes an evaluation of the airplane's handling qualities in icing 

conditions to be required, the effect on ice accretions from the associated change in 

airplane attitude should be evaluated. Impingement and icing limits on airfoils will 

change, for example, and there now may be accretion sites on the airframe that were 
AC 25-XX

Appendix 4

A4-6 

previously free of ice. 

g. Center of Gravity Envelope Increase. 

Generally, the same guidance used for gross weight increases can be used for center-ofgravity envelope increases. The one exception is that a more forward center-of-gravity 

limit may make an airplane more susceptible to ICTS. For airplanes with unpowered, 

reversible elevators, or for airplanes with propellers, the applicant should address this by 

flight testing. An analysis may be acceptable for other airplanes. 

h. Flight Envelope Increase. 

If the applicant applies for an increase in maximum operating altitude, the applicant should 

demonstrate the following: 

(1) The IPS operating pressures (for pneumatic systems) or temperatures (for thermal 

air) by dry air testing; and

(2) Stall characteristics with ice accretions. 

The effect of increased cruise airspeeds and increased altitudes that could affect windshield 

ice accretion, and adequacy of the windshield heat, should be addressed. 

i. Turboprop Conversion. 

If the ice protection systems 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 lines, resulting in failure of some or all zones of the 

pneumatic system. The applicant must show that the pneumatic deicing system will 

continue to function in icing conditions. 

The pneumatic deicer operating pressure may also decrease at lower engine RPMs. A 

minimum engine RPM for acceptable pneumatic operating pressure, which should allow for 

descent, should be established and published in the airplane flight manual. 

j. Modifications to Lifting and Control Surfaces. 

If lifting and control surfaces are modified, critical ice accretions may have to be re-defined, 

especially if the changes affect operational wing angle of attack. This redefinition includes

consideration of pre-activation, inter-cycle, residual, and runback icing. Stall strips are 

collectors of ice and are an example of an area where leading edge ice accretions should be 

re-defined. If ice accretions change because of modifications, or if the modifications could 

affect control power or hinge moments, AC 25.21-1 provides guidance on flight testing. 

k. Installation of Vortex Generators. 

Depending on their location, vortex generators may accumulate ice. The applicant should 
AC 25-XX

Appendix 4

A4-7 

provide data on expected ice accretions. Flight conditions to consider are hold, descent, and 

approach. Substantiation of the effects on stall speeds, stall characteristics, and stability and 

control should be provided. 

l. Modifications to Ice Protection Systems.

If ice protection systems are modified, critical ice accretions may have to be re-defined. 

This redefinition would include consideration of pre-activation, inter-cycle, residual, and 

runback icing. If the critical levels of ice accretions are changed, or if the modifications to 

the ice protection systems could affect control power or hinge moments, the applicant 

should perform flight testing with simulated ice accretions. AC 25.21-1 provides guidance 

on flight testing.

AC 25-XX

Appendix 5

A5-1 

Appendix 5

CAPABILITIES OF ENGINEERING TOOLS FOR COMPLIANCE WITH APPENDIX O 

REQUIREMENTS OF PART 25

1. Assessment of Engineering Tool Capabilities

This appendix is the result of a 2009 evaluation of engineering tool capabilities for predicting

Appendix O ice accretions and icing effects. This appendix provides guidance on the use of 

such tools based on their state of development in 2009. In the main body of this AC, for 

purposes of plain language, the term supercooled large drop was spelled out. In this appendix, 

however, because the term appears so frequently, its acronym, SLD, will be used. 

Table 1 of this appendix graphically illustrates results of an assessment of the capabilities of 

engineering icing tools. As described in the legend to that table, the capability of each tool for 

the various applications is classified as green, yellow, or orange,. As discussed in the main 

body of this AC, reliance upon available simulation methods combined with engineering 

judgment will be required for finding compliance with the Appendix O requirements of part 25. 

Even though a simulation tool type may be classified as green, an applicant must ensure that the 

specific tool is appropriate for its application and is used in a conservative manner, including 

consideration of critical-case icing conditions. General and specific concerns that should be 

considered are discussed later in this appendix. 

Section 2 of this appendix provides guidance on the tools, their capabilities, and best practices 

for tool use with respect to Appendix O conditions. Section 3 discusses means of compliance 

for specific airplane components. Sections 4-6 discuss means of compliance for each of the 

three certification options given in § 25.1420. Section 7 discusses means of compliance for 

air-data sensors and windshields (§§ 25.1323, 25.1325, and 25.773). Section 8 defines 

standard roughness levels. 

While Table 1 illustrates the current state of simulation capabilities, we expect that capabilities 

will improve in the future. We encourage applicants and research agencies to develop and 

validate the engineering tools currently classified in Table 1 as yellow and orange. 
AC 25-XX

Appendix 5

A5-2 

Table 1

Assessment of Appendix O Engineering Tool Capabilities Wing Tail Radome Thermal (protected area) Thermal (Aft of protected area) Mechanical (protected area) Mechanical (aft of protected area) Fluid Freezing Point Depressant

Instrument (performance)Instrument (performance)

Icing Tunnels * * * *

Codes ** ** **

Tankers

Icing Tunnels * * * *

Codes ** ** **

Tankers

Icing Tunnels *

Codes ** **

Tankers

Icing Tunnels *

Codes ** **

Tankers

LEGEND Updated FEB 2009

*

**

FZRA

MVD < 40μm

Unprotected Areas

Visual Cues

(Reference Surface)

Instrument

(position or

installation effects)

Protected Areas

FZDZ

MVD < 40μm

FZDZ

MVD > 40μm

Instrument

(position or

Non-lifting Surfaces

installation effects)

(antenna, inlets, external 

modifications)

It may be possible to test small-scale installation effects, but large-scale installations are not currently feasible

Current 2-D capabilities exist with large droplet effects, but limitations exist in the use of 3-D codes for simulation of Appendix O effects

This testing capability exists today and is suitable to be an element of a means of compliance, or is readily achievable based on current experience

This testing capability is possible, but has not been demonstrated, or there is limited or no validation.

This testing capability is unknown, or does not currently exist

Detection Methods Air Data Sensors

FZRA

MVD > 40μm

This evaluation of icing test tool capabilities is predominantly based on assessment of National 

Aeronautics and Space Administration (NASA) LEWICE codes (2-D and 3-D), the NASA 

Icing Research Tunnel (IRT) facilities, and existing icing tanker information such as SAE ARP 

5904, Airborne Icing Tankers. International icing simulation codes and facilities were 

considered where information was available. The US Air Force icing tanker that was used for 

SLD investigations during the 1990s on some commercial airplanes is currently not operational 

and its capabilities were not assessed. 

The main body of this AC states that flight testing in natural Appendix O icing conditions 

should not be necessary if at least two methods of predicting Appendix O ice accretions are 
AC 25-XX

Appendix 5

shown to provide similar results. Table 1 illustrates that, for some Appendix O conditions, few 

of the engineering tools are classified as green or yellow for use as a means of compliance.

Radomes, for example, have no engineering tools classified as green. In other cases, such as 

areas aft of protected areas, there is only one simulation tool that is classified as green. 

For testing in freezing rain (FZRA), there are very few engineering tools classified as 

green.This would indicate that the primary method for showing compliance available at this 

time appears to be natural icing flight testing. In some cases, engineering judgment may allow 

other methods of compliance, even though not validated, when used in a fashion similar to that

recommended for freezing drizzle (FZDZ). However, until the engineering tools become more 

mature, flight tests in natural Appendix O icing conditions will likely be necessary to achieve 

the confidence needed for certifying airplanes for unrestricted flight in Appendix O or a portion 

of Appendix O conditions (i.e., compliance with § 25.1420(a)(3) or § 25.1420(a)(2)). 

For airplanes certificated to detect Appendix O icing conditions and safely exit all icing 

conditions (i.e., compliance with § 25.1420(a)(1)), it may be acceptable to use one tool alone, 

even if classified as yellow or orange, without natural icing flight tests, if that tool is used in a 

conservative manner. In cases where there are two green tools available, the applicant should 

use both tools to verify results, as discussed in the main body of this AC.

The number of tests may be reduced through the use of conservative test methods and criticalpoint analyses. However, validation of the test methods should be accomplished until 

confidence is provided 

2. Capabilities & Limitations of Engineering Tools and SLD Measurement 

Instrumentation

Icing Tunnel Appendix O Simulations

A tunnel that uses a single pressure source for spray bars cannot simultaneously produce th

large and small drop distributions (bi-model distributions) defined in Appendix O. 

However, NASA has developed a technique called “sequencing” that alternates large and 

small drop sprays to simulate Appendix O conditions. 

When icing tunnels are used as an element of the means of compliance— 

e 

• Scale effects must be considered for tunnel blockage effects for all tests.

 

• For FZDZ with a median-volume diameter (MVD)<40μm, the cloud drop 

distributions in the IRT are similar to existing Appendix C calibrations. 

• If there are concerns about the bi-modal distribution affecting performance of ice 

protection systems, sequencing should be considered. Sequencing of freezing 

drizzle conditions f has been demonstrated in the IRT for unprotected surfaces. The 

sequencing technique approximates drop distributions found in natural conditions 

A5-3 
AC 25-XX

Appendix 5

A5-4 

(insert reference # for NASA sequencing paper). It results in rougher textures than 

Appendix C ice shapes. 

• Ice shapes in unprotected areas are generally considered critical from the perspective 

of producing the largest disruption to the airflow. With a large ice shape, details on 

the impingement limit characteristics, which may have a less critical effect, may not 

be essential. However, sequencing between large and small drop conditions may be 

necessary if standard sprays2 do not produce the drop distribution appropriate for 

simulation of the conditions desired. In cases where the impingement limit 

characteristics are important, sequencing may be necessary. 

• In general, sequencing produces rougher textures than a standard spray. 

• Sequencing may not be an appropriate way to test thermal systems because the rate of 

mass flow per unit area (i.e. mass flux) of the incoming water is not the same for the 

small and large drop sprays. 

• If sequencing is used to test deicing systems, the ratio of sequencing time to shed 

cycles should be evaluated to ensure that sequencing does not inappropriately affect 

the ice shape. 

Experimental tests of methods for designing subscale hybrid airfoils with full-scale leading 

edges to determine leading edge ice shapes for large-chord airfoil sections has been 

accomplished in icing wind tunnels3,4. These techniques have been used successfully for 

Appendix C icing conditions. Adapting them to icing tests for SLD conditions requires an 

evaluation of the areas of interest for impingement analysis and analysis of the flow-field to 

determine if scale conditions aft of the leading edge can be met considering the compromises 

necessary in design variables on circulation, velocity distribution and impingement 

characteristics. Although we anticipate that the hybrid airfoil design technique would be 

applicable for SLD conditions, such use has not been tested and validated to date. 

Computational Fluid Dynamic Tools

Computational fluid dynamic (CFD) tools—the “codes” referred to in the table—are used 

extensively in certification for Appendix C conditions. CFD tools can provide valuable 

information on impingement limits, icing limits, ice size, ice shape, and ice thickness for 

Appendix O conditions. Some validation of collection efficiency and ice shapes has been 

 

2 The phrase “standard spray” method refers to using the IRT nozzles in off-design conditions to generate larger drops for SLD 

conditions. 

3 Saeed, Farooq, Selig, Michael S. and Bragg, Michael B., "Design of Subscale Airfoils with Full-Scale Leading Edges for Ice 

Accretion Testing," Journal of Aircraft, Vol. 34, No.1, 1997.

4 Saeed, Farooq, Selig, Michael S., Bragg Michael B., and Addy, Harold E. Jr., "Experimental Validation of the Hybrid Airfoil 

Design Procedure for Full-Scale Ice Accretion Simulation,” AIAA Paper No. 98-0199, Reno, NV, January 12-15, 1998.
AC 25-XX

Appendix 5

A5-5 

accomplished for FZDZ; there have been no validation exercises for FZRA.

CFD tools have been devised to predict, using mathematical calculation, ice accretion on an 

airplane in SLD conditions and the behavior of various types of ice protection systems as a 

result of those accretions. Besides being useful for assessing effects on protected surfaces,

these tools can also account for the possibility of SLD ice impingement beyond the ice 

protection system limits, as well as for possible water runback. No current CFD method, 

however, can identify the breakup of water into rivulets, roughness formation, or ice sliding 

that may occur under these circumstances. Thus analysis of the regions behind those that 

are protected by ice protection systems requires some combination of CFD results, empirical 

data and test results (if available), and engineering judgment. This usually consists of 

determining the extent of possible ice formation using some criteria from the computational 

analysis, such as ice extent, impingement limits, or some minimum ice thickness level. The 

resulting ice shape would include intercycle or residual ice on the protected region and ice 

formed aft of the protected region. This result is then combined with guidance on ice 

roughness levels, such as described elsewhere in this document, to produce a rough ice 

region that can be evaluated in wind tunnel testing or flight tests.

Information can be calculated for drop trajectories for evaluating sensor locations and 

potential visual cues. 

Many non-lifting surfaces require testing with 3-D computer codes. At the current time, 

many 3-D codes do not have large drop effects (such as splashing and break-up). Even 

without large drop effects, however, 3-D codes can offer information on impingement 

limits. Although 3-D codes may generate physical models and correlations that can support 

analysis of large drop icing, the capabilities of performing analyses with 3-D SLD CFD 

tools have not yet been evaluated. There are codes that may have this potential, but no 

guidance can be offered at this time for their use.

Some codes have limited capabilities with short-chord geometries (e.g., antennas or struts) 

for Appendix C icing conditions. These limitations are expected to be similar for large drop 

icing and are typically addressed with empirical methods or icing tunnels. However, for 

non-lifting surfaces, conservative assessments may be acceptable, such as assuming drop 

impingement on the full frontal area, assuming the collection efficiency is one, and 

approximating the shapes appropriate for the temperature (glaze, rime, etc.).

Icing Tankers

Icing tankers have been used extensively by some manufacturers for Appendix C icing 

certifications. Icing tankers typically have a limited plume size and have been used 

primarily for localized icing effects, such as ice shedding, and for assessing the thermal 

performance of anti-ice systems. 

Current tankers do have some limited capabilities to produce freezing-drizzle-sized drops, 

but they cannot produce the distribution effects. Current tankers do not produce freezing 

rain distributions either, and the feasibility of producing such conditions is likely limited by 
AC 25-XX

Appendix 5

A5-6 

drop break-up (due to deceleration effects) and the ability to sub-cool the large drops within 

a workable flight envelope. Additionally, drop sorting effects are likely, because of higher 

fall rates of large drops within an Appendix O distribution. 

Instrumentation

When making in-situ measurements of Appendix O conditions, it is important to note that 

technology to make such measurements is rapidly changing. It is essential to consult 

experts in all phases of the measurement program, including those aware of the latest 

problems and strengths of each probe. You should use instrumentation suited to the task. 

Instrumentation must be mounted in appropriate locations on the aircraft, where the 

measurements are not affected by the airflow. For certification purposes, the 

instrumentation must be calibrated. An often-overlooked aspect of a measurement program 

is the need to calibrate the instruments. Appropriate software and analysis techniques are 

also essential, because complicated algorithms are often needed in the analysis.

Instruments are required to measure particle concentrations as a function of size over the 

complete size range—2 μm to at least 1,500 μm—including cloud drops and precipitation 

drops. This may require more than one probe. Liquid water content and ice water content 

(IWC) measurements obtained by integrating 2-D images from spectral measurements 

generally have larger errors than those obtained from probes specifically designed to make 

such measurements. Consequently, it is recommended that probes designed to measure 

LWC and, if necessary, IWC, directly be used, recognizing that some hot-wire devices 

detect larger drops (>50μm) with reduced efficiency. Mixed-phase clouds can occur 

frequently, so it is necessary to be able to discriminate between ice and liquid particles, 

especially for sizes larger than 50μm, so that ice particles are not incorrectly identified as 

SLDs. For detect-and-exit airplanes (those certifying to §25.1420(a)(1)), it may not be 

necessary to measure IWC directly; but for airplanes using natural icing SLD flight tests to 

certify for a portion of Appendix O (§25.1420(a)(2)) or for unrestricted operations 

(§25.1420(a)(3)), IWC must be determined in order to assess the SLD conditions. 

3. Component Evaluations

Lifting Surfaces 

This paragraph is applicable to anti-icing systems aft of protected areas and to deicing 

systems both on and aft of protected areas. 

For detect-and-exit airplanes (§ 25.1420(a)(1)) in freezing drizzle conditions, icing tunnels 

alone may be used to develop ice shapes, provided that the model appropriately represents 

the airfoil beyond the FZDZ icing limit. Roughness may be evaluated in icing tunnel testing 

and replicated on the ice shapes for flight testing. The standard spray2

 method should be 

used for anti-icing systems because of the varying mass flux of incoming water associated 

with sequencing. For deicing systems, it is acceptable to use either the standard spray or the 

sequencing technique.
AC 25-XX

Appendix 5

A5-7 

Radomes

Most radomes are too large to fit into existing icing wind tunnels. Additionally, 

computational analysis of radomes typically would require 3-D codes. Many 3-D codes do 

not have large drop effects and if they do not, freezing drizzle ice shapes cannot be 

simulated. (All 3-D codes have capabilities for testing impingement limits, however.) 

Radome ice shapes have been developed in the past for Appendix C icing conditions using 

analysis and observed ice shapes from Appendix C flight tests (typically holding ice 

shapes). 

For detect-and-exit airplanes (§ 25.1420(a)(1)) in freezing drizzle conditions, one method of 

compliance would be to modify Appendix C ice shapes to account for the larger 

impingement regions produced in FZDZ as predicted by the 3-D codes. You may use CFD 

codes to predict ice thickness. Ice roughness should be in accordance with the section 

headed Standard Roughness Levels for Appendix O Ice Shapes later in this 

appendix. The radome ice tested should reflect the total mass associated with the icing 

exposures for § 25.1420(a)(1) airplanes, which are defined in part II of Appendix O . 

Non-Lifting Surfaces (Antennas, Enhanced Vision Cameras, Struts, Auxiliary 

Inlets)

For non-lifting surfaces that do not require use of 3-D codes, 2-D codes in combination with 

icing tunnels are available as a means of compliance. However, many non-lifting surfaces 

require the use of 3-D codes. At the current time, many 3-D codes do not have large drop 

effects, although some codes may have this potential. 

For detect-and-exit airplanes (§ 25.1420(a)(1)), if the non-lifting surface is not critical from 

an engine ingestion or airframe damage perspective, 3-D codes may provide sufficient 

information for compliance. However, for more critical surfaces, until large drop effects in 

3-D codes are validated, icing tunnels alone may be used to develop ice shapes. 

Ice Detection Methods

Different types of ice detection require assessment of their capabilities in large drop 

conditions, based upon their sensing technology. Vibrating probe type ice detectors which 

detect ice accretion on a probe through a decrease in the probe’s vibration frequency, may 

experience increased response time in large drop exposures because splashing and 

aerodynamic forces, particularly in near freezing temperatures, can cause water to shed from

sensing surfaces. This physical behavior may also occur with other types of probes. 

While CFD can determine whether the large drops impact the ice detection surface, 

available CFD codes cannot accurately predict aerodynamic forces that cause drop 

shedding, or freezing fraction effects which may delay freezing. Therefore, use of CFD 

alone is not acceptable for showing that ice detectors function in large drop conditions. 

When possible, effects of installation position should be evaluated with a combination of 

codes and icing tunnels. Devices mounted on smaller surfaces could be assessed in an icing 
AC 25-XX

Appendix 5

A5-8 

tunnel. However, if the device is mounted on the fuselage, and tunnel blockage effects 

would preclude a meaningful icing tunnel test, then CFD codes that adequately predict the 

shadowing and concentration effects may be used to verify that the equipment is properly 

located.

Because of the lack of engineering tools for FZRA, primary and advisory ice detectors used 

for compliance with § 25.1420(c) will require validation in natural large drop conditions to 

substantiate that the detectors function in all Appendix O conditions, unless ground testing 

with both FZDZ and FZRA drops representative of Appendix O distributions and 

temperatures can be substantiated. However, if visual cues are certificated as the primary 

means of compliance with § 25.1420(a)(1)(i) and the airplane is equipped with an ice 

detector system that is not required for compliance with § 25.1420(a)(1)(i) or § 25.1420(c), 

then the ice detector need not be tested in natural Appendix O conditions. Certification of 

visual cues for detect-and-exit airplanes is discussed later in this appendix under the heading 

Certification for Detect and Exit - § 25.1420(a)(1). 

 

Air Data Sensors

Air Data Sensor Position Installation Effects 

When possible, you should evaluate the effects of installation position with a 

combination of CFD codes and icing tunnels. Devices mounted on smaller surfaces 

could be assessed in an icing tunnel. However, if the device is mounted on the fuselage 

and tunnel blockage effects would preclude a meaningful icing tunnel test, then codes 

that adequately predict the shadowing and concentration effects are acceptable as the 

only method for compliance with installation location requirements.

Air Data Sensor Performance Effects 

Icing tunnels alone may be used to determine the performance of air-data sensors in

Appendix O icing conditions in compliance with §§ 25.1323, 25.1324, and 25.1325. 

For sensors with collection efficiencies approaching "1," if performance has been shown 

in FZDZ conditions, then a qualitative analysis based upon water-catch ratios may be 

used for extrapolation to FZRA conditions. 

If the Appendix C or mixed-phase or ice crystal conditions are shown to be more critical 

for air-data sensors than the SLD environment, the number of tests may be reduced. 

However, the test methods should be validated until confidence is provided. In some 

cases, such as that of wing leading-edge-mounted lift transducers, icing tunnel tests may 

still be necessary.
AC 25-XX

Appendix 5

A5-9 

4. Certification for Detect and Exit - §25.1420(a)(1)

Detect and Exit Freezing Drizzle (FZDZ)<40μm

FZDZ conditions with an MVD less than 40μm are similar to existing Appendix C 

distributions with the exception of a small percentage of the mass in drops larger than typical 

Appendix C conditions. As a result, many of the current Appendix C simulation methods 

can support compliance. The small percentage of large drops in this distribution can affect 

the impingement limits and increase the water catch.

If visual cues are used for compliance with § 25.1420(a)(1)(i), it may be possible to use 

codes in combination with icing tunnels to verify the visual cues. Visual cues should not be 

based on only one engineering method; a second, correlating method should be used. 

Detect and Exit FZDZ>40μm 

Where the capabilities of the tools available for FZDZ>40μm are the same as for 

FZDZ<40μm, the applicant may use similar means of compliance that are adjusted for the 

FZDZ>40μm icing conditions. The tool capabilities are different for mechanical deicing 

system protected areas and areas aft of the protected areas. 

Detect and Exit Freezing Rain (FZRA) (MVD<40μm & MVD>40μm)

The capabilities of the tools for FZRA are limited. To simulate accretions on unprotected 

surfaces and aft of protected areas, CFD codes may be used to determine the difference in 

impingement region between FZRA and FZDZ. The increase in impingement area can then 

be simulated using a standard roughness in that region. For areas where a ridge of ice may 

form, a simulated ridge may be developed using a height developed analytically based upon 

local water catch. Ridge location could be developed from FZDZ tunnel tests, and the 

height would be modified based upon the ratio of local water catch (determined with CFD) 

between FZDZ and FZRA. 

Note: Icing tunnels are classified as yellow when used for testing for FZDZ >40μm

because use of the tunnels appears feasible but has not been demonstrated. Icing tunnel 

tests alone are acceptable for the development of ice shapes for deicing system protected 

areas and areas aft of the mechanically protected areas. Sequencing or standard 

distributions are acceptable, but the ratio of sequencing time to shed cycles should be 

examined.
AC 25-XX

Appendix 5

A5-10 

Other concerns: 

• Thermal Ice Protection Systems—Analyses to assess water catch and

melting/evaporation rates are acceptable for determining the capabilities of thermal 

systems in FZRA, provided that the analyses are based upon the validated results of 

the system capabilities performed for Appendix C and FZDZ. Any additional ice 

that may form on runback ice shapes in freezing rain should be accounted for by 

analysis of the runback volume. Potential roughness effects ahead of the runback 

should be addressed. 

• Mechanical Ice Protection Systems — For assessing mechanical ice protection 

system performance in FZRA, it is acceptable to use the same pre-activation and 

intercycle and residual ice shapes as for FZDZ. The limits of accretion should be 

determined using CFD tools.

 

• Validation of Visual Cues — Use of qualitative analysis is acceptable for assessing 

whether visual cues used for FZDZ will function in FZRA conditions. 

5. Certification for a Portion of Appendix O - § 25.1420(a)(2)

Current technology does not support distinguishing between FZDZ and FZRA in flight. Because 

of this, airplanes should not be certificated for compliance with § 25.1420(a)(2) based upon the 

boundaries between FZDZ and FZRA. Certification to § 25.1420(a)(2) is discussed in the main 

body of this AC; however, there are concerns about the ability of applicants to define ice shapes 

which distinguish between the approved portions and the unapproved portions of Appendix O 

with the current simulation tools. As a result, certification for a portion of Appendix O will be 

challenging and will require close coordination with certifying authorities.

Certification for a portion of Appendix O that considers phase of flight (e.g., takeoff, holding), as 

discussed in the body of this AC in paragraph 8(b), may be feasible. Any method of certification 

for a portion of Appendix O should be included as part of the certification planning and will 

require approval from the cognizant certifying authority. 

In cases where only one engineering tool or none have been validated as capable of simulating 

FZDZ or FZRA, the means of compliance should include flight tests in measured Appendix O 

icing conditions to verify ice accretions. 

6. Certification for Unrestricted Operations - § 25.1420(a)(3)

The use of simulation tools as described for detect-and-exit airplanes is also acceptable for 

showing compliance for airplanes certificated in accordance with § 25.1420(a)(3). However, in 

cases where only one engineering tool (or none) has been validated as capable of simulating 

FZDZ or FZRA, the means of compliance should include flight tests in measured Appendix O 
AC 25-XX

Appendix 5

A5-11 

icing conditions to verify ice accretions. 

7. Compliance with §§ 25.1323, 25.1324, 25.1325, and 25.773

Compliance with §§ 25.1323, 15.1324, 25.1325 and 25.773 for Appendix C and 

Appendix O Conditions 

Analysis of exposures to Appendix C and Appendix O conditions should consider holding 

operations consistent with the applicable “Holding Ice” definition contained in Part II of 

those appendices. 

Compliance with §§ 25.1323 and 25.1324 for Mixed-Phase and Ice Crystal 

Conditions 

Analysis of exposures to mixed-phase and ice crystal conditions should consider the 

horizontal extents defined in the rules. 

8. Standard Roughness Levels for Appendix O Ice Shapes

Ice shapes for Subpart B of part 25 testing are typically based upon icing tunnel tests or CFD 

computations or both. Current CFD programs do not provide roughness information. 

Roughness levels and aft extent of roughness for Appendix O ice shapes should be determined 

from icing tunnels or tanker testing, if the capabilities exist. However, when the empirical 

capabilities do not exist, the roughness levels (or equivalents) as defined in Table 2, below, may 

be used. Note that roughness levels for Appendix C ice shapes are discussed in AC 25-25X. 
AC 25-XX

Appendix 5

A5-12 

Table 2

Appendix O Icing Standard Roughness

Ice Type Roughness Height (mm) Percent of 

Surface 

Covered

Notes

FZDZ & FZRA, thin 

accretions;

thickness < 3mm (0.12 

inch)

1.5 to 2 mm (0.06 to 0.08 inch)

or

16 to 20 grit sandpaper

Particle density

to cover 50% to 

70% of total 

area

Use to simulate 

pre-detection, initial 

accretions, or roughness on 

computed ice shapes

FZDZ

thickness > 3 mm (0.12 

inch) 3 to 6 mm (0.12 to 0.24 inch)

Mean particle size ~4.5mm

Intercycle, residual, 

unprotected surfaces

FZRA

thickness ≥ 3 mm (0.12 

inch) and ≤ 6 mm (0.24 

inch)

FZRA

thickness ≥ 6 mm (0.24 

inch)

6 to 8 mm (0.24 to 0.31 inch)

Mean particle size ~7 mm

Notes:

1. The simulated roughness elements should approximate roughness elements observed in icing tunnels 

or natural icing. Smooth and spherical elements should not be used because they may result in nonconservative aerodynamic results.

2. For computed ice shapes, the roughness simulation should be extended aft to the limits of predicted 

accretion (where the ice accretion thickness is calculated as 0.015").