Advisory

Circular

Subject: PERFORMANCE AND 

HANDLING CHARACTERISTICS IN 

ICING CONDITIONS 

Date: DRAFT

Initiated By: ANM-110 

AC No: 25-25X

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

compliance with the performance and handling requirements for flight in the icing conditions 

defined in Appendices C and O of Title 14, Code of Federal Regulations (14 CFR) part 25. Part 

25 contains the airworthiness standards for transport category airplanes. 

2. APPLICABILITY. 

a. The guidance provided in this document is directed towards airplane manufacturers, 

modifiers, foreign regulatory authorities, and Federal Aviation Administration (FAA) 

transport airplane type certification engineers, flight test pilots, and their designees. 

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

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

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

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

not mandatory, they are derived from extensive FAA and industry experience in determining 

compliance with the relevant regulations. 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. 

3. Cancellation. Advisory Circular 25-25 Change 1, Performance and Handling 

Characteristics in the Icing Conditions Specified in Part 25, Appendix C, dated xxxx is

cancelled.

Ali Bahrami

Manager, Transport Airplane Directorate

Aircraft Certification Service

7/30/08 AC 25-25X

ii

CONTENTS

Paragraph Page

1. REQUIREMENTS AND GUIDANCE. 1

a. Overview.

b. Proof of Compliance, § 25.21(g).

c. Propeller Speed and Pitch Limits, § 25.33.

d. Performance – General, § 25.101.

e. Stall Speed, § 25.103.

f. Failure Conditions, § 25.1309.

g. Flight-Related Systems.

h. Airplane Flight Manual, § 25.1581 through § 25.1587.

2. ACCEPTABLE MEANS OF COMPLIANCE—GENERAL. 11

a. Overview.

b. Flight Testing.

c. Wind Tunnel Testing and Analysis.

d. Engineering Simulator Testing and Analysis.

e. Engineering Analysis.

f. Ancestor Airplane Analysis.

3. ACCEPTABLE MEANS OF COMPLIANCE—FLIGHT TEST

PROGRAM.

15

a. Overview.

b. Stall Speed, § 25.103.

c. Accelerate-Stop Distance, § 25.109.

d. Takeoff Path, § 25.111.

e. Landing Climb: All-Engines-Operating, § 25.119.

f. Climb: One-Engine-Inoperative, § 25.121.

g. En Route Flight Paths, § 25.123.

h. Landing, § 25.125.

i. Controllability and Maneuverability – General, § 25.143.

j. Longitudinal Control, § 25.145.

k. Directional and Lateral Control, § 25.147.

l. Trim, § 25.161

m. Stability – General, § 25.171.

n. Demonstration of Static Longitudinal Stability, § 25.175.

o. Static Lateral – Directional Stability, § 25.177.

p. Dynamic Stability, § 25.181.

q. Stall Demonstration (§ 25.201)/Stall Characteristics,

§ 25.203.

r. Stall Warning, § 25.207.

s. Wind Velocities, § 25.237.
7/30/08 AC 25-25X

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t. Vibration and Buffeting, § 25.251.

u. Natural Icing Conditions, § 25.1419(b).

v. Failure Conditions, § 25.1309.

APPENDIX 1 

AIRFRAME ICE ACCRETIONS

1. OVERVIEW. A1-1

2. ICE ACCRETIONS WITH AN OPERATIVE ICE PROTECTION 

SYSTEM.

A1-3

a. All Flight Phases Except Takeoff.

b. Takeoff Phase.

c. Ice Accretion Before Activation and Effective System 

Operation.

3. ICE ACCRETED DURING ICE PROTECTION SYSTEM 

FAILURE.

A1-6

a. Unprotected Parts.

b. Protected Parts Following System Failure.

4. ADDITIONAL GUIDANCE FOR APPENDIX O ICE 

ACCRETIONS.

A1-7

a. Ice Accretion in Appendix O Conditions Before those 

Conditions have been Detected by the Flightcrew and 

Actions Taken, in Accordance with the AFM, to Either Exit 

All Icing Conditions or Continue Flight in Appendix O Icing 

Conditions.

b. Ice Accretions for Encounters with Appendix O Icing 

Conditions Beyond those for which the Airplane is Certified 

to Operate.

c. Ice Accretions for Encounters with Appendix O

Atmospheric Icing Conditions in Which the Airplane is 

Certified to Operate.

APPENDIX 2 

SIMULATED ICE ACCRETIONS

1. OVERVIEW. A2-1

2. SHAPE AND TEXTURE OF SIMULATED ICE ACCRETION. A2-1

3. SANDPAPER ICE. A2-2
7/30/08 AC 25-25X

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APPENDIX 3 

DESIGN FEATURES

1. AIRPLANE CONFIGURATION AND ANCESTRY. A3-1

a. Size.

b. Ancestors.

2. WING. A3-1

a. Airfoil Type.

b. Leading Edge Devices.

c. Lateral Control System Effectiveness.

3. EMPENNAGE. A3-2

a. Tailplane Design Philosophy.

b. Horizontal Stabilizer Design.

c. Control Surface Actuation.

d. Control Surface Size.

e. Vertical Stabilizer and Rudder Design.

4. AERODYNAMIC BALANCING OF FLIGHT CONTROL 

SURFACES.

A3-2

5. ICE PROTECTION/DETECTION SYSTEM. A3-3

a. Wing Ice Protection/Detection.

b. Tail Ice Protection/Detection.

APPENDIX 4 

EXAMPLES OF AIRPLANE FLIGHT MANUAL LIMITATIONS 

AND OPERATING PROCEDURES FOR OPERATIONS IN 

SUPERCOOLED LARGE DROP ICING CONDITIONS

1. AIRPLANE APPROVED FOR FLIGHT IN APPENDIX C ICING 

CONDITIONS BUT NOT APPROVED FOR FLIGHT IN 

APPENDIX O ICING CONDITIONS.

A4-1

a. AFM Limitations.

b. AFM Operating Procedures (Normal Procedures Section).

c. Flightcrew Operating Manual Operating Procedures.

2. AIRPLANE APPROVED FOR FLIGHT IN APPENDIX C ICING 

CONDITIONS AND FREEZING DRIZZLE CONDITIONS OF 

APPENDIX O BUT NOT APPROVED FOR FLIGHT IN 

FREEZING RAIN CONDITIONS OF APPENDIX O

A4-2

a. AFM Limitations.

b. AFM Operating Procedures (Normal Procedures Section).
7/30/08 AC 25-25X

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c. Flightcrew Operating Manual Operating Procedures.

3. AIRPLANE APPROVED FOR FLIGHT IN APPENDIX C AND 

APPENDIX O ICING CONDITIONS EXCEPT FOR EN ROUTE 

AND HOLDING FLIGHT PHASES IN APPENDIX O ICING 

CONDITIONS.

A4-3

a. AFM Limitations.

b. AFM Operating Procedures (Normal Procedures Section).

c. Flightcrew Operating Manual Operating Procedures.

4. AIRPLANE APPROVED FOR FLIGHT IN APPENDIX C ICING 

CONDITIONS AND A PORTION OF APPENDIX O ICING 

CONDITIONS.

A4-4

a. AFM Limitations.

b. AFM Operating Procedures (Normal Procedures Section).

c. Flightcrew Operating Manual Operating Procedures.

APPENDIX 5 

RELATED REGULATIONS AND

ADVISORY CIRCULARS

Regulations.

Advisory Circulars.

APPENDIX 6 

ACRONYMS AND DEFINITIONS

TABLES

1. Means for Showing Compliance 12

2. Controllability & Maneuverability—Trim Speeds 20

3. Holding Scenario—Maneuvers 36

4. Approach/Landing Scenario—Maneuvers 37
7/30/08 AC 25-25X

1

1. REQUIREMENTS AND GUIDANCE.

a. Overview. 

(1) For certification for flight in the icing conditions described in Appendix C of part 

25, § 25.21(g)(1) requires that an airplane meet certain performance and handling 

qualities requirements while operating in the icing environment defined in Appendix C. 

In addition, § 25.1420 requires applicants to consider icing conditions beyond those 

covered by Appendix C for airplanes with a maximum takeoff weight less than 60,000 

pounds or with reversible flight controls. The additional icing conditions that must be 

considered for these airplanes are the supercooled large drop icing conditions defined in 

Appendix O. Appendix 1 of this AC provides detailed guidance for determining ice 

accretions in both Appendix C and Appendix O icing conditions that can be used for 

showing compliance. 

(2) For airplanes with a maximum takeoff weight less than 60,000 pounds or with 

reversible flight controls to be certified for flight in icing conditions, § 25.1420 requires

applicants to choose to do one of the following: 

(a) Not seek approval for flight in the supercooled large drop atmospheric icing 

conditions defined in Appendix O.

(b) Seek approval for flight in only a portion of Appendix O icing conditions.

(c) Seek approval for flight throughout the entire Appendix O atmospheric icing 

envelope. 

(3) For airplanes with a maximum takeoff weight equal to or greater than 60,000 

pounds that do not have reversible flight controls, applicants are not required to consider 

the icing conditions of Appendix O. For these airplanes, applicants may disregard any 

compliance guidance provided in this AC related to Appendix O icing conditions.

(4) Because an airplane may encounter supercooled large drop icing conditions at any 

time while flying in icing conditions, certain safety requirements must be met for the 

supercooled large drop icing conditions of Appendix O even if the airplane will not be 

certified for flight in the complete range of Appendix O atmospheric icing conditions. 

Section 25.21(g)(2) requires the stall speed (§ 25.103), landing climb (§ 25.119), and 

landing (§ 25.125) requirements to be met in supercooled large drop atmospheric icing 

conditions beyond those the airplane will be certified for. Compliance with these 

requirements plus the requirements for flight in Appendix C icing conditions are 

intended to provide adequate performance capability for a safe exit from all icing 

conditions after an encounter with supercooled large drop atmospheric icing conditions 

beyond those the airplane is certified for. 
7/30/08 AC 25-25X

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(5) If the airplane is not to be certified for flight in all of the supercooled large drop

icing conditions of Appendix O, there must be a means of indicating when the airplane 

has encountered icing conditions beyond those it is certified for. See AC 25.1419/1420

for guidance on acceptable means of detecting and indicating when the airplane has 

encountered icing conditions beyond those it is certified for. The applicant should 

provide procedures in the airplane flight manual to enable a safe exit from all icing 

conditions after an encounter with icing conditions beyond those the airplane is certified

for.

(6) To certify an airplane for operations in Appendix O icing conditions only for certain 

flight phase(s), the applicant should define the flight phase(s) for which approval is 

sought in a way that will allow a flightcrew to easily determine whether the airplane is 

operating inside or outside its certified icing envelope. The critical ice accretion or 

accretions used to show compliance with the applicable requirements should cover the 

range of airplane configurations, operating speeds, angles-of-attack, and engine thrust or 

power settings that may be encountered during that phase of flight (not just at the 

conditions specified in the part 25 subpart B requirements). For the ice accretion 

scenarios defined in paragraph 4c(3) of Appendix 1 to this AC, the applicable flight

phases are take-off (including the ground roll, take-off, and final take-off segments), en

route, holding, and approach/landing (including both the approach and landing 

segments).

(7) Ice accretions used to show compliance with the applicable subpart B regulations 

should be consistent with the extent of the desired certification for flight in icing 

conditions. Appendices C and O define the ice accretions, as a function of flight phase, 

that must be considered for certification approval for flight in those icing conditions. 

Any of the applicable ice accretions (or a composite accretion representing a 

combination of accretions) may be used to show compliance with a particular subpart B

requirement if it is either the ice accretion identified in the requirement or one shown to 

be more conservative than that. In addition, the ice accretion with the most adverse 

effect on handling characteristics may be used for compliance with the airplane

performance requirements if each difference in performance is conservatively taken into 

account. Ice accretion(s) used to show compliance should take into account the speeds, 

configurations (including configuration changes), angles of attack, power or thrust 

settings, etc. for the flight phases and icing conditions they are intended to cover. For 

example, if the applicant desires certification for flight in the supercooled large drop

icing conditions of Appendix O in addition to those of Appendix C, compliance with the 

applicable subpart B requirements may be shown using the most critical of the Appendix 

C and Appendix O ice accretions.

(8) Certification experience has shown that it is usually unnecessary to consider ice 

accumulation on the propeller, induction system, or engine components of an 

inoperative engine when demonstrating airplane handling qualities. Similarly, for ice 

accreted anywhere on the airplane, the mass of any accreted ice need not normally be 

considered. 
7/30/08 AC 25-25X

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(9) Requirements and guidance for flight in icing conditions also apply to operation of 

the airplane after leaving icing conditions until it is determined that no ice accretions 

remain on the airplane’s critical surfaces. 

(10) Ice-contaminated tailplane stall (ICTS) refers to a phenomenon identified as a 

causal factor in several airplane incidents and accidents. It results from airflow 

separation on the lower surface of the tailplane because ice is present. ICTS can occur if 

the angle-of-attack of the horizontal tailplane exceeds its stall angle-of-attack. Even 

very small quantities of ice on the tailplane leading edge can significantly reduce the 

angle-of-attack at which the tailplane stalls. An increase in tailplane angle-of-attack, 

which may lead to a tailplane stall, can result from changes in airplane configuration (for 

example, extending flaps, which increases the downwash angle at the tail or the pitch 

trim required) or flight conditions (a high approach speed, gusts, or maneuvering, for 

example). An ICTS is characterized by reduction or loss of pitch control or pitch 

stability while in, or soon after leaving, icing conditions. A flight test procedure for 

determining susceptibility to ICTS is presented in paragraph 3i(4), Low g Maneuvers 

and Sideslips, of this AC. 

(a) For airplanes with unpowered longitudinal control systems, the pressure 

differential between the upper and lower surfaces of the stalled tailplane may result 

in a high elevator hinge moment, forcing the elevator trailing edge down. This 

elevator hinge moment reversal can be of sufficient magnitude to cause the 

longitudinal control (for example, the control column) to suddenly move forward 

with a force beyond the capability of the flightcrew to overcome. On some 

airplanes, ICTS has been caused by a lateral flow component coming off the 

vertical stabilizer, as may occur in sideslip conditions or because of a wind gust 

with a lateral component. 

(b) Aerodynamic effects of reduced tailplane lift should be considered for all 

airplanes, including those with powered controls. Airplanes susceptible to this 

phenomenon are those having a near zero or negative tailplane stall margin with 

tailplane ice contamination. 

(11) There have been airplane controllability incidents in icing conditions as a result of

ice on unprotected leading edges of extended trailing edge flaps or flap vanes. The 

primary safety concern illustrated by these incidents is the potential for controllability 

problems due to the accretion of ice on trailing edge flap or flap vane leading edges 

while extending flaps in icing conditions. The flight tests specified in Table 4 of this 

AC, in which handling characteristics are tested at each flap position while ice is being 

accreted in natural icing conditions, are intended to investigate this safety concern. 

Unless controllability concerns arise from these tests, it is not necessary to conduct flight 

tests with simulated ice accretions on the extended trailing edge flap or flap vanes or to 

include extended trailing edge flap or flap vane ice accretions when evaluating airplane 

performance with flaps extended.
7/30/08 AC 25-25X

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(12) Supercooled large drop icing conditions, or runback ice in any icing condition, can 

cause a ridge of ice to form aft of the protected area on the upper surface of the wing. 

This can lead to separated airflow over the aileron. Ice-induced airflow separation

upstream of the aileron can have a significant effect on aileron hinge moment. 

Depending on the extent of the separated flow and the design of the flight control 

system, ice accretion upstream of the aileron may lead to aileron hinge moment reversal, 

reduced aileron effectiveness, and aileron control reversal. Although airplanes with deicing boots and unpowered aileron controls are most susceptible to this problem, all 

airplanes should be evaluated for roll control capability in icing conditions. Acceptable 

flight test procedures for checking roll control capability are presented in paragraphs 

3i(3), 3o, and 3q(2)(e) of this AC and consist of bank-to-bank roll maneuvers, steady 

heading sideslips, and rolling maneuvers at stall warning speed.

(13) Appendix 5 contains related regulations and advisory circulars. Appendix 6

contains acronyms and definitions used in this AC.

b. Proof of Compliance, § 25.21(g).

(1) You may show compliance with certification requirements for flight in icing 

conditions by any of the means discussed in section 2, Acceptable Means of 

Compliance – General, paragraph a, Overview, of this AC.

(2) Certification experience has shown that, for airplanes of conventional design

applying for flight in icing conditions, it is usually not necessary to perform additional 

detailed substantiation of compliance with requirements of the following sections of part 

25:

§ 25.23 Load distribution limits

§ 25.25 Weight limits

§ 25.27 Center of gravity limits

§ 25.29 Empty weight and corresponding center of gravity

§ 25.31 Removable ballast

§ 25.231 Longitudinal stability and control

§ 25.233 Directional stability and control

§ 25.235 Taxiing condition

§ 25.253(a) and (b) High-speed characteristics
7/30/08 AC 25-25X

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§ 25.255 Out-of-trim characteristics

(3) If different stall warning system or stall identification system activation settings are 

used for flight in icing conditions (for example, if the stall warning or stall identification 

system activation settings are changed when the ice protection system is activated), it is 

acceptable to return to the non-icing settings when the critical wing surfaces are free of 

ice. The applicant should validate that the means for determining when the critical wing 

surfaces are free of ice accretions is reliable under all expected operating conditions.

c. Propeller Speed and Pitch Limits, § 25.33. Certification experience in meeting 

§ 25.33 has shown that it may be necessary to impose additional propeller speed limits for 

operations in icing conditions. 

d. Performance – General, § 25.101.

(1) This regulation states that the propulsive power or thrust available for each flight 

condition must be appropriate to the airplane operating limitations and normal 

procedures for flight in icing conditions. 

(2) In general, propulsive power or thrust available may be determined by suitable 

analysis, substantiated when required by appropriate flight tests. An example would be

when determining the power or thrust available for showing compliance with § 25.119. 

The following aspects should be considered:

(a) Operation of induction system ice protection. 

(b) Operation of propeller ice protection. 

(c) Operation of engine ice protection.

(d) Operation of airframe ice protection. 

(3) The applicant should consider the following when determining the change in 

performance due to flight in icing conditions:

(a) Thrust loss due to ice accretion on propulsion system components, including 

the engine induction system and other engine components, and the propeller spinner 

and blades, with normal operation of the ice protection system.

(b) Incremental airframe drag due to ice accretion with normal operation of the ice 

protection system.
7/30/08 AC 25-25X

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(c) Changes in operating speeds provided in the airplane flight manual (AFM) for 

flight in icing conditions. 

(4) Certification experience has shown that a suitable conservative analysis or flight test 

can determine any increase in drag (or decrease in thrust) resulting from ice 

accumulation on the landing gear, propeller, induction system, and engine components. 

Certification experience has also shown that runback ice may be critical for propellers, 

so runback ice on the propeller should be addressed. Research has shown that ice 

accretions on propellers, and resulting thrust loss, may be larger in Appendix O

(supercooled large drop) icing conditions than in Appendix C icing conditions for some 

designs. This may call for airplane performance checks in natural icing conditions, icing 

tanker tests, icing wind tunnel tests, or the use of an assumed (conservative) loss in 

propeller efficiency. Testing should include a range of outside air temperatures, 

including warmer (near freezing) temperatures that could result in runback icing.

(5) Apart from using appropriate speed adjustments to account for operation in icing 

conditions, all changes in the procedures established for takeoff or go-around should be 

agreed to by the responsible aircraft certification office (ACO). 

e. Stall Speed, § 25.103. Certification experience in meeting this requirement has 

shown that, for airplanes of conventional design, it is not necessary to separately determine 

the effects of Mach number on stall speeds for the airplane with ice accretions. 

f. Failure Conditions, § 25.1309. 

(1) The applicant should analyze failure modes of the ice protection system and 

resulting effects on airplane handling and performance in accordance with § 25.1309. In 

determining the probability of a failure condition, the applicant should assume that the 

probability of entering the icing conditions defined in part 25 Appendix C is one. As 

explained in AC 25.1419/1420 , on an annual basis, the average probability of 

encountering the icing conditions defined in Appendix O may be assumed to be 1 X 10-2

per flight hour. This probability should not be reduced on a phase-of-flight basis. The 

ice accretion to use for failures of the ice protection system is defined in Appendix 1, 

paragraph 3 of this AC. 

NOTE: This guidance is not intended to apply to failures of other 

systems (or the engines) that may indirectly affect airplane handling 

and performance in icing conditions due to any effects of those 

failures on an otherwise normally functioning ice protection system. 

In such situations, the ice protection system itself is considered to be 

operating normally, although performance of its anti-ice or deice 

function may be degraded.
7/30/08 AC 25-25X

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(2) For probable failure conditions of the ice protection system that are: 

• Not annunciated to the flightcrew, or 

• Annunciated to the flightcrew, but the associated procedure does not require 

exiting the icing conditions, 

the guidance in this AC for a normal (that is, non-failure) condition is applicable with 

the failure ice accretion.

(3) For probable ice protection failure conditions annunciated to the flightcrew, when

the associated operating procedure requires the airplane to leave the icing conditions as 

soon as practicable, the following applies: The applicant should show that, with the ice 

accretion resulting from the failure, the airplane’s performance and handling 

characteristics are adequate as determined by a system safety analysis in accordance with 

§ 25.1309. The operating procedures and related speeds may restrict the airplane’s 

operating envelope, but the size of the restricted envelope should be consistent with the 

safety analysis.

(4) For failure conditions that are improbable but not extremely improbable, the 

analysis and substantiation of continued safe flight and landing required by § 25.1309 

should take into consideration whether the failure is annunciated, and what associated 

operating procedures and speeds would be used following the failure condition. 

g. Flight-Related Systems. The applicant should consider flight-related systems 

aspects when determining compliance with the subpart B flight requirements for flight in 

icing conditions. For example, the following considerations may be relevant.

(1) Ice protection systems may not adequately perform their anti-ice or deice functions 

at some engine power or thrust settings. This may result in a need to establish a 

minimum useable power or thrust setting for operation in icing conditions, and this can 

affect descent or approach capabilities. The applicant should also consider the effect of 

power or thrust settings in determining the applicable ice accretions. For example, at 

low engine power or thrust, a thermal bleed air system may not be able to evaporate the 

liquid water completely after melting the ice, resulting in the potential for runback ice.

(2) Ice may block control surface gaps, or freeze seals, causing increased control forces, 

control restrictions, or blockage.

(3) Ice may block unprotected inlets and vents. This may affect flight controls or the 

propulsive thrust available, as well as powerplant control and cooling and aerodynamic 

drag. 
7/30/08 AC 25-25X

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(4) Airspeed, altitude, or angle-of-attack sensing errors may occur due to ice accretion 

forward of the sensors (for example, radome ice). Control feel systems that use dynamic 

pressure inputs may also be affected.

(5) There may be an effect on operation of stall warning and stall identification reset 

features for flight in icing conditions, including effects of failure to operate.

(6) Operation of icing condition sensors, ice accretion sensors, and automatic or manual 

activation of ice protection systems may be affected.

(7) Flight guidance and automatic flight control systems operation may be affected. See 

AC 25.1329-1B, Approval of Flight Guidance Systems, for guidance on compliance 

with § 25.1329 for flight in icing conditions, including stall and maneuverability

evaluations with the airplane under flight guidance system control.

(8) There may be an effect on installed thrust. Consideration of installed thrust includes 

consideration of ice protection systems operation when establishing acceptable power or 

thrust setting procedures, and when evaluating control, stability, temperature lapse rates, 

rotor speed margins, temperature margins, automatic takeoff thrust control system 

(ATTCS) operation, and power or thrust lever angle functions. 

h. Airplane Flight Manual, § 25.1581 through § 25.1587. 

(1) Section 25.1581 states that an airplane flight manual (AFM) must be furnished with 

each airplane and must contain information necessary for safe operation because of 

design, operating, or handling characteristics.

(2) To comply with this requirement, the AFM Limitations section should include the 

following: 

(a) The limitations required to ensure safe operation in icing conditions.

(b) Performance limitations. These should be presented for flight in icing and

reflect all effects on lift, drag, thrust, and operating speeds related to operating in 

icing conditions. These limitations may be presented in the AFM performance

section and included as limitations by specific reference in the AFM Limitations 

section.

(c) All airspeed limitations associated with flight in icing conditions, such as the 

minimum airspeed for each normal airplane configuration in icing conditions.

(d) As applicable, a statement similar to the following: 
7/30/08 AC 25-25X

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In icing conditions, this airplane must be operated and its ice 

protection systems used as described in the operating 

procedures section of this manual. Where specific operational 

speeds and performance information have been established for 

such conditions, this information must be used. 

(e) For turbojet airplanes without leading edge high-lift devices, unless the 

applicant shows that the airplane retains adequate stall and stall warning margins 

during takeoff with residual ice contamination, or that such contamination would be 

otherwise detected and removed before takeoff, statements similar to the following: 

Takeoff may not be initiated unless the flightcrew verifies that 

a visual and tactile (hands on surface) check of the wing 

upper surfaces and leading edges has been accomplished, and 

the wing has been found to be free of frost, ice, or snow in 

conditions conducive to ice/frost/snow formation. Conditions 

conducive to ice/frost/snow formation exist whenever the 

outside air temperature is below 6 degrees C (42 degrees F) 

and any of the following applies: 

• Visible moisture is present in the air or on the wing,

• The difference between the dew point temperature 

and the outside air temperature is less than 3 degrees 

C (5 degrees F), or

• Standing water, slush, ice, or snow is present on 

taxiways or runways.

1 Residual ice contamination is contamination that is difficult to detect 

through visual observation alone. If deicing is performed before takeoff, 

residual ice contamination is the contamination that may remain after deicing,

or, the contamination that may form after deicing. For the purpose of tests or 

analyses, sandpaper ice can be used to evaluate the effects of residual ice on 

stall and stall warning speed margins. 

2 Stall and stall warning speed margins are considered adequate if the stall 

speed does not increase by more than the greater of 3 knots calibrated airspeed 

(CAS) or 3 percent of reference stall speed (VSR) and compliance with 

§ 25.207(e) and (f) can be shown with residual ice contamination on the wing 

leading edge and upper surface. Potential means for increasing stall and stall 

warning speed margins, if necessary, include reducing the peak angle of attack 

reached during takeoff by using increased rotation and takeoff safety speeds 

(V2), reducing the takeoff rotation pitch rate, or reducing the target pitch 

attitude. If any of these means are used, the effect on takeoff performance (that
7/30/08 AC 25-25X

10

is, takeoff run, takeoff distance, accelerate-stop distance) should be assessed 

and accounted for.

3 An acceptable means for showing that residual ice contamination could be 

detected and removed without a visual and tactile check would be to show that 

on an airplane operated in accordance with § 121.629(c), a wing ice protection 

system or primary wing ice detection system can be used while the airplane is 

on the ground. In this case, the AFM Limitations section should include a 

statement similar to the following: 

The wing ice protection system or ice detection 

system [whichever is applicable] must be 

operating until immediately before starting the 

takeoff roll whenever conditions conducive to

ice/frost/snow formation exist.

(4) To comply with § 25.1583(e), Kinds of operation, the AFM Limitations section 

should clearly identify the extent of each approval to operate in icing conditions, 

including the extent of any approval to operate in the supercooled large drop 

atmospheric icing conditions defined in part 25 Appendix O. 

(5) For airplanes not certified to operate throughout the atmospheric icing envelope of 

part 25 Appendix O for every flight phase, the Limitations section of the AFM should 

also identify the means for detecting when the certified icing conditions have been 

exceeded and state that intentional flight, including takeoff and landing, into these 

conditions is prohibited. A requirement to exit all icing conditions must be included if 

icing conditions for which the airplane is not certified are encountered. 

(6) To comply with § 25.1585, the AFM operating procedures should include the 

following information: 

(a) Flight in icing conditions operating procedures should include normal 

operation of the ice protection system and operation of the airplane following ice 

protection system failures. All changes in procedures for other airplane system 

failures that affect the capability of the airplane to operate in icing conditions 

should be included. 

(b) Normal operating procedures should reflect the procedures used to certify the 

airplane for flight in icing conditions. These include configurations, speeds, ice 

protection system operation, and powerplant and systems operation for takeoff, 

climb, cruise, descent, holding, go-around, and landing. For airplanes not certified 

for flight in all of the supercooled large drop atmospheric icing conditions defined 

in Appendix O to part 25, procedures should be provided for safely exiting all icing 

conditions if the airplane encounters Appendix O icing conditions that exceed the 

icing conditions the airplane is certified for.
7/30/08 AC 25-25X

11

(c) For turbojet airplanes without leading edge high-lift devices, unless the 

applicant shows that the airplane retains sufficient stall and stall warning margins 

during takeoff with residual ice contamination, the AFM normal operating 

procedures section should contain a statement similar to the following, along with 

the procedures to be followed to ensure that such contamination is detected and 

removed before takeoff. 

WARNING

Minute amounts of ice or other contamination on the 

leading edges or wing upper surfaces can result in a 

stall without warning, leading to loss of control on 

takeoff.

(d) Non-normal operating procedures should include procedures to be followed in 

the event of annunciated ice protection system failures and suspected unannunciated 

ice protection system failures. If flight in icing conditions results in any changes to 

other non-normal procedures contained in the AFM, these changes should also be 

included. 

(7) Performance information, derived in accordance with subpart B of part 25, should 

be provided in the AFM for all relevant phases of flight in accordance with § 25.1587.

(8) Examples of AFM limitations and operating procedures are contained in Appendix 

4.

2. ACCEPTABLE MEANS OF COMPLIANCE – GENERAL.

a. Overview. 

(1) This section describes acceptable methods and procedures that an applicant may use 

to show that an airplane meets the performance and handling requirements of subpart B 

in the atmospheric icing conditions of Appendices C and O to part 25. 

(2) The applicant should show compliance with § 25.21(g)(1) by one or more of the 

methods listed in this section as agreed to by the responsible aircraft certification office 

(ACO). 

(3) The compliance process should address all phases of flight, including takeoff, 

climb, cruise, holding, descent, landing, and go-around, as appropriate to the airplane 

type, considering its typical operating regime and the extent of its certification approval 

for operation in the atmospheric icing conditions of Appendix O to part 25. 

(4) The design features described in Appendix 3 of this AC should be considered 

when determining the extent of the substantiation program. 
7/30/08 AC 25-25X

12

(5) Appropriate means for showing compliance include the items listed in Table 1, 

below. These compliance means are explained in more detail in following sections of 

this AC. 

TABLE 1

Means for Showing Compliance

ITEM DESCRIPTION

Flight Testing Flight testing in dry air using simulated ice 

accretions or with ice accretions created in 

natural icing conditions.

Wind Tunnel Testing and 

Analysis

An analysis of results from wind tunnel tests 

with simulated or actual ice accretions.

Engineering Simulator Testing 

and Analysis

An analysis of results from engineering 

simulator tests.

Engineering Analysis An analysis that may include the results from 

any of the other means of compliance as well 

as the use of engineering judgment. 

Ancestor Airplane Analysis An analysis of results from a closely-related 

ancestor airplane (an airplane model from

which this airplane model is a derivative).

(6) Various factors affecting ice accretion on the airframe are discussed in Appendix 1

of this AC.

(7) An acceptable method for developing simulated ice accretions is given in 

Appendix 2 of this AC. 

b. Flight Testing. 

(1) Overview. Section 21.35 requires applicants to perform all flight tests that the 

Administrator finds necessary to show compliance with the applicable requirements of 

part 25.

(a) The extent of the flight test program should consider results of certification 

flight tests conducted with the uncontaminated airplane (this is the airplane without 

ice accretions, or the “clean” airplane). Design features of the airplane as discussed 

in Appendix 3 of this AC should also be considered. 
7/30/08 AC 25-25X

13

(b) It is not necessary to repeat, this time with ice accretions, the extensive 

performance and handling characteristics test program that is conducted with the 

uncontaminated airplane. A suitable program sufficient to demonstrate compliance 

with the requirements can be established from experience with airplanes of similar 

size, and from review of design features of the ice protection system, control 

system, wing, horizontal, and vertical stabilizer, and the performance and handling 

characteristics of the uncontaminated airplane. In particular, it is not necessary to 

investigate all weight and center-of-gravity combinations when results from the 

uncontaminated airplane clearly indicate the most critical combination to be tested. 

It is not necessary to investigate the flight characteristics of the airplane with ice 

accretions at high altitude (that is, above the highest altitude for the atmospheric 

icing envelope specified in Appendices C and O to part 25). An example of an 

acceptable flight test program is provided in paragraph 3, Acceptable Means of 

Compliance—Flight Test Program, of this AC.

(c) Certification experience has shown that flight tests are usually necessary to 

evaluate the consequences of ice protection system failures on handling 

characteristics and performance, and to demonstrate that the airplane is capable of 

continued safe flight and landing.

(2) Flight Testing Using Approved Simulated Ice Accretions. 

(a) Tests for performance and handling in icing conditions may be based on flight 

testing performed in dry air using simulated ice accretions that have been agreed to 

by the responsible ACO. 

(b) Additional limited flight tests should be conducted in natural icing conditions, 

as discussed in paragraph 2b(3)(b), below. 

(3) Flight Testing In Natural Icing Conditions.

(a) If the applicant uses flight testing with ice accretions obtained in natural 

atmospheric icing conditions as the primary means of showing compliance, all

relevant meteorological conditions (including liquid water content (LWC) and 

median volumetric diameter (MVD)) should be measured and recorded. The tests 

should ensure good coverage of the atmospheric icing conditions of part 25 

Appendices C and O (consistent with the extent of the certification approval sought 

for operation in Appendix O icing conditions) and, in particular, the critical 

conditions for the airplane. The conditions for accumulating ice (including the 

icing atmosphere, airplane configuration, speed, and duration of exposure) should 

be agreed to by the responsible ACO. 

(b) If the applicant uses flight testing with simulated ice accretions as the primary 

means of showing compliance, additional limited flight tests should be conducted 

with ice accretions obtained in natural icing conditions. The objective of these tests 
7/30/08 AC 25-25X

14

is to corroborate the simulated ice accretions, and the handling characteristics and 

performance results obtained in flight testing with the simulated ice accretions. It is 

not necessary to measure the atmospheric icing characteristics (for example, liquid 

water content and mean effective drop diameter) of these flight test icing 

conditions. For derivative airplanes with aerodynamic characteristics and ice 

protection systems similar to the ancestor airplane, it may not be necessary to carry 

out additional flight tests in natural icing conditions if such tests have already been 

performed with the ancestor airplane. Depending on the extent of the Appendix O

icing conditions that certification is being sought for, and the means used for 

showing compliance with the performance and handling characteristics 

requirements, it may also not be necessary to conduct flight tests in the natural icing

conditions of Appendix O. See AC 25.-1419/1420 for guidance on when it is 

necessary to conduct flight tests in the natural atmospheric icing conditions of 

Appendix O.

c. Wind Tunnel Testing and Analysis. To help substantiate acceptable performance 

and handling characteristics, the applicant may use analysis of results of dry air wind tunnel 

testing of models with the simulated ice accretions defined in Appendices C and O.

d. Engineering Simulator Testing and Analysis. Results of an engineering 

simulator airplane analysis that includes effects of ice accretions defined in Appendices C

and O may be used to help substantiate acceptable handling characteristics. Data used to 

model the effects of ice accretions for the engineering simulator may be based on results of 

dry air wind tunnel tests, flight tests, or computational analysis, or on engineering judgment.

e. Engineering Analysis. To help substantiate acceptable performance and handling 

characteristics, the applicant may use an engineering analysis that includes effects of the ice 

accretions defined in Appendices C and O. Effects of the ice accretions used in this analysis 

may be determined by analyzing results of dry air wind tunnel tests, flight tests, 

computational analyses or engineering simulator analyses, or by engineering judgment.

f. Ancestor Airplane Analysis. 

(1) To help substantiate acceptable performance and handling characteristics, the 

applicant may use an analysis of an ancestor airplane that includes effects of ice 

accretions as defined in Appendices C and O. This analysis should consider the 

similarity of the airplane configuration, operating envelope, performance and handling 

characteristics, and ice protection system of the ancestor airplane to the one being 

certified. 

(2) The analysis may include flight test data, dry air wind tunnel test data, icing tunnel 

test data, engineering simulator analysis, service history, or engineering judgment. 
7/30/08 AC 25-25X

15

3. ACCEPTABLE MEANS OF COMPLIANCE – FLIGHT TEST PROGRAM.

a. Overview.

(1) This section provides guidance for developing an acceptable flight test program. 

Such a test program may be used if the applicant selects flight testing as the primary 

means for showing compliance with the requirements for airplane performance and 

handling qualities for flight in icing conditions and if that method of showing 

compliance is agreed to by the responsible ACO.

(2) Section 25.21(a)(1) states that compliance with each requirement of subpart B must 

be shown by tests upon an airplane of the type for which certification is requested, or by 

calculations based on, and equal in accuracy to, the results of testing. In accordance 

with § 25.21(a)(1), calculations, if used, must provide at least the same degree of 

confidence of compliance as testing would provide.

(3) Ice accretions for each flight phase are defined in Appendices C and O to part 25. 

Additional guidance for determining the applicable ice accretions is provided in 

Appendix 1 to this AC.

(4) This test program is based on the assumption that the applicant will choose to use 

the holding ice accretion for the majority of the testing, assuming that it is the most 

conservative ice accretion. However, in accordance with part II(a) of Appendix C to 

part 25 and part II(d) of Appendix O, if the holding ice accretion is not as conservative 

as the ice accretion appropriate to the flight phase, then the ice accretion appropriate to 

the flight phase and icing conditions of Appendix C or O, or a more conservative ice 

accretion, must be used. In general, the applicant may choose to use either a

conservative ice accretion or the specific ice accretion appropriate to the particular phase 

of flight and icing conditions. 

(5) For the approach and landing configurations, in accordance with the guidance 

provided in paragraph 1a(11) of this AC, the flight tests in natural icing conditions 

specified in Table 4 of this AC are usually sufficient to evaluate whether ice accretions 

on trailing edge flaps adversely affect airplane performance or handling qualities. If 

these tests show that airplane performance or handling qualities are adversely affected, 

additional tests may be necessary to show compliance with the airplane performance and 

handling qualities requirements.

(6) Unless otherwise specified, the speeds (for example, VSR, VREF, V2, etc.) referenced 

in the flight tests described in this AC refer to the speeds used with the appropriate ice 

accretion on the airplane.
7/30/08 AC 25-25X

b. Stall Speed, § 25.103.

(1) The stall speed for intermediate high lift configurations (for takeoff configurations, 

for example) can normally be obtained by interpolation. However, additional tests may 

be necessary if— 

• A stall identification system (for example, a stick pusher) activation point is set 

as a function of the high lift configuration, or 

• The activation point is reset for icing conditions, or 

• Significant configuration changes occur with extension of trailing edge flaps 

(such as extension of wing leading edge high lift devices). 

(2) Acceptable Test Program. The following specifications represent an example 

of an acceptable test program subject to the provisions outlined above. 

16

Maneuvers

(a) Load the airplane to a forward center-of-gravity position appropriate to the 

airplane configuration. 

(b) Conduct the test at the stall test altitude used in non-icing tests. 

(c) Trim at an initial speed of 1.13 to 1.30 VSR. Decrease speed at a rate not to 

exceed 1 knot per second until an acceptable stall identification is obtained. 

Perform this maneuver with the following ice accretions: 

1 In high lift devices retracted configuration—final takeoff ice.

2 In high lift devices retracted configuration—en route ice.

3 In holding configuration—holding ice. 

4 In lowest lift takeoff configuration—holding ice. 

5 In highest lift takeoff configuration—takeoff ice.

6 In highest lift landing configuration—holding ice. 
7/30/08 AC 25-25X

17

c. Accelerate–Stop Distance, § 25.109. To comply with this requirement, the effect 

of any increase in V1 due to takeoff in icing conditions may be determined by a suitable 

analysis. 

d. Takeoff Path, § 25.111. In accordance with § 25.105(a), the applicant should conduct 

takeoff evaluations to substantiate the speed schedule and distances for takeoff in icing 

conditions if the following applies— 

VSR in the configuration defined by § 25.121(b) with the takeoff ice accretion exceeds 

VSR for the same configuration without ice accretions by more than the greater of 3 

knots or 3 percent. 

Any need for a takeoff speed increase, and any effects of thrust loss or drag increase on the 

takeoff path, may be determined by a suitable analysis.

e. Landing Climb: All–Engines–Operating, § 25.119. The following specifications 

represent an example of an acceptable test program for showing compliance with § 25.119. 

Maneuvers

 (1) The holding ice accretion should be used. 

 (2) The airplane should be loaded to a forward center-of-gravity position appropriate to 

the airplane configuration. 

 (3) The airplane should be in the highest lift landing configuration, at a landing climb 

speed no greater than VREF.

(4) Stabilize the airplane at the specified speed and conduct 2 climbs or drag polar 

checks as agreed to by the responsible ACO. 
7/30/08 AC 25-25X

18

f. Climb: One-Engine-Inoperative, § 25.121. The following specifications represent 

an example of an acceptable test program for showing compliance with § 25.121. 

Maneuvers

(1) The airplane should be loaded to a forward center-of-gravity position appropriate to 

the configuration. 

(2) Stabilize the airplane at the specified speed with one engine inoperative (or 

simulated inoperative if all relevant effects of an inoperative engine can be taken into 

account) and conduct 2 climbs or drag polar checks in each of the following airplane 

configurations, with the specified ice accretions, to substantiate the asymmetric drag 

increment as agreed to by the responsible ACO. 

(a) High lift devices retracted configuration, final takeoff climb speed–—final 

takeoff ice.

(b) Lowest lift takeoff configuration, landing gear retracted, V2 climb speed–—

takeoff ice.

(c) Approach configuration appropriate to the highest lift landing configuration, 

landing gear retracted, approach climb speed–—holding ice. 

 

 

 
7/30/08 AC 25-25X

19

g. En Route Flight Paths, § 25.123. The following specifications represent an 

example of an acceptable test program to show compliance with § 25.123. 

Maneuvers

(1) The en route ice accretion should be used. 

(2) The airplane should be loaded to a forward center-of-gravity position appropriate to 

the airplane configuration. 

(3) The airplane should be in the en route configuration at the en route climb speed.

(4) Stabilize the airplane at the specified speed with one engine inoperative (or 

simulated inoperative if all relevant effects of an inoperative engine can be taken into 

account) and conduct 2 climbs or drag polar checks to substantiate the asymmetric drag 

increment as agreed to by the responsible ACO. 

h. Landing, § 25.125. To comply with this regulation, the effect of any landing speed 

increase on the landing distance may be determined by a suitable analysis. 

i. Controllability and Maneuverability – General, § 25.143. 

 (1) A qualitative and quantitative evaluation is usually necessary to show compliance 

with the controllability and maneuverability requirements of § 25.143. Depending on 

how clearly compliance is demonstrated, or whether the force limits or stick force per g 

limits of § 25.143 are being approached, additional substantiation may be necessary to 

ensure that the airplane complies. 
7/30/08 AC 25-25X

20

(2) The following specifications represent an example of an acceptable test program to 

evaluate general controllability and maneuverability, subject to the provisions outlined 

above.

Maneuvers

(a) The holding ice accretion should be used. 

(b) The airplane should be loaded to a medium to light weight, with an aft 

center-of-gravity position, and symmetric fuel loading. 

(c) In the configurations listed in Table 2, below, trim the airplane at the specified 

speeds and conduct the following maneuvers: 

1 30-degree banked turns left and right with rapid reversals; 

2 A pull-up to 1.5 g (except that this may be limited to 1.3 g at VREF), and a 

pushover to 0.5 g (except that the pushover is not required at VMO and VFE); 

and

3 Deployment and retraction of deceleration devices. 

TABLE 2

Controllability & Maneuverability – Trim Speeds

Configuration Trim Speed

High lift devices retracted configuration 1.3 VSR, 

and

VMO or 250 KIAS, whichever is 

less

Lowest lift takeoff configuration 1.3 VSR, 

and 

VFE or 250 KIAS, whichever is less 

Highest lift landing configuration VREF,

and 

VFE or 250 KIAS, whichever is less
7/30/08 AC 25-25X

21

VSR – Reference Stall Speed

VMO – Maximum operating limit speed

KIAS – Knots indicated air speed

VFE – Maximum flap extended speed

VREF – Reference landing speed

(d) In the lowest lift takeoff configuration, at the greater of 1.13 VSR or V2MIN, with 

one engine inoperative (simulated), conduct 30-degree banked turns left and right 

with normal turn reversals and, in wings-level flight, a 5-knot speed decrease and 

increase. 

(e) Conduct an approach and go-around with all engines operating using the 

airplane flight manual (AFM) approach and go-around procedure. 

(f) Conduct an approach and go-around with one engine inoperative (simulated) 

using the AFM one-engine-inoperative approach and go-around procedure. 

(g) Conduct an approach and landing using the AFM approach and landing 

procedure. In addition, you should demonstrate satisfactory controllability during a 

landing at VREF minus 5 knots. These tests should be done at heavy weight and 

forward center-of-gravity. 

(h) Conduct an approach and landing with one engine inoperative (or with one 

engine simulated to be inoperative if all effects associated with an engine failure 

can be taken into account) using the procedure provided in the AFM. 

(3) Evaluation of Lateral Control Characteristics. Aileron hinge moment 

reversal and other lateral control anomalies have been identified as causal factors in 

icing accidents and incidents. The following maneuvers, along with the following two 

evaluations, is intended to determine susceptibility of the airplane to aileron hinge 

moment reversals or other adverse effects on lateral control characteristics due to ice 

accretion.

Evaluations

• Evaluate lateral controllability during deceleration to the stall warning speed 

(covered in paragraph 3q(2)(e) of this AC), and 

• Evaluate static lateral-directional stability (covered in paragraph 3o of this AC).
7/30/08 AC 25-25X

22

Maneuvers

(a) For each of the test conditions specified in paragraph (b) below, perform the 

maneuvers described in paragraph (b)1 through (b)6 below. 

(b) 

• Test condition 1 – Holding configuration, holding ice accretion, 

maximum landing weight, forward center-of-gravity position, at minimum 

holding speed (highest expected holding angle-of-attack)

• Test condition 2 – Landing configuration, holding ice accretion, 

medium to light weight, forward center-of-gravity position, at VREF

(highest expected landing approach angle-of-attack).

1 Establish a 30-degree banked level turn in one direction. 

2 Using a step input of approximately 1/3 full lateral control deflection, roll 

the airplane in the other direction.

3 Maintain the control input as the airplane passes through a wings level 

attitude. 

4 At approximately 20 degrees of bank, apply a step input in the opposite 

direction to approximately 1/3 full lateral control deflection. 

5 Release the control input as the airplane passes through a wings level 

attitude.

6 Repeat this test procedure with 2/3 and up to full lateral control deflection 

unless the roll rate or structural loading is judged excessive. It should be 

possible to readily arrest and reverse the roll rate using only lateral control 

input, and the lateral control force should not reverse with increasing control 

deflection.

(4) Low g Maneuvers and Sideslips. The maneuvers in (c) below represent an 

example of an acceptable test program for showing compliance with controllability 

requirements in low g maneuvers and in sideslips to evaluate susceptibility to icecontaminated tailplane stall.
7/30/08 AC 25-25X

23

(a) Section 25.143(i)(2), states:

It must be shown that a push force is required throughout a 

pushover maneuver down to a zero g load factor, or to the 

lowest load factor obtainable if limited by elevator power

or other design characteristic of the flight control system.

It must be possible to promptly recover from the maneuver 

without exceeding a pull control force of 50 pounds; . . 

An example of another design characteristic of the flight control system that 

would limit the lowest obtainable load factor would be a g-limiting envelope 

protection system.

(b) For sideslips, § 25.143(i)(3) requires that:

Any changes in force that the pilot must apply to the pitch 

control to maintain speed with increasing sideslip angle

must be steadily increasing with no force reversals,

unless the change in control force is gradual and easily 

controllable by the pilot without using exceptional 

piloting skill, alertness, or strength. (See paragraph 

3o(1) of this AC for lateral-directional aspects). 

Abrupt changes in the control force characteristic, unless so small as to be 

unnoticeable, would not be considered compliant with the requirement that the 

force be steadily increasing. A gradual change in control force is a change that is 

not abrupt and does not have a steep gradient. It can be easily managed by a pilot 

of average skill, alertness, and strength. Control forces in excess of those 

permitted by § 25.143(c) would be considered excessive.

(c) Test maneuvers used to show compliance with the regulatory requirements

quoted in paragraphs (a) and (b) above should be conducted using the following 

ice accretions, configurations, and procedures: 
7/30/08 AC 25-25X

24

Maneuvers

1 The holding ice accretion should be used. For airplanes with unpowered 

elevators, these tests should also be performed with sandpaper ice.

2 The airplane should be loaded to a medium to light weight, at the most 

critical center-of-gravity position, with symmetric fuel loading, and should be 

in the highest lift landing configuration. 

3 Test maneuver for showing compliance with § 25.143(i)(2). Start 

with the airplane in trim, or as near as possible to being in trim, at the most 

critical of the trim speeds specified below. 

• Trim speed 1.23 VSR, target speed not more than 1.23 VSR, or 

• Trim speed VFE, target speed not less than VFE - 20 knots.

Pull up to a suitable pitch attitude, then 

• Push over in a continuous maneuver (without changing trim) to reach 

zero g normal load factor or, if limited by elevator control authority (or 

other design characteristic), the lowest load factor obtainable at the 

target speed, as the airplane’s pitch attitude passes approximately 

through level flight (that is, through the horizon). 

• Perform this maneuver with idle power or thrust and with go-around

power or thrust.

4 Test maneuver for showing compliance with § 25.143(i)(3).

Conduct steady heading sideslips to full rudder input, 180 pounds rudder 

force, or full lateral control authority (whichever comes first) at a trim speed 

of 1.23 VSR and with the power or thrust needed for a minus 3 degrees flight 

path angle. 
7/30/08 AC 25-25X

25

(5) Ice Accretion Prior to Activation and Operation of the Ice Protection 

System. Following is an example of an acceptable test program for showing 

compliance with §§ 25.143(j). 

Maneuvers

For the configurations, speeds, and power settings listed below, with the ice accretion 

specified in the requirement, trim the airplane at the specified speed, conduct a pull-up 

maneuver to 1.5 g and pushover maneuver to 0.5 g, and show that longitudinal control 

forces do not reverse. 

(a) High lift devices retracted configuration (or holding configuration if different), 

holding speed, power or thrust for level flight. 

(b) Landing configuration, VREF for non-icing conditions, power or thrust for 

landing approach. If necessary, limit the pull-up maneuver to the point at which 

stall warning occurs. 

j. Longitudinal Control, § 25.145. 

(1) No specific quantitative evaluations are required for demonstrating compliance with 

§§ 25.145(b) and (c). Qualitative evaluations should be combined with the other 

testing. Review results of tests on the uncontaminated airplane for any cases of 

marginal compliance. All tests showing marginal compliance should be repeated with 

ice accretions on the airplane. 
7/30/08 AC 25-25X

26

(2) Acceptable Test Program. The following specifications represent an example 

of an acceptable test program for compliance with § 25.145(a).

Maneuvers

(a) The holding ice accretion should be used.

(b) The airplane is at a medium to light weight, aft center-of-gravity position, with 

symmetric fuel loading. 

(c) In the configurations listed below, trim the airplane at 1.3 VSR. Reduce speed 

approximately 1 knot per second using elevator control to 1 second past stall 

warning and demonstrate prompt recovery to the trim speed using elevator control. 

1 High lift devices retracted configuration, maximum continuous power or 

thrust. 

2 Maximum lift landing configuration, maximum continuous power or thrust. 

k. Directional and Lateral Control, § 25.147. To show compliance with § 25.147,

applicants should combine qualitative evaluations with the other testing (for example, the 

roll tests described in paragraphs 3i(2)(c)1 and 3i(2)(d)) of this AC). Review the results of

the tests on the uncontaminated airplane for any cases of marginal compliance. All tests 

showing marginal compliance should be repeated with ice accretions on the airplane. 

l. Trim, § 25.161. 

(1) To show compliance with this requirement, the applicant should combine 

qualitative evaluations with the other testing. Review results of tests on the 

uncontaminated airplane for any cases of marginal compliance. All tests showing 

marginal compliance should be repeated with ice accretions on the airplane. In 

addition, a specific check should be made to demonstrate compliance with 

§ 25.161(c)(2).
7/30/08 AC 25-25X

27

(2) Acceptable Test Program. The following specifications represent an example 

of an acceptable test program for compliance with § 25.161(c)(2).

Maneuvers

(a) The holding ice accretion should be used.

(b) The airplane is at the most critical landing weight, with a forward center of 

gravity position and symmetric fuel loading. 

(c) In the maximum lift landing configuration, trim the airplane at the most critical 

of: 

1 1.3 VSR1 at idle power or thrust; or 

2 VREF with the power or thrust for a minus 3 degrees flight path angle. 

m. Stability – General, § 25.171. To show compliance with this requirement,

applicants should combine qualitative evaluations with the other testing. Each tendency to 

change speed when trimmed and a need for frequent trim inputs should be specifically 

investigated. 

n. Demonstration of Static Longitudinal Stability, § 25.175. 

(1) To show compliance with § 25.175, the applicant should test each of the following 

cases. In general, it is not necessary to test in the cruise configuration at low speed 

(§ 25.175(b)(2)) or in the cruise configuration with landing gear extended 

(§ 25.175(b)(3)); nor is it necessary to test at high altitude. Maximum speed for 

substantiation of stability characteristics in icing conditions (as prescribed by 

§ 25.253(c)) is the lowest of—

• 300 knots CAS; 

• VFC; or 

• A speed at which it is demonstrated that the airframe will be free of ice 

accretion due to the effects of increased dynamic pressure.
7/30/08 AC 25-25X

28

(2) Acceptable Test Program. The following specifications represent an example 

of an acceptable test program for demonstrating static longitudinal stability.

Maneuvers

(a) The holding ice accretion should be used.

(b) The airplane is at a high landing weight, aft center-of-gravity position, with 

symmetric fuel loading. 

(c) In the configurations listed below, trim the airplane at the specified speed. The 

power or thrust should be set and stability demonstrated over the speed ranges as 

stated in §§ 25.175(a) through (d), as applicable. 

1 Climb: With high lift devices retracted, trim at the speed for best rate of

climb, except that the speed need not be less than 1.3 VSR. 

2 Cruise: With high lift devices retracted, trim at VMO or 250 knots CAS, 

whichever is lower. 

3 Approach: With the high lift devices in the approach position appropriate 

to the highest lift landing configuration, trim at 1.3 VSR. 

4 Landing: With the highest lift landing configuration, trim at 1.3 VSR. 

o. Static Lateral–Directional Stability, § 25.177. 

(1) Use steady heading sideslips to show compliance with the directional and lateral 

stability requirements. The maximum sideslip angles obtained should be recorded and 

may be used to substantiate a crosswind value for landing (see paragraph 3s of this AC). 

Section 25.177(c) requires directional and lateral control movements and forces to be 

substantially proportional to the angle of sideslip without reversing.
7/30/08 AC 25-25X

29

(2) Acceptable Test Program. The following specifications represent an example 

of an acceptable test program for demonstrating static directional and lateral stability.

Maneuvers

(a) The holding ice accretion should be used.

(b) The airplane is at a medium to light weight, aft center-of-gravity position, with 

symmetric fuel loading. 

(c) In the configurations listed below, trim the airplane at the specified speed and 

conduct steady heading sideslips to full rudder authority, 180 pounds of rudder 

pedal force, or full lateral control authority, whichever comes first. 

1 High lift devices retracted configuration: Trim at best rate-of-climb speed, 

but need not be less than 1.3 VSR. 

2 Lowest lift takeoff configuration: Trim at the all-engines-operating initial 

climb speed. 

3 Highest lift landing configuration: Trim at VREF. 

p. Dynamic Stability, § 25.181. If the uncontaminated airplane has no borderline

compliance test results for § 25.181, dynamic stability tests with ice accretions on the 

airplane are not necessary. The applicant should combine qualitative evaluations with other 

testing. Each difficulty in achieving precise attitude control and each tendency to sustain 

oscillations in turbulence should be investigated. 

q. Stall Demonstration, § 25.201/Stall Characteristics, § 25.203. 

(1) The applicant should conduct sufficient stall testing to demonstrate that the stall 

characteristics comply with requirements. In general, it is not necessary to conduct a 

stall program that encompasses all weights, center-of-gravity positions, altitudes, high 

lift configurations, deceleration device configurations, straight and turning flight 

attitudes, and thrust or power settings. The applicant can establish a reduced test matrix 

based on a review of the stall characteristics of the uncontaminated airplane. However, 

additional tests may be necessary if: 
7/30/08 AC 25-25X

30

(a) The stall characteristics with ice accretion show a significant difference from 

those on the uncontaminated airplane, 

(b) The testing indicates borderline compliance, or

(c) The activation point of a stall identification system (for example, a stick 

pusher) is reset for icing conditions. 

(2) Acceptable Test Program. Turning flight stalls at decelerations greater than 

1 knot per second are not required. Note that slow decelerations (much slower than 

1 knot per second) may be critical on airplanes with anticipation logic in their stall 

protection system or on airplanes with low directional stability, where large sideslip 

angles could develop. The following specifications represent an example of an 

acceptable test program subject to the provisions outlined above. 
7/30/08 AC 25-25X

31

Maneuvers

(a) The holding ice accretion should be used. 

(b) The airplane should be loaded to a medium to light weight, aft center-ofgravity position, with symmetric fuel loading. 

(c) The tests should be conducted at the normal stall test altitude. 

(d) In the configurations listed below, trim the airplane at the same initial stall 

speed factor used for stall speed determination. For power on stalls, use the power 

setting as defined in § 25.201(a)(2), but with ice accretions on the airplane. 

Decrease speed at a rate not to exceed 1 knot per second to stall identification and 

recover using the same recovery maneuver as for the uncontaminated airplane. 

1 High lift devices retracted configuration: Straight/Power Off,

Straight/Power On, Turning/Power Off, Turning/Power On. 

2 Lowest lift takeoff configuration: Straight/Power On, Turning/Power Off. 

3 Highest lift takeoff configuration: Straight/Power Off, Turning/Power On. 

4 Highest lift landing configuration: Straight/Power Off, Straight/Power On, 

Turning/Power Off, Turning/Power On. 

(e) For the configurations listed in paragraph 3q(2)(d)1 and 4, and each other 

configuration if deemed more critical, in 1 knot/second deceleration rates down to 

stall warning with wings level and power off, roll the airplane left and right up to 

10 degrees of bank using the lateral control.

r. Stall Warning, § 25.207. 

(1) To show compliance with § 25.207, the applicant should assess stall warning in 

conjunction with stall speed testing and stall demonstration/characteristics testing 

(§§ 25.103, 25.201, and 25.203, and paragraphs 3b and 3q of this AC, respectively), and 

in tests with faster entry rates. 
7/30/08 AC 25-25X

32

(2) Normal Ice Protection System Operation. The following specifications 

represent an example of an acceptable test program for stall warning in slow-down turns 

of at least 1.5 g and at entry rates of at least 2 knots per second:

Maneuvers

(a) The holding ice accretion should be used. 

(b) The airplane should be loaded to a medium to light weight, aft center-of-gravity 

position, with symmetric fuel loading. 

(c) The test should be conducted at the normal stall test altitude. 

(d) In the configurations listed below, trim the airplane at 1.3 VSR with the power 

or thrust necessary to maintain straight level flight. Maintain the trim power or 

thrust during the test demonstrations. Increase speed as necessary before

establishing at least 1.5 g and a deceleration of at least 2 knots per second. 

Decrease speed until 1 second after stall warning and recover using the same 

recovery maneuver as for the uncontaminated airplane. 

1 High lift devices retracted configuration; 

2 Lowest lift takeoff configuration; and

3 Highest lift landing configuration. 
7/30/08 AC 25-25X

33

(3) Ice Accretion Prior to Activation and Operation of the Ice Protection 

System. The following represents an acceptable means for showing compliance with 

§ 25.207(h).

Maneuvers

(a) In the configurations listed in paragraphs 1 and 2 below, with the ice accretion 

specified in the requirement, trim the airplane at 1.3 VSR. 

1 High lift devices retracted configuration: Straight Flight/Power Off. 

2 Landing configuration: Straight Flight/Power Off. 

(b) At deceleration rates of up to 1 knot per second, reduce the speed until

1 second past stall warning, and demonstrate that stalling can be prevented using 

the same recovery maneuver as used for the uncontaminated airplane, without 

encountering any adverse characteristics (rapid wing roll-off, for example). If stall 

warning for the airplane with ice accretion is provided by a different means than it 

is for the airplane without ice accretion, the following requirements must be met. 

Section 25.207(h)(2)(ii) requires that stalling be preventable even if the pilot takes

no recovery action for at least 3 seconds after stall warning. Section § 25.207(i)

requires that satisfactory stall characteristics be demonstrated. 

s. Wind Velocities, § 25.237. To show compliance with this requirement:

(1) Crosswind landings with the landing ice accretion should be evaluated on an 

opportunity basis. 

(2) Results of the steady heading sideslip tests with landing ice may be used to establish 

the safe crosswind component of this requirement. If the flight test data show that the 

maximum sideslip angle demonstrated is similar to that shown with the uncontaminated

airplane, and the flight characteristics (control forces and deflections, for example) are 

similar, then the non-contaminated airplane crosswind component test result is 

considered valid for icing conditions. 

(3) If results of the comparison discussed in paragraph 3s(2) are not clearly similar, and 

in the absence of a more extensive analysis, the applicant may use a conservative 
7/30/08 AC 25-25X

34

analysis based on results of the steady heading sideslip tests to establish the safe 

crosswind component value. The crosswind value may be estimated from: 

( )

1.5

sin * sideslip angle VCW = VREF

where: 

VCW is the crosswind component, 

VREF is the landing reference speed appropriate to a minimum landing weight, and 

sideslip angle is that demonstrated at VREF (see paragraph 3o of this AC). 

t. Vibration and Buffeting, § 25.251. To show compliance with this requirement:

(1) Combine qualitative evaluations with the other testing, including speeds up to the 

maximum speed obtained in the longitudinal stability tests (see paragraph 3n of this 

AC). 

(2) Demonstrate that the airplane is free from harmful vibration due to residual ice 

accumulation. This may be done in conjunction with the natural icing tests. 

(3) Evaluate an airplane with pneumatic deicing boots up to VDF/MDF both with the 

deicing boots operating (including deicing boot inflation cycles) and not operating. It is 

not necessary to do this demonstration with ice accretions on the airplane. 

u. Natural Icing Conditions, § 25.1419(b). 

(1) To show compliance with this requirement, the applicant should perform additional 

flight testing.

(a) Whether flight testing has been performed with artificial or simulated ice 

accretions or in natural icing conditions, additional limited flight testing described 

in this section should be conducted in the natural atmospheric icing conditions

specified in Appendix C to part 25, and, if necessary, in the icing conditions 

described in Appendix O. (AC 25.-1419/1420 provides guidance on when it is 

necessary to perform flight testing in the natural atmospheric icing conditions of 

Appendix O.). If flight testing with simulated ice accretions is the primary means 

for showing compliance, the objective of the tests described in this section is to 

corroborate the handling characteristics and performance results obtained in the 

flight testing with simulated ice accretions. Validating all aspects of predicted ice 

accretions through natural icing flight tests is not required. The flight testing 

should confirm the general physical characteristics and location of the ice 

accretions and their effect on airplane performance and handling characteristics. 

The applicant should make at least a qualitative assessment to confirm that the 

artificial ice accretions are conservative relative to ice accretions resulting from

natural atmospheric icing conditions, and that ice in natural atmospheric icing 

conditions does not accrete in unexpected places.
7/30/08 AC 25-25X

35

(b) It is acceptable for some ice to be shed during the testing due to air loads or 

wing flexure, etc. However, the applicant should make an attempt to accomplish 

the test maneuvers as soon as possible after exiting the icing cloud in order to 

minimize atmospheric influences on ice shedding. 

(c) Acceptable Test Program. During each of the maneuvers specified in 

paragraph 3u(2), below, the behavior of the airplane should be consistent with that 

obtained with simulated ice accretions. There should be no unusual control 

responses or uncommanded airplane motions. Additionally, during the level turns 

and bank-to-bank rolls, there should be no buffeting or stall warning. 

Maneuvers

(2) Ice Accretion Maneuvers. 

(a) Holding scenario. 

1 The maneuvers specified in Table 3, below, should be carried out with the 

following ice accretions representative of normal operation of the ice protection 

system: 

(aa) Ice on unprotected parts—A target accretion thickness 

equivalent to the 45-minute dry air ice accretions on an unprotected part 

of the wing should be the objective. (A thickness of 2 inches is normally 

a minimum value, unless a lesser value is agreed to with the responsible 

aircraft certification office (ACO)). 

(bb) Ice on protected parts—The ice accretion thickness should be 

that resulting from normal operation of the ice protection system. 

2 For airplanes with control surfaces that may be susceptible to jamming due 

to ice accretion (for example, elevator horns exposed to the air flow), the 

holding speed that is critical with respect to this ice accretion should be used.
7/30/08 AC 25-25X

36

TABLE 3

Holding Scenario – Maneuvers

Airplane 

Configuration

Center of 

Gravity

Position

Trim 

Speed Maneuver

Flaps up, 

Gear up

Any position 

in the aft 

range

Holding, 

except 

1.3 VSR

for the 

stall 

maneuver

Level, 40° banked turn; 

Bank-to-bank rapid roll, 30° - 30°; 

Speedbrake extension, retraction; 

Full straight stall (1 knot/second 

deceleration rate, wings level, power 

off).

Flaps in 

intermediate 

positions, 

gear up

Any position 

in the aft 

range

1.3 VSR Deceleration to the speed reached 

3 seconds after activation of stall 

warning in a 1 knot/second 

deceleration.

Landing flaps, 

gear down

Any position 

in the aft 

range

VREF Level, 40° banked turn; 

Bank-to-bank rapid roll, 30° - 30°; 

Speedbrake extension, retraction (if 

approved); 

Full straight stall (1 knot/second 

deceleration rate, wings level, power 

off).

(b) Approach/Landing Scenario. 

1 The maneuvers specified in Table 4, below, should be carried out with 

successive accretions in different configurations on unprotected surfaces. 

2 Each test condition should be accomplished with the ice accretion that exists 

at that point. 

3 The final ice accretion (Test Condition 3) represents the sum of the amounts 

that would accrete during a normal descent from holding to landing in icing 

conditions. 
TABLE 4

Approach/Landing Scenario - Maneuvers

37

Test 

condition

Ice 

accretion 

thickness 

(*)

Airplane 

configuration

Center of 

Gravity

Position

Trim 

speed Maneuver

AC 25-25

X

— First 0.5 in Flaps up, 

gear up

Any 

position in 

the aft 

range

Holding No specific test

1 Additional 

0.25 in 

(0.75 in. 

total)

First 

intermediate 

flaps, 

gear up

Any 

position in 

the aft 

range

Holding, 

except 

1.3 VSR for the 

deceleration 

maneuver

Level 40° banked turn, 

Bank-to-bank rapid roll, 30°- 30°, 

Speed brake extension and retraction (if approved), 

Deceleration to the speed reached 3 seconds after activation 

of 

stall warning in a 1 knot/second deceleration.

2 Additional 

0.25 in 

(1.00 in. 

total)

Further 

intermediate 

flaps, 

gear up (as 

applicable)

Any 

position in 

the aft 

range

1.3 VSR Bank-to-bank rapid roll, 30° - 30°, 

Speed brake extension and retraction (if approved), 

Deceleration to the speed reached 3 seconds after activation of 

stall warning in a 1 knot/second deceleration.

3 Additional 

0.25 in

(1.25 in. 

total)

Landing flaps, 

gear down

Any 

position in 

the aft 

range

VREF Bank-to-bank rapid roll, 30° - 30°, 

Speed brake extension and retraction (if approved), Bank to 

40°,

Full straight stall (1 knot/second deceleration rate, wings level, 

power off).

 (*) The indicated thickness is that accumulated on the parts of the unprotected airfoil most likely to accumulate ice. 

7/30/08 AC 25-25X

38

(3) For airplanes with unpowered elevator controls, unless the applicant can adequately 

substantiate the critical simulated ice accretion used to demonstrate compliance with the 

controllability requirement, the pushover test of paragraph 3i(4)(c)3 should be repeated 

with a thin accretion of natural ice. 

(4) Existing propeller speed limits or, if required, revised propeller speed limits for 

flight in icing, should be verified by flight tests in natural icing conditions. 

v. Failure Conditions, § 25.1309. To show compliance with this requirement:

(1) For failure conditions that are annunciated to the flightcrew, the applicant may take

credit for flightcrew action to follow the established operating procedures provided in 

the airplane flight manual (AFM). 

(2) Acceptable Test Program. In addition to a general qualitative evaluation, the

applicant should carry out the following test program for the most critical probable 

failure condition for which the associated procedure requires the airplane to exit the 

icing condition. The test program should be modified as necessary to reflect the specific 

operating procedures.

Maneuvers

(a) The ice accretion is defined as a combination of the following: 

1 Ice on unprotected surfaces—The holding ice accretion described in 

Appendix 1, paragraph 2(a)(3) of this AC;

2 Ice on normally protected surfaces that are no longer 

protected—The failure ice accretion described in Appendix 1, paragraph 

3(b) of this AC; and

3 Ice on normally protected surfaces that are still protected

following segmental failure of a cyclical deice system—The ice 

accretion that will form during the rest time of the deice system following the 

critical failure condition.
7/30/08 AC 25-25X

39

(b) The airplane should be loaded to a medium to light weight, at aft center-ofgravity position, with symmetric fuel loading. 

(c) In the configurations listed in paragraphs 1 through 3 below, trim the airplane 

at the specified speed. Conduct 30-degree banked turns left and right with normal 

reversals. Conduct a pull-up maneuver to 1.5 g and a pushover maneuver to 0.5 g.

1 High lift devices retracted configuration (or holding configuration if 

different): Holding speed, power or thrust for level flight. In addition, deploy 

and retract the deceleration devices.

2 Approach configuration: Approach speed, power or thrust for level flight. 

3 Landing configuration: Landing speed, power or thrust for landing 

approach (limit pull-up to 1.3 g). In addition, conduct steady heading sideslips 

to the angle of sideslip appropriate to the airplane type and the AFM landing 

procedure.

(d) In the configurations listed in 1 and 2 below, trim the airplane at the estimated 

1.3 VSR. Decrease speed at approximately 1 knot per second until 1 second after 

stall warning, and demonstrate prompt recovery using the same recovery maneuver 

as for the uncontaminated airplane. It is acceptable for stall warning to be provided

by a different means (for example, by the behavior of the airplane rather than by 

stick shaker) for failure cases not considered probable.

1 High lift devices retracted configuration: Straight Flight/Power Off. 

2 Landing configuration: Straight Flight/Power Off. 

(e) Conduct an approach and go-around with all engines operating using the AFM 

approach and go-around procedure. 

(f) Conduct an approach and landing with all engines operating (unless the oneengine-inoperative condition results in a more critical probable failure condition) 

using the appropriate AFM approach and landing procedure. 

(3) For improbable failure conditions, flight testing may be required to demonstrate that 

the effect on safety of flight (as measured by degradation in flight characteristics) supports the 

system safety analysis, or to verify results of analyses or wind tunnel tests. The extent of each

required flight testing should be similar to that described in paragraph 3v(2), above, or as agreed 

to by the responsible ACO for the specific failure condition. 
7/30/08 AC 25-25X

Appendix 1

A1-1

APPENDIX 1

AIRFRAME ICE ACCRETIONS

1. Overview. 

a. In accordance with § 25.1419, each airplane certified for flight in icing conditions must 

be capable of safely operating in the continuous maximum and intermittent maximum icing 

conditions of Appendix C. Therefore, at a minimum, certification for flight in icing 

conditions must include consideration of ice accretions that can occur in Appendix C icing 

conditions.

b. In accordance with § 25.1420(a)(1), each airplane with a maximum takeoff weight less 

than 60,000 pounds or with reversible flight controls certified for flight in icing conditions 

must, at a minimum, be capable of safely operating: 

(1) In the atmospheric icing conditions of Appendix C to part 25, and

(2) After encountering the atmospheric icing conditions of Appendix O, and 

subsequently while exiting all icing conditions. 

Therefore, for airplanes with a maximum takeoff weight less than 60,000 pounds or with 

reversible flight controls, certification for flight in icing conditions must consider ice 

accretions that can occur during flight in Appendix C icing conditions and during detection 

and exiting of Appendix O icing conditions.

c. In accordance with § 25.1420(a)(2), an airplane with a maximum takeoff weight less than 

60,000 pounds or with reversible flight controls may also be certified for operation in a 

portion of the atmospheric icing conditions of Appendix O to part 25. In that case, the 

airplane must also be capable of operating safely after encountering, and while exiting, 

atmospheric icing conditions in the portion of Appendix O for which operation is not 

approved. Ice accretions used for certification must consider:

(1) Operations in Appendix C icing conditions, 

(2) Operations in the Appendix O icing conditions for which approval is sought, and 

(3) Detection and exiting of the Appendix O icing conditions beyond those for which 

approval is sought. 

d. In accordance with § 25.1420(a)(3), in addition to being certified for flight in Appendix 

C conditions, an airplane with a maximum takeoff weight less than 60,000 pounds or with 

reversible flight controls may be certified for operation throughout the atmospheric icing 
7/30/08 AC 25-25X

Appendix 1

A1-2

conditions of Appendix O to part 25. Certification for flight throughout the atmospheric 

icing conditions of Appendix O must consider ice accretions resulting from: 

(1) Operations in Appendix C icing conditions, and

(2) Operations in Appendix O icing conditions. 

e. The part 25 subpart B airplane performance and handling characteristics requirements 

identify the specific ice accretions that apply in showing compliance. In accordance with 

Appendix C, part II(b) and Appendix O, part II(e), to reduce the number of ice accretions 

used for demonstrating compliance, the applicant may use any of the applicable ice 

accretions (or a composite accretion representing a combination of accretions) to show 

compliance with a particular subpart B requirement if that accretion is either the ice 

accretion identified in the requirement or is shown to be more conservative than the ice

accretion identified in the requirement. In addition, the ice accretion with the most adverse 

effect on handling characteristics may be used for compliance with the airplane performance 

requirements if any difference in performance is conservatively taken into account. Ice 

accretion(s) used to show compliance should take into account the speeds, configurations 

(including configuration changes), angles of attack, power or thrust settings, etc. for the 

flight phases and icing conditions they are intended to cover.

f. The applicant should determine the most critical ice accretion in terms of handling 

characteristics and performance for each flight phase. Parameters to be considered include: 

• Flight conditions (for example, airplane configuration, speed, angle-of-attack, 

altitude) and 

• Atmospheric icing conditions for which certification is desired (for example, 

temperature, liquid water content (LWC), mean effective drop diameter (MED), 

drop median volume diameter (MVD)).

g. For each phase of flight, the shape, chordwise and spanwise, and the roughness of the 

shapes, considered in selection of a critical ice shape should accurately reflect the full range 

of atmospheric icing conditions for which certification is desired in terms of MED, LWC , 

MVD, and temperature during the respective phase of flight. Justification and selection of 

the most critical ice shape for each phase of flight should be agreed to by the responsible 

ACO. 

h. See Appendix R of FAA AC 20-73A, Aircraft Ice Protection, for additional detailed 

information about determining the applicable critical ice accretion (shape and roughness).
7/30/08 AC 25-25X

Appendix 1

A1-3

2. ICE ACCRETIONS WITH AN OPERATIVE ICE PROTECTION SYSTEM.

a. All Flight Phases Except Takeoff. 

(1) For unprotected parts, the ice accretion to be considered should be determined in 

accordance with Appendices C and O to part 25. 

(2) Unprotected parts consist of the unprotected airfoil leading edges and all 

unprotected airframe parts on which ice may accrete. The effect of ice accretion on 

protuberances such as antennas or flap hinge fairings need not normally be investigated. 

However, airplanes characterized by unusual unprotected airframe protuberances, for 

example fixed landing gear, large engine pylons, or exposed control surface horns or 

winglets may experience significant additional effects, which should therefore be taken 

into consideration. 

(3) For holding ice, the applicant should determine the effect of a 45-minute hold in 

continuous maximum icing conditions. The analysis should assume that the airplane 

remains in a rectangular “race track” pattern, with all turns being made within the icing 

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

some previous airplane certification programs, the maximum pinnacle height was 

limited to 3 inches. This method of compliance may continue to be accepted for 

follow-on products if service experience has been satisfactory, and the designs are 

similar enough to conclude that the previous experience is applicable. The applicant 

should substantiate the critical MED, LWC, and temperature resulting in the formation 

of an ice accretion critical to the airplane’s performance and handling qualities. The 

shape and texture of the ice are important and should be agreed to by the responsible 

ACO. 

(4) For protected parts, the ice protection systems are normally assumed to be operative. 

However, the applicant should consider the effect of ice accretion on or behind the 

protected surfaces that results from the rest time of a deicing cycle. Performance may 

be established on the basis of a representative intercycle ice accretion for normal 

operation of the deicing system. (The applicant should also consider effects of each

residual ice accretion that is not shed). The average drag increment determined over the 

deicing cycle may be used for performance calculations. Consider the effect of— 

(a) Runback ice that occurs on or downstream of the protected surface (including 

de-icing boots). 

(b) Ice accretion prior to activation and operation of the ice protection system. 

(See paragraph 2(c) of this appendix.)
7/30/08 AC 25-25X

Appendix 1

A1-4

b. Takeoff Phase. 

(1) For both unprotected and protected parts, the ice accretion identified in 

Appendices C and O to part 25 for the takeoff phase may be determined by calculation, 

assuming the following.

• Airfoils, control surfaces and, if applicable, propellers are free from frost, 

snow, or ice at the start of the takeoff; 

• The ice accretion starts at liftoff; 

• The airplane is at the critical ratio of power/thrust-to-weight; 

• Failure of the critical engine occurs at VEF; and

• The flightcrew activates the ice protection system in accordance with an AFM 

procedure, except that after beginning the takeoff roll no flightcrew action to 

activate the ice protection system should be assumed to occur until the airplane 

is 400 feet above the takeoff surface. 

(2) The ice accretions identified in Appendices C and O to part 25 for the takeoff phase 

are: 

• Takeoff ice. The most critical ice accretion on unprotected surfaces, and any 

ice accretion on protected surfaces appropriate to normal ice protection system 

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

assuming accretion starts at liftoff in the takeoff maximum icing conditions of 

Appendix C, part I(c) to part 25. 

• Final takeoff ice. The most critical ice accretion on unprotected surfaces, 

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

protection system operation, between 400 feet and the height at which the 

transition to the en route configuration and speed is completed, or 1,500 feet 

above the takeoff surface, whichever is higher, assuming accretion starts at 

liftoff. 

c. Ice Accretion Before Activation and Effective System Operation. 

(1) When considering ice accretion before the ice protection system has been activated 

and is performing its intended function, you should take into account the means of 

activating the ice protection system and the system response time. System response 

time is defined as the time interval between activation of the system and its effective 

operation (for example, for a thermal ice protection system used for de-icing, the time 

to heat the surface and perform its de-icing function). If activation of the ice protection 
7/30/08 AC 25-25X

Appendix 1

A1-5

system depends on flightcrew recognition of icing conditions or response to a cockpit 

annunciation, appropriate delays in identifying the icing conditions and activating the 

ice protection system should be taken into account. For the icing conditions of 

Appendix C, the airplane should be assumed to be in continuous maximum icing 

conditions during the time between entering the icing conditions and effective operation 

of the ice protection system. 

(2) Section 25.1419(e) requires one of the following three methods for detecting icing 

and activating the airframe ice protection system:

(a) A primary ice detection system that automatically activates or that alerts the 

flightcrew to activate the airframe ice protection system;

(b) A definition of 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; or

(c) Identification of conditions conducive to airframe icing as defined by an 

appropriate static or total air temperature and visible moisture for use by the 

flightcrew to activate the airframe ice protection system.

(3) The following guidance should be used to determine the ice accretion on the 

unprotected and protected aerodynamic surfaces before activation and normal system 

operation of the ice protection system.

(a) If the ice protection system activates automatically after annunciation from a 

primary ice detection system, the assumed ice accretion should take into account 

the time it takes for automatic activation of the ice protection system and the time 

it takes for the system to perform its intended function. The assumed ice accretion 

can be determined as follows:

1 The ice accretion on the protected surfaces corresponding to the time 

between entry into the icing conditions and activation of the system, plus

2 The ice accretion during the system response time. 

(b) If ice protection system activation depends on pilot action following 

annunciation from a primary ice detection system, the assumed ice accretion 

should take into account flightcrew delays in activating the ice protection system 

and the time it takes for the system to perform its intended function. The assumed

ice accretion can be determined as follows: 

1 The ice accretion corresponding to the time between entry into the icing 

conditions and annunciation from the primary ice detection system, plus
7/30/08 AC 25-25X

Appendix 1

A1-6

2 The ice accretion corresponding to 10 additional seconds of operation in 

icing conditions, plus

3 The ice accretion during the system response time. 

(c) If ice protection system activation depends on the flightcrew visually

recognizing the first indication of ice accretion on a reference surface (for example, 

an ice accretion probe) combined with an advisory ice detection system, the 

assumed ice accretion should take into account flightcrew delays in detecting the 

accreted ice and in activating the ice protection system, and the time it takes for the 

system to perform its intended function. This may be determined as follows: 

1 The ice accretion that would be easily recognizable by the flightcrew under 

all foreseeable conditions (for example, at night in clouds) as it corresponds to 

the first indication of ice accretion on the reference surface, plus

2 The ice accretion equivalent to 30 seconds of operation in the continuous 

maximum icing conditions of Appendix C, part I(a) to part 25, plus

3 The ice accreted during the system response time. 

(d) If ice protection system activation depends on pilot identification of icing 

conditions (as defined by an appropriate static or total air temperature in 

combination with visible moisture conditions) with or without an advisory ice 

detector, the assumed ice accretion should take into account flightcrew delays in 

recognizing the presence of icing conditions and flightcrew delays in activating the 

ice protection system, and the time it takes for the system to perform its intended 

function. This may be determined as follows: 

1 The ice accretion equivalent to 30 seconds of operation in the continuous 

maximum icing conditions of Appendix C, part I(a) to part 25, plus

2 The ice accretion during the system response time.

3. ICE ACCRETED DURING ICE PROTECTION SYSTEM FAILURE.

a. Unprotected Parts. The same accretion as in paragraph 2(a), above, is applicable. 

b. Protected Parts Following System Failure. The assumed ice accretion for 

failures of the ice protection system is defined as follows:

(1) If the failure condition is not annunciated, the ice accretion on normally protected 

parts where the ice protection system has failed should be assumed to be the same as the 

accretion specified for unprotected parts. 
7/30/08 AC 25-25X

Appendix 1

A1-7

(2) If the failure condition is annunciated and the associated AFM procedure does not 

require the airplane to exit icing conditions, the ice accretion on normally protected 

parts where the ice protection system has failed should be assumed to be the same as the 

accretion specified for unprotected parts. 

(3) If the failure condition is annunciated and the associated AFM procedure requires 

the airplane to exit icing conditions as soon as possible, the ice accretion on normally 

protected parts where the ice protection has failed should be assumed to be one-half of 

the accretion specified for unprotected parts unless another value is agreed to by the 

responsible ACO. 

4. ADDITIONAL GUIDANCE FOR APPENDIX O ICE ACCRETIONS

a. Ice Accretion in Appendix O Conditions Before those Conditions Have 

Been Detected by the Flightcrew. This ice accretion, defined as pre-detection ice in 

Appendix O, part II(b)(5), refers to the ice accretion existing at the time the flightcrew 

becomes aware that they are in Appendix O icing conditions and have taken action to begin

exiting from all icing conditions. 

(1) Both direct entry into Appendix O icing conditions and entry into Appendix O icing 

conditions from flight in Appendix C icing conditions should be considered. 

(2) The time that the applicant should assume it will take to detect Appendix O icing 

conditions exceeding those for which the airplane is certified should be based on the 

means of detection. AC 25.1419/1420 provides guidance for certifying the detection 

means. In general, we expect that the time to detect exceedance icing conditions may 

be significantly longer for a detection means relying on the flightcrew seeing and 

recognizing a visual icing cue than it is for an ice detection system that provides an 

attention-getting alert to the flightcrew.

(3) Visual detection requires time for accumulation on the reference surface(s) of 

enough ice to be reliably identified by either pilot in all atmospheric and lighting 

conditions. Time between pilot scans of reference surface(s) should be considered.

(a) The amount of ice needed for reliable identification is a function of the

distinguishing characteristics of the ice (for example, size, shape, contrast 

compared to the surface feature that it is adhered to), the distance from the pilots 

(for example, windshield vs. engine vs. wingtip), and the relative viewing angle 

(location with respect to the pilots’ primary fields of view).

(b) Pilot scan time of the reference surface(s) will be influenced by many factors. 

Such factors include phase of flight, workload, frequency of occurrence of 

Appendix O conditions, pilot awareness of the possibility of supercooled large 

drop conditions, and ease of seeing the reference surface(s). The infrequency of 
7/30/08 AC 25-25X

Appendix 1

A1-8

Appendix O conditions (approximately 1 in 100 to 1 in 1000, on average in all 

worldwide icing encounters) and the high workload associated with some phases of 

flight in instrument conditions (for example, approach and landing) justify using a 

conservative estimate for the time between pilot scans.

(c) In the absence of specific studies or tests validating visual detection times, the 

following times should be used for visual detection of exceedance icing conditions 

following accumulation of enough ice to be reliably identified by either pilot in all 

atmospheric and lighting conditions:

1 For a visual reference located on or immediately outside a cockpit window 

(for example, ice accretions on side windows, windshield wipers, or icing 

probe near the windows) – 3 minutes.

2 For a visual reference located on a wing, wing mounted engine, or wing tip 

– 5 minutes.

b. Ice Accretions for Encounters with Appendix O Conditions Beyond those 

in Which the Airplane is Certified to Operate.

(1) Use the ice accretions in Table 1, below, to evaluate compliance with the applicable 

subpart B requirements for operating safely after encountering Appendix O atmospheric 

icing conditions for which the airplane is not approved, and then safely exiting all icing 

conditions.

(2) These ice accretions apply when the airplane is not certified for flight in any portion 

of Appendix O atmospheric icing conditions, when the airplane is certified for flight in 

only a portion of Appendix O conditions, and for any flight phase for which the airplane

is not certified for flight throughout the Appendix O icing envelope.

(3) Table 1 shows the scenarios to be used for determining ice accretions for 

certification testing of encounters with Appendix O conditions beyond those in which 

the airplane is certified to operate (for detecting and exiting those conditions):
7/30/08 AC 25-25X

Appendix 1

A1-9

Table 1

Flight 

Phase/Condition -

Appendix O Detect-and-Exit Ice Accretion

Ground Roll No accretion

Takeoff No accretion1

Final Takeoff No accretion1

En Route En Route Detect-and-Exit Ice

Combination of:

(1) Either Appendix C en route ice or Appendix O en route ice 

for which the airplane is approved, whichever is applicable, 

(2) Pre-detection ice, 

(3) Accretion from one standard cloud horizontal extent (17.4 

nautical miles) in Appendix O conditions for which the airplane 

is not approved, and 

(4) Accretion from one standard cloud horizontal extent (17.4 

nautical miles) in Appendix C continuous maximum icing 

conditions.

Holding Holding Detect-and-Exit Ice

Combination of: 

(1) Either Appendix C holding ice or Appendix O holding ice for 

which the airplane is approved, whichever is applicable, 

(2) Pre-detection ice, 

(3) Accretion from one standard cloud horizontal extent (17.4 

nautical miles) in Appendix O conditions for which the airplane 

is not approved, and 

(4) Accretion from one standard cloud horizontal extent (17.4 

nautical miles) in Appendix C continuous maximum icing 

conditions. 
7/30/08 AC 25-25X

Appendix 1

A1-10

Flight 

Phase/Condition -

Appendix O Detect-and-Exit Ice Accretion

The total time in icing conditions need not exceed 45 minutes.

Approach Approach Detect-and-Exit Ice

The more critical of holding detect-and-exit ice and the 

combination of: 

(1) Ice accreted during a descent in the cruise configuration 

from the maximum vertical extent of the Appendix C maximum 

continuous icing conditions or the Appendix O icing 

environment for which the airplane is approved, whichever is 

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

transition to the approach configuration is made, 

(2) Pre-detection ice, and 

(3) Ice accreted at 2,000 feet above the landing surface while 

transiting one standard cloud horizontal extent (17.4 nautical 

miles) in Appendix O conditions for which the airplane is not 

approved and one standard cloud horizontal extent (17.4 

nautical miles) in Appendix C continuous maximum icing 

conditions.

Landing Landing Detect-and-Exit Ice

The more critical of holding detect-and-exit ice and the 

combination of: 

(1) Either Appendix C or Appendix O approach and landing ice 

for which the airplane is approved, whichever is applicable, 

(2) Pre-detection ice, and 

(3) Ice accreted during an exit maneuver beginning with the 

minimum climb gradient specified in § 25.119 from a height of 

200 feet above the landing surface and transiting through one 

standard cloud horizontal extent (17.4 nautical miles) in 

Appendix O conditions for which the airplane is not approved, 

and one standard cloud horizontal extent (17.4 nautical miles) 

in Appendix C continuous maximum icing conditions.

For the purposes of defining the landing detect-and-exit ice 
7/30/08 AC 25-25X

Appendix 1

A1-11

Flight 

Phase/Condition -

Appendix O Detect-and-Exit Ice Accretion

shape, the Appendix C approach and landing ice is defined as 

the ice accreted during:

• A descent in the cruise configuration from the maximum 

vertical extent of the Appendix C maximum continuous 

icing environment to 2,000 feet above the landing 

surface, 

• A transition to the approach configuration and 

maneuvering for 15 minutes at 2,000 feet above the 

landing surface, and

• A descent from 2,000 feet to 200 feet above the landing 

surface with a transition to the landing configuration.

Ice Accretion Before 

the Ice Protection 

System Has Been 

Activated and is 

Performing its 

Intended Function

Ice accreted on protected and unprotected surfaces during the 

time it takes for icing conditions (either Appendix C or Appendix 

O) to be detected, the ice protection system to be activated, 

and the ice protection system to become fully effective in 

performing its intended function.

Ice Accretion in 

Appendix O

Conditions Before

Those Conditions 

Have Been Detected 

by the Flightcrew and 

Actions Taken, in 

Accordance With the 

AFM, to Either Exit 

All Icing Conditions 

or Continue Flight in 

Appendix O Icing 

Conditions

Ice accreted on protected and unprotected surfaces during:

• The time it takes to detect and identify Appendix O

conditions (based on the method of detection) beyond 

those in which the airplane is certified to operate, and 

• The time it takes the flight crew to refer to and act on 

procedures, including coordinating with Air Traffic 

Control, to exit the icing conditions beyond those in 

which the airplane is certified to operate. 

A minimum time period of two minutes should be used as the 

time needed for the flightcrew to refer to and act on the 

procedures to exit the icing conditions after the Appendix O

icing conditions are recognized.

Failures of the Ice 

Protection System

No accretion2
7/30/08 AC 25-25X

Appendix 1

A1-12

Notes:

1 Takeoff is not permitted when Appendix O conditions beyond those in which the

airplane is certified to operate exist in the vicinity of departure airport.

2 It is not necessary to consider an unintentional encounter with Appendix O icing 

conditions beyond those in which the airplane is certified to operate while operating with 

a failed ice protection system.

c. Ice Accretions for Encounters with Appendix O Atmospheric Icing 

Conditions in Which the Airplane is Certified to Operate.

(1) The applicant should use the ice accretions in Table 2 to evaluate compliance with 

the applicable subpart B requirements for operating safely in the Appendix O

atmospheric icing conditions for which the airplane is approved.

(2) The decision about which ice accretions to use should include consideration of 

combinations of Appendix C and Appendix O icing conditions within the scenarios 

defined in paragraph 4c(3) of this appendix. For example, flight in Appendix O

conditions may result in ice accumulating, and potentially forming a ridge, behind a 

protected surface. Once this accretion site has been established, flight in Appendix C 

icing conditions for the remaining portion of the applicable flight phase scenario may 

result in a more critical additional accretion than would occur for continued flight in 

Appendix O icing conditions.

(3) Table 2 shows the scenarios the applicant should use for determining ice accretions 

for certification for flight in the icing conditions of Appendix O to part 25.

Table 2

Flight 

Phase/Condition

Appendix O Ice Accretion

Ground Roll No accretion

Takeoff Takeoff Ice

Ice accretion occurring between liftoff and 400 feet above the 

takeoff surface assuming ice accretion starts at liftoff.

Final Take-off Final Take-off Ice
7/30/08 AC 25-25X

Appendix 1

A1-13

Flight 

Phase/Condition

Appendix O Ice Accretion

Ice accretion occurring between a height of 400 ft above the 

takeoff surface and the height at which the transition to the 

en-route configuration and speed is completed, or 1500 feet 

above the takeoff surface, whichever is higher, assuming ice 

accretion starts at liftoff.

En Route En Route Ice

Ice accreted during the en route phase of flight.

Holding Holding Ice

Ice accreted during a 45 minute hold with no reduction for 

horizontal cloud extent (that is, the hold is conducted entirely 

within the 17.4 nautical mile standard cloud extent).

Approach Approach Ice

More critical ice accretion of: 

(1) Ice accreted during a descent in the cruise configuration 

from the maximum vertical extent of the Appendix O icing 

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

by—

• transition to the approach configuration and 

• maneuvering for 15 minutes at 2,000 feet above the 

landing surface; and 

(2) Holding ice (if the airplane is certified for holding in 

Appendix O conditions).

Landing Landing Ice

More critical ice accretion of: 

(1) Approach ice plus ice accreted during descent from 2,000 

feet above the landing surface to 200 feet above the landing 

surface with—

• a transition to the landing configuration, followed by 

• a go-around maneuver beginning with the minimum 

climb gradient specified in § 25.119 from 200 feet to 
7/30/08 AC 25-25X

Appendix 1

A1-14

Flight 

Phase/Condition

Appendix O Ice Accretion

2,000 feet above the landing surface, 

• holding for 15 minutes at 2,000 feet above the landing 

surface in the approach configuration, and 

• a descent to the landing surface in the landing 

configuration, and 

(2) Holding ice (if the airplane is certified for holding in 

Appendix O conditions).

Ice Accretion Before 

the Ice Protection 

System has been 

Activated and is 

Performing its Intended 

Function

Ice accreted during the time it takes for the flightcrew to 

recognize icing conditions and activate the ice protection 

system, plus the time for the ice protection system to perform 

its intended function.

Ice Accretion in 

Appendix O Conditions 

Before those 

Conditions have been 

Detected by the 

Flightcrew and Actions 

Taken, in Accordance 

With the AFM, to Either 

Exit All Icing 

Conditions or Continue 

Flight in Appendix O

Icing Conditions

Ice accreted during the time it takes for the flightcrew to detect 

Appendix O conditions and refer to and initiate associated 

procedures, and any time it takes for systems to perform their 

intended functions (if applicable). Pre-detection ice need not 

be considered if there are no specific crew actions or systems 

changes associated with flight in Appendix O conditions.

Failures of the Ice 

Protection System

Same criteria as for Appendix C (see paragraph 3 of this 

appendix), but in Appendix O conditions.
7/30/08 AC 25-25X

Appendix 2

A2-1

APPENDIX 2

SIMULATED ICE ACCRETIONS

1. OVERVIEW.

a. In general, the simulated ice accretions used for flight testing should be those that have 

the most adverse effects on handling characteristics. However, if analytical data show that 

other reasonably expected ice accretions could be generated that could have a more adverse 

effect on performance, then the ice accretion having the most adverse effect on handling 

characteristics may be used for performance tests only if each difference in performance can 

be conservatively taken into account.

b. Simulated accretions should be representative of natural icing conditions in terms of 

location, general shape, thickness, and texture. The determination of the form and surface 

texture of the ice accretion (see paragraph 2, below) should be agreed to by the responsible 

ACO as being representative of natural ice accretion. 

c. Sandpaper ice is addressed in paragraph 3, below. 

2. SHAPE AND TEXTURE OF SIMULATED ICE ACCRETION. 

a. The shape and texture of the simulated ice accretion should be established and 

substantiated by agreed-upon methods. Common practices used to accomplish this include: 

• Use of computer codes, 

• Flight in measured natural icing conditions, 

• Icing wind tunnel tests, and 

• Flight in a controlled simulated icing cloud (for example, from an icing tanker). 

b. Unless another texture is substantiated by the applicant and agreed to by the ACO, the 

applicant should use an ice accretion roughness height of 3 mm with a particle density of 

8 to 10/cm2

.
7/30/08 AC 25-25X

Appendix 2

A2-2

3. SANDPAPER ICE. 

a. Sandpaper ice is the most critical thin, rough layer of ice. Carborundum sandpaper 

no. 40 (40-grit carborundum sandpaper) has been used in past certification programs to 

represent sandpaper ice. However, as detailed in Appendix R of AC 20-73A, the uniformly 

distributed roughness of carborundum grit may not result in aerodynamic effects similar to 

those of the actual intercycle ice surface roughness. The applicant should validate use of 

uniformly distributed roughness to simulate sandpaper ice, particularly for intercycle ice 

accretions.

b. Sandpaper ice is not relevant to the supercooled large drop icing conditions of Appendix 

O to part 25.

c. The spanwise and chordwise coverage of simulated ice should be consistent with the 

areas of ice accretion determined for the conditions of part 25, Appendix C, except that, for 

the zero-g pushover maneuver of paragraph 3i(4)(c)3 of this AC, the sandpaper ice may be 

restricted to the horizontal stabilizer if this can be shown to be conservative.

d. Appendix C, part II(a) requires applicants to use the most critical ice accretion to show 

compliance with the applicable subpart B airplane requirements for performance and 

handling characteristics in icing conditions. When determining the most critical ice 

accretion, the applicant must consider the full range of atmospheric icing conditions of part I 

of Appendix C as well as the characteristics of the ice protection system, per § 25.21(g)(1) 

and Appendix C, part II(a). This would include consideration of sandpaper ice (as well as 

any other type of ice accretion that may occur in the applicable atmospheric icing 

conditions), taking into account the operating characteristics of the ice protection system and 

the flight phase. Sandpaper ice can be particularly critical for showing compliance with 

either § 25.143(i)(2) or (j)(2). If sandpaper ice can be present, it must be considered in 

showing compliance with both of these requirements.
7/30/08 AC 25-25X

Appendix 3

A3-1

APPENDIX 3

DESIGN FEATURES

1. AIRPLANE CONFIGURATION AND ANCESTRY. An airplane’s size is an important 

design characteristic that can affect performance, controllability, and maneuverability in icing 

conditions. The safety record of the airplane’s closely-related ancestors is another aspect that 

may be taken into consideration.

a. Size. The size of an airplane determines the sensitivity of its flight characteristics to ice 

thickness and roughness. The relative effect of a given ice height (or ice roughness height) 

decreases as airplane size increases. 

b. Ancestors. If a closely-related ancestor airplane was certified for flight in icing 

conditions, that airplane’s safety record may be used on a similarity basis when evaluating

the subject airplane’s general capabilities for flight in icing conditions.

2. WING. Design features of a wing that can affect performance, controllability, and 

maneuverability include airfoil type, leading edge devices, and lateral control system 

effectiveness. 

a. Airfoil Type. Aerodynamic effects of ice accretions result mainly from the effects of 

the ice accretion on the behavior of the airfoil’s boundary layer. The boundary layer is the 

layer of air close to the surface of the airfoil that is moving across the airfoil at a velocity 

lower than the freestream velocity, that is, the velocity of the airfoil. Ice accretions that

occur in areas favorable to keeping the boundary layer attached to the aircraft surface will 

result in effects that are less aerodynamically adverse than ice accretions that occur in areas 

less favorable to attached boundary layer conditions. Ice shapes that build up in areas of 

local airflow deceleration (positively increasing surface pressure), or result in conditions 

unfavorable to keeping attached flow conditions, as the airflow negotiates the ice surface, 

will result in the most adverse effects.

b. Leading Edge Devices. The presence of a leading edge device (such as a slat) 

reduces the percentage decrease in CLmax** due to ice by increasing the overall level of CL

***. 

Movement of a slat so that a gap forms may improve the situation further. Leading edge 

devices can also reduce loss in angle of attack at stall due to ice. 

c. Lateral Control System Effectiveness. The applicant can evaluate effectiveness of 

the lateral control system in icing conditions by comparing it with the lateral control system 

in closely-related ancestor airplanes, with consideration of the design of aerodynamic 

balance surfaces. 

 ** Maximum lift coefficient. *** Lift coefficient.
7/30/08 AC 25-25X

Appendix 3

A3-2

3. EMPENNAGE. The effects of size and airfoil type also apply to the horizontal and vertical 

tails. Other design features include tailplane design philosophy, horizontal stabilizer design, 

control surface actuation, control surface size, and vertical stabilizer and rudder design. Since 

tails are usually not equipped with leading edge devices, the effects of ice on tail aerodynamics 

are similar to those on a wing with no leading edge devices. However, these effects usually 

result in changes to airplane handling and/or control characteristics rather than degraded 

performance.

a. Tailplane Design Philosophy. The tailplane may be designed and sized to provide 

full functionality in icing conditions without ice protection, or it may be designed with a 

deicing or anti-icing system. 

b. Horizontal Stabilizer Design. Cambered airfoils and/or trimmable stabilizers may 

reduce the susceptibility to and consequences of elevator hinge moment reversal due to iceinduced tailplane stall. 

c. Control Surface Actuation. Hydraulically powered irreversible elevator controls are 

usually not affected by ice-induced aerodynamic hinge moment reversal. 

d. Control Surface Size. For mechanical elevator controls, the size of the surface 

significantly affects the control force resulting from an ice-induced aerodynamic hinge 

moment reversal. Small surfaces are less susceptible to control difficulties for given hinge 

moment coefficients. 

e. Vertical Stabilizer and Rudder Design. Effectiveness of the vertical stabilizer and 

rudder in icing conditions can be evaluated by comparison with the vertical stabilizer and 

rudder in closely-related ancestor airplanes, with consideration of the design of aerodynamic 

balance surfaces. 

4. AERODYNAMIC BALANCING OF FLIGHT CONTROL SURFACES. 

a. The aerodynamic balance of unpowered or boosted reversible flight control surfaces is 

an important design feature to consider. The applicant should carefully evaluate the design 

to account for effects of ice accretion on flight control system hinge moment 

characteristics. Closely balanced controls may be vulnerable to overbalance in icing. The 

effect of ice in front of the control surface, or on the surface, may upset the balance of hinge 

moments, leading to either increased positive or negative force gradients. 

b. The aerodynamic balancing of unpowered or boosted reversible flight control surfaces is 

particularly important with respect to lateral flight control systems when large aileron hinge 

moments are balanced by equally large hinge moments on the opposite aileron. Any 

asymmetric disturbance in flow that affects this critical balance can lead to a sudden 

uncommanded deflection of the control. This auto deflection, in extreme cases, may be to 

the control stops. 
7/30/08 AC 25-25X

Appendix 3

A3-3

5. ICE PROTECTION/DETECTION SYSTEM. The ice protection/detection system design 

philosophy may include design features that reduce ice accretion on the wing and/or tailplane.

a. Wing Ice Protection/Detection.

(1) A primary ice detection system that automatically activates a wing deicing 

system may ensure that there is no significant ice accretion on wings that are 

susceptible to performance losses with small amounts of ice. 

(2) If portions of the wing leading edge are not protected, the part that is protected 

may be selected to provide good handling characteristics at stall, with an acceptable 

performance degradation. 

b. Tail Ice Protection/Detection.

(1) A primary ice detection system may automatically activate a tailplane deicing 

system on airplanes that do not have visible cues for system operation. 

(2) An ice protection system on the unshielded aerodynamic balances of airplanes 

with unpowered reversible controls can reduce the risk of ice-induced aerodynamic 

hinge moment reversals. 

7/30/08 AC 25-25X

Appendix 4

A4-1

APPENDIX 4

EXAMPLES OF AIRPLANE FLIGHT MANUAL LIMITATIONS AND OPERATING 

PROCEDURES FOR OPERATIONS IN SUPERCOOLED LARGE DROP ICING 

CONDITIONS

1. AIRPLANE APPROVED FOR FLIGHT IN APPENDIX C ICING CONDITIONS BUT 

NOT APPROVED FOR FLIGHT IN APPENDIX O ICING CONDITIONS.

a. AFM Limitations.

Intentional flight, including takeoff and landing, into freezing drizzle or 

freezing rain conditions is prohibited. If freezing drizzle or freezing rain 

conditions are encountered, or if {insert cue description here}, immediately 

request priority handling from air traffic control to facilitate a route or altitude 

change to exit all icing conditions. Stay clear of all icing conditions for the 

remainder of the flight, including landing, unless it can be determined that ice 

accretions no longer remain on the airframe.

b. AFM Operating Procedures (Normal Procedures Section).

Freezing drizzle and freezing rain conditions are severe icing conditions for 

this airplane. Intentional flight, including takeoff and landing, into freezing 

drizzle or freezing rain conditions is prohibited. A flight delay or diversion to 

an alternate airport is required if these conditions exist at the departure or 

destination airports.

{insert cue description here} is one indication of severe icing for this airplane.

If severe icing is encountered, immediately request priority handling from air 

traffic control to facilitate a route or altitude change to exit all icing 

conditions. Stay clear of all icing conditions for the remainder of the flight, 

including landing, unless it can be determined that ice accretions no longer 

remain on the airframe.

c. Flightcrew Operating Manual Operating Procedures.

Warning: Severe icing may result from environmental conditions outside of 

those for which this airplane is certified. Intentional flight into severe icing 

conditions may result in ice build-up on protected surfaces exceeding the 

capability of the ice protection system, or in ice forming aft of the protected 

surfaces. This ice may not be shed when using the ice protection systems, and 

may seriously degrade performance and controllability of the airplane.
7/30/08 AC 25-25X

Appendix 4

A4-2

Operations in icing conditions were evaluated as part of the certification 

process for this airplane. Freezing drizzle and freezing rain conditions were 

not evaluated and are considered severe icing conditions for this airplane.

Intentional flight, including takeoff and landing, into freezing drizzle or 

freezing rain conditions is prohibited. A flight delay or diversion to an 

alternate airport is required if these conditions exist at the departure or 

destination airports. {insert cue description here}is an indication of severe 

icing conditions that exceed those for which this airplane is certified. If severe 

icing is encountered, immediately request priority handling from air traffic 

control to facilitate a route or altitude change to exit all icing conditions. Stay 

clear of all icing conditions for the remainder of the flight, including landing, 

unless it can be determined that ice accretions no longer remain on the 

airframe.

2. AIRPLANE APPROVED FOR FLIGHT IN APPENDIX C ICING CONDITIONS AND 

FREEZING DRIZZLE CONDITIONS OF APPENDIX O BUT NOT APPROVED FOR

FLIGHT IN FREEZING RAIN CONDITIONS OF APPENDIX O.

a. AFM Limitations.

Intentional flight, including takeoff and landing, into freezing rain conditions 

is prohibited. If freezing rain conditions are encountered, or if {insert cue

description here}, immediately request priority handling from air traffic 

control to facilitate a route or altitude change to exit all icing conditions. Stay 

clear of all icing conditions for the remainder of the flight, including landing, 

unless it can be determined that ice accretions no longer remain on the 

airframe.

b. AFM Operating Procedures (Normal Procedures Section).

Freezing rain conditions are severe icing conditions for this airplane. 

Intentional flight, including takeoff and landing, into freezing rain conditions 

is prohibited. A flight delay or diversion to an alternate airport is required if 

these conditions exist at the departure or destination airports.

{insert cue description here} is one indication of severe icing for this airplane. 

If severe icing is encountered, immediately request priority handling from air 

traffic control to facilitate a route or altitude change to exit all icing 

conditions. Stay clear of all icing conditions for the remainder of the flight, 

including landing, unless it can be determined that ice accretions no longer 

remain on the airframe.
7/30/08 AC 25-25X

Appendix 4

A4-3

c. Flightcrew Operating Manual Operating Procedures.

Warning: Severe icing may result from environmental conditions outside of 

those for which this airplane is certified. Intentional flight into severe icing 

conditions may result in ice build-up on protected surfaces exceeding the 

capability of the ice protection system, or may result in ice forming aft of the 

protected surfaces. This ice may not be shed when using the ice protection 

systems, and may seriously degrade the performance and controllability of the 

airplane.

Operations in icing conditions were evaluated as part of the certification 

process for this airplane. Freezing rain conditions were not evaluated and are

considered severe icing conditions for this airplane.

Intentional flight, including takeoff and landing, into freezing rain conditions 

is prohibited. A flight delay or diversion to an alternate airport is required if 

these conditions exist at the departure or destination airports. {insert cue 

description here}is an indication of severe icing conditions that exceed those 

for which this airplane is certified. If severe icing is encountered, immediately 

request priority handling from air traffic control to facilitate a route or altitude 

change to exit all icing conditions. Stay clear of all icing conditions for the 

remainder of the flight, including landing, unless it can be determined that ice 

accretions no longer remain on the airframe.

3. AIRPLANE APPROVED FOR FLIGHT IN APPENDIX C AND APPENDIX O ICING 

CONDITIONS EXCEPT FOR EN ROUTE AND HOLDING FLIGHT PHASES IN 

APPENDIX O ICING CONDITIONS.

a. AFM Limitations.

Intentional holding or en route flight into freezing drizzle or freezing rain

conditions is prohibited. If freezing drizzle or freezing rain conditions are

encountered during a hold (in any airplane configuration) or in the en route

phase of flight (climb, cruise, or descent with high lift devices and gear 

retracted), or if {insert cue description here}, immediately request priority 

handling from air traffic control to facilitate a route or altitude change to exit 

all icing conditions. Stay clear of all icing conditions for the remainder of the 

flight, including landing, unless it can be determined that ice accretions no 

longer remain on the airframe.

b. AFM Operating Procedures (Normal Procedures Section).

Freezing drizzle and freezing rain conditions encountered during a hold (in 

any airplane configuration) or in the en route phase of flight (climb, cruise, or
7/30/08 AC 25-25X

Appendix 4

A4-4

descent with high lift devices and gear retracted) are severe icing conditions 

for this airplane. Intentional holding or en route flight into freezing drizzle or

freezing rain conditions is prohibited.

{insert cue description here} is one indication of severe icing for this airplane. 

If severe icing is encountered, immediately request priority handling from air 

traffic control to facilitate a route or altitude change to exit all icing 

conditions. Stay clear of all icing conditions for the remainder of the flight, 

including landing, unless it can be determined that ice accretions no longer 

remain on the airframe.

c. Flightcrew Operating Manual Operating Procedures.

Warning: Severe icing may result from environmental conditions outside of 

those for which this airplane is certified. Intentional flight into severe icing 

conditions may result in ice build-up on protected surfaces exceeding the 

capability of the ice protection system, or in ice forming aft of the protected 

surfaces. This ice may not be shed when using the ice protection systems, and 

may seriously degrade the performance and controllability of the airplane.

Operations in icing conditions were evaluated as part of the certification 

process for this airplane. En route (climb, cruise, and descent with high left 

devices and gear retracted) and holding flight (in any airplane configuration) 

in freezing drizzle and freezing rain conditions were not evaluated and are 

considered severe icing conditions for this airplane.

Intentional holding or en route flight into freezing drizzle or freezing rain

conditions is prohibited. {insert cue description here}is an indication of 

severe icing conditions that exceed those for which the airplane is certified. If 

severe icing is encountered, immediately request priority handling from air 

traffic control to facilitate a route or altitude change to exit all icing 

conditions. Stay clear of all icing conditions for the remainder of the flight, 

including landing, unless it can be determined that ice accretions no longer 

remain on the airframe.

4. AIRPLANE APPROVED FOR FLIGHT IN APPENDIX C ICING CONDITIONS AND 

A PORTION OF APPENDIX O ICING CONDITIONS.

a. AFM Limitations.

Intentional flight, including takeoff and landing, into {insert pilot usable

description here} conditions is prohibited. If {insert pilot usable description 

here} conditions are encountered, or if {insert cue description here}, 

immediately request priority handling from air traffic control to facilitate a 

route or altitude change to exit all icing conditions. Stay clear of all icing 
7/30/08 AC 25-25X

Appendix 4

A4-5

conditions for the remainder of the flight, including landing, unless it can be 

determined that ice accretions no longer remain on the airframe.

b. AFM Operating Procedures (Normal Procedures Section).

{insert pilot usable description here} are severe icing conditions for this

airplane. Intentional flight, including takeoff and landing, into {insert pilot

usable description here} conditions is prohibited. A flight delay or diversion 

to an alternate airport is required if these conditions exist at the departure or

destination airports.

{insert cue description here} is one indication of severe icing for this airplane. 

If severe icing is encountered, immediately request priority handling from air 

traffic control to facilitate a route or altitude change to exit all icing 

conditions. Stay clear of all icing conditions for the remainder of the flight, 

including landing, unless it can be determined that ice accretions no longer 

remain on the airframe.

c. Flightcrew Operating Manual Operating Procedures.

Warning: Severe icing may result from environmental conditions outside of 

those for which this airplane is certified. Intentional flight into severe icing 

conditions may result in ice build-up on protected surfaces exceeding the 

capability of the ice protection system, or may result in ice forming aft of the 

protected surfaces. This ice may not be shed when using the ice protection 

systems, and may seriously degrade the performance and controllability of the 

airplane.

Operations in icing conditions were evaluated as part of the certification 

process for this airplane. {insert pilot usable description here} were not 

evaluated and are considered severe icing conditions for this airplane.

Intentional flight, including takeoff and landing, into {insert pilot usable

description here} is prohibited. A flight delay or diversion to an alternate 

airport is required if these conditions exist at the departure or destination 

airports. {insert cue description here}is an indication of severe icing 

conditions that exceed those for which this airplane is certified. If severe icing 

is encountered, immediately request priority handling from air traffic control 

to facilitate a route or altitude change to exit all icing conditions. Remain 

clear of all icing conditions for the remainder of the flight, including landing, 

unless it can be determined that ice accretions no longer remain on the 

airframe.

7/30/08 AC 25-25X

Appendix 5

A5-1

APPENDIX 5

RELATED REGULATIONS AND ADVISORY CIRCULARS

Regulations. The following sections of Title 14, Code of Federal Regulations (14 CFR), 

part 25 are referenced in this AC. The full text of 14 CFR can be downloaded from the Internet 

at http://www.gpoaccess.gov/nara. A paper copy may be ordered from the Government Printing 

Office (GPO), Superintendent of Documents, Attn: New Orders, PO Box 371954, Pittsburgh, 

PA 15250-7954. 

§ 25.21 Proof of compliance

§ 25.23 Load distribution limits

§ 25.25 Weight limits

§ 25.27 Center of gravity limits

§ 25.29 Empty weight and corresponding center of gravity

§ 25.31 Removable ballast

§ 25.33 Propeller speed and pitch limits

§ 25.101 Performance – General

§ 25.103 Stall speed

§ 25.105 Takeoff

§ 25.107 Takeoff speeds

§ 25.109 Accelerate-stop distance

§ 25.111 Takeoff path

§ 25.119 Landing climb: All-engines-operating

§ 25.121 Climb: One-engine-inoperative

§ 25.123 En route flight paths

§ 25.125 Landing

§ 25.143 Controllability and Maneuverability – General

§ 25.145 Longitudinal control

§ 25.147 Directional and lateral control

§ 25.161 Trim

§ 25.171 Stability – General

§ 25.175 Demonstration of static longitudinal stability

§ 25.177 Static lateral-directional stability

§ 25.181 Dynamic stability

§ 25.201 Stall demonstration

§ 25.203 Stall characteristics

§ 25.207 Stall warning

§ 25.231 Longitudinal stability and control
7/30/08 AC 25-25X

Appendix 5

A5-2

§ 25.233 Directional stability and control

§ 25.235 Taxiing condition

§ 25.237 Wind velocities

§ 25.251 Vibration and buffeting

§ 25.253 High-speed characteristics

§ 25.255 Out-of-trim characteristics

§ 25.1309 Equipment, systems, and installations

§ 25.1329 Flight guidance system

§ 25.1419 Ice protection

§ 25.1420 Supercooled large drop icing conditions

§ 25.1581 Airplane Flight Manual – General

§ 25.1583 Operating limitations

§ 25.1585 Operating procedures

§ 25.1587 Performance information

Part 25, Appendix C

Part 25, Appendix O

Advisory Circulars. The following ACs are related to the guidance contained in this AC. An 

electronic copy of the following ACs can be downloaded from the Internet at http://rgl.faa.gov. 

A paper copy may be ordered from the U.S. Department of Transportation, Subsequent 

Distribution Office, M-30, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, MD 

20795.

AC 20-73A Aircraft Ice Protection

AC 25-7A Flight Test Guide for Certification of Transport Category Airplanes

AC 25.1309-1A System Design and Analysis

AC 25.1329-1B Approval of Flight Guidance Systems

AC 25.-1419/1420 Certification of Transport Category Airplanes for Flight in 

Icing Conditions

NOTE: This list represents the latest version of each AC at the time of 

publication. Individual ACs may be updated in the future, so that this list may 

reference a specific version of an AC that has been superseded. You should 

always use the latest version of each AC, even though this document may 

reference a previous version.
7/30/08 AC 25-25X

Appendix 6

A6-1

APPENDIX 6

ACRONYMS AND DEFINITIONS

AC Advisory Circular.

ACO Aircraft Certification Office.

AFM Airplane Flight Manual.

ATTCS Automatic Takeoff Thrust Control System.

CAS Calibrated Airspeed.

CFR Code of Federal Regulations.

FAA Federal Aviation Administration.

ICTS Ice-Contaminated Tailplane Stall.

KIAS Knots Indicated Airspeed.

LWC Liquid Water Content.

MED Mean Effective Diameter.

MVD Median Volumetric Diameter.

CL Lift Coefficient.

CLMAX Maximum Lift Coefficient.

Trim A flight condition in which the aerodynamic moment acting 

about the axis of interest is zero. In the absence of an 

external disturbance no control input is needed to maintain 

the flight condition. 

V1 The maximum speed in the takeoff at which the pilot must 

take the first action (for example, apply brakes, reduce 

thrust, deploy speed brakes) to stop the airplane within the 

accelerate-stop distance. V1 also means the minimum 

speed in the takeoff, following a failure of the critical engine 

at VEF, at which the pilot can continue the takeoff and 

achieve the required height above the takeoff surface within 

the takeoff distance.

V2 Takeoff Safety Speed. (The target speed to be reached by 

the time the airplane is 35 feet above the takeoff surface.) 

V2MIN Minimum Takeoff Safety Speed. 

VCW Crosswind Component of Wind Speed.

VDF/MDF Demonstrated Flight Diving Speed.
7/30/08 AC 25-25X

Appendix 6

A6-2

VEF Engine Failure Speed. The speed at which the critical 

engine is assumed to fail during takeoff.

VFC Maximum Speed for Stability Characteristics.

VFE Maximum Flaps Extended Speed.

VMO Maximum Operating Limit Speed.

VREF Landing Reference Speed

VS 1-g 1-g Stall Speed. The calibrated airspeed at which 

aerodynamic forces alone can support the airplane in 1-g 

flight.

VSR Reference Stall Speed. VSR may not be less than VS 1-g. 

For airplanes with a device that abruptly pushes the nose 

down at a selected angle of attack, (for example, a stick 

pusher), VSR may not be less than 2 knots or 2 percent, 

whichever is greater, above the speed at which the device 

operates.

VSR1 Reference Stall Speed in a specific configuration.