# Advisory
Circular

Subject: PERFORMANCE AND
HANDLING CHARACTERISTICS IN
ICING CONDITIONS Date: DRAFT Initiated By: ANM

- 110 AC No: 25

- 25 X

1. PURPOSE

. This advisory c ircular (AC) describes an acceptable means of sh owing 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 reg ulations. 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 rel evant 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 Service7/30/08 AC 25

- 25X

ii 

### C ONTENTS

Paragraph Page

1. RE QUIREMENTS 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. Engin eering Analysis

. f. Ancestor Airplane Analysis

.

3. ACCEPTABLE MEANS OF COMPLIANCE

— FLIGHT TEST

P ROGRAM

. 15

a. Overview.

b. Stall Speed

, § 25.103

. c. Accelerate

- Stop Distance

, § 25.109

. d. Takeoff Path

, § 25.111

. e. Landing Climb: All

- E ngines

- 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. Direction al 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. Sta ll Demonstration (§ 25.201)/Stall Characteristics

, § 25.203

. r. Stall Warning

, § 25.207

. s. Wind Velocities

, § 25.237

.7/30/08 AC 25

- 25X

iii t. Vibration and Buffeting

, § 25.251

. u. Natural Icing Conditions

, § 25.1419(b)

. v. Failure Conditions

, § 25.1309

.

AP PENDIX 1

AIRFRAME ICE ACCRETION S

1. OVERVIE W

. A1

- 1

2. ICE ACCRETIONS WITH AN OPERATIVE ICE PROTEC TION
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 SYSTE M
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 i n Appendix O Conditions Before t hose
Conditions h ave b een Detected b y t he Flightcrew and Actions Taken, i n Accordance w ith t he AFM, t o Either Exit All Icing Conditions or Continue Flight i n Appendix O Icing
Conditions

. b. Ice Accretions f or Encounters w ith Appendix O Icing
Conditions Beyond t hose f or w hich t he Airplane i s Certified to Operate

. c. Ice Accretions f or Encounters w ith Appendix O Atmospheric Icing Conditions i n Which t he Airplane i s Certified t o Operate

.

APPENDIX 2

SIMULATED ICE ACCRETIONS

1. OVERVIEW. A2

- 1

2. SHAPE AND TEXTURE OF SIMULATED ICE ACCRET ION

. A2

- 1

3. SANDPAPER ICE

. A2

- 27/30/08 AC 25

- 25X

iv APPENDIX 3

DESIGN FEATURES

1. AIRPLANE CONFIGURATI ON AND ANCESTRY

. A3

- 1

a. Size

. b. Ancestors

.

2. WING

. A3

- 1

a. Airfoil Type

. b. Leading Edg e Device s

. c. Lateral Control System Effectiveness.

3. EMPENNAGE

. A3

- 2

a. Tail plane Design Philosophy

. b. Horizontal Stabilizer Design

. c. Control Surface Actuation

. d

. Control Surface Size

. e

. Vertical Stabilizer and Rudder Design

.

4. AERODYNAMIC BALANCIN G OF FLIGHT CONTROL
SURFACES

. A3

- 2

5. ICE PROTECTION/DETEC TION SYSTEM

. A3

- 3

a. Wing Ice Protection/Detection

. b. Tail Ice Protection/Detection

.

APPENDIX 4

EXAMPLES OF AIRPLANE FLIGHT MANUAL LIMITATIONS

AND OPER ATING PROCEDURES FOR OPERATIONS IN
SUPERCOOLED LARGE DROP ICING CONDITIONS

1

. AIRPLANE APPROVED FO R FLIGHT IN APPENDIX C ICING
CONDITIONS BUT NOT A PPROVED FOR FLIGHT I N
APPENDIX O ICING CONDITIONS

. A4

- 1

a. AFM Limitations

. b. AFM Operating Proce dures (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 CONDI TIONS OF APPENDIX O A4

- 2

a. AFM Limitations

. b. AFM Operating Procedures (Normal Procedures Section)

.7/30/08 AC 25

- 25X

v c. Flightcrew Operating Manual Operating Procedures

.

3

. AIRPLANE APPROVED FOR FLIGHT IN APPENDIX C AND

APPENDIX O ICING CONDITIONS EXCEPT FO R 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 APP ENDIX 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

ACRO N YMS AND DEFINITIONS

TABLES

1. Means for Showing Compliance 1 2

2. Controllability & Maneuverability

— Trim Speeds 20
3. Holding Scenario

— Maneuvers 3 6

4. Approach/Landing Scenario

— Maneuvers 3 77/30/08 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 h andling qualit ies 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

. Appen dix 1 of this AC provides detailed guidance for det ermining 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 t o 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 o f 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 superco oled 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

. C ompliance 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

condit ions after an encounter with supercooled large drop atmospheric icing conditions beyond those the airplane is certified for

.7/30/08 AC 25

- 25X

2 ( 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 indicat ing 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 applica nt should provide p rocedures in the a irplane f light m anual 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 env elope. 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 dur ing that phase of flight (not just at the conditions specified in the part 25 s ubpart 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 rol l, take

- off

, and final take

- off segments), en route, holding, and approach/landing (including both the approach and landing segments).

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

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

conditions. Appendi c es 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.

A ny of the applicable i ce accretions (or a composite accretion representing a

combination of accretions) may be used to show compliance with a particular s ubpart B requirement if it is either the ice accretion identified in the requirement or one shown to

be more conservative th an 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. I ce 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 ap plicant desires certification for flight in the supercooled large drop icing conditions of Appendix O in addition to those of A ppendix C, compliance with the applicable subpart B requirements may be shown using the most critical of the A ppendix

C and Appen dix 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 demonstrat ing airplane handling qualities. S imilarly, for ice accreted anywhere on the airplane, the mass of any accreted ice need not normally be

considered.7/30/08 AC 25

- 25X

3 ( 9 ) R equirements 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 presen t

. 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 significantl y reduce t he 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 increas es 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 ICT S 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 lo wer surfaces of the stalled tailplane may result in a high elevator hinge momen t

, 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 colum n) 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 ic e contamination.

( 1 1 ) There have been a irplane controllability incidents in icing conditions as a result of ice on unprotected leading edges of extended trailing edge flap s or flap vane s

. The primary safety concern illus trated 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 hand ling 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

t ests with simulated ice accretions on the extended trailing edge flap or flap vane s 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

4 (12) Supercooled large drop icing conditi ons
, 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 ai leron control reversal. Although airplanes with de

icing 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 c hecking roll control capability are presented in parag raphs 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 regulation s 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 s ection 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 additi onal 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

§ 25.3 1 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 characteristics7/30/08 AC 25

- 25X

5 § 25.255 Out

- of

- trim characteristics

(3) I f 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
accepta ble to return to the non

- icing settings when the critical wing surfaces are free of ice. The applicant should validate that the means for determin ing when the critical wing surfaces are free of ice accretions is reliable under all expected operating condi tions.

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 t he 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 thr ust 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 consid ered:

(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 followin g 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) I ncremental airframe drag due to ice accretion with normal operation of the ice protection system.7/30/08 AC 25

- 25X

6 (c) Changes in operating speeds provided in the a irplane f light m anual (AFM) for flight in ic ing conditions.

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

can determine any incre ase in drag (or decre ase 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 large r 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 assume d (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 us ing appropriate speed adjustments to account fo r operation in icing conditions, all changes in the procedures established for takeoff or go

- around should be agreed to by the responsible a ircraft c ertification o ffice (ACO)

.

e. Stall Speed

, § 25.103. Certification experience in meeting this requireme nt 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 f ailure 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 s hould assume that the probability of entering the ici ng 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 s hould 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

sys tems (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

7 (2) For probable failure conditions of the ice prote ction system that are:

• N ot annunciated to the flightcrew, or

• A nnunciated to the flightcrew

, but the associated procedure doe s not require exiting the icing conditions,

the guidance in this AC for a normal ( that is , non

- failure) condition is applicabl e 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 a s practicable, the fol lowing applies : The applicant should show that

, with the ice

accretion resulting from the failure

, the airplane’s performance and handling

characteristics are adequate as deter mined 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 substant iation 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 Sy stems. The applicant should conside r 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) I ce protection system s 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 appr oach capabilities. T he applicant should also consider the effect of power or thrust setting s 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 wate r completely after melting the ice, resulting in the potential for runback ice.

(2) Ice may block control surface gaps, or freez e seals

, causing increased control forces, control restrictions, or blockage.

(3) Ice may block unprotected inlets and vent s . 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

8 (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 o peration of stall warning and stall identification reset features for flight in icing conditions, including effects of fail ure to operate.

(6) O peration 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

- 1 B , 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 i nstalled 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 spee d margins, temperature margins, a utomatic t akeoff t hrust c ontrol s ystem (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

followin g:

(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

m inimum airspeed for each normal airplane configuration in icing conditions.

(d) A s applicable, a statement similar to the following:7/30/08 AC 25

- 25X

9 In icing conditions

, th is airplane must be operated and its ice protection systems used as described in the operati ng 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 :

• V isible moisture is present in the air or on the wi ng
,

• T he difference between the dew point temperature and the outside air temperature is less than 3 degrees C (5 degrees F) , or

• S tanding water

, slush, ice

, or snow is present on taxiways or runways

.

1 Residual ice contamination is contamination that i s 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 test s or analys e s, 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 k nots calibrated airspeed ( CAS ) or 3 percent of reference stall speed ( V SR ) 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

warn ing speed margins, if necessary, include reducing the peak angle of attack reached during takeoff by using increased rotation and takeoff safety speeds ( V 2 ) , reducing the takeoff rotation pitch rate, or reducing the target pitch

attitude. If any of these means are used, the effect on takeoff performance ( that7/30/08 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 wit hout 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 Limitat ions section should include a

statement similar to the following :

T he 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 e xtent of each approval to operate in icing conditions, including the extent of any approval to operate in the s upercooled 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 sh oul d also identify the means for detecting when the certified icing conditions have been

exceeded and state that int entional flight, including take off and landing, into these conditions is prohibited. A requirement to exit all icing conditions must be includ ed if icing conditions for which the airplane is not certified are encountered

.

( 6 ) To comply with § 25. 1585 , the AFM o perating p rocedures should include the following information :

(a) F light 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

sh ould be included.

(b) N ormal operating procedures should reflect the procedures used to certify the airplane for flight in icing conditions. Th ese 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 re tains 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 c ontamination 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 operati ng 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 procedur es 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 o f 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 meet s the performance and handling requirements of subpart B in the atmospheric icing conditions of A ppendi ces C and O to part 25.

(2) The applicant should show c ompliance with § 25.21(g) (1) by one or more of the methods listed in this section as agreed t o 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 ext ent 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 f rom which this airplane model is a derivative)

.

(6) Various factors affect ing ice accretion on the airframe are discussed in Appe ndix 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 n ecessary 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 accretio ns , or the “clean” airplane )

. D esign 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 tes t program that is conducted with the

uncontaminated a irplane. 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 pr otection 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 wh en 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 altitud e for the atmospheric icing envelope specified in A ppendices 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 ha s 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) Addi tional 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 cover age of the atmospheric icing conditions of part 25

A ppendi ces 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 conditi ons for accum u lating 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 o f showing compliance, additional limited flight tests should be conducted with ice accretions obtained in natural icing conditions. The objective of these tests7/30/08 AC 25

- 25X

14 is to corroborate the simulated ice accretions, and the handling characteristics and performan ce 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 a ncestor 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

. T o he lp substantiate acceptable performance and handling characteristics , the applicant may use a nalysis of results of dry air wind tunnel testing of models with the simulated ice accretions defined in A ppendi ces C and O

.

d. Engineering Simulator Testing and Analysis. R esults of an engineering

simulator airplane analysis that includes effects of ice accretions defined in A ppendi ces C

and O may be used to help substantiate acceptable handling characteristics. D ata 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. T o help substantiate acceptable performance and handling

characteristics , t he applicant may use a n engineering analysis that includes effects of the ice

accretions defined in A ppendi ces C and O

. E ffects of the ice accretions used in this analysis may be determined by analyzing results of dry air wind tunnel tests, flight tests, computational analys e s or engineering simulator analys e s
, or by engineering judgment.

f. Ancestor Airplane Analysis

.

(1) T o help substantiate acceptable performance and handling characteristics , the applicant may use a n analysis of an ancestor airpl ane that includes effect s of ice

accretions as defined in A ppendi ces 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 ancest or 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 req uirements 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 deg ree 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 th is 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, i n accordance with part II(a) of Appendix C t o 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 A ppendix C or O

, or a more cons ervative 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 con figurations, 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 af fect 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 requirem ents.

( 6 ) Unless otherwise specified, the speeds ( for example , V SR , V REF , V 2 , 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 s tall 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) activatio n point is set as a function of the high lift configuration, or

• T he activation point is reset for icing conditions, or

• S ignificant configuration changes occur with extension of trailing edge flaps (such as extension of wing leading edge high lift devic es)

.

(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 t he airplane to a forward center -of-gravity position appropriate to the airplane configuration.

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

.

(c) Trim at an initial speed of 1.13 to 1.30 V SR

. D ecrease s peed at a rate not to

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

Perform this maneuver with the following ice accretions :

1 In h igh lift devices retracted configuration

— final takeoff ice.

2 In h igh lift devices retracted configuration

— en route ice.

3 In h olding configuration—holding ice.

4 In l owest l ift takeoff configuration—holding ice.

5 In h ighest lift takeoff configuration

— takeoff ice.

6 In h ighest lift landing configuration —holding ice.7/30/08 AC 25

- 25X

17 c. Accelerate

– Stop Distance , § 25.109. To comply with this requirement, t he effect of any increase in V 1 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 takeof f in icing

conditions if the following applies

—

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

V SR for the same configuration without ice accretions by more than the gr eater 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 f orward center -of-gravity position appropriate to the airplane configuration.

(3) The a irplane should be in the highest lift landing configuration, at a landing climb speed no greater than V REF

.

(4) S tabilize 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 f orward center -of-gravity position approp riate to the configuration.

(2) S tabilize 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, V 2 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 f orward 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) S tabilize the airplane at the specified speed with one engine inoperati ve (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, t he 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 evaluati on 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 provision s outlined

above

.

Maneuvers

(a) The h olding ice accretion should be used

.

(b) The a irplane should be loaded to a m edium to light weight, with an aft

center

- of

- gravity position, and symmetric fuel loading.

(c) In the configurations li sted in Table 2

, below, trim the airplane at the specified speeds and conduct the following maneuvers:

1 30

- degree bank ed turn s 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 V REF ), and a pushover to 0.5 g (except that the pushover is not required at V MO and V FE );

and

3 Deploy ment and retract ion of deceleration devices.

TABLE 2

Controllability & Maneuverability

– Trim Speeds

Configuration Trim Speed

High lift devices retracte d configuration 1.3 V SR , and
V MO or 250 KIAS, whichever is
less

Lowest lift takeoff configuration 1.3 V SR , and
V FE or 250 KIAS, whichever is less

Highest lift landing configuration V REF , and
V FE or 250 KIAS, whichever is less7/30/08 AC 25

- 25X

21 V SR

– Reference Stal l Speed

V MO

– Maximum operating limit speed

KIAS

– Knots indicated air speed

V FE

– Maximum flap extended speed

V REF

– Reference landing speed

(d ) In the l owest lift takeoff configuration , a t the greater of 1.13 V SR or V 2MIN , with one engine inoperativ e (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 m anua l ( 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 app roach and landing procedure. In addition, you should demonstrate satisfactory controllability during a landing at V REF 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. Ailero n hinge moment

reversal and other lateral control anomalies have been identified as causal factors in icing accidents and incidents. The following maneuver s , 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

• E valuat e lateral controllability during deceleration to the stall warning speed ( covered in paragraph 3q(2)(e) of this AC), a nd

• E valuat e static lateral

- directional stability ( covered in paragraph 3 o of this AC).7/30/08 AC 25

- 25X

22 Maneuver s

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

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

.

(b)

• Test condition 1

– H olding 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

– L anding configuration, holding ice accr etion, medium to light weight, forward center

- of

- gravity position, at V REF (highest expected landing approach angle

- of

- attack).

1 E stablish a 30

- degree bank ed 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 l ateral 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 excessi ve. 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) b elow 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 ice

contaminated tailplane stall.7/30/08 AC 25

- 25X

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

I

t 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 possibl e 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

pr otection 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 increas ing with no force reversals

, unless the cha nge 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 ma naged 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) T est 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 h olding ice accretion should be used . For airplanes with unpowered elevators, these tests should also be performed with san dpaper ice.

2 The airplane should be loaded to a m edium 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 compl iance with § 25.143(i)(2 )

. Start

w ith the airplane in trim, or as near as possible to being in trim, at the most
critical of the trim speed s specified below

.

• Trim speed 1.23 V SR , target speed not more than 1.23 V SR , or

• Trim speed V FE , target speed no t less than V FE - 20 knots.

P ull up to a suitable pitch attitude, then

• P ush 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 lowes t 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 ma neuver 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 V SR 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. F ollowing is an example of an acceptable test program for showing compliance with §§ 25.143(j)

.

Maneuvers

For the configuration s , speeds, and power settings listed below, with the ice accretion specified in the requirement, t rim 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

f

orces 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, V REF 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). Q ualitative evaluations should be combined with the other testing. Review r esults of tests on the uncontaminated airplane for any cases of marginal compliance. All test s 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 a irplane is at a m edium t o light weight

, aft center

- of

- gravity position, with symmetric fuel loading.

(c) In the configurations listed below, trim the airplane at 1.3 V SR . 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. Direc tional and Lateral Control

, § 25.147

. To show compliance with § 25.147, applicants should combine q ualitative 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 r esults of

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

q ualitative evaluations with the other testing. Review r esults of tests on the

uncontaminated airplane for any cases of marginal compliance. All test s showing marginal compliance should be repeated with ice accretions on the airp lane. 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 a irplane is at the m ost 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 V SR1 at idle power or thrust; or

2 V REF 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 co mbine q ualitative 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 com pliance with § 25.175, the applicant should test e ach 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. M aximum speed for substantiation of stability characteristics in icing conditions (as prescribed by § 25.253(c)) is the lowe st of

—

• 300 knots CAS ;

• V FC ; 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 demonstrati ng static longitudinal stability

.

Maneu vers

(a) The holding ice accretion should be used

.

(b) The airplane is at a h igh 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 ne ed not be less than 1.3 V SR

.

2 Cruise: With high lift devices retracted, trim at V MO 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 V SR

.

4 Landing: With the highest lift landing configuration, trim at 1.3 V SR

.

o. Static Lateral

– Directional Stability

, § 25.177.

(1) U s e steady heading sideslips to show compliance with the directional and lateral stability requirement s . 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 d irectional and lateral control movements and forces to be

subs tantially 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 demonstrat ing static directional and lateral stability

.

Maneuvers

(a) The holding ice accretion should be used

.

(b) The airplane is at a m edium 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

conduc t 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 V SR

.

2 Lowest lift takeoff configuration: Trim at the all

- engines

- operating initial climb speed.

3 Highest lift landing configuration: Trim at V REF

.

p. Dynamic Stability

, § 25.181

. If the uncontaminated airplane has no borderline complia nce test results for § 25.181

, dynamic stability tests with ice accretions on the airplane are not necessary

. The applicant should combine q ualitative 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 s ufficient stall testing to demonstrate that the stall characteristics comply with re quirements. 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

b ased 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) T he stall characteristics with ice accretion show a significant difference from

those on the uncontaminated airplane,

(b) The testing indicates borderline compliance, or

(c) T he 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 s low decelerations (much slower than

1 knot per second) may be critical on airplanes with anticipation logic in their stall
prote ction 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) T he h olding ice accretion should be used

.

(b) The airplane should be loaded to a m edium to light weight, aft center

- of

gravity position, with symmetric fuel loading.

(c) The tests should be conducted at the n ormal 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 sp eed 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 s tall warning in conjunction with stall speed tes ting 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

re present 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 h olding ice accretion should be used

.

(b) The airplane should be loaded to a m edium to light weight, aft center

- of

- gravity position, with symmetric fuel loading.

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

(d) In the configurations listed below, trim the airplane at 1.3 V SR with the p ower 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 spee d 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 configura tion.7/30/08 AC 25

- 25X

33 (3) Ice Accretion Prior to Activation and Operation of the Ice Protection System. The following represent s 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 V SR .

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

2 Landing configuration: Straight Flight /Power Off.

(b) At decelerati on 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 characteristi cs (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 take s

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) R esults of the steady heading sideslip tests with landing ice may be used to establish the safe crosswind component of t h is 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

- co ntaminated airplane crosswind component test result is

considered valid for icing conditions

.

(3) If r esults 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 conservative7/30/08 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:



## ( )

5

. 1 sin
* angle sideslip V V REF

CW =

where:

V CW is the crosswind component,

V REF is the landing reference speed appropriate to a minimum landing weight, and sideslip angle is that demonstrated at V REF (see paragraph 3o of this AC).

t. Vibration and Buffeting

, § 25.251. To show compliance with this requirement:

(1) C omb ine q ualitative 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) D emonstrate that the airplane is free from harmful vibration due to residual i ce accumulation. This may be done in conjunction with the natural icing tests.

(3) E valuate a n airplane with pneumatic deicing boots up to V DF / M DF both with the deicing boots operating ( including deicing boot inflation cycles) and not operating. It i s 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 A ppendix C to pa rt 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 simulate d 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. Validat ing all aspects of predicted ice

accretions through n atural 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 handlin g characteristics. The applicant should make a t 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 atmospher ic 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 a s 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 wi th 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 o n unpr otected 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 res ponsible 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 susceptibl e 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
Gravit y Position Trim Speed Maneuver

Flaps up, Gear up Any position
in the aft

range Holding, except
1.3 V SR 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 V SR Deceleration to the speed reached
3 seconds after activation of stall warning in a 1 knot/second
deceleration.

Landing flap s, gear down Any position
in the aft

range V REF 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) Approa ch/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

- Maneuvers37 Tes t

con
dition Ice
accretion
thickness (*) Airplane configuration Center of Gravity
Position Trim speed ManeuverAC25-25X — First 0.5 in Flaps up,

gear up Any posi tion 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 V SR for the

deceleration

maneuver Level 40° banked turn,

Bank

- to

- bank rapid rol l, 30°

- 30°,

Speed brake extension and retraction (if approved),

Deceleration to the speed reached 3 seconds after activation

of

stall w arning 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 V SR 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 decelerati on.

3 Additional

0.25 in (1.25 in. total) Landing flaps, gear down Any position in

the aft

range V REF 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 adequ ately

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 condition s that are annunciated to the flightcrew, the applicant may tak e 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 qualitat ive 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 necessa ry to reflect the specific operating procedures

.

Maneuvers

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

1 Ice o n unprotected surfaces

— T he holding ice accretion described in Appendix 1 , paragraph 2(a) (3) of this AC;

2 Ice o n normally protected surfac es that are no longer protected

— T he failure ice accretion described in Appendix 1 , paragraph 3(b) of this AC; and

3 Ice o n normally protected surfaces that are still protected following segmental failure of a cyclical deice system

— T he 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 m edium to light weight, at aft center

- of

gravity position, with symmetri c 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 mane uver 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: Approa ch 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 t ype and the AFM landing procedure.

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

1.3 V SR

. Decrease speed at approximately 1 knot per second until 1 second after stall warning, and demonstrate prompt recovery u sing 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 proced ure.

(f) Conduct an approach and landing with all engines operating (unless the one

engine

- inoperative condition results in a more critical probable failure condition) using the appropriate AFM approach and landing procedure.

(3) For improb able 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. T he 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 ACCRETION S

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

condi tions must include consideration of ice accretions that can occur in A ppendix 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, a t a minimum, be capable of safely operating:

(1) I n the atmospheric icing conditions of A ppendix C to part 25, and

(2 ) A fter encountering the atmospheric icing conditions of Appendix O

, and subsequently while exitin g all icing conditions.

Therefore, for airplane s with a maximum takeoff weight less than 60,000 pounds or with reversible flight controls, ce rtification 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 op eration 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 whi ch operation is not approved. Ice accretions used for c ertification must consider :

( 1 ) O perations in A ppendix C icing conditions,

( 2 ) O perations 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 certifi ed for flight in Appendix C conditions, an airplane with a maximum takeoff weight less than 60,000 pounds or wit h reversible flight controls may be certified for operation throughout the atmospheric icing7/30/08 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 fro m :

( 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 showin g compliance. In accordance with Appendix C, part II(b) and Appendix O , part II( e ), t o 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 s ubpart 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. I ce accretion(s) used to s how 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 determi ne t he most critical ice accretion in terms of handling characteristics and performance for each flight phase. P arameters to be considered include:

• F light conditions ( for example , airplane configuration, speed, angle

- of

- attack, altitude) and

• A tmospheric 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 dete rmining 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 determi ned in

accordance with A ppendices 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 antenna s 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 signif icant additional effects, which should therefore be taken

into consideration.

(3) For holding ice, t he applicant should determine the effect of a 45

- minute hold in continuous maximum icing conditions. The analysis should assume that the airplane

rema ins 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

l imited 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 result ing 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 t he rest time of a deicing cycle. P erformance may be established on the basis of a representative intercycle ice accretion for normal operation of the deicing system. ( The applicant should also c onsider effects of each residual ice accretion that is not shed). The average drag increment d etermined 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 ope ration 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

A ppendi ces C and O to part 25 for the takeoff phase may be determined by calculation, assuming the following.

• A irfoils , control surfaces and, if applicable, propellers are free from frost, snow, or ice at the start of the takeoff;

• T he ice accretion starts at liftoff;

• T he airplane is at the critical ratio of power/thrust

- t o

- weight;

• F ailure of the critical engine occurs at V EF ; and

• The flightcrew activates the ice protection system in accordance with an AFM procedure, except that af ter beginning the takeoff roll no flightcrew action to activate the ice protection system s hould be assumed to occur until the airplane

is 400 feet above the takeoff surface.

(2) The ice accretions identified in A ppendi ces 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

transi tion 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 r esponse 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 protection7/30/08 AC 25

- 25X Appendix 1

A1

- 5 system dep ends 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, t he 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 activat ing 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 air frame 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

unpr otected and protected aerodynamic surfaces before activation and normal system operation of the ice protection system

.

( a ) If the ice protection system activat es automatic ally after annunciation from a primary ice detection system , the assumed ice ac cretion 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 T he ice accret ion on the protected surfaces corresponding to the time between entry into the icing conditions and activation of the system, plus

2 The i ce accretion during the system response time.

( b ) If ice protection system activati on depends on pilot act ion 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 T he ice accretion corresponding to the time between entry into the icing conditions and annunciation from the primary ice detection system, plus7/30/08 AC 25

- 25X Appendix 1

A1

- 6 2 T he ice accretion corresponding to 10 additional seconds of operation in icing conditions, plus

3 The i ce accretion during the system response time.

( c ) If ice protection system activati on depends on the flightcrew visual ly recogni zing the first indication of ice accretion on a reference sur face ( 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 T he ice accretion that would be easily recognizable by the flightcrew under all foreseeable conditions ( for example , at night in clouds) as it correspond s to
the first indication of ice accretion on the reference surface, plus

2 T he 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 i ce accret ed during the syste m response time.

( d ) If ice protection system activati on 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 ic e 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

fun ction . This may be determined as follows :

1 T he 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 i ce 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 syst em is defined as follows :

(1) I f 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) I f 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 a s the accretion specified for unprotected parts.

(3) I f 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 i ce 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 i n 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

become s aware that they are in Appendi x 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 g uidance for certif ying the det ection

means

. In general, we expect that the time to detect exceedance icing conditions may be significantly longer for a detection means rel ying on the flightcrew see ing and recogniz ing a visual icing cue than it is for an ice detection system that provi des 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 . T ime between pilot sca ns 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 adh ered 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 b y many factors.

S uch 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 of7/30/08 AC 25

- 25X Appendix 1

A1

- 8 App endix 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 est imate 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 b e 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 win dows)

– 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 t he ic e accretions in Table 1, below, to evaluate compliance with the applicable

s ubpart 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

cond itions.

( 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 ph ase for which the airplane is not certified for flight throughout the Appendix O icing envelope.

( 3 ) T able 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 accretion 1

Final Takeoff No accreti on 1

En R oute En Route Detect

- and

- Exit Ice

Combinati on of:

(1) Either Appendix C en route ice or Appendix O en route ice for which the airplane is approved, whichever is applicable,

(2) P re

- detection ice,

(3) A ccretion from one standard cloud horizonta l extent (17.4 nautical miles) in Appendix O conditions for which the airplane is not approved , and

(4) A ccretion from one standard cloud horizontal extent (17.4 nautical miles) in Appendix C continuous maximum icing
conditions.

Holding Holding Detect

- an d

- 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) P re

- detection ice,

(3) A ccretion from one standard cloud horizontal extent (17.4 nautical miles) in Appendix O conditions for which the airplane is not approved , and

(4) A ccretion 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 minu tes.

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) P re

- detection ice, and

(3) I ce 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 con tinuous 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) P re

- detection ice, and

(3) I ce 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 exten t (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 ice7/30/08 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

sur face,

• A t ransition 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 Bef ore
t he Ice Protection System Has Been
Activated a nd i s Performing i ts
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 s ystem to be activated, and the ice protection system to become fully effective in performing its intended function.

Ice Accretion i n Appendix O
Conditions Before
Those Conditions
Have Been Detected
b y t he Flightcrew and Actions Taken, i n Accordance With t he AFM, t o Either Exit All Icing Conditions or Continue Flight i n Appendix O Icing
Conditions Ice accreted on protected and unprotected surfaces during :

• T he time it takes to detect and identify Appendix O conditions (based on the method of detection) beyo nd
those in which the airplane is certified to operate, and

• The time it takes t he 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 accretion 27/30/08 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 A ppendix 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 t he ice accretions in Table 2 to evaluate compliance with

the applicable s ubpart B requirements for operating safely in the Appendix O atmospheric icing conditions for which the airplane is approved.

(2) The dec ision 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 acc umulating , 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 cri tical additional accretion than would occur for continued flight in Appendix O icing conditions.

(3) T able 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 Ice7/30/08 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, assum ing 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

w ithin the 17.4 nautical mile standard cloud extent).

Approach Approach Ice

Mo re 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 Lan ding Ice

Mo re 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 to7/30/08 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 t he landing configuration

, and

(2) Holding ice (if the airplane is certified for holding in Appendix O conditions).

Ice Accretion Before

t he Ice Protection System h as b een Activated a nd i s Performing i ts Intended
Function Ice accreted during the time it t akes 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 i n Appendix O Conditions
Before t hose
Conditions h ave b een Detected b y t he Flightcrew and Actions Taken, i n Accordance
With t he AFM, t o Either Exit All Icing
Conditions or Continue Flight i n 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 c onditions.

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, t he simulated ice accretions used for flight testing should be those that have

the most adverse effects on handling characteristics

. However, i f analytical data show that other reasonably expected ice accretions could be generated that could have a more adverse

effect on performance , the n 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. S imulated accretions should be representative of natura l 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 i ce 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:

• U se of computer codes,

• F light in measured natural icing conditions,

• I cing wind tunnel tests, and

• Fl ight in a controlled simulated icing cloud ( for example , from an icing tanker).

b. Unless another tex ture is substantiated by the applicant and agreed to by the ACO, the applicant should use a n ice accretion roughness height of 3 mm with a particle density of

8 to 10/cm 2

.7/30/08 AC 25

- 25X Appendix 2

A2

- 2 3

. SANDPAPER ICE.

a. Sandpaper ice is the most critical thin, rough layer o f 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, e xcept 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 crit ical ice accretion to show compliance with the applicable subpart B airplane requirements for performance and handling characteristics in icing conditions. When determin ing the most critical ice

accretion , the applicant must consider the full range of atm ospheric 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) o r (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

A ppendix 3

A3

- 1 APPENDIX 3

DESIGN FEATURES

1

. AIRPLANE CONFIGURATION AND ANCESTRY

. An airplane’s size is an important design characteristic th at can affect performance, controllability, and maneuverability in icing

condition s

. T he 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 whe n evaluat ing the subject airplane’s general capabilities for flight in icing conditions.

2

. WING. Design features of a wing that can affect performance, co ntrollability, 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 ar eas 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 u p 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 Ed ge Device s

. The presence of a leading edge device (such as a slat) reduces the percentage decrease in C Lmax ** due to ice by increasing the overall level of C L ***

. Movement of a slat so that a gap forms may improve the situation further. Leading ed ge dev ices can also reduce loss in angle of attack at stall due to ice.

c

. Lateral Control System Effectiveness

. The applicant can evaluate e ffectiveness of the lateral control system in icing conditions by compari ng it with the lateral control system

in cl osely

- related ancestor airplanes

, with consideration of the design of aerodynamic

balance surfaces

.

** Maximum li ft coefficient.

*** Lift coefficient.7/30/08 AC 25

- 25X

A ppendix 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 phil osophy , 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. Tail plane Design Philosophy

. The tailplane may be designed and sized to p rovide 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 conse quences of elevator hinge moment reversal due to ice

induced 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

m oment coefficients.

e. Vertical Stabilizer and Rudder Design. E ffectiveness 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 co nsideration 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 a pplicant 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 su rface, 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 r espect 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 deflectio n of the control. This auto deflection, in extreme cases, may be to the control stops.7/30/08 AC 25

- 25X

A ppendix 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 t ailplane.

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 a mounts 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 unpowe red reversible controls can reduce the risk of ice

- induced aerodynamic hinge moment reversal s

.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

. AIRPLA NE APPROVED FOR FLIGHT IN A PPENDIX 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. I f 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 ic ing 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 P rocedures ( N ormal Procedures S ection)

.

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 a t 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 c hange 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 P roced ures

.

Warning: Severe icing may result from environmental conditions outside of

those for which th is 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 evaluat ed 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 dri zzle 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 t hat exceed those for which th is 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 condition s 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 A PPROVED FOR

FLI GHT 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 descriptio n 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 determin ed that ice accretions no longer remain on the

airframe.

b. AFM Operating Procedures (Normal Procedures S ection)

.

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 seve re 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 P rocedures

.

Warning: Severe icing may result from environmental conditions outside of

those for which th is airplane is certified. Inten tional 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 prot ection 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

consi dered 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 th is airplane is certified. If severe icing is encountered, immediately request priority handling from air traffic control to faci litate 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 proh ibited. 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 h ere}, 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 P roce dures (Normal Procedures S ection)

.

Freezing drizzle and freezing rain conditions encountered during a hold (in

any airplane configuration) or in the en route phase of flight (c limb, cruise, or7/30/08 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 on e 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 t he 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 P rocedures

.

Warning: Severe icing may result from environmental conditions outsid e of

those for which th is 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 (cli mb, 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

.

Intenti onal 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 encountere d, 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 tha t 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 icing7/30/08 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 P rocedures S ection)

.

{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 a lternate 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 airf rame.

c. Flightcrew Operating Manual Operating P rocedures

.

Warning: Severe icing may result from environmental conditions outside of

those for which th is airplane is certified. Intentional flight into severe icing

conditions may result in ice build

- u p 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 contro llability 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.

Int entional 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 des cription here}is an indication of severe icing

conditions that exceed those for which th is airplane is certified. If severe icing is encountered, immediately request priority handling from air traffic control

to facilitate a route or altitude change to ex it 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 CIRCULA RS

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 P ropeller speed and pitch limits

§ 25.101 Performance

– General

§ 25.103 Stall speed

§ 25.105 Takeoff

§ 25.107 Takeoff speeds

§ 25.109 Accelerate

- s top 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 Lan ding

§ 25.143 Controllabili ty and Maneuverability

– General

§ 25.145 Long itudinal control

§ 25.147 Directional and lateral control

§ 25.161 Trim

§ 25.171 Stability

– General

§ 25.175 Demonstration o f static longitudinal stability

§ 25.177 Stati c lateral

- directional stability

§ 25.181 Dynamic stability

§ 25.201 St all demonstration

§ 25.203 Stall character istics

§ 25.207 Stall warning

§ 25.231 Lon gitudinal stability and control7/30/08 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 buffet ing

§ 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

§ 25.158 1 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 contain ed 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

- 73 A 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

- 1 B Approval of Flight Guidance Systems

AC 25

. - 14 19/1420 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. I ndividual ACs may be updated in the future, so that this list may

reference a s pecific 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 Airspee d

. LWC Liquid Water Content

. MED Mean Effective Diameter

. MVD Median Volumetric Diameter

. C L Lift Coefficient

. C LMAX Maximum Lift Coefficient

.

Trim A flight condition in which the aerodynamic moment acting
about the axis of interest is zero. In th e absence of an
external disturbance no control input is needed to maintain the flight condition

. V 1 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 t he airplane within the
accelerate

- stop distance. V 1 also means the minimum speed in the takeoff, following a failure of the critical engine
at V EF, at which the pilot can continue the takeoff and achieve the required height above the takeoff surface withi n the takeoff distance.

V 2 Takeoff Safety Speed. (The target speed to be reached by the time the airplane is 35 feet above the takeoff surface.)

V 2MIN Minimum Takeoff Safety Speed.

V CW Crosswind Component of Wind Speed

. V DF /M DF Demonstrated Fligh t Diving Speed

.7/30/08 AC 25

- 25X Appendix 6

A6

- 2 V EF Engine Failure Speed. The speed at which the critical engine is assumed to fail during takeoff

. V FC Maximum Speed for Stability Characteristics

. V FE Maximum Flaps Extended Speed

. V MO Maximum Operating Limit Speed

. V REF Landing Ref erence Speed

V S 1

- g 1

- g Stall Speed. The calibrated airspeed at which aerodynamic forces alone can support the airplane in 1

- g flight

. V SR Reference Stall Speed. V SR may not be less than V S 1

- g . For airplanes with a device that abruptly pushes the nos e down at a selected angle of attack, (for example, a stick pusher), V SR may not be less than 2 knots or 2 percent, whichever is greater, above the speed at which the device operates.

V SR1 Reference Stall Speed in a specific configuration.