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2.3 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.
2.4 Performance—General, § 25.101.
2.4.1 This regulation states that the propulsive power or thrust available for each flight condition must be appropriate to the airplane operating limitations and normal procedures for flight in icing conditions.
2.4.2 In general, propulsive power or thrust available may be determined by suitable analysis, substantiated when required by appropriate flight tests. An example would be when determining the power or thrust available for showing compliance with § 25.119. The following aspects should be considered:
2.4.2.1 Operation of induction system ice protection.
2.4.2.2 Operation of propeller ice protection.
2.4.2.3 Operation of engine ice protection.
2.4.2.4 Operation of airframe ice protection.
2.4.3 The applicant should consider the following when determining the change in performance due to flight in icing conditions:
2.4.3.1 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.
2.4.3.2 Incremental airframe drag due to ice accretion with normal operation of the ice protection system.
2.4.3.3 Changes in operating speeds provided in the AFM for flight in icing conditions.
2.4.4 Certification experience has shown that a suitable conservative analysis or flight test can determine any increase in drag (or decrease in thrust) resulting from ice accumulation on the landing gear, propeller, induction system, and engine components.
Certification experience has also shown that runback ice may be critical for propellers, so runback ice on the propeller should be addressed. Research has shown that ice accretions on propellers, and resulting thrust loss, may be larger in Appendix O SLD icing conditions than in Appendix C icing conditions for some designs. Applicants may conduct airplane performance checks in natural icing conditions, icing tanker tests, icing wind tunnel tests, or propose the use of an assumed (conservative) loss in
2-8 propeller efficiency. Testing should include a range of outside air temperatures, including warmer (near freezing) temperatures that could result in runback icing.
2.4.5 Apart from using appropriate speed adjustments to account for operation in icing conditions, all changes in the procedures established for takeoff or go-around should be agreed to by the responsible aircraft certification office (ACO).
2.5 Stall Speed, § 25.103.
Certification experience in meeting this requirement has shown that, for airplanes of conventional design, the effects of Mach number on stall speeds is unaffected by the presence of ice accretions.
2.6 Failure Conditions, § 25.1309.
2.6.1 The applicant should analyze failure modes of the ice protection system and resulting effects on airplane handling and performance in accordance with § 25.1309. In determining the probability of a failure condition, the applicant should assume that the probability of entering the icing conditions defined in part 25, Appendix C, is “1.” As explained in AC 25-28 and AC 25.1419-2, Compliance with the Ice Protection Requirements of § 25.1419(e), (f), (g), and (h), dated October 27, 2009, on an annual basis, the average probability of encountering the icing conditions defined in Appendix O may be assumed to be 1 x 10-2 per flight hour. This probability should not be reduced on a phase-of-flight basis. The ice accretion to use for failures of the ice protection system is defined in paragraph A.3 of appendix A of this AC.
Note: This guidance is not intended to apply to failures of other systems (or the engines) that may indirectly affect airplane handling and performance in icing conditions due to any effects of those failures on an otherwise normally functioning ice protection system. In such situations, the ice protection system itself is considered to be operating normally, although performance of its anti-ice or deice function may be degraded.
2.6.2 The guidance in this AC for a normal (that is, non-failure) condition is applicable, with the failure ice accretion, for probable failure conditions of the ice protection system that are (1) not annunciated to the flightcrew, or (2) annunciated to the flightcrew, but the associated procedure does not require exiting icing conditions.
2.6.3 For probable ice protection failure conditions annunciated to the flightcrew, when the associated operating procedure requires the airplane to leave the icing conditions as soon as practicable, the following applies: The applicant should show that, with the ice accretion resulting from the failure, the airplane’s performance and handling characteristics are adequate as determined by a system safety analysis in accordance with § 25.1309. The operating procedures and related speeds may restrict the airplane’s operating envelope, but the size of the restricted envelope should be consistent with the safety analysis.
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2.6.4 For failure conditions that are improbable but not extremely improbable, the analysis and substantiation of continued safe flight and landing required by § 25.1309 should take into consideration whether the failure is annunciated, and what associated operating procedures and speeds would be used following the failure condition.
2.7 Flight-Related Systems.
The applicant should consider flight-related systems aspects when determining compliance with the subpart B flight requirements for flight in icing conditions. For example, the following considerations may be relevant.
2.7.1 Ice protection systems may not adequately perform their anti-ice or deice functions at some engine power or thrust settings. This may result in a need to establish a minimum useable power or thrust setting for operation in icing conditions, and this can affect descent or approach capabilities. The applicant should also consider the effect of power or thrust settings in determining the applicable ice accretions. For example, at low engine power or thrust, a thermal bleed air system may not be able to evaporate the liquid water completely after melting the ice, resulting in the potential for runback ice.
2.7.2 Ice may block control surface gaps, or freeze seals, causing increased control forces, control restrictions, or blockage.
2.7.3 Ice may block unprotected inlets and vents. This may affect flight controls or the propulsive thrust available, as well as powerplant control and cooling and aerodynamic drag.
2.7.4 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.
2.7.5 There may be an effect on operation of stall warning and stall identification reset features for flight in icing conditions, including effects of failure to operate.
2.7.6 Operation of icing condition sensors, ice accretion sensors, and automatic or manual activation of ice protection systems may be affected.
2.7.7 Flight guidance and automatic flight control systems operation may be affected. See AC 25.1329-1C, Approval of Flight Guidance Systems, dated October 27, 2014, 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.
2.7.8 There may be an effect on installed thrust. Consideration of installed thrust includes consideration of ice protection systems operation when establishing acceptable power or thrust setting procedures, and when evaluating control, stability, temperature lapse rates, rotor speed margins, temperature margins, automatic takeoff thrust control system (ATTCS) operation, and power or thrust lever angle functions.
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2.8 Airplane Flight Manual, § 25.1581 through § 25.1587.
2.8.1 Section 25.1581 states that an AFM must be furnished with each airplane and must contain information necessary for safe operation because of design, operating, or handling characteristics. To comply with this requirement, the AFM Limitations section should include the following:
2.8.1.1 The limitations required to ensure safe operation in icing conditions.
2.8.1.2 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.
2.8.1.3 All airspeed limitations associated with flight in icing conditions, such as the minimum airspeed for each normal airplane configuration in icing conditions.
2.8.1.4 As applicable, a statement similar to the following:
“In icing conditions, this airplane must be operated and its ice protection systems used as described in the operating procedures section of this manual. Where specific operational speeds and performance information have been established for such conditions, this information must be used.”
2.8.1.5 For turbojet airplanes without leading edge high-lift devices, unless all ice contamination would otherwise be detected and removed from the wing leading edges and upper surfaces 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 °C (42 °F) and any of the following exists:
• Visible moisture is present in the air or on the wing.
• The difference between the dew point temperature and the outside air temperature is less than 3 °C (5 °F).
• Standing water, slush, ice, or snow is present on taxiways or runways.”
Note: An acceptable means for showing that residual ice contamination could be detected and removed without needing 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
2-11 AFM Limitations section should include a statement similar to the following:
“The [wing ice protection system or ice detection system–applicant to identify which one] must be operating after deicing until immediately before starting the takeoff roll whenever conditions conducive to ice/frost/snow formation exist.”
2.8.2 To comply with § 25.1583(e), Kinds of operation, the AFM Limitations section should clearly identify the extent of each approval to operate in icing conditions, including the extent of any approval to operate in the SLD atmospheric icing conditions defined in part 25, Appendix O.
2.8.3 For airplanes not certified to operate throughout the atmospheric icing envelope of part 25, Appendix O, for every flight phase, the Limitations section of the AFM should also identify the means for detecting when the certified icing conditions have been exceeded and state that intentional flight, including takeoff and landing, into these conditions is prohibited. A requirement to exit all icing conditions must be included if icing conditions for which the airplane is not certified are encountered.
2.8.4 To comply with § 25.1585, the AFM operating procedures should include the following information:
2.8.4.1 Flight in icing conditions operating procedures should include both (1) normal operation of the ice protection system, and (2) operation of the airplane following ice protection system failures. All changes in procedures for other airplane system failures that affect the capability of the airplane to operate in icing conditions should be included.
2.8.4.2 Normal operating procedures should reflect the procedures used to certify the airplane for flight in icing conditions. These include configurations, speeds, ice protection system operation, and powerplant and systems operation for takeoff, climb, cruise, descent, holding, go-around, and landing. For airplanes not certified for flight in all of the SLD atmospheric icing conditions defined in part 25, Appendix O, procedures should be provided for safely exiting all icing conditions if the airplane encounters Appendix O icing conditions that exceed the icing conditions for which the airplane is certified.
2.8.4.3 For turbojet airplanes without leading edge high-lift devices, the AFM normal operating procedures section should contain a statement similar to the following, along with procedures to ensure that such contamination is detected and removed before takeoff:
“WARNING Minute amounts of ice or other contamination on the leading edges or wing upper surfaces can result in a stall without warning, leading to loss of control on takeoff.”
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2.8.4.4 Non-normal operating procedures should include procedures to be followed in the event of annunciated ice protection system failures and suspected unannunciated ice protection system failures. If flight in icing conditions results in any changes to other non-normal procedures contained in the AFM, these changes should also be included.
2.8.5 Performance information, derived in accordance with part 25, subpart B, should be provided in the AFM for all relevant phases of flight in accordance with § 25.1587.
2.8.6 Examples of AFM limitations and operating procedures for operations in SLD icing conditions (i.e., Appendix O) are contained in appendix D of this AC.
3-13 CHAPTER 3. ACCEPTABLE MEANS OF COMPLIANCE—GENERAL
3.1 Overview.
3.1.1 This chapter describes acceptable methods and procedures that an applicant may use to show that an airplane meets the airplane performance and handling characteristics requirements of subpart B in the atmospheric icing conditions of part 25, Appendices C and O.
3.1.2 The applicant should show compliance by one or more of the methods listed in this chapter as agreed to by the responsible ACO.
3.1.3 The compliance process should address all phases of flight, including takeoff, climb, cruise, holding, descent, approach, 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 part 25, Appendix O.
3.1.4 The design features described in appendix C of this AC should be considered when determining the extent of the substantiation program.
3.1.5 Appropriate means for showing compliance include the items listed in table 3-1 below.
These compliance means are explained in more detail in following chapters of this AC.
Table 3-1. Means for Showing Compliance Item Description Flight Testing Flight testing in dry air using simulated ice accretions or with ice accretions created in natural icing conditions.
Wind Tunnel Testing and Analysis An analysis of results from wind tunnel tests with simulated or actual ice accretions.
Engineering Simulator Testing and Analysis An analysis of results from engineering simulator tests.
Engineering Analysis An analysis that may include the results from any of the other means of compliance as well as the use of engineering judgment.
Ancestor Airplane Analysis An analysis of results from a closely related ancestor airplane (an airplane model from which this airplane model is a derivative).
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3.1.6 Means of compliance guidance for determining the applicable ice accretion on the airframe is provided in appendix A of this AC.
3.1.7 Guidance for developing simulated ice accretions is given in appendix B of this AC.
3.2 Flight Testing.
3.2.1 Overview.
3.2.1.1 Section 21.35 requires applicants to perform all flight tests that the Administrator finds necessary to show compliance with the applicable requirements of part 25.
3.2.1.2 The extent of the flight test program should consider results of certification flight tests conducted with the uncontaminated airplane. This is the airplane without ice accretions, or the “clean” airplane. Design features of the airplane as discussed in appendix C of this AC should also be considered.
3.2.1.3 It is not necessary to repeat, this time with ice accretions, the extensive airplane performance and handling characteristics test program that is conducted with the uncontaminated airplane. A suitable program sufficient to demonstrate compliance with the requirements can be established from experience with airplanes of similar size, and from review of design features of the ice protection system, control system, wing, horizontal and vertical stabilizer, and the performance and handling characteristics of the uncontaminated airplane. In particular, it is not necessary to investigate all weight and center-of-gravity combinations when results from the uncontaminated airplane clearly indicate the most critical combination to be tested. It is also not usually necessary to investigate the flight characteristics of the airplane with ice accretions at high altitude (that is, above the highest altitude for the atmospheric icing envelope specified in part 25, Appendices C and O) if the ice protection system continues to provide the same level of protection as at lower altitudes.
3.2.1.4 Certification experience has shown that flight tests are usually necessary to evaluate the consequences of ice protection system failures on airplane performance and handling characteristics, and to demonstrate that the airplane is capable of continued safe flight and landing.
3.2.1.5 An example of an acceptable flight test program is provided in chapter 4, Acceptable Means of Compliance—Flight Test Program, of this AC.
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3.2.2 Flight Testing Using Approved Simulated Ice Accretions.
3.2.2.1 Tests for airplane performance and handling characteristics 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.
3.2.2.2 Additional limited flight tests should be conducted in natural icing conditions, as discussed in paragraph 3.2.3.2 below.
3.2.3 Flight Testing in Natural Icing Conditions.
3.2.3.1 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 volume diameter (MVD) of the cloud drops) should be measured and recorded. (The Appendix C icing envelopes use the term mean effective diameter (MED) rather than MVD, but this equates to MVD for the instrumentation used and the drop size distribution assumed at the time those envelopes were established.) The tests should ensure good coverage of the atmospheric icing conditions of part 25, Appendices C and O (consistent with the extent of the certification approval sought for operation in Appendix O icing conditions) and, in particular, the critical conditions for the airplane. The conditions for accumulating ice (including the icing atmosphere, airplane configuration, speed, and duration of exposure) should be agreed to by the responsible ACO.
3.2.3.2 If the applicant uses flight testing with simulated ice accretions as the primary means of showing compliance, additional limited flight tests should be conducted with ice accretions obtained in natural icing conditions. The objective of these tests is to corroborate the simulated ice accretions, and the handling characteristics and performance results obtained in flight testing with the simulated ice accretions. It is not necessary to measure the atmospheric icing characteristics (for example, LWC and MVD) of these flight test icing conditions. For derivative airplanes with aerodynamic characteristics and ice protection systems similar to the ancestor airplane, it may not be necessary to carry out additional flight tests in natural icing conditions if such tests have already been performed with the ancestor airplane. Depending on the extent of the Appendix O icing conditions for which certification is being sought, 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 25-28 for guidance on when it is necessary to conduct flight tests in the natural atmospheric icing conditions of Appendix O.
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3.3 Wind Tunnel Testing and Analysis.
To help substantiate acceptable performance and handling characteristics, the applicant may use analysis of results of dry air wind tunnel testing of models with the simulated ice accretions defined in Appendices C and O.
3.4 Engineering Simulator Testing and Analysis.
Results of an engineering simulator airplane analysis that includes effects of ice accretions defined in Appendices C and O may be used to help substantiate acceptable handling characteristics. Data used to model the effects of ice accretions for the engineering simulator may be based on results of dry air wind tunnel tests, flight tests, computational analysis, or on engineering judgment.
3.5 Engineering Analysis.
To help substantiate acceptable performance and handling characteristics, the applicant may use an engineering analysis that includes effects of the ice accretions defined in Appendices C and O. Effects of the ice accretions used in this analysis may be determined by analyzing results of dry air wind tunnel tests, flight tests, computational analyses, engineering simulator analyses, or by engineering judgment.
3.6 Ancestor Airplane Analysis.
3.6.1 To help substantiate acceptable performance and handling characteristics, the applicant may use an analysis of an ancestor airplane that includes effects of ice accretions as defined in Appendices C and O. This analysis should consider the similarity of the airplane configuration, operating envelope, performance and handling characteristics, and ice protection system of the ancestor airplane to the one being certified.
3.6.2 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.
4-17 CHAPTER 4. ACCEPTABLE MEANS OF COMPLIANCE—FLIGHT TEST PROGRAM
4.1 Overview.
4.1.1 This chapter provides guidance for developing an acceptable flight test program. Such a test program may be used if the applicant selects flight testing as the primary means for showing compliance with the requirements for airplane performance and handling qualities for flight in icing conditions and if that method of showing compliance is agreed to by the responsible ACO.
4.1.2 Section 25.21(a)(1) states that compliance with each requirement of subpart B must be shown by tests upon an airplane of the type for which certification is requested, or by calculations based on, and equal in accuracy to, the results of testing. In accordance with § 25.21(a)(1), calculations, if used, must provide at least the same degree of confidence of compliance as testing would provide.
4.1.3 Ice accretions for each flight phase are defined in part 25, part II of Appendices C and O. Additional guidance for determining the applicable ice accretions is provided in appendix A of this AC.
4.1.4 This example test program uses the holding ice accretion for the majority of the testing, assuming that it is the most conservative ice accretion. However, in accordance with part 25, Appendix C, part II(b), and part 25, Appendix O, part II(e), if the holding ice accretion is not as conservative as the ice accretion appropriate to the flight phase, then the ice accretion appropriate to the flight phase and icing conditions of Appendix C or O, as applicable, or a more conservative ice accretion, must be used. In general, the applicant may choose to use either a conservative ice accretion or the specific ice accretion appropriate to the particular phase of flight and icing conditions.
4.1.5 For the approach and landing configurations, in accordance with the guidance provided in paragraph 2.1.11 of this AC, the flight tests in natural icing conditions specified in table 4-5 of this AC are usually sufficient to evaluate whether ice accretions on trailing edge flaps adversely affect airplane performance or handling qualities. If these tests show that airplane performance or handling qualities are adversely affected, additional tests may be necessary to show compliance with the airplane performance and handling qualities requirements.
4.1.6 Unless otherwise specified, the speeds (for example, VSR, VREF, V2, etc.) referenced in the flight tests described in this AC refer to the speeds used with the appropriate ice accretion on the airplane.
4.2 Stall Speed, § 25.103.
4.2.1 The stall speed for intermediate high lift configurations (for takeoff configurations, for example) can normally be obtained by interpolation. However, additional tests may be necessary if—
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4.2.1.1 A stall identification system (for example, a stick pusher) activation point is set as a function of the high lift configuration,
4.2.1.2 The activation point is reset for icing conditions, or
4.2.1.3 Significant configuration changes occur with extension of trailing edge flaps (such as extension of wing leading edge high lift devices).
4.2.2 Acceptable Test Program.
The following specifications represent an example of an acceptable test program subject to the provisions outlined paragraph 4.2.1 of this AC.
Maneuvers
4.2.2.1 Load the airplane to a forward center-of-gravity position appropriate to the airplane configuration.
4.2.2.2 Conduct the test at the stall test altitude used in non-icing tests.
4.2.2.3 Trim at an initial speed of 1.13 to 1.30 VSR. Decrease speed at a rate not to exceed 1 knot per second until an acceptable stall identification is obtained. Perform this maneuver with the following ice accretions:
4.2.2.3.1 In high lift devices retracted configuration—final takeoff ice.
4.2.2.3.2 In high lift devices retracted configuration—en route ice.
4.2.2.3.3 In holding configuration—holding ice.
4.2.2.3.4 In lowest lift takeoff configuration—holding ice.
4.2.2.3.5 In highest lift takeoff configuration—takeoff ice.
4.2.2.3.6 In highest lift landing configuration—holding ice.
4.3 Accelerate–Stop Distance, § 25.109.
To comply with this requirement, the effect of any increase in V1 for taking off in icing conditions may be determined by a suitable analysis.
4.4 Takeoff Path, § 25.111.
In accordance with § 25.105(a), the applicant should conduct takeoff evaluations to substantiate the speed schedule and distances for takeoff in icing conditions if the following applies: