2-3 CHAPTER 2. REQUIREMENTS AND GUIDANCE
2.1 Overview.
2.1.1 For certification for flight in icing conditions, § 25.21(g)(2) requires that an airplane meet certain performance and handling qualities requirements while operating in the icing environment defined in part 25, 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 (SLD) icing conditions defined in part 25, Appendix O. Appendix A of this AC provides detailed guidance for determining ice accretions in both Appendix C and Appendix O icing conditions that can be used for showing compliance.
2.1.2 For airplanes with a maximum takeoff weight less of than 60,000 pounds or with reversible flight controls to be certified for flight in icing conditions, § 25.1420 requires applicants to do one of the following:
2.1.2.1 Not seek approval for flight in the SLD atmospheric icing conditions defined in Appendix O.
2.1.2.2 Seek approval for flight in only a portion of Appendix O icing conditions.
2.1.2.3 Seek approval for flight throughout the entire Appendix O atmospheric icing envelope.
2.1.3 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. The guidance provided in this AC related to Appendix O icing conditions does not apply to those airplanes.
2.1.4 Because an airplane may encounter SLD icing conditions at any time while flying in icing conditions, certain safety requirements must be met for the SLD icing conditions of Appendix O for airplanes with a maximum takeoff weight less than 60,000 pounds or with reversible flight controls, even if the airplane will not be certified for flight in the complete range of Appendix O atmospheric icing conditions. For those airplanes that will not be certified for flight in all icing conditions defined in Appendix O, § 25.21(g)(3) requires certain part 25, subpart B, requirements to be met in SLD atmospheric icing conditions beyond those for which the airplane will be certified.
Compliance with these requirements in addition to the requirements for flight in Appendix C icing conditions are intended to provide adequate airplane performance and handling qualities for a safe exit from all icing conditions after an encounter with SLD atmospheric icing conditions beyond those for which the airplane will be certified.
2.1.5 If an airplane with a maximum takeoff weight less than 60,000 pounds or with reversible flight controls is not to be certified for flight in all of the SLD icing
2-4 conditions of Appendix O, then § 25.1420 requires a means of indicating when the airplane has encountered icing conditions beyond those for which it is certified. See AC 25-28, Compliance of Transport Category Airplanes for Flight in Icing Conditions, dated October 27, 2014, for guidance on acceptable means of detecting and indicating when the airplane has encountered icing conditions beyond those for which it is certified. The applicant should provide procedures in the airplane flight manual (AFM) that will enable the flightcrew to make a safe exit from all icing conditions after an encounter with icing conditions beyond those for which the airplane is certified.
2.1.6 To certify an airplane with a maximum takeoff weight less than 60,000 pounds or with reversible flight controls for operations in Appendix O icing conditions only for certain flight phase(s), the applicant should define the flight phase(s) for which approval is sought in a way that will allow a flightcrew to easily determine whether the airplane is operating inside or outside its certified icing envelope. The critical ice accretion or accretions used to show compliance with the applicable requirements should cover the range of airplane configurations, operating speeds, angles-of-attack, and engine thrust or power settings that may be encountered during that phase of flight (not just at the conditions specified in the part 25, subpart B, requirements). For the ice accretion scenarios defined in paragraph A.4.3.3 of Appendix A of this AC, the applicable flight phases are takeoff (including the ground roll, takeoff, and final takeoff segments), en route, holding, and approach/landing (including both the approach and landing segments).
2.1.7 Ice accretions used to show compliance with the applicable subpart B regulations should be consistent with the extent of the desired certification for flight in icing conditions. Appendices C and O define the ice accretions, as a function of flight phase, that must be considered for certification approval for flight in those icing conditions.
Any of the applicable ice accretions (or a composite accretion representing a combination of accretions) may be used to show compliance with a particular subpart B requirement if it is either the ice accretion identified in the requirement or one shown to be more conservative than that. In addition, the ice accretion with the most adverse effect on handling characteristics may be used for compliance with the airplane performance requirements if each difference in performance is conservatively taken into account. Ice accretion(s) used to show compliance should take into account the speeds, configurations (including configuration changes), angles-of-attack, power or thrust settings, etc. for the flight phases and icing conditions they are intended to cover. For example, if the applicant desires certification for flight in the SLD icing conditions of Appendix O in addition to those of Appendix C, compliance with the applicable subpart B requirements may be shown using the most critical of the Appendix C and Appendix O ice accretions.
2.1.8 Certification experience has shown that it is usually unnecessary to consider ice accumulation on the propeller, induction system, or engine components of an inoperative engine when demonstrating airplane handling qualities. Similarly, for ice accreted anywhere on the airplane, the mass of any accreted ice need not normally be considered.
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2.1.9 Requirements and guidance for flight in icing conditions also apply to operation of the airplane after leaving icing conditions until it is determined that no ice accretions remain on the airplane’s critical surfaces.
2.1.10 Ice-contaminated tailplane stall (ICTS) refers to a phenomenon identified as a causal factor in several airplane incidents and accidents. It results from airflow separation on the lower surface of the tailplane because ice is present. ICTS can occur if the angle-ofattack of the horizontal tailplane exceeds its stall angle-of-attack. Even very small quantities of ice on the tailplane leading edge can significantly reduce the angle-of-attack at which the tailplane stalls. An increase in tailplane angle-of-attack, which may lead to a tailplane stall, can result from changes in airplane configuration (for example, extending flaps, which increases the downwash angle at the tail or the pitch trim required) or flight conditions (a high approach speed, gusts, or maneuvering, for example). An ICTS is characterized by reduction or loss of pitch control or pitch stability while in, or soon after leaving, icing conditions. A flight test procedure for determining susceptibility to ICTS is presented in paragraph 4.9.4, Low g Maneuvers and Sideslips, of this AC.
2.1.10.1 For airplanes with unpowered longitudinal control systems, the pressure differential between the upper and lower surfaces of the stalled tailplane may result in a high elevator hinge moment, forcing the elevator trailing edge down. This elevator hinge moment reversal can be of sufficient magnitude to cause the longitudinal control (for example, the control column) to suddenly move forward with a force beyond the capability of the flightcrew to overcome. On some airplanes, ICTS has been caused by a lateral flow component coming off the vertical stabilizer, as may occur in sideslip conditions or because of a wind gust with a lateral component.
2.1.10.2 Aerodynamic effects of reduced tailplane lift should be considered for all airplanes, including those with powered controls. Airplanes susceptible to this phenomenon are those having a near zero or negative tailplane stall margin with tailplane ice contamination.
2.1.11 There have been airplane controllability incidents in icing conditions as a result of ice on unprotected leading edges of extended trailing edge flaps or flap vanes. The primary safety concern illustrated by these incidents is the potential for controllability problems due to the accretion of ice on trailing edge flap or flap vane leading edges while extending flaps in icing conditions. The flight tests specified in table 4-5 of this AC, in which handling characteristics are tested at each flap position while ice is being accreted in natural icing conditions, are intended to investigate this safety concern.
Unless controllability concerns arise from these tests, it is not necessary to conduct flight tests with simulated ice accretions on the extended trailing edge flap or flap vanes or to include extended trailing edge flap or flap vane ice accretions when evaluating airplane performance with flaps extended.
2.1.12 SLD icing conditions, or runback ice in any icing condition, can cause a ridge of ice to form aft of the protected area on the upper surface of the wing. This can lead to
2-6 separated airflow over the aileron. Ice-induced airflow separation upstream of the aileron can have a significant effect on aileron hinge moment. Depending on the extent of the separated flow and the design of the flight control system, ice accretion upstream of the aileron may lead to aileron hinge moment reversal, reduced aileron effectiveness, and aileron control reversal. Although airplanes with deicing boots and unpowered aileron controls are most susceptible to this problem, all airplanes should be evaluated for roll control capability in icing conditions. Acceptable flight test procedures for checking roll control capability are presented in paragraphs 4.9.3, 4.15, and 4.17.2.5 of this AC and consist of bank-to-bank roll maneuvers, steady heading sideslips, and rolling maneuvers at stall warning speed.
2.2 Proof of Compliance, § 25.21(g).
2.2.1 You may show compliance with certification requirements for flight in icing conditions by any of the means discussed in chapter 3, Acceptable Means of Compliance–General, paragraph 3.1, Overview, of this AC.
2.2.2 Certification experience has shown that, for airplanes of conventional design applying for flight in icing conditions, it is usually not necessary to perform additional detailed substantiation of compliance with requirements of the following sections of part 25:
• § 25.23, Load distribution limits.
• § 25.25, Weight limits.
• § 25.27, Center of gravity limits.
• § 25.29, Empty weight and corresponding center of gravity.
• § 25.31, Removable ballast.
• § 25.231, Longitudinal stability and control.
• § 25.233, Directional stability and control.
• § 25.235, Taxiing condition.
• § 25.253(a) and (b), High-speed characteristics.
• § 25.255, Out-of-trim characteristics.
2.2.3 If different stall warning system or stall identification system activation settings are used for flight in icing conditions (for example, if the stall warning or stall identification system activation settings are changed when the ice protection system is activated), it is acceptable to return to the non-icing settings when the critical wing surfaces are free of ice. The applicant should validate that the means for determining when the critical wing surfaces are free of ice accretions is reliable under all expected operating conditions.
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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.