Appendix A A-1 APPENDIX A. AIRFRAME ICE ACCRETIONS A.1 Overview.
A.1.1 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 part 25, Appendix C. Therefore, at a minimum, certification for flight in icing conditions must include consideration of ice accretions that can occur in Appendix C icing conditions.
A.1.2 In accordance with § 25.1420(a)(1), each airplane with a maximum takeoff weight less than 60,000 pounds or with reversible flight controls certified for flight in icing conditions must, at a minimum, be capable of safely operating: (1) in the atmospheric icing conditions of part 25, Appendix C, and (2) after encountering the atmospheric icing conditions of part 25, Appendix O, and while subsequently exiting all icing conditions. Therefore, for airplanes with a maximum takeoff weight less than 60,000 pounds or with reversible flight controls, certification for flight in icing conditions must consider, at a minimum, ice accretions that can occur during flight in Appendix C icing conditions and during detection and exiting of Appendix O icing conditions.
A.1.3 In accordance with § 25.1420(a)(2), an airplane with a maximum takeoff weight less than 60,000 pounds or with reversible flight controls may also be certified for operation in a portion of the atmospheric icing conditions of part 25, Appendix O. In that case, the airplane must also be capable of operating safely after encountering, and while exiting, atmospheric icing conditions in the portion of Appendix O for which operation is not approved. Ice accretions used for certification must consider— A.1.3.1 Operations in Appendix C icing conditions, A.1.3.2 Operations in the Appendix O icing conditions for which approval is sought, and A.1.3.3 Detection and exiting of the Appendix O icing conditions beyond those for which approval is sought.
A.1.4 In accordance with § 25.1420(a)(3), in addition to being certified for flight in Appendix C conditions, an airplane with a maximum takeoff weight less than 60,000 pounds or with reversible flight controls may be certified for operation throughout the atmospheric icing conditions of part 25, Appendix O. Certification for flight throughout the atmospheric icing conditions of Appendix O must consider ice accretions resulting from— A.1.4.1 Operations in Appendix C icing conditions, and A.1.4.2 Operations in Appendix O icing conditions.
Appendix A A-2 A.1.5 The part 25, subpart B, airplane performance and handling characteristics requirements identify the specific ice accretions that apply in showing compliance. In accordance with Appendix C, part II(b), and Appendix O, part II(e), to reduce the number of ice accretions used for demonstrating compliance, the applicant may use any of the applicable ice accretions (or a composite accretion representing a combination of accretions) to show compliance with a particular subpart B requirement if that accretion is either the ice accretion identified in the requirement or is shown to be more conservative than the ice accretion identified in the requirement. In addition, the ice accretion with the most adverse effect on handling characteristics may be used for compliance with the airplane performance requirements if any difference in performance is conservatively taken into account. Ice accretion(s) used to show compliance should take into account the speeds, configurations (including configuration changes), angles-of-attack, power or thrust settings, etc., for the flight phases and icing conditions they are intended to cover.
A.1.6 The applicant should determine the most critical ice accretion in terms of handling characteristics and performance for each flight phase. Parameters to be considered include— A.1.6.1 Flight conditions (for example, airplane configuration, speed, angle-of-attack, altitude), and A.1.6.2 Atmospheric icing conditions for which certification is desired (for example, temperature, liquid water content (LWC), drop mean effective diameter (MED), and drop median volume diameter (MVD)).
A.1.7 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. Justification and selection of the most critical ice shape for each phase of flight should be agreed to by the responsible ACO.
A.1.8 See Appendix R of AC 20-73A, Aircraft Ice Protection, dated August 16, 2009, for additional detailed information about determining the applicable critical ice accretion (shape and roughness).
A.2 Ice Accretions With an Operative Ice Protection System.
A.2.1 All Flight Phases Except Takeoff.
A.2.1.1 For unprotected parts, the ice accretion to be considered should be determined in accordance with part 25, Appendices C and O.
A.2.1.2 Unprotected parts consist of the unprotected airfoil leading edges and all unprotected airframe parts on which ice may accrete. The effect of ice accretion on protuberances such as antennas or flap hinge fairings need
Appendix A A-3 not normally be investigated. However, airplanes characterized by unusual unprotected airframe protuberances, for example fixed landing gear, large engine pylons, or exposed control surface horns or winglets may experience significant additional effects, which should therefore be taken into consideration.
A.2.1.3 For holding ice, the applicant should determine the effect of a 45-minute hold in continuous maximum icing conditions. The analysis should assume that the airplane remains in a rectangular “race track” pattern, with all turns being made within the icing cloud. Therefore, no horizontal extent correction should be used for this analysis. For some previous airplane certification programs, the maximum pinnacle height was limited to 3 inches. This method of compliance may continue to be accepted for follow-on products if service experience has been satisfactory, and the designs are similar enough to conclude that the previous experience is applicable. The applicant should substantiate the critical MED, LWC, and temperature resulting in the formation of an ice accretion critical to the airplane’s performance and handling qualities. The shape and texture of the ice are important and should be agreed to by the responsible ACO.
A.2.1.4 In accordance with the definitions of holding ice, approach ice, and landing ice in Appendices C and O, these flight segments are considered contiguous and sequential. Unless there is a procedure to deice surfaces that have previously accreted ice in the holding flight phase, approach ice is normally the same accretion as the holding ice accretion plus any additional ice accreted during the transition to the approach configuration.
Similarly, unless there is a procedure to deice surfaces before landing, the landing ice accretion is normally the same accretion as the approach ice accretion plus any additional ice accreted during transition to the final landing configuration. A total time of 45 minutes can be used for the combination of hold, approach, and landing ice.
A.2.1.5 For protected parts, the ice protection systems are normally assumed to be operative. However, the applicant should consider the effect of ice accretion on or behind the protected surfaces that results from the rest time of a deicing cycle. Performance may be established on the basis of a representative intercycle ice accretion for normal operation of the deicing system. (The applicant should also consider effects of each residual ice accretion that is not shed). The average drag increment determined over the deicing cycle may be used for performance calculations. Consider the effect of— A.2.1.5.1 Runback ice that occurs on or downstream of the protected surface (including deicing boots), and A.2.1.5.2 Ice accretion prior to activation and operation of the ice protection system.
(See paragraph A.2.3 of this appendix.)
Appendix A A-4 A.2.2 Takeoff Phase.
A.2.2.1 For both unprotected and protected parts, the ice accretion identified in part 25, Appendices C and O, for the takeoff phase may be determined by calculation, assuming the following:
A.2.2.1.1 Airfoils, control surfaces, and propellers if applicable are free from frost, snow, or ice at the start of the takeoff.
A.2.2.1.2 The ice accretion starts at the end of the takeoff distance.
A.2.2.1.3 The airplane is at the critical ratio of power/thrust-to-weight.
A.2.2.1.4 Failure of the critical engine occurs at VEF.
A.2.2.1.5 The flightcrew activates the ice protection system in accordance with an AFM procedure; except that after beginning the takeoff roll, no flightcrew action to activate the ice protection system should be assumed to occur until the airplane is 400 feet above the takeoff surface.
A.2.2.2 The ice accretions identified in part 25, Appendix C, for the takeoff phase are takeoff ice and final takeoff ice.
A.2.2.2.1 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 the end of the takeoff distance and 400 feet above the takeoff surface, assuming accretion starts at the end of the takeoff distance in the takeoff maximum icing conditions of part 25, Appendix C, part I(c).
A.2.2.2.2 Final takeoff ice: The most critical ice accretion on unprotected surfaces, and any ice accretion on the protected surfaces appropriate to normal ice protection system operation, between 400 feet and the height at which the transition to the en route configuration and speed is completed, or 1,500 feet above the takeoff surface, whichever is higher, assuming accretion starts at the end of the takeoff distance in the takeoff maximum icing conditions of part 25, Appendix C, part I(c).
A.2.2.3 The final takeoff ice accretion includes the takeoff ice accretion since these flight segments are contiguous and sequential. For both flight path segments, ice begins accreting at the end of the takeoff distance. The reference to a height of 400 feet above the takeoff surface in these definitions is only used to separate the two takeoff segments. It does not mean that ice accreted before reaching 400 feet above the takeoff surface may be ignored when determining the final takeoff ice accretion.
A.2.2.4 The takeoff ice and final takeoff ice accretions for Appendix O are the same as those for Appendix C, except that the icing conditions are defined in part 25, Appendix O, part I.
Appendix A A-5 A.2.3 Ice Accretion Before Activation and Effective System Operation.
A.2.3.1 When considering the ice accretion before the ice protection system has been activated and is performing its intended function, you should take into account the means of activating the ice protection system and the system response time. However, if artificial stall warning is provided and the point at which stall warning is initiated changes when the ice protection system is activated, then the pre-activation ice accretion used to evaluate the “clean” stall warning schedule does not need to include consideration of the ice protection system response time. System response time is defined as the time interval between activation of the system and its effective operation (for example, for a thermal ice protection system used for deicing, the time to heat the surface and perform its deicing function). If activation of the ice protection system depends on flightcrew recognition of icing conditions or response to a cockpit annunciation, appropriate delays in identifying the icing conditions and activating the ice protection system should be taken into account. For the icing conditions of Appendix C, the airplane should be assumed to be in continuous maximum icing conditions during the time between entering the icing conditions and effective operation of the ice protection system.
A.2.3.2 Section 25.1419(e) requires one of the following three methods for detecting icing and activating the airframe ice protection system:
A.2.3.2.1 A primary ice detection system that automatically activates or that alerts the flightcrew to activate the airframe ice protection system.
A.2.3.2.2 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.
A.2.3.2.3 Identification of conditions conducive to airframe icing as defined by an appropriate static or total air temperature and visible moisture for use by the flightcrew to activate the airframe ice protection system.
A.2.3.3 The following guidance should be used to determine the ice accretion on the unprotected and protected aerodynamic surfaces before activation and normal system operation of the ice protection system:
A.2.3.3.1 If the ice protection system activates automatically after annunciation from a primary ice detection system, the assumed ice accretion should take into account the time it takes for automatic activation of the ice protection system and the time it takes the system to perform its intended function.
The assumed ice accretion can be determined as follows: (1) the ice accreted on the protected surfaces during the time between entry into the icing conditions and activation of the system, plus (2) the ice accreted during the system response time.
Appendix A A-6 A.2.3.3.2 If ice protection system activation depends on pilot action following annunciation from a primary ice detection system, the assumed ice accretion should include flightcrew delays in activating the ice protection system and the time it takes the system to perform its intended function.
The assumed ice accretion should be determined as follows: (1) the ice accreted during the time between entry into the icing conditions and annunciation from the primary ice detection system, plus (2) the ice accreted during 10 additional seconds of operation in icing conditions, plus (3) the ice accreted during the system response time.
A.2.3.3.3 If ice protection system activation depends on the flightcrew visually recognizing the first indication of ice accretion on a reference surface (for example, an ice accretion probe) combined with an advisory ice detection system, the assumed ice accretion should take into account flightcrew delays in detecting the accreted ice and in activating the ice protection system, and the time it takes for the system to perform its intended function. This may be determined as follows: (1) the ice accretion on the aerodynamic surfaces corresponding to the ice accretion on the reference surface that would be easily recognizable by the flightcrew under all foreseeable conditions (for example, at night in clouds), plus (2) the ice accreted during 30 additional seconds of operation in icing conditions, plus (3) the ice accreted during the system response time.
A.2.3.3.4 If ice protection system activation depends on pilot identification of icing conditions (as defined by an appropriate static or total air temperature in combination with visible moisture conditions) with or without an advisory ice detector, the assumed ice accretion should take into account flightcrew delays in recognizing the presence of icing conditions and flightcrew delays in activating the ice protection system, and the time it takes for the system to perform its intended function. This may be determined as follows: (1) the ice accreted during 30 seconds of operation in icing conditions, plus (2) the ice accreted during the system response time.
A.3 Ice Accreted During Ice Protection System Failure.
A.3.1 Unprotected Parts.
The same accretion as in paragraph A.2 of this appendix is applicable.
A.3.2 Protected Parts Following System Failure.
The assumed ice accretion for failures of the ice protection system is defined as follows:
A.3.2.1 If the failure condition is not annunciated, the ice accretion on normally protected parts where the ice protection system has failed should be assumed to be the same as the accretion specified for unprotected parts.
A.3.2.2 If the failure condition is annunciated and the associated AFM procedure does not require the airplane to exit icing conditions, the ice accretion on
Appendix A A-7 normally protected parts where the ice protection system has failed should be assumed to be the same as the accretion specified for unprotected parts.
A.3.2.3 If the failure condition is annunciated and the associated AFM procedure requires the airplane to exit icing conditions as soon as possible, the ice accretion on normally protected parts where the ice protection has failed should be assumed to be one-half of the accretion specified for unprotected parts unless another value is agreed to by the responsible ACO.
A.4 Additional Guidance for Appendix O Ice Accretions.
A.4.1 Ice Accretion in Appendix O Conditions before those Conditions have been Detected by the Flightcrew.
A.4.1.1 This ice accretion, defined as pre-detection ice in Appendix O, part II(b)(5), refers to the ice accretion existing at the time the flightcrew becomes aware that they are in Appendix O icing conditions and have taken action to begin exiting from all icing conditions.
A.4.1.2 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.
A.4.1.3 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 25-28 provides guidance for certifying the detection means. In general, we expect that the time to detect exceedance icing conditions may be significantly longer for a detection means relying on the flightcrew seeing and recognizing a visual icing cue than it is for an ice detection system that provides an attentiongetting alert to the flightcrew.
A.4.1.4 Visual detection requires time for accumulation on the reference surface(s) of enough ice to be reliably identified by either pilot in all atmospheric and lighting conditions. Time between pilot scans of reference surface(s) should be included.
A.4.1.4.1 The amount of ice needed for reliable identification is a function of the distinguishing characteristics of the ice (for example, size, shape, contrast compared to the surface feature that it is adhered to), the distance from the pilots (for example, windshield vs. engine vs. wingtip), and the relative viewing angle (location with respect to the pilots’ primary fields of view).
A.4.1.4.2 Pilot scan time of the reference surface(s) will be influenced by many factors. Such factors include phase of flight, workload, frequency of occurrence of Appendix O conditions, pilot awareness of the possibility of
Appendix A A-8 SLD conditions, and ease of seeing the reference surface(s). The infrequency of Appendix O conditions (approximately 1 in 100 to 1 in 1000, on average in all worldwide icing encounters) and the high workload associated with some phases of flight in instrument conditions (for example, approach and landing) justify using a conservative estimate for the time between pilot scans.
A.4.1.4.3 In the absence of specific studies or tests validating visual detection times, the following times should be used for visual detection of exceedance icing conditions following accumulation of enough ice to be reliably identified by either pilot in all atmospheric and lighting conditions: (1) three minutes for a visual reference located on or immediately outside a cockpit window (for example, ice accretions on side windows, windshield wipers, or icing probe near the windows), and (2) Five minutes for a visual reference located on a wing, wing-mounted engine, or wing tip.
A.4.2 Ice Accretions for Encounters with Appendix O Conditions Beyond those in Which the Airplane is Certified to Operate.
A.4.2.1 Use the ice accretions in table A-1 below, to evaluate compliance with the applicable subpart B requirements for operating safely after encountering Appendix O atmospheric icing conditions for which the airplane is not approved, and then safely exiting all icing conditions.
A.4.2.2 These ice accretions apply when the airplane is not certified for flight in any portion of Appendix O atmospheric icing conditions, when the airplane is certified for flight in only a portion of Appendix O conditions, and for any flight phase for which the airplane is not certified for flight throughout the Appendix O icing envelope.
A.4.2.3 Table A-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).
Table A-1. Appendix O Detect-and-Exit Ice Accretions per Flight Phase Flight Phase/ Condition Appendix O Detect-and-Exit Ice Accretion Ground Roll No accretion Takeoff No accretiona Final Takeoff No accretiona
Appendix A A-9 Flight Phase/ Condition Appendix O Detect-and-Exit Ice Accretion En Route En Route Detect-and-Exit Ice Combination of—
(1) Either Appendix C en route ice or Appendix O en route ice for which the airplane is approved, whichever is applicable,
(2) Pre-detection ice,
(3) Accretion from one standard cloud horizontal extent (17.4 nautical miles) in Appendix O conditions for which the airplane is not approved, and
(4) Accretion from one standard cloud horizontal extent (17.4 nautical miles) in Appendix C continuous maximum icing conditions.
Holding Holding Detect-and-Exit Ice Combination of—
(1) Either Appendix C holding ice or Appendix O holding ice for which the airplane is approved, whichever is applicable,
(2) Pre-detection ice,
(3) Accretion from one standard cloud horizontal extent (17.4 nautical miles) in Appendix O conditions for which the airplane is not approved, and
(4) Accretion from one standard cloud horizontal extent (17.4 nautical miles) in Appendix C continuous maximum icing conditions.
The total time in icing conditions need not exceed 45 minutes.
Approach Approach Detect-and-Exit Ice The more critical of holding detect-and-exit ice and the combination of—
(1) Ice accreted during a descent in the cruise configuration from the maximum vertical extent of the Appendix C maximum continuous icing conditions or the Appendix O icing environment for which the airplane is approved, whichever is applicable, to 2,000 feet above the landing surface, where transition to the approach configuration is made,
(2) Pre-detection ice, and
(3) Ice accreted at 2,000 feet above the landing surface while transiting one standard cloud horizontal extent (17.4 nautical miles) in Appendix O conditions for which the airplane is not approved and one standard cloud horizontal extent (17.4 nautical miles) in Appendix C continuous maximum icing conditions.
Appendix A A-10 Flight Phase/ Condition Appendix O Detect-and-Exit Ice Accretion Landing Landing Detect-and-Exit Ice The more critical of holding detect-and-exit ice and the combination of—
(1) Either Appendix C or Appendix O approach and landing ice for which the airplane is approved, whichever is applicable,
(2) Pre-detection ice, and
(3) Ice accreted during an exit maneuver beginning with the minimum climb gradient specified in § 25.119 from a height of 200 feet above the landing surface and transiting through one standard cloud horizontal extent (17.4 nautical miles) in Appendix O conditions for which the airplane is not approved, and one standard cloud horizontal extent (17.4 nautical miles) in Appendix C continuous maximum icing conditions.
For the purposes of defining the landing detect-and-exit ice shape, the Appendix C approach and landing ice is defined as the ice accreted during—
• A descent in the cruise configuration from the maximum vertical extent of the Appendix C maximum continuous icing environment to 2,000 feet above the landing surface,
• A transition to the approach configuration and maneuvering for 15 minutes at 2,000 feet above the landing surface, and
• A descent from 2,000 feet to 200 feet above the landing surface with a transition to the landing configuration.
Ice Accretion Before the Ice Protection System Has Been Activated and is Performing its Intended Function Ice accreted on protected and unprotected surfaces during the time it takes for icing conditions (either Appendix C or Appendix O) to be detected, the ice protection system to be activated, and the ice protection system to become fully effective in performing its intended function. (Note: If artificial stall warning is provided and the initiation point of that warning changes when the ice protection system is activated, this ice accretion does not need to include consideration of the time it takes for the ice protection system to be effective in performing its intended function.)
Appendix A A-11 Flight Phase/ Condition Appendix O Detect-and-Exit Ice Accretion Ice Accretion in Appendix O Conditions Before Those Conditions Have Been Detected by the Flightcrew and Actions Taken, in Accordance With the AFM, to Either Exit All Icing Conditions or Continue Flight in Appendix O Icing Conditions Ice accreted on protected and unprotected surfaces during—
• The time it takes to detect and identify Appendix O conditions (based on the method of detection) beyond those in which the airplane is certified to operate, and
• The time it takes the flightcrew to refer to and act on procedures, including coordinating with Air Traffic Control, to exit all icing conditions.
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 all icing conditions after the Appendix O icing conditions are recognized.
Failures of the Ice Protection System No accretionb a Intentional flight, including takeoff, is not permitted into Appendix O conditions beyond those in which the airplane is certified to operate.
b It is not necessary to consider an unintentional encounter with Appendix O icing conditions beyond those for which the airplane is certified to operate while operating with a failed ice protection system.
A.4.3 Ice Accretions for Encounters with Appendix O Atmospheric Icing Conditions in Which the Airplane is Certified to Operate.
A.4.3.1 The applicant should use the ice accretions in table A-2 to evaluate compliance with the applicable subpart B requirements for operating safely in the Appendix O atmospheric icing conditions for which the airplane is approved.
A.4.3.2 The decision about which ice accretions to use should include consideration of combinations of Appendix C and Appendix O icing conditions within the scenarios defined in paragraph A.4.3.3 of this appendix. For example, flight in Appendix O conditions may result in ice accumulating, and potentially forming a ridge, behind a protected surface.
Once this accretion site has been established, flight in Appendix C icing
Appendix A A-12 conditions for the remaining portion of the applicable flight phase scenario may result in a more critical additional accretion than would occur for continued flight in Appendix O icing conditions.
A.4.3.3 Table A-2 shows the scenarios the applicant should use for determining ice accretions for certification for flight in the icing conditions of part 25, Appendix O.
Table A-2. Appendix O Ice Accretion per Flight Phase Flight Phase/Condition Appendix O Ice Accretion Ground Roll No accretion Takeoff Takeoff Ice Ice accretion occurring between the end of the takeoff distance and 400 feet above the takeoff surface assuming ice accretion starts at the end of the takeoff distance.
Final Takeoff Final Takeoff Ice Ice accretion occurring between a height of 400 feet above the takeoff surface and the height at which the transition to the en route configuration and speed is completed, or 1500 feet above the takeoff surface, whichever is higher, assuming ice accretion starts at the end of the takeoff distance.
En Route En Route Ice Ice accreted during the en route phase of flight.
Holding Holding Ice Ice accreted during a 45 minute hold with no reduction for horizontal cloud extent (that is, the hold is conducted entirely within the 17.4 nautical mile standard cloud extent).