Appendix 1 – Airframe Ice Accretion

ED Decision 2016/010/R

A1.1   General.

a.       In accordance with CS 25.1419, each aeroplane certified for flight in icing conditions must be capable of safely operating in the continuous maximum and intermittent maximum icing conditions of Appendix C. Therefore, at a minimum, certification for flight in icing conditions must include consideration of ice accretions that can occur in Appendix C icing conditions.

b.       In accordance with CS 25.1420(a)(1), each aeroplane certified for flight in icing conditions must, at a minimum, be capable of safely operating:

i.        In the atmospheric icing conditions of Appendix C to CS-25, and

ii.       After encountering the atmospheric icing conditions of Appendix O, and subsequently while exiting all icing conditions.

Therefore, at a minimum, certification for flight in icing conditions must consider ice accretions that can occur during flight in Appendix C icing conditions and during detection and exiting of Appendix O icing conditions.

c.       In accordance with CS 25.1420(a)(2), an aeroplane may also be certified for operation in a portion of the atmospheric icing conditions of Appendix O to CS-25. In that case, the aeroplane 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:

i.        Operations in Appendix C icing conditions,

ii.       Operations in the Appendix O icing conditions for which approval is sought, and

iii.      Detection and exiting of the Appendix O icing conditions beyond those for which approval is sought.

d.       In accordance with CS 25.1420(a)(3), in addition to being certified for flight in Appendix C conditions, an aeroplane may be certified for operation throughout the atmospheric icing conditions of Appendix O to CS-25. Certification for flight throughout the atmospheric icing conditions of Appendix O must consider ice accretions resulting from:

i.        Operations in Appendix C icing conditions, and

ii.       Operations in Appendix O icing conditions.

e.       The CS-25 subpart B aeroplane 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 aeroplane performance requirements if any difference in performance is conservatively taken into account. Ice accretion(s) used to show compliance should take into account the speeds, configurations (including configuration changes), angles of attack, power or thrust settings, etc. for the flight phases and icing conditions they are intended to cover.

f.        The applicant should determine the most critical ice accretion in terms of handling characteristics and performance for each flight phase. Parameters to be considered include:

—         flight conditions (for example, aeroplane configuration, speed, angle-of-attack, altitude) and

—         atmospheric icing conditions for which certification is desired (for example, temperature, liquid water content (LWC), mean effective drop diameter (MED), drop median volume diameter (MVD)).

                   If a comparative analysis (refer to AMC 25.1420(f)) is used as the means of compliance with the CS-25 certification specifications relative to the Appendix O icing conditions, the most critical ice accretions determined for Appendix C icing conditions are acceptable.

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 Agency.

h.       See Appendix R of FAA Advisory Circular AC 20-73A, Aircraft Ice Protection, for additional detailed information about determining the applicable critical ice accretion (shape and roughness).

A1.2   Operative Ice Protection System.

A1.2.1 All flight phases except take-off.

A1.2.1.1 For unprotected parts, the ice accretion to be considered should be determined in accordance with Appendices C and O to CS-25.

A1.2.1.2 Unprotected parts consist of the unprotected aerofoil leading edges and all unprotected airframe parts on which ice may accrete. The effect of ice accretion on protuberances such as antennae or flap hinge fairings need not normally be investigated. However aeroplanes that are characterised by unusual unprotected airframe protuberances, e.g. fixed landing gear, large engine pylons, or exposed control surface horns or winglets, etc., may experience significant additional effects, which should therefore be taken into consideration.

A1.2.1.3 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 aeroplane 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 aeroplane certification programs, the maximum pinnacle height was limited to 75 mm (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 mean effective drop diameter, liquid water content, and temperature that result in the formation of an ice accretion that is critical to the aeroplane’s performance and handling qualities. The shape and texture of the ice are important and should be agreed with the Agency.

A1.2.1.4 For protected parts, the ice protection systems are normally assumed to be operative. However, the applicant should consider the effect of ice accretion on the protected surfaces that result from:

a.       The rest time of a de-icing cycle. Performance may be established on the basis of a representative intercycle ice accretion for normal operation of the de-icing system (consideration should also be given to the effects of any residual ice accretion that is not shed.) The average drag increment determined over the de-icing cycle may be used for performance calculations.

b.      Runback ice which occurs on or downstream of the protected surface.

c.       Ice accretion prior to activation and normal operation of the ice protection system (see paragraph A1.2.3, below).

A1.2.2 Take-off phase.

A1.2.2.1 For both unprotected and protected parts, the ice accretion identified in Appendix C and Appendix O to CS-25 for the take-off phase may be determined by calculation, assuming the following:

—         aerofoils, control surfaces and, if applicable, propellers are free from frost, snow, or ice at the start of the take-off;

—         the ice accretion starts at the end of the take-off distance

—         the critical ratio of thrust/power-to-weight;

—         failure of the critical engine occurs at VEF; and

—         flight crew activation of the ice protection system in accordance with an AFM procedure, except that after commencement of the take-off roll no flight crew action to activate the ice protection system should be assumed to occur until the aeroplane is 122 m (400 ft) above the take-off surface.

A1.2.2.2 The ice accretions identified in Appendix C and Appendix O to CS-25 for the take-off phase are:

—         "Take-off ice": The most critical ice accretion between the end of the take-off distance and 122 m (400 ft) above the takeoff surface, assuming accretion starts at the end of the take-off distance in the icing environment.

—         "Final Take-off ice": The most critical ice accretion between 122 m (400 ft) ) and the height at which the transition to the en route configuration and speed is completed, or 457 m (1500 ft) above the take-off surface, whichever is higher, assuming accretion starts at the end of the take-off distance in the icing environment.

A1.2.3 Ice accretion prior to activation and normal system operation.

A1.2.3.1 When considering ice accretion before the ice protection system has been activated and is performing its intended function, the means of activating the ice protection system and the system response time should be taken into account. System response time is defined as the time interval between activation of the system and its effective operation (for example, for a thermal ice protection system used for de-icing, the time to heat the surface and perform its de-icing function).

If activation of the ice protection system depends on flight crew 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 aeroplane 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.

A1.2.3.2 For an aeroplane certified in accordance with CS 25.1420 (a)(2) or (a)(3), the requirements of CS 25.1419 (e), (f), (g), and (h) must be met for the icing conditions defined in Appendix O in which the aeroplane is certified to operate.

         CS 25.1419(e) requires one of the following three methods for detecting icing and activating the airframe ice protection system:

(a)     A primary ice detection system that automatically activates or that alerts the flight crew to activate the airframe ice protection system; or

(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 flight crew to activate the airframe ice protection system; or

(c)      Identification of conditions conducive to airframe icing as defined by an appropriate static or total air temperature and visible moisture for use by the flight crew to activate the airframe ice protection system.

A1.2.3.3 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.       If the ice protection system activates automatically after annunciation from a primary ice detection system, the assumed ice accretion should take into account the time it takes for automatic activation of the ice protection system and the time it takes for the system to perform its intended function. The assumed ice accretion can be determined as follows:

i.        The ice accretion on the protected surfaces corresponding to the time between entry into the icing conditions and activation of the system, plus

ii.       The ice accretion during the system response time.

b.       If ice protection system activation depends on pilot action following annunciation from a primary ice detection system, the assumed ice accretion should take into account flight crew 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:

i.        The ice accretion corresponding to the time between entry into the icing conditions and annunciation from the primary ice detection system, plus

ii.       The ice accretion corresponding to 10 additional seconds of operation in icing conditions, plus

iii.      The ice accretion during the system response time.

c.       If ice protection system activation depends on the flight crew 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 flight crew 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:

i.        The ice accretion that would be easily recognizable by the flight crew under all foreseeable conditions (for example, at night in clouds) as it corresponds to the first indication of ice accretion on the reference surface, plus

ii.       the ice accretion equivalent to 30 seconds of operation in icing conditions, plus

iii.      the ice accreted during the system response time.

d.       If ice protection system activation depends on pilot identification of icing conditions (as defined by an appropriate static or total air temperature in combination with visible moisture conditions) with or without an advisory ice detector, the assumed ice accretion should take into account flight crew delays in recognizing the presence of icing conditions and flight crew 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:

i.        the ice accretion equivalent to 30 seconds of operation in icing conditions, plus

ii.       the ice accretion during the system response time.

A1.3   Ice Protection System Failure Cases.

A1.3.1 Unprotected parts. The same accretion as in paragraph A1.2.1 is applicable.

A1.3.2 Protected parts following system failure. "Failure Ice" is defined as follows:

A1.3.2.1 In the case where the failure condition is not annunciated, the ice accretion on normally protected parts where the ice protection system has failed should be the same as the accretion specified for unprotected parts.

A1.3.2.2 In the case where the failure condition is annunciated and the associated procedure does not require the aeroplane to exit icing conditions, the ice accretion on normally protected parts where the ice protection system has failed should be the same as the accretion specified for unprotected parts.

A1.3.2.3 In the case where the failure condition is annunciated and the associated procedure requires the aeroplane to exit icing conditions as soon as possible, the ice accretion on normally protected parts where the ice protection has failed, should be taken as one-half of the accretion specified for unprotected parts unless another value is agreed by the Agency.

A1.4   Additional guidance for Appendix O ice accretions.

A1.4.1 Ice Accretion in Appendix O Conditions Before those Conditions Have Been Detected by the Flight crew.

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 flight crew become aware that they are in Appendix O icing conditions and have taken action to begin exiting from all icing conditions.

a.       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.

b.       The time that the applicant should assume it will take to detect Appendix O icing conditions exceeding those for which the aeroplane is certified should be based on the means of detection. AMC 25.1419 and AMC 25.1420 provide guidance for certifying the detection means. In general, the Agency expects that the time to detect exceedance icing conditions may be significantly longer for a detection means relying on the flight crew seeing and recognizing a visual icing cue than it is for an ice detection system that provides an attention-getting alert to the flight crew.

c.       Visual detection requires time for accumulation on the reference surface(s) of enough ice to be reliably identified by either pilot in all atmospheric and lighting conditions. Time between pilot scans of reference surface(s) should be considered.

i.        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).

ii.       Pilot scan time of the reference surface(s) will be influenced by many factors. Such factors include phase of flight, workload, frequency of occurrence of Appendix O conditions, pilot awareness of the possibility of supercooled large drop conditions, and ease of seeing the reference surface(s). The infrequency of Appendix O conditions (approximately 1 in 100 to 1 in 1 000, 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.

iii.      In the absence of specific studies or tests validating visual detection times, the following times should be used for visual detection of exceedance icing conditions following accumulation of enough ice to be reliably identified by either pilot in all atmospheric and lighting conditions:

1.       For a visual reference located on or immediately outside a cockpit window (for example, ice accretions on side windows, windshield wipers, or icing probe near the windows) – 3 minutes.

2.       For a visual reference located on a wing, wing mounted engine, or wing tip – 5 minutes.

A1.4.2 Ice Accretions for Encounters with Appendix O Conditions Beyond those in Which the Aeroplane is Certified to Operate.

a.       Use the ice accretions in Table 1, below, to evaluate compliance with the applicable CS-25 subpart B requirements for operating safely after encountering Appendix O atmospheric icing conditions for which the aeroplane is not approved, and then safely exiting all icing conditions.

b.       The ice accretions of Table 1 apply when the aeroplane is not certified for flight in any portion of Appendix O atmospheric icing conditions, when the aeroplane is certified for flight in only a portion of Appendix O conditions, and for any flight phase for which the aeroplane is not certified for flight throughout the Appendix O icing envelope.

c.       Table 1 shows the scenarios to be used for determining ice accretions for certification testing of encounters with Appendix O conditions beyond those in which the aeroplane is certified to operate (for detecting and exiting those conditions):

Table 1

Notes:

¹        Intentional flight, including Take-off, is not permitted into Appendix O conditions beyond those in which the aeroplane is certified to operate.

²         It is not necessary to consider an unintentional encounter with Appendix O icing conditions beyond those in which the aeroplane is certified to operate while operating with a failed ice protection system.

A1.4.3 Ice Accretions for Encounters with Appendix O Atmospheric Icing Conditions in Which the Aeroplane is Certified to Operate.

a.       The applicant should use the ice accretions in Table 2 to evaluate compliance with the applicable CS-25 subpart B requirements for operating safely in the Appendix O atmospheric icing conditions for which approval is sought.

b.       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 A1.4.3(c) of this appendix. For example, flight in Appendix O conditions may result in ice accumulating, and potentially forming a ridge, behind a protected surface. Once this accretion site has been established, flight in Appendix C icing conditions for the remaining portion of the applicable flight phase scenario may result in a more critical additional accretion than would occur for continued flight in Appendix O icing conditions.

c.       Table 2 shows the scenarios the applicant should use for determining ice accretions for certification for flight in the icing conditions of Appendix O to CS-25.

Table 2

[Amdt 25/3]

[Amdt 25/16]

[Amdt 25/18]