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).
Appendix A A-13 Flight Phase/Condition Appendix O Ice Accretion Approach Approach Ice More critical ice accretion of—
(1) Ice accreted during a descent in the cruise configuration from the maximum vertical extent of the Appendix O icing environment to 2,000 feet above the landing surface, followed by—
• Transition to the approach configuration, and
• Maneuvering for 15 minutes at 2,000 feet above the landing surface; and
(2) Holding ice (if the airplane is certified for holding in Appendix O conditions).
Landing Landing Ice More critical ice accretion of—
(1) Approach ice plus ice accreted during descent from 2,000 feet above the landing surface to 200 feet above the landing surface with—
• A transition to the landing configuration, followed by
• A go-around maneuver beginning with the minimum climb gradient specified in § 25.119 from 200 feet to 2,000 feet above the landing surface,
• Holding for 15 minutes at 2,000 feet above the landing surface in the approach configuration, and
• A descent to the landing surface in the landing configuration; and
(2) Holding ice (if the airplane is certified for holding in Appendix O conditions).
Ice Accretion Before the Ice Protection System has been Activated and is Performing its Intended Function Ice accreted during the time it takes for the flightcrew to recognize icing conditions and activate the ice protection system, plus the time for the ice protection system to perform its intended function.
Appendix A A-14 Flight Phase/Condition Appendix O 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 during the time it takes for the flightcrew to detect Appendix O conditions and refer to and initiate associated procedures, and any time it takes for systems to perform their intended functions (if applicable). Predetection ice need not be considered if there are no specific crew actions or systems changes associated with flight in Appendix O conditions.
Failures of the Ice Protection System Same criteria as for Appendix C (see paragraph A.3 of this appendix), but in Appendix O conditions.
Appendix B B-1 APPENDIX B. SIMULATED ICE ACCRETIONS B.1 Overview.
B.1.1 In general, the simulated ice accretions used for flight testing should be those that have the most adverse effects on handling characteristics. However, if analytical data show that other reasonably expected ice accretions could be generated that could have a more adverse effect on performance, then the ice accretion having the most adverse effect on handling characteristics may be used for performance tests only if each difference in performance can be conservatively taken into account.
B.1.2 Simulated accretions should be representative of natural icing conditions in terms of location, general shape, thickness, and texture. The determination of the form and surface texture of the ice accretion (see paragraph B.2 below) should be agreed to by the responsible ACO as being representative of natural ice accretion.
B.1.3 Sandpaper ice is addressed in paragraph B.3 below.
B.2 Shape and Texture of Simulated Ice Accretion.
B.2.1 The shape and texture of the simulated ice accretion should be established and substantiated by agreed-upon methods. Common practices used to accomplish this include—
• Use of computer codes,
• Flight in measured natural icing conditions,
• Icing wind tunnel tests, and
• Flight in a controlled simulated icing cloud (for example, from an icing tanker).
B.2.2 Unless another texture is substantiated by the applicant and agreed to by the ACO, the applicant should use an ice accretion roughness height of 3 mm with a particle density of 8 to 10/cm2.
B.3 Sandpaper Ice.
B.3.1 Sandpaper ice is the most critical thin, rough layer of ice. Carborundum sandpaper No. 40 (40-grit carborundum sandpaper) has been used in past certification programs to represent sandpaper ice. However, as detailed in Appendix R of AC 20-73A, the uniformly distributed roughness of carborundum grit may not result in aerodynamic effects similar to those of the actual intercycle ice surface roughness. The applicant should validate use of uniformly distributed roughness to simulate sandpaper ice, particularly for intercycle ice accretions.
Appendix B B-2 B.3.2 Because sandpaper ice must be considered in the basic icing certification within the Appendix C environmental icing envelope, it does not need to be considered for certification of flight in Appendix O icing conditions.
B.3.3 The spanwise and chordwise coverage of simulated ice should be consistent with the areas of ice accretion determined for the conditions of part 25, Appendix C, except that, for the zero-g pushover maneuver of paragraph 4.9.4.3.3 of this AC, the sandpaper ice may be restricted to the horizontal stabilizer if this can be shown to be conservative.
B.3.4 Appendix C, part II(a) requires applicants to use the most critical ice accretion to show compliance with the applicable subpart B airplane requirements for performance and handling characteristics in icing conditions. When determining the most critical ice accretion, the applicant must consider the full range of atmospheric icing conditions of Appendix C, part I, as well as the characteristics of the ice protection system, in accordance with § 25.21(g)(2) and Appendix C, part II(a). This would include consideration of sandpaper ice (as well as any other type of ice accretion that may occur in the applicable atmospheric icing conditions), taking into account the operating characteristics of the ice protection system and the flight phase. Sandpaper ice can be particularly critical for showing compliance with either § 25.143(i)(2) or (j)(2). If sandpaper ice can be present, it must be considered in showing compliance with both of these requirements.
Appendix C C-1 APPENDIX C. DESIGN FEATURES C.1 Airplane Configuration and Ancestry.
An airplane’s size is an important design characteristic that can affect performance, controllability, and maneuverability in icing conditions. The safety record of the airplane’s closely related ancestors is another aspect that may be taken into consideration.
C.1.1 Size.
The size of an airplane determines the sensitivity of its flight characteristics to ice thickness and roughness. The relative effect of a given ice height (or ice roughness height) decreases as airplane size increases.
C.1.2 Ancestors.
If a closely related ancestor airplane was certified for flight in icing conditions, that airplane’s safety record may be used on a similarity basis when evaluating the subject airplane’s general capabilities for flight in icing conditions.
C.2 Wing.
Design features of a wing that can affect performance, controllability, and maneuverability include airfoil type, leading edge devices, and lateral control system effectiveness.
C.2.1 Airfoil Type.
Aerodynamic effects of ice accretions result mainly from the effects of the ice accretion on the behavior of the airfoil’s boundary layer. The boundary layer is the layer of air close to the surface of the airfoil that is moving across the airfoil at a velocity lower than the freestream velocity, that is, the velocity of the airfoil. Ice accretions that occur in areas favorable to keeping the boundary layer attached to the aircraft surface will result in effects that are less aerodynamically adverse than ice accretions that occur in areas less favorable to attached boundary layer conditions. Ice shapes that build up in areas of local airflow deceleration (positively increasing surface pressure), or result in conditions unfavorable to keeping attached flow conditions, as the airflow negotiates the ice surface, will result in the most adverse effects.
C.2.2 Leading Edge Devices.
The presence of a leading edge device (such as a slat) reduces the percentage decrease in the maximum lift coefficient (CLMAX) due to ice by increasing the overall level of the lift coefficient (CL). Movement of a slat so that a gap forms may improve the situation further. Leading edge devices can also reduce loss in angle-of-attack at stall due to ice.
Appendix C C-2 C.2.3 Lateral Control System Effectiveness.
The applicant can evaluate effectiveness of the lateral control system in icing conditions by comparing it with the lateral control system in closely related ancestor airplanes, with consideration of the design of aerodynamic balance surfaces.
C.3 Empennage.
The effects of size and airfoil type also apply to the horizontal and vertical tails. Other design features include tailplane design philosophy, horizontal stabilizer design, control surface actuation, control surface size, and vertical stabilizer and rudder design. Since tails are usually not equipped with leading edge devices, the effects of ice on tail aerodynamics are similar to those on a wing with no leading edge devices. However, these effects usually result in changes to airplane handling and/or control characteristics rather than degraded performance.
C.3.1 Tailplane Design Philosophy.
The tailplane may be designed and sized to provide full functionality in icing conditions without ice protection, or it may be designed with a deicing or anti-icing system.
C.3.2 Horizontal Stabilizer Design.
Cambered airfoils and/or trimmable stabilizers may reduce the susceptibility to and consequences of elevator hinge moment reversal due to ice-induced tailplane stall.
C.3.3 Control Surface Actuation.
Hydraulically powered irreversible elevator controls are usually not affected by ice-induced aerodynamic hinge moment reversal.
C.3.4 Control Surface Size.
For mechanical elevator controls, the size of the surface significantly affects the control force resulting from an ice-induced aerodynamic hinge moment reversal. Small surfaces are less susceptible to control difficulties for given hinge moment coefficients.
C.3.5 Vertical Stabilizer and Rudder Design.
Effectiveness of the vertical stabilizer and rudder in icing conditions can be evaluated by comparison with the vertical stabilizer and rudder in closely related ancestor airplanes, with consideration of the design of aerodynamic balance surfaces.
C.4 Aerodynamic Balancing of Flight Control Surfaces.
C.4.1 The aerodynamic balance of unpowered or boosted reversible flight control surfaces is an important design feature to consider. The applicant should carefully evaluate the design to account for effects of ice accretion on flight control system hinge moment characteristics. Closely balanced controls may be vulnerable to overbalance in icing.
The effect of ice in front of the control surface, or on the surface, may upset the balance of hinge moments, leading to either increased positive or negative force gradients.