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.
Appendix C C-3 C.4.2 The aerodynamic balancing of unpowered or boosted reversible flight control surfaces is particularly important with respect to lateral flight control systems when large aileron hinge moments are balanced by equally large hinge moments on the opposite aileron.
Any asymmetric disturbance in flow that affects this critical balance can lead to a sudden uncommanded deflection of the control. This auto deflection, in extreme cases, may be to the control stops.
C.5 Ice Protection/Detection System.
The ice protection/detection system design philosophy may include design features that reduce ice accretion on the wing and/or tailplane.
C.5.1 Wing Ice Protection/Detection.
C.5.1.1 A primary ice detection system that automatically activates a wing deicing system may ensure that there is no significant ice accretion on wings that are susceptible to performance losses with small amounts of ice.
C.5.1.2 If portions of the wing leading edge are not protected, the part that is protected may be selected to provide good handling characteristics at stall, with an acceptable performance degradation.
C.5.2 Tail Ice Protection/Detection.
C.5.2.1 A primary ice detection system may automatically activate a tailplane deicing system on airplanes that do not have visible cues for system operation.
C.5.2.2 An ice protection system on the unshielded aerodynamic balances of airplanes with unpowered reversible controls can reduce the risk of iceinduced aerodynamic hinge moment reversals.
Appendix D D-1 APPENDIX D. EXAMPLES OF AIRPLANE FLIGHT MANUAL LIMITATIONS AND
OPERATING PROCEDURES FOR OPERATIONS IN SUPERCOOLED LARGE DROP ICING
CONDITIONS D.1 Airplane Approved for Flight in Appendix C Icing Conditions but not Approved for Flight in Appendix O Icing Conditions.
D.1.1 AFM Limitations.
Intentional flight, including takeoff and landing, into supercooled large drop (SLD) icing conditions, which includes freezing drizzle or freezing rain, is prohibited. If freezing drizzle or freezing rain conditions are encountered, or if {placeholder for applicant to insert description of pilot cues here}, immediately request priority handling from air traffic control to facilitate a route or altitude change to exit all icing conditions.
Stay clear of all icing conditions for the remainder of the flight, including landing, unless it can be determined that ice accretions no longer remain on the airframe.
D.1.2 AFM Operating Procedures (Normal Procedures Section).
Supercooled large drop (SLD) icing conditions, including freezing drizzle and freezing rain conditions, are severe icing conditions for this airplane. Intentional flight, including takeoff and landing, into SLD icing conditions is prohibited. A flight delay or diversion to an alternate airport is required if these conditions exist at the departure or destination airports.
{Placeholder for applicant to insert description of pilot cues here} is one indication of severe icing for this airplane. If severe icing is encountered, immediately request priority handling from air traffic control to facilitate a route or altitude change to exit all icing conditions. Stay clear of all icing conditions for the remainder of the flight, including landing, unless it can be determined that ice accretions no longer remain on the airframe.
D.1.3 Flightcrew Operating Manual Operating Procedures.
Warning: Hazardous icing effects may result from environmental conditions outside of those for which this airplane is certified. Flight into unapproved icing conditions may result in ice build-up on protected surfaces exceeding the capability of the ice protection system, or in ice forming aft of the protected surfaces. This ice might not be shed when using the ice protection systems, and may seriously degrade performance and controllability of the airplane.
Operations in icing conditions were evaluated as part of the certification process for this airplane. Supercooled large drop (SLD) icing conditions, including freezing drizzle and freezing rain, were not evaluated and are considered severe icing conditions for this airplane.
Intentional flight, including takeoff and landing, into SLD icing conditions, including freezing drizzle or freezing rain, is prohibited. A flight delay or diversion to an alternate airport is required if these conditions exist at the departure or destination airports.
{Placeholder for applicant to insert description of pilot cues here} is an indication of
Appendix D D-2 severe icing conditions that exceed those for which this airplane is certified. If severe icing is encountered, immediately request priority handling from air traffic control to facilitate a route or altitude change to exit all icing conditions. Stay clear of all icing conditions for the remainder of the flight, including landing, unless it can be determined that ice accretions no longer remain on the airframe.
D.2 Airplane Approved for Flight in Appendix C Icing Conditions and Freezing Drizzle Conditions of Appendix O but not Approved for Flight in Freezing Rain Conditions of Appendix O.
D.2.1 AFM Limitations.
Intentional flight, including takeoff and landing, into freezing rain conditions is prohibited. If freezing rain conditions are encountered, or if {placeholder for applicant to insert description of pilot cues here}, immediately request priority handling from air traffic control to facilitate a route or altitude change to exit all icing conditions. Stay clear of all icing conditions for the remainder of the flight, including landing, unless it can be determined that ice accretions no longer remain on the airframe.
D.2.2 AFM Operating Procedures (Normal Procedures Section).
Freezing rain conditions are severe icing conditions for this airplane. Intentional flight, including takeoff and landing, into freezing rain conditions is prohibited. A flight delay or diversion to an alternate airport is required if these conditions exist at the departure or destination airports.
{Placeholder for applicant to insert description of pilot cues here} is one indication of severe icing for this airplane. If severe icing is encountered, immediately request priority handling from air traffic control to facilitate a route or altitude change to exit all icing conditions. Stay clear of all icing conditions for the remainder of the flight, including landing, unless it can be determined that ice accretions no longer remain on the airframe.
D.2.3 Flightcrew Operating Manual Operating Procedures.
Warning: Hazardous icing effects may result from environmental conditions outside of those for which this airplane is certified. Flight into unapproved icing conditions may result in ice build-up on protected surfaces exceeding the capability of the ice protection system, or may result in ice forming aft of the protected surfaces. This ice might not be shed when using the ice protection systems, and may seriously degrade the performance and controllability of the airplane.
Operations in icing conditions, including freezing drizzle, were evaluated as part of the certification process for this airplane. Freezing rain conditions were not evaluated and are considered severe icing conditions for this airplane.
Intentional flight, including takeoff and landing, into freezing rain conditions is prohibited. A flight delay or diversion to an alternate airport is required if these conditions exist at the departure or destination airports. {Placeholder for applicant to insert description of pilot cues here} is an indication of severe icing conditions that
Appendix D D-3 exceed those for which this airplane is certified. If severe icing is encountered, immediately request priority handling from air traffic control to facilitate a route or altitude change to exit all icing conditions. Stay clear of all icing conditions for the remainder of the flight, including landing, unless it can be determined that ice accretions no longer remain on the airframe.
D.3 Airplane Approved for Flight in Appendix C and Appendix O Icing Conditions Except for En Route and Holding Flight Phases in Appendix O Icing Conditions.
D.3.1 AFM Limitations.
Intentional holding or en route flight into supercooled large drop (SLD) icing conditions, including freezing drizzle or freezing rain, is prohibited. If SLD icing conditions, including freezing drizzle or freezing rain, are encountered during a hold (in any airplane configuration) or in the en route phase of flight (climb, cruise, or descent with high lift devices and gear retracted), or if {placeholder for applicant to insert description of pilot cues here} is present, immediately request priority handling from air traffic control to facilitate a route or altitude change to exit all icing conditions. Stay clear of all icing conditions for the remainder of the flight, including landing, unless it can be determined that ice accretions no longer remain on the airframe.
D.3.2 AFM Operating Procedures (Normal Procedures Section).
Supercooled large drop (SLD) icing conditions, including freezing drizzle and freezing rain, encountered during a hold (in any airplane configuration) or in the en route phase of flight (climb, cruise, or descent with high lift devices and gear retracted) are severe icing conditions for this airplane. Intentional holding or en route flight in SLD icing conditions, including freezing drizzle or freezing rain, is prohibited.
{Placeholder for applicant to insert description of pilot cues here} is one indication of severe icing for this airplane. If severe icing is encountered, immediately request priority handling from air traffic control to facilitate a route or altitude change to exit all icing conditions. Stay clear of all icing conditions for the remainder of the flight, including landing, unless it can be determined that ice accretions no longer remain on the airframe.
D.3.3 Flightcrew Operating Manual Operating Procedures.
Warning: Hazardous icing effects may result from environmental conditions outside of those for which this airplane is certified. Flight into unapproved icing conditions may result in ice build-up on protected surfaces exceeding the capability of the ice protection system, or in ice forming aft of the protected surfaces. This ice might not be shed when using the ice protection systems, and may seriously degrade the performance and controllability of the airplane.
Operations in icing conditions were evaluated as part of the certification process for this airplane. En route (climb, cruise, and descent with high left devices and gear retracted) and holding flight (in any airplane configuration) in supercooled large drop (SLD) icing
Appendix D D-4 conditions, including freezing drizzle and freezing rain, were not evaluated and are considered severe icing conditions for this airplane.
Intentional holding or en route flight in SLD icing conditions, including freezing drizzle or freezing rain, is prohibited. {Placeholder for applicant to insert description of pilot cues here} is an indication of severe icing conditions that exceed those for which the airplane is certified. If severe icing is encountered, immediately request priority handling from air traffic control to facilitate a route or altitude change to exit all icing conditions. Stay clear of all icing conditions for the remainder of the flight, including landing, unless it can be determined that ice accretions no longer remain on the airframe.
D.4 Airplane Approved for Flight in Appendix C Icing Conditions and a Portion of Appendix O Icing Conditions.
D.4.1 AFM Limitations.
Intentional flight, including takeoff and landing, into {Placeholder for applicant to insert pilot usable description here} conditions is prohibited. If {insert pilot usable description here} conditions are encountered, or if {insert cue description here}, immediately request priority handling from air traffic control to facilitate a route or altitude change to exit all icing conditions. Stay clear of all icing conditions for the remainder of the flight, including landing, unless it can be determined that ice accretions no longer remain on the airframe.
D.4.2 AFM Operating Procedures (Normal Procedures Section).
{Placeholder for applicant to insert description for pilots to use here} are severe icing conditions for this airplane. Intentional flight, including takeoff and landing, into {insert pilot usable description here} conditions is prohibited. A flight delay or diversion to an alternate airport is required if these conditions exist at the departure or destination airports.
{Placeholder for applicant to insert description of pilot cues here} is one indication of severe icing for this airplane. If severe icing is encountered, immediately request priority handling from air traffic control to facilitate a route or altitude change to exit all icing conditions. Stay clear of all icing conditions for the remainder of the flight, including landing, unless it can be determined that ice accretions no longer remain on the airframe.
D.4.3 Flightcrew Operating Manual Operating Procedures.
Warning: Hazardous icing effects may result from environmental conditions outside of those for which this airplane is certified. Flight into unapproved icing conditions may result in ice build-up on protected surfaces exceeding the capability of the ice protection system, or may result in ice forming aft of the protected surfaces. This ice might not be shed when using the ice protection systems, and may seriously degrade the performance and controllability of the airplane.
Appendix D D-5 Operations in icing conditions were evaluated as part of the certification process for this airplane. {Placeholder for applicant to insert description for pilots to use here} were not evaluated and are considered severe icing conditions for this airplane.
Intentional flight, including takeoff and landing, into {Placeholder for applicant to insert description for pilots to use here} is prohibited. A flight delay or diversion to an alternate airport is required if these conditions exist at the departure or destination airports. {Placeholder for applicant to insert description of pilot cues here} is an indication of severe icing conditions that exceed those for which this airplane is certified. If severe icing is encountered, immediately request priority handling from air traffic control to facilitate a route or altitude change to exit all icing conditions. Remain clear of all icing conditions for the remainder of the flight, including landing, unless it can be determined that ice accretions no longer remain on the airframe.
Appendix E E-1 APPENDIX E. RELATED REGULATIONS AND ADVISORY CIRCULARS E.1 Regulations.
The following sections of Title 14, Code of Federal Regulations (14 CFR) part 25 are referenced in this AC. The full text of these regulations can be downloaded from the U.S. Government Printing Office e-CFR. A paper copy may be ordered from the Government Printing Office, Superintendent of Documents, Attn: New Orders, PO Box 371954, Pittsburgh, PA, 15250-7954.
Table E-1. Related Regulations 14 CFR Title § 25.21 Proof of compliance § 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 § 25.33 Propeller speed and pitch limits § 25.101 Performance—General § 25.103 Stall speed § 25.105 Takeoff § 25.107 Takeoff speeds § 25.109 Accelerate-stop distance § 25.111 Takeoff path § 25.119 Landing climb: All-engines-operating § 25.121 Climb: One-engine-inoperative § 25.123 En route flight paths
Appendix E E-2 14 CFR Title § 25.125 Landing § 25.143 Controllability and Maneuverability—General § 25.145 Longitudinal control § 25.147 Directional and lateral control § 25.161 Trim § 25.171 Stability—General § 25.175 Demonstration of static longitudinal stability § 25.177 Static lateral-directional stability § 25.181 Dynamic stability § 25.201 Stall demonstration § 25.203 Stall characteristics § 25.207 Stall warning § 25.231 Longitudinal stability and control § 25.233 Directional stability and control § 25.235 Taxiing condition § 25.237 Wind velocities § 25.251 Vibration and buffeting § 25.253 High-speed characteristics § 25.255 Out-of-trim characteristics § 25.1309 Equipment, systems, and installations § 25.1329 Flight guidance system § 25.1419 Ice protection § 25.1420 Supercooled large drop icing conditions § 25.1581 Airplane Flight Manual—General
Appendix E E-3 14 CFR Title § 25.1583 Operating limitations § 25.1585 Operating procedures § 25.1587 Performance information Part 25, Appendix C Part 25, Appendix O Supercooled Large Drop Icing Conditions E.2 Advisory Circulars.
The following ACs are related to the guidance in this AC. Table E-2 identifies the latest version of each AC at the time of publication. If any AC is revised after publication of this AC, you should refer to the latest version for guidance. You can find the latest version at the FAA website.
Table E-2. Related Advisory Circulars Number Title Date AC 20-73A Aircraft Ice Protection August 16, 2006 AC 25-7C Flight Test Guide for Certification of Transport Category Airplanes October 16, 2012 AC 25.1309-1A System Design and Analysis June 21, 1988 AC 25.1329-1C Approval of Flight Guidance Systems October 27, 2014 AC 25.1419-2 Compliance with the Ice Protection Requirements of §§ 25.1419(e), (f), (g), and (h) October 27, 2009 AC 25-28 Compliance of Transport Category Airplanes with Certification Requirements for Flight in Icing Conditions October 27, 2014
Appendix F F-1 APPENDIX F. ACRONYMS Table F-1. Acronyms, Terms, and Definitions Acronym/Term Definition AC Advisory Circular ACO Aircraft Certification Office AFM Airplane Flight Manual ATTCS Automatic Takeoff Thrust Control System CAS Calibrated Airspeed CFR Code of Federal Regulations FAA Federal Aviation Administration ICTS Ice-Contaminated Tailplane Stall KIAS Knots Indicated Airspeed LWC Liquid Water Content MED Mean Effective Diameter MVD Median Volume Diameter CL Lift Coefficient CLMAX Maximum Lift Coefficient SLD Supercooled Large Drop Trim A flight condition in which the aerodynamic moment acting about the axis of interest is zero. In the absence of an external disturbance, no control input is needed to maintain the flight condition.