0 U.S. Department Advisory of Transportation Federal Aviation Administration Circular Subject: Compliance of Transport Category AC No: 25-28 Date: 10/27/14 Airplanes with Certification Requirements for Flight in Icing Conditions Initiated By: ANM-110 This advisory circular (AC) describes an acceptable means for showing compliance with the supercooled large drop condition requirements of§ 25.773, Pilot compartment view,§ 25.1323, Airspeed indicating system,§ 25.1324, Angle of attack system, and§ 25.1325, Static pressure systems, as well as the requirements of§ 25 .1419, Ice protection, and § 25.1420, Supercooled large drop icing conditions, of Title 14, Code of Federal Regulations (14 CFR) part 25. Part 25 contains the certification requirements for transport category airplanes. The compliance means described in this document are intended as guidance. They are meant to supplement the engineering judgment that must form the basis of any compliance findings for the supercooled large drop icing conditions in§§ 25.773, 25.1323, 25.1324, and 25.1325, as well as the compliance findings for§§ 25.1419 and 25.1420.
If you have suggestions for improving this AC, you may use the feedback form at the end of this AC.
Michael Kaszycki Acting Manager, Transport Airplane Directorate Aircraft Certification Service
Contents Paragraph Page ii
1 Purpose1
2 Applicability1
3 Cancellation3
4 Background3
5 Definition of Terms3
5.1 Advisory Ice Detection System3
5.2 Airframe Icing3
5.3 Anti-Icing3
5.4 Appendix C Icing Conditions4
5.5 Appendix O Icing Conditions4
5.6 Artificial Ice4
5.7 Automatic Cycling Mode4
5.8 Deicing4
5.9 Drizzle Drop4
5.10 Freezing Drizzle (FZDZ)4
5.11 Freezing Precipitation4
5.12 Freezing Rain (FZRA)4
5.13 Glaciated Icing Conditions4
5.14 Ice Protection System (IPS)4
5.15 Icing Conditions4
5.16 Icing Conditions Detector5
5.17 Irreversible Flight Controls5
5.18 Liquid Water Content (LWC)5
5.19 Mean Effective Diameter (MED)5
5.20 Median Mass Dimension (MMD)5
5.21 Median Volume Diameter (MVD)5
5.22 Migrating Ice5
5.23 Mixed Phase Icing Conditions5
5.24 Monitored Surface6
5.25 Primary Ice Detection System6
Contents (continued) Paragraph Page iii
5.26 Rain Drop6
5.27 Reference Surface6
5.28 Reversible Flight Controls6
5.29 Runback Ice6
5.30 Simulated Ice7
5.31 Static Air Temperature7
5.32 Substantiated Visual Cue7
5.33 Supercooled Large Drops (SLD)7
5.34 Supercooled Water7
5.35 Water Catch7
6 Icing Envelopes7
6.1 The Appendix O Environment7
6.2 Use of Icing Envelopes10
7 Certification Plan13
8 Analysis14
8.1 Analytical Simulation Methods14
8.2 Substantiating Icing Simulation Analytical Tools14
8.3 Analysis of Areas and Components to be Protected14
8.4 Flutter Analysis15
8.5 Power Sources15
8.6 Failure Analyses15
8.7 Similarity Analyses18
8.8 Impingement Limit Analyses19
8.9 Ice Shedding Analyses19
8.10 Propeller Deicing Analysis20
8.11 Airspeed Indication and Static Pressure System Ice Protection20
8.12 Stall Warning System and Angle-of-Attack Ice Protection21
8.13 Runback Ice22
9 Simulated Ice Shapes and Roughness22
10 Critical Ice Shapes23
Contents (continued) Paragraph Page iv
11 Compliance Tests (Flight/Simulation)23
12 Certification with § 25.142032
12.1 Certification with §§ 25.1420(a)(1) and 25.1420(a)(2)32
12.2 Certification to § 25.1420(a)(3)37
13 Pneumatic Deicer Boots37
14 Emergency and Abnormal Operating Conditions37
15 Ice and Icing Conditions Detection Systems37
15.1 Compliance with § 25.1419(e)(1) and (e)(2)37
15.2 Compliance with § 25.1419(e)(3)41
15.3 Compliance with § 25.1419(f)42
15.4 Compliance with § 25.1419(g)42
15.5 Compliance with § 25.1420(a)(1) or (a)(2)43
16 Performing Intended Functions in Icing44
16.1 Engines and Equipment44
16.2 Engine Alternate Induction Air Sources44
16.3 Fuel System Venting44
16.4 Landing Gear44
16.5 Primary and Secondary Flight Control Surfaces44
16.6 Ram Air Turbine44
17 Ice Inspection Light(s)44
18 Caution Information45
19 Airplane Flight Manual46
19.1 Operating Limitations Section46
19.2 Operating Procedures Section46
20 Guidance for Amended Type Certificates and Supplemental Type Certificates48
21 Instructions for Continued Airworthiness48
Appendix A. Related Regulations and DocumentsA-1
Appendix B. History of Icing Certification RulesB-1
Appendix C. Example of Certification to 25.1420(a)(1) Using Visual CuesC-1
Contents (continued) Paragraph Page v Appendix D. Guidance for Amended Type Certificates and Supplemental Type Certificates .. D-1 Appendix E. Capabilities of Engineering Tools for Compliance with Part 25, Appendix O
RequirementsE-1
Appendix F. AcronymsF-1
Tables
Table 1. Freezing Drizzle Drop Size Distributions Represented Using 10 Bins11
Table 2. Freezing Rain Drop Size Distributions Represented Using 10 Bins12
Table A-1. Related RegulationsA-1
Table A-2. Related Advisory CircularsA-3
Table A-3. Related Industry DocumentsA-5
Table E-1. Assessment of Appendix O Engineering Tool CapabilitiesE-2
Table E-2. Appendix O Icing Standard RoughnessE-12
Table F-1. Acronyms and DefinitionsF-1
1 1 PURPOSE.
1.1 This AC describes an acceptable means for showing compliance with the supercooled large drop condition requirements of § 25.773, Pilot compartment view, § 25.1323, Airspeed indicating system, § 25.1324, Angle of attack system, and § 25.1325, Static pressure systems, as well as the requirements of § 25.1419, Ice protection, and § 25.1420, Supercooled large drop icing conditions, of 14 CFR part 25. Part 25 contains the certification requirements for transport category airplanes. The compliance means described in this document are intended as guidance. They are meant to supplement the engineering judgment that must form the basis of any compliance findings for the supercooled large drop icing conditions in §§ 25.773, 25.1323, 25.1324, and 25.1325, as well as the compliance findings for §§ 25.1419 and 25.1420.
1.2 The following appendices appear at the end of this AC:
• Appendix A, Related Regulations and Documents.
• Appendix B, History of Icing Certification Rules.
• Appendix C, Example of Certification to § 25.1420(a)(1) Using Visual Cues.
• Appendix D, Guidance for Amended Type Certificates and Supplemental Type Certificates.
• Appendix E, Capabilities of Engineering Tools for Compliance with Appendix O Requirements of Part 25.
• Appendix F, Acronyms.
2 APPLICABILITY.
2.1 This AC provides guidance directed to airplane manufacturers, modifiers, foreign regulatory authorities, and Federal Aviation Administration (FAA) airplane type certification engineers and their designees. This guidance applies to certification of part 25 transport category airplanes for flight in icing conditions.
2.2 This material in this AC is neither mandatory nor regulatory in nature and does not constitute a regulation. It describes acceptable means, but not the only means, for demonstrating compliance with the applicable regulations. The FAA will consider other methods of demonstrating compliance that an applicant may elect to present. While these guidelines are not mandatory, they are derived from extensive FAA and industry experience. If, however, we become aware of circumstances that convince us that following this AC would not result in compliance with the applicable regulations, we will not be bound by the terms of this AC, and we may require additional substantiation or design changes as a basis for finding compliance.
2.3 This material in this AC does not change or create any additional regulatory requirements, nor does it authorize changes in, or permit deviations from, regulatory requirements.
2.4 The guidance provided in this AC applies to certification of part 25 transport category airplanes for flight in icing conditions. If certification for flight in icing conditions is desired, the airplane must be able to safely operate throughout the icing envelope defined in 14 CFR part 25, Appendix C. Additional icing requirements in part 25 apply to airplanes for which at least one of the following applies—
• The airplane’s gross weight is less than 60,000 lbs, or
• The airplane is equipped with reversible flight controls.
2.5 Such an airplane, besides being able to operate safely in Appendix C icing conditions, must also be able to safely operate in or exit the icing conditions defined by 14 CFR part 25, Appendix O, Supercooled Large Drop Icing Conditions. The applicant, however, has several certification options available for Appendix O icing conditions.
The airplane can be certified for—
2.5.1 The ability to detect Appendix O conditions and safely exit all icing conditions,
2.5.2 The ability to operate safely throughout a portion of Appendix O icing conditions and safely exit all icing conditions when that portion of Appendix O is exceeded, or
2.5.3 The ability to operate safely throughout all Appendix O icing conditions.
2.6 Sections 25.1419 and 25.1420 provide specific airframe requirements for certificating an airplane to operate in the icing conditions defined in Appendices C and O.
Additionally, for other parts of the airplane (i.e., engine, engine inlet, propeller), there are more specific icing requirements and associated guidance. These are contained in the regulations and ACs listed in Appendix A of this AC. Some icing-related regulations must be complied with even if the airplane is not certificated for flight in icing. The following are examples:
• Section 25.629(d)(3).
• Section 25.903.
• Section 25.975.
• Section 25.1093.
• Section 25.1323(i).
• Section 25.1324.
• Section 25.1325(b).
2.7 This AC contains information on flight tests for ice protection certification. Additional information on flight tests for showing compliance with the airplane performance and handling qualities requirements for icing certification may be found in AC 25-25A, Performance and Handling Characteristics in Icing Conditions. AC 20-73A, Aircraft Ice Protection, also contains flight-in-icing approval guidance. The guidance in this AC is applicable to new type certificates (TCs), supplemental type certificates (STCs), and amendments to existing TCs for airplanes certified under part 4b of the Civil Air
3 Regulations (CAR) and under part 25, for which approval under the provisions of § 25.1419 is desired.
3 CANCELLATION.
This AC cancels AC 25.1419-1A, Certification of Transport Category Airplanes for Flight in Icing Conditions, dated May 7, 2004, and AC 25.1419-2, Compliance with the Ice Protection Requirements of §§ 25.1419(e), (f), (g), and (h), dated October 27, 2009.
4 BACKGROUND.
Before 1953, airplanes were certificated under part 04 of the Civil Air Regulations (CARs). The first icing rule, section 04.5814, stated that if deicer boots were installed on an airplane, positive means for deflation of all wing boots was required. There were no other references to an airplane ice protection system (IPS) in part 04. Part 4b of the CAR was codified on December 31, 1953, and it was subsequently recodified into 14 CFR part 25, effective February 1, 1965. Over the years, as our knowledge has increased about icing conditions and their effects on safe operation of airplanes, we have made changes to the regulations. A detailed history of icing certification requirements can be found in Appendix B of this AC.
5 DEFINITION OF TERMS.
For purposes of this AC, the following definitions are used.
Note: These definitions of terms are intended for use only with respect to §§ 25.1419 and 25.1420.
5.1 Advisory Ice Detection System.
A system that advises the flightcrew of the presence of ice accretion or icing conditions.
An advisory ice detection system can only be used in conjunction with other means (most commonly, visual observation by the flightcrew) to determine the need for, or timing of, activating the anti-icing or deicing system. When using an advisory ice detection system, the flightcrew is responsible for monitoring icing conditions or ice accretion as defined in the airplane flight manual (AFM), typically using total air temperature and visible moisture criteria or visible ice accretion. The flightcrew is responsible for activating the anti-icing or deicing system(s).
5.2 Airframe Icing.
Ice accretion on the airplane, except for on the propulsion system.
5.3 Anti-Icing.
Prevention of ice accretions on a protected surface either by evaporating the impinging water, or allowing the impinging water to run back and off the protected surface or freeze on non-critical areas.
5.4 Appendix C Icing Conditions.
The environmental conditions defined in Appendix C of 14 CFR part 25.
5.5 Appendix O Icing Conditions.
The environmental conditions defined in Appendix O of 14 CFR part 25.
5.6 Artificial Ice.
Real ice formed by artificial or simulated means, such as by a spray rig in a tunnel or behind an icing tanker.
5.7 Automatic Cycling Mode.
A mode of operation of the airframe deicing system that provides repetitive cycles of the system without the need for the pilot to select each cycle. This is generally done with a timer, and there may be more than one timing mode.
5.8 Deicing.
Removing or the process of removing an ice accretion after it has formed on a surface.
5.9 Drizzle Drop.
A drop of water measuring 100 to 500 micrometers, or microns (µm), (0.1 – 0.5 mm) in diameter.
5.10 Freezing Drizzle (FZDZ).
Supercooled drizzle drops that remain in liquid form and freeze on contact with objects colder than 0 °C.
5.11 Freezing Precipitation.
Any form of supercooled liquid precipitation that freezes on impact with the ground or exposed objects colder than 0 °C—freezing rain or freezing drizzle.
5.12 Freezing Rain (FZRA).
Supercooled rain drops that remain in liquid form and freeze on contact with objects colder than 0 °C.
5.13 Glaciated Icing Conditions.
Icing conditions in which only ice crystals are present.
5.14 Ice Protection System (IPS).
A system that protects certain critical airplane parts from ice accretion.
5.15 Icing Conditions.
The presence of atmospheric moisture and temperature conducive to airplane icing.
5.16 Icing Conditions Detector.
A device that detects the presence of atmospheric moisture and temperature conducive to airplane icing.
5.17 Irreversible Flight Controls.
Flight controls in the normal operating configuration that have force or motion originating at the airplane’s control surface (through aerodynamic loads, static imbalance, or trim or servo tab inputs, for example) that is transmitted to the actuator and its mounting. The force or motion cannot be transmitted directly back to the flight deck controls. This term refers to flight controls in which all of the force necessary to move the pitch, roll, or yaw control surfaces is provided by hydraulic or electric actuators, the motion of which is controlled by signals from the flight deck controls.
5.18 Liquid Water Content (LWC).
The total mass of water contained in liquid drops within a unit volume of air, usually given in units of grams of water per cubic meter (g/m3).
5.19 Mean Effective Diameter (MED).
The calculated drop diameter that divides the total liquid water content present in the drop size distribution in half. Half the water volume will be in larger drops and half the volume in smaller drops. This value is calculated, as opposed to measuring actual drop size. The MED is based on an assumed Langmuir drop size distribution. The fact that it is a calculated measurement is how it differs from median volume diameter, which is based on actual drop size.
5.20 Median Mass Dimension (MMD).
The particle size (sphere or equivalent mass) that divides the total ice mass present in an ice particle distribution in half. Half the ice mass is in larger particles and half the ice mass is in smaller particles.
5.21 Median Volume Diameter (MVD).
The drop diameter that divides the total liquid water content present in the drop distribution in half. Half the water volume will be in larger drops and half the volume in smaller drops. This value is obtained by actual drop size measurements.
5.22 Migrating Ice.
Ice resulting from the down-stream migration and build up of ice from wet protected surfaces to areas that cool sufficiently for the migrating ice to adhere to the surface.
5.23 Mixed Phase Icing Conditions.
Icing conditions in which both ice crystals and supercooled liquid water drops are present.
5.24 Monitored Surface.
The surface of concern regarding the ice hazard (for example, the leading edge of a wing). Ice accretion on the monitored surface may be measured directly or correlated to ice accretion on a reference surface.
5.25 Primary Ice Detection System.
A detection system used to determine when the IPS must be activated. This system announces the presence of ice accretion or icing conditions, and it may also provide information to other airplane systems. A primary automatic system automatically activates the anti-icing or deicing IPS. A primary manual system requires the flightcrew to activate the anti-icing or deicing IPS on indication from the primary ice detection system.
5.26 Rain Drop.
A drop of water greater than 500 µm (0.5 mm) in diameter.
5.27 Reference Surface.
The observed surface used as a reference for the presence of ice on the monitored surface. The reference surface may be observed directly or indirectly. Ice must occur on the reference surface before—or at the same time—it appears on the monitored surface.
Examples of reference surfaces include windshield wiper blades or bolts, windshield posts, ice evidence probes, the propeller spinner, and the surface of ice detectors. The reference surface may also be the monitored surface.
5.28 Reversible Flight Controls.
Flight controls in the normal operating configuration that have force or motion originating at the airplane’s control surface (through aerodynamic loads, static imbalance, or trim or servo tab inputs, for example) that is transmitted back to flight deck controls. This term refers to flight deck controls connected to the pitch, roll, or yaw control surfaces by direct mechanical linkages, cables, or push-pull rods in such a way that pilot effort produces motion or force about the hinge line.
5.28.1 Aerodynamically Boosted Flight Controls.
Reversible flight control systems that employ a movable tab on the trailing edge of the main control surface linked to the pilot’s controls or to the structure in such a way as to produce aerodynamic forces that move, or help to move, the surface. Among the various forms are flying tabs, geared or servo tabs, and spring tabs.
5.28.2 Power-Assisted Flight Controls.
Reversible flight control systems in which some means is provided, usually a hydraulic actuator, to apply force to a control surface in addition to that supplied by the pilot to enable large surface deflections to be obtained at high speeds.
5.29 Runback Ice.
Ice formed from the freezing or refreezing of water leaving one area on an airplane surface that is above freezing temperature and flowing downwind to another area that is
7 sufficiently cooled for freezing (or refreezing) to take place. This ice type is frequently an unwanted product of thermal deicing systems.
5.30 Simulated Ice.
Ice shapes fabricated from wood, epoxy, or other materials by any construction technique.
5.31 Static Air Temperature.
The air temperature that would be measured by a temperature sensor that is not in motion in relation to that air. This temperature is also referred to in other documents as “outside air temperature,” “true outside temperature,” or “ambient temperature.”
5.32 Substantiated Visual Cue.
Ice accretion on the airplane’s protected surface or on a reference surface that has been demonstrated by testing to correlate with ice accretion on a monitored surface.
5.33 Supercooled Large Drops (SLD).
Supercooled liquid water that includes freezing rain or freezing drizzle.
5.34 Supercooled Water.
Liquid water at a temperature below the freezing point of 0 °C.
5.35 Water Catch.
The amount of water that strikes an airplane surface.
6 ICING ENVELOPES.
6.1 The Appendix O Environment.
6.1.1 The conditions included in the part 25, Appendix C, icing envelopes are unchanged by the addition of Appendix O. For details on the Appendix C icing envelopes, see AC 20-73A, Aircraft Ice Protection, and FAA Technical Report DOT/FAA/CT-88/8-1, Aircraft Icing Handbook. Appendix O was developed to define a representative icing environment for supercooled large drops. Supercooled large drops, which include freezing drizzle and freezing rain conditions, are not included in Appendix C.
6.1.2 Section 25.1420 requires that the airplane operate safely in the supercooled large drop icing conditions defined in part 25, Appendix O. This section is applicable to airplanes for which either of the following applies—
6.1.2.1 The airplane’s maximum gross weight is less than 60,000 pounds, or
6.1.2.2 The airplane is equipped with reversible flight controls.
6.1.3 Section 25.1420 requires compliance with § 25.1420(a)(1), (a)(2), or (a)(3). When complying with § 25.1420(a)(1), the applicant must provide a method for detecting that