15.5.3 Aerodynamic Performance Monitor.
A flightcrew-alerting system using pressure probes and signal processors could be developed for quantifying pressure fluctuations in the flow field from contamination over the wing surface. This technology does exist, but full development is necessary before incorporating it into the flightcrew-alerting system.
16 PERFORMING INTENDED FUNCTIONS IN ICING.
All systems and components of the basic airplane should continue to function as intended when operating in an icing environment. This includes the following:
16.1 Engines and Equipment.
Engines and equipment (such as generators and alternators operating under maximum ice protection load) should be monitored during icing tests for adequate cooling (§ 25.1041) and found acceptable for this operation (§§ 25.1093 and 25.1301).
16.2 Engine Alternate Induction Air Sources.
Engine alternate induction air sources should remain functional in an icing environment.
16.3 Fuel System Venting.
Fuel system venting should not be adversely affected by ice accumulation.
16.4 Landing Gear.
A retractable landing gear should operate as intended after an icing encounter.
16.5 Primary and Secondary Flight Control Surfaces.
Primary and secondary flight control surfaces should remain operational after exposure to icing conditions. The applicant should demonstrate either that aerodynamic balance surfaces are not subject to icing throughout the airplane’s operating envelope (weight, center of gravity, and speed), or that no ice accumulation on these surfaces interferes with or limits actuation of the control for these surfaces, including retraction of flaps for a safe go-around from the landing configuration.
16.6 Ram Air Turbine.
The ram air turbine should remain operational. It does not need to be tested in natural icing conditions if tested in the icing wind tunnel.
17 ICE INSPECTION LIGHT(S).
Unless operations at night in icing conditions are prohibited by an operating limitation, § 25.1403 requires that a means be provided, during flight at night, to illuminate or otherwise determine ice formation on parts of the wings that are critical from the standpoint of ice accumulations.
17.1 If the flightcrew cannot see the wings, one acceptable means of compliance with this regulation would be to install an ice evidence probe in a position where the flightcrew
45 can observe ice accumulation. The applicant should substantiate that formation of ice on this device precedes formation of ice on parts of the wings critical from the standpoint of ice accumulation, or occurs simultaneously with it. Consideration should be given to the need for illuminating the ice evidence probe.
17.2 Ice inspection lights should be evaluated both in and out of clouds during night flight to determine that the component of interest is adequately illuminated without excessive glare, reflections, or other distractions to the flightcrew. These tests may be accomplished both in and out of clouds during the airplane certification flight tests.
Typically, airplane-mounted illumination has been used to comply with this regulation.
If a hand-held flashlight is being considered for inspection illumination, the impact that use of a flashlight has on the associated flightcrew workload should be considered.
Previously, use of a hand-held flashlight has not been acceptable because of the associated increase in flightcrew workload. The appropriate manual should identify the ice characteristics that the flightcrew is expected to observe, as well as the action that the flightcrew must perform if such ice is observed.
18 CAUTION INFORMATION.
Section 25.1419(c) requires that caution information be provided to alert the flightcrew when the IPS is not functioning normally. In this context, caution information is considered to be a general term referring to an alert rather than referring specifically to a caution level alert. Crew alerting should be provided for failure conditions of the IPS in accordance with §§ 25.1309(c) and 25.1322. It should be assumed that icing conditions exist during the failure event. In accordance with § 25.1419(c), the decision to provide an alert must not be based on the numerical probability of the failure event. However, the type of alert provided should be based on the failure effects and necessary crew action to be performed in response. Section 25.1420 describes requirements that are in addition to the requirements in § 25.1419 for certain airplanes and does not contain a requirement complementary to § 25.1419(c). Instead, it relies on compliance with § 25.1309(c) to ensure that adequate warning is provided to the flightcrew of unsafe system operating conditions. Warning information required by § 25.1309(c), to alert the flightcrew of unsafe system operating conditions, is applicable to design features installed to meet the additional requirements in § 25.1420 and must be provided in accordance with § 25.1322.
18.1 Sensor(s) used to identify a failure condition should be evaluated to ensure that they are properly located to obtain accurate data on the failure of the IPS.
18.2 The indication system should not be designed so that it could give the flightcrew a false indication that the system is functioning normally because of a lack of an alert. The applicant should submit data to substantiate that this could not happen. For example, if a pneumatic deicing system (boots) requires a specific minimum pressure and pressure rise rate to adequately shed ice, an alert should be provided if that minimum pressure and pressure rise rate are not attained. Without an alert, the flightcrew may erroneously believe that the boots are operating normally when, in fact, they might not be inflating with sufficient pressure or with a sufficient inflation rate to adequately shed ice. The
46 applicant should also consider the need for an alert about ice forming in the pneumatic system that can result in low pneumatic boot pressures or an inadequate pressure rise rate.
19 AIRPLANE FLIGHT MANUAL.
The AFM should provide the pilot with the information needed to operate the IPS, including the following:
19.1 Operating Limitations Section.
19.1.1 Establishment of all systems limitations when operating in icing conditions. These limitations should address airframe, engine, and equipment IPSs, and other systems that may need to be specifically configured for operation in icing conditions. Appropriate icing conditions definitions may need to be provided to ensure proper operation of the related IPS.
19.1.2 Limitations on operating time for ice protection equipment, if these limitations are based on fluid anti-icing/deicing systems capacities and flow rates.
19.1.3 Airplane speed limitations (if any).
19.1.4 Environmental limitations for equipment operations as applicable (for example, minimum temperature or maximum altitude for boot operation). The AFM should include appropriate limitations to be observed in the event that icing conditions are encountered when the IPS cannot be activated.
19.1.5 Engine limitations (power or speed) required for proper operation of the IPS.
19.1.6 A list of required placards.
19.1.7 If compliance is based on § 25.1420(a)(1) or § 25.1420(a)(2), the Limitations section must include a requirement that the flightcrew exit all (Appendix O and C) icing conditions immediately on recognition that the Appendix O icing conditions exceed the certification boundary. The Limitations section must reflect the certification assumptions such as phase of flight, freezing drizzle, freezing rain, etc.
19.2 Operating Procedures Section.
19.2.1 Section 25.1585 requires that operating procedures be provided in the AFM for—
19.2.1.1 Normal procedures peculiar to the particular airplane type or model that are to be carried out in connection with routine operations,
19.2.1.2 Non-normal procedures for cases of malfunction and failure conditions involving use of special systems or the alternative use of regular systems, and
19.2.1.3 Emergency procedures for foreseeable but unusual situations in which immediate and precise action by the flightcrew may be expected to substantially reduce the risk of catastrophe.
19.2.2 The operating procedures required by § 25.1585 should include all preflight actions necessary to minimize the potential of en route emergencies associated with the IPS.
System components should be described with sufficient clarity and depth that the pilot can understand their function. Unless flightcrew actions are accepted as normal airmanship, the appropriate procedures should be included in the FAA-approved AFM, AFM revision, or AFM supplement. These procedures should include proper pilot response to cockpit warnings, a means to identify system failures, and safe use of the system(s).
19.2.3 Procedures should be provided to optimize airplane operation while in icing conditions, including climb, holding, and approach configurations and speeds.
19.2.4 Emergency or abnormal procedures, including procedures to be followed when IPSs fail and/or warning or monitor alerts occur, should be provided.
19.2.5 For fluid anti-icing/deicing systems, information and method(s) for determining the remaining flight operation time should be provided.
19.2.6 For airplanes that cannot supply adequate electrical power for all systems at low engine speeds, load-shedding instructions should be provided to the pilot for approach and landing in icing conditions if pilot action is necessary.
19.2.7 For compliance with § 25.1420(a)(1) or § 25.1420(a)(2):
19.2.7.1 The method used to determine when icing conditions must be exited has to be provided, as well as appropriate failure indications and flightcrew procedures.
19.2.7.2 The operating procedures section should contain guidance for exiting the icing environment.
19.2.7.3 For an airplane certified to Appendix O for certain phases of flight, information defining the restricted phases should be provided. This information should include all airplane configurations associated with the restricted phases of flight, such as flap position, gear extension, or airspeed.
19.2.8 Section 25.1419(h) requires that procedures be established for operating the IPS, including activation and deactivation. Procedures for IPS deactivation must be consistent with the § 25.1419(e) requirements for activation of the IPS. The exact timing of deactivation should consider the type of IPS (e.g., deicing, anti-icing, or running wet) and all delays in deactivation necessary to ensure that residual ice is minimized. Pneumatic boots should be operated for three complete cycles following the absence of the cues used for activation. However, if the airplane’s stall protection
48 system reverts from an icing schedule to a non-icing schedule when the airframe IPS is deactivated, AFM procedures ought to state that the airframe IPS should not be deactivated until the flightcrew is certain that the critical wing surfaces are free of ice.
For example:
19.2.8.1 If the airplane is certificated in accordance with § 25.1419(e)(2), pneumatic deicing boots may be deactivated following completion of three boot cycles after both the substantiated visual cues and the advisory ice detection system no longer indicate ice accretion.
19.2.8.2 If the airplane is certificated in accordance with § 25.1419(e)(3), pneumatic deicing boots may be deactivated following completion of three boot cycles after leaving icing conditions, even if residual ice is observed on the boots. The boots must be activated on reentry into icing conditions.
19.2.8.3 If the airplane’s stall protection system reverts from an icing schedule to a non-icing schedule when the airframe IPS is deactivated, AFM procedures should state that the airframe IPS should not be deactivated until the flightcrew is certain that the critical wing surfaces are free of ice.
19.2.9 The following statement should be included in the operating procedures section of the AFM:
“Convective clouds above the freezing altitude that are vigorously increasing in size should be avoided because they may contain icing conditions exceeding those for which this airplane has been certified.”
20 GUIDANCE FOR AMENDED TYPE CERTIFICATES AND SUPPLEMENTAL TYPE CERTIFICATES.
General guidance for amended TCs and STCs can be found in AC 21.101-1A, Establishing the Certification Basis of Changed Aeronautical Products. As stated in its “APPLICABILITY” section, the guidance in that AC applies to any STC or amended TC on an airplane for which the applicant seeks approval under provisions of §§ 25.1419 and 25.1420. Increases in gross weight, increases in engine power, and propeller changes could affect approval for flight in icing and these areas should be evaluated using AC 21.101-1A as the method of compliance. An applicant wishing to use an alternate means of compliance must consult the appropriate ACO. For additional guidance, see Appendix D of this AC.
21 INSTRUCTIONS FOR CONTINUED AIRWORTHINESS.
Instructions for Continued Airworthiness (ICA) are required in accordance with 14 CFR
21.50(b) and 25.1729. ICA should be prepared in accordance with part 25, Appendix H.
At a minimum, the following should be addressed:
21.1 Basic description of the IPS operation, components, and installation.
21.2 Servicing information, such as fluid type and quantities.
21.3 Location of panels used for inspection, and/or instructions to access components.
21.4 Scheduling information for each applicable component including cleaning, inspecting, adjusting, testing, and lubricating.
21.5 Overhaul or replacement periods for components, if any.
21.6 Instructions for removing and replacing components.
21.7 Precautions, such as toxicity of freezing point depressant fluid or flammability of some cements.
21.8 Cleaning instructions, such as only soap and warm water for pneumatic deicing boots.
21.9 Limitations on the materials that can be applied, such as ice adhesion inhibitors, rubber preservatives, cosmetic coatings.
21.10 Recommendations on the frequency of ice adhesion inhibitors being used.
21.11 Special equipment or materials, such solvents, cements, edge filler, conductive edge sealer, or rollers for pneumatic boot installation.
Appendix A A-1 Appendix A. Related Regulations and Documents A.1 REGULATIONS.
Table A-1 identifies the sections of 14 CFR part 25 that 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 A-1. Related Regulations 14 CFR Title § 25.21 Proof of compliance § 25.103 Stall speed § 25.105 Takeoff § 25.107 Takeoff speeds § 25.111 Takeoff path § 25.119 Landing climb: All-engines-operating § 25.121 Climb: One-engine-inoperative § 25.123 En route flight paths § 25.125 Landing § 25.143 General (Controllability and Maneuverability) § 25.207 Stall warning § 25.237 Wind velocities § 25.251 Vibration and buffeting § 25.253 High-speed characteristics § 25.629 Aeroelastic stability requirements § 25.773 Pilot compartment view
Appendix A A-2 14 CFR Title § 25.875 Reinforcement near propellers § 25.929 Propeller deicing § 25.975 Fuel tank vents and carburetor vapor vents § 25.1091 Air induction § 25.1093 Induction system icing protection
25.1301 Function and installation (Equipment) § 25.1305 Powerplant instruments § 25.1309 Equipment, systems, and installations § 25.1316 Electrical and electronic system lightning protection § 25.1321 Arrangement and visibility (Instruments: Installation) § 25.1322 Flightcrew alerting § 25.1323 Airspeed indicating system § 25.1324 Angle of attack system § 25.1325 Static pressure systems § 25.1403 Wing icing detection lights § 25.1419 Ice protection § 25.1420 Supercooled large drop icing conditions § 25.1455 Draining of fluids subject to freezing § 25.1585 Operating procedures Appendix C to part 25 Paragraph K25.1.3(a) of Appendix K to part 25 Operations in icing conditions (Extended Operations (ETOPS) Appendix O to part 25 Supercooled Large Drop Icing Conditions
Appendix A A-3 A.2 ADVISORY CIRCULARS.
Table A-2 identifies the ACs that are related to the guidance in this AC. The table lists 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 A-2. Related Advisory Circulars Number Title Date AC 20-73A Aircraft Ice Protection August 16, 2006 AC 20-115C Airborne Software Assurance July 19, 2003 AC 20-117 Hazards Following Ground Deicing and Ground Operations in Conditions Conducive to Aircraft Icing December 17, 1982 AC 20-147A Turbojet, Turboprop, and Turbofan Engine Induction System Icing and Ice Ingestion October 22, 2014 AC 21-16G RTCA Document DO-160E versions D, E, F, and G, “Environmental Conditions and Test Procedures for Airborne Equipment” June 22, 2011 AC 21-40A Guide for Obtaining a Supplemental Type Certificate September 27, 2007 AC 21.101-1A Establishing the Certification Basis of Changed Aeronautical Products September 3, 2010 AC 25-11B Electronic Flight Displays October 7, 2014 AC 25-22 Certification of Transport Airplane Mechanical Systems March 14, 2000 AC 25-25A Performance and Handling Characteristics in the Icing Conditions October 27, 2014 AC 25.629-1B Aeroelastic Stability Substantiation of Transport Category Airplanes October 27, 2014 AC 25.1309-1A System Design Analysis June 21, 1988
Appendix A A-4 A.3 OTHER FAA DOCUMENTS.
A.3.1 FAA Technical Report DOT/FAA/CT-88/8-1, FAA Aircraft Icing Handbook, issued March 1991, updated September 1993.
A.3.2 FAA Technical Report DOT/FAA/AR-09/10, Data and Analysis for the Development of an Engineering Standard for Supercooled Large Drop Conditions, dated March 2009.
A.3.3 FAA Policy No. ANE-2003-35-1-RO, Policy for Propeller Ice Protection Equipment, dated March 2, 2005.
A.4 INDUSTRY DOCUMENTS.
Table A-3 identifies the industry documents that are related to the guidance in this AC.
A.4.1 You can purchase the following RTCA, Inc. (formerly the Radio Technical Commission for Aeronautics) documents by mail from RTCA Inc., 1150 18th Street NW., Suite 910, Washington, DC 20036; by faxing the Document Order Form to
(202) 833-9434; or from the RTCA website.
A.4.2 You can purchase the following SAE International (formerly Society of Automotive Engineers) documents by mail from SAE Customer Service, 400 Commonwealth Drive, Warrendale, PA, 15096; by faxing your order information to (724) 776-0790; or from the SAE website. Please note that the time of publication of this AC, the SAE documents have not been updated to reflect supercooled large drop analyses and testing but provide good reference material.
Appendix A A-5 Table A-3. Related Industry Documents Document Title Date RTCA DO-178C Software Considerations in Airborne Systems and Equipment Certification December 13, 2011 RTCA DO-160G Environmental Conditions and Test Procedures for Airborne Equipment December 8, 2010 RTCA DO-254 Design Assurance Guidance for Airborne Electronic Hardware April 19, 2000 SAE AIR1168/4 Ice, Rain, Fog, and Frost Protection Issued July 1, 1989, Reaffirmed June 2004 SAE ARP5624 Aircraft Inflight Icing Terminology Issued March 3, 2008, Reaffirmed April 2013 SAE ARP5903 Droplet Impingement and Ice Accretion Computer Codes Issued October 16, 2003, Reaffirmed December 2009 SAE ARP5904 Airborne Icing Tankers Issued October 21, 2002, Reaffirmed October 2012 SAE ARP5905 Calibration and Acceptance of Icing Wind Tunnels Issued September 18, 2003, Reaffirmed December 2009 SAE AS393B Airspeed Tubes Electrically Heated Issued August 14, 2002, Reaffirmed February 2008 SAE AS403A Stall Warning Instrument Issued July 15, 1958, Reaffirmed February 2008 SAE AS5498 (EUROCAE ED-103) Minimum Operational Performance Specification for Inflight Icing Detection Systems Issued October 1, 2001, Reaffirmed December 2009 SAE AS8006 Minimum Performance Standard for Pitot and Pitot-Static Tubes Issued April 1, 1988, Reaffirmed May 1993
Appendix B B-1 Appendix B. History of Icing Certification Rules B.1 CIVIL AIR REGULATIONS.
B.1.1 Before 1953, airplanes were certificated under part 04 of the civil air regulations (CARs). Section 04.5814 required that if deicer boots were installed on an airplane, positive means must be provided for deflation of all wing boots. There were no other references to an airplane ice protection system (IPS) in Part 04.
B.1.2 Part 4b of the CAR was codified on December 31, 1953. The requirement for positive means of deflating deicer boots was incorporated, without change, into § 4b.640.
Section 4b.640 stated that, “When an ice protection system is installed, it shall be of an approved type. If pneumatic boots are used, at least two independent sources of power and a positive means for the deflation of the boots shall be provided.”
Section 4b.351(b)(ii) provided requirements for pilot compartment vision in icing conditions. Section 4b.406 provided propeller-deicing requirements. Section 4b.461 gave induction system deicing and anti-icing requirements. Section 4b.612(a)(5) required that the airspeed indicating system be provided with a heated pitot tube or equivalent means of preventing malfunctioning due to icing.
B.1.3 Amendment 4b-2, effective August 25, 1955, introduced icing envelopes similar to those in the current Appendix C to part 25. The graphs added by Amendment 4b-21 were identical in substance and format to those describing the current Appendix C icing envelopes with a few exceptions. Those envelopes were defined by descriptions of the liquid water content (LWC), the mean effective diameter of drops, the temperature, and the horizontal and vertical extent of the supercooled icing cloud environment. In the figures introduced by Amendment 4b-2, the units of distances shown on the graphs were expressed in statute miles instead of nautical miles. The LWC in the Amendment 4b-2 figures and the current Appendix C envelopes is identical, however, if the correction for the difference between nautical and statute miles is made. There are two significant differences between the Amendment 4b-2 icing envelopes and those described in the current Appendix C.
B.1.3.1 The minimum mean effective diameter in the intermittent maximum conditions was 20 µm. The current diameter is 15 µm.
B.1.3.2 The minimum cloud horizontal extent in the chart titled Intermittent Maximum Atmospheric Icing Conditions Variation of LWC Factor with Cloud Horizontal Extent was 1.5 statute miles. The current horizontal extent is 2.6 nautical miles (3.0 statute miles).
1 4b-24a, 4b-24b, 4b-24c, 4b-25a, 4b-25b, and 4b-25c