Subject: CERTIFICATION OF TRANSPORT
CATEGORY AIRPLANES FOR FLIGHT IN
ICING CONDITIONS Date: 5/7/04 Initiated By: ANM-112
AC No: 25.1419-1A Change:
1. PURPOSE. This advisory circular (AC) provides guidance for certification of airframe ice protection systems on transport category airplanes. While this is the primary focus of this AC, the guidance also supplements similar guidance provided in other AC’s concerning icing requirements for other parts of the airplane.
2. APPLICABILITY.
a. The guidance provided in this document is directed to airplane manufacturers, modifiers, foreign regulatory authorities, and Federal Aviation Administration (FAA) transport airplane type certification engineers and their designees.
b. This material 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 in determining compliance with the relevant regulations. On the other hand, if 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.
c. This material does not change, create any additional, authorize changes in, or permit deviations from, regulatory requirements.
3. CANCELLATION. Advisory Circular 25.1419-1, dated 8/18/99, is canceled.
4. ICING REQUIREMENTS. The guidance provided herein applies to the approval of the installation and operation of ice protection systems in the icing environment defined in 14 CFR part 25, Appendix C. If certification for flight in icing conditions is desired, the airplane must be able to safely operate throughout the icing envelope of Appendix C. Section 25.1419 sets forth the specific airframe requirements for demonstrating compliance with the icing conditions defined in Appendix C. Additionally, for other parts of the airplane (i.e., engine, engine inlet, propeller) there are more specific icing requirements and associated guidance defined in the
ii
referenced regulations and advisory circulars listed in paragraph 3 below. Some of the icingrelated regulations must be complied with, even if the airplane is not certificated for flight in icing (e.g., §§ 25.629(d)(3), 25.903, 25.975, 25.1093, 25.1323(e), and 25.1325(b)). This AC contains limited information on flight tests for icing certification; additional information may be found in Advisory Circular 25-7A, referenced in paragraph 3 of this AC. The guidance provided in this AC is applicable to new Type Certificates (TC's), Supplemental Type Certificates (STC's), and amendments to existing TC's for airplanes certified under Part 4b of the Civil Aviation Regulations (CAR) and Part 25, for which approval under the provisions of § 25.1419 is desired.
5. RELATED REGULATIONS AND DOCUMENTS.
a. Regulations.
§ 25.629 - Aeroelastic stability requirements § 25.773(b)(1)(ii) - Pilot compartment view § 25.903 - Engines § 25.929 - Propeller deicing § 25.975 - Fuel tank vents and carburetor vapor vents § 25.1093 - Induction system icing protection § 25.1323(e) - Airspeed indicating system § 25.1325(b) - Static pressure systems § 25.1326 - Pitot heat indication system § 25.1403 - Wing icing detection lights § 25.1585(a)(6) - Operating procedures § 33.68 - Induction system icing § 33.77 - Foreign object ingestion § 33.78 - Rain and hail ingestion
b. Advisory Circulars. The AC's listed below may be obtained from the U.S. Department of Transportation, Subsequent Distribution Office, M-30, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, MD 20795:
AC 20-73 Aircraft Ice Protection, dated 4/21/71.
AC 25-7A Flight Test Guide for Certification of Transport Category Airplanes, dated 3/31/98.
AC 25-11 Transport Category Airplane Electronic Display Systems, dated July 16, 1987
AC 25.629-1A Flutter Substantiation of Transport Category Airplanes, dated 7/23/98.
iii AC 25.1309-1A System Design Analysis, dated 6/21/88.
AC 20-117A Hazards Following Ground Deicing and Ground Operations in Conditions Conducive to Aircraft Icing, dated 12/17/82.
AC 20-115B Radio Technical Commission for Aeronautics, Inc. (RTCA) Document RTCA/DO-178B, dated 1/11/93.
AC 120-42A Extended Range Operation with Two-Engine Airplanes (ETOPS), dated 12/30/88
Note: AC 25-7A is available from the Superintendent of Documents, U.S. Government Printing Office, Washington D.C. 20402 (stock number 050-007-01214-4; $33.00).
6. RELATED READING MATERIAL.
a. RTCA/DO-178B, “Software Considerati ons in Airborne Systems and Equipment Certification,” published December 1, 1992, provides an acceptable means of compliance for software used in airborne systems and equipment. This document may be obtained from the RTCA Inc., 1140 Connecticut Avenue NW., Suite 1020, Washington, D.C. 20036.
b. FAA Technical Report DOT/FAA/CT-88/8-1, "Aircraft Icing Handbook" published March 1991, updated September 1993, includes reference material on ground and airborne icing facilities, simulation procedures, and analytical techniques. This document represents all types and classes of aircraft and is intended as a working tool for the designer and analyst of ice protection systems.
c. FAA Technical Report ADS-4, “Engineering Summary of Airframe Icing Technical Data,” and Report No. FAA-RD-77-76, “Engineering Summary of Powerplant Icing Technical Data,” provide technical information on airframe and engine icing conditions, and methods of detecting, preventing, and removing ice accretion on airframes and engines in flight. Although most of the information contained in ADS-4 and FAA-RD-77-76 is still valid, some is outdated.
More usable information is now available through recent research and experience and is included in the Aircraft Icing Handbook referenced in paragraph b. above.
Note: The FAA technical reports listed above can be obtained from the National Technical Information Service (NTIS), Department of Commerce, 5285 Port Royal Road, Springfield, Virginia 22161.
iv
d. RTCA/DO160D, “Environmental Conditions and Test Procedures for Airborne Equipment.” This document may be obtained from the RTCA Inc., 1140 Connecticut Avenue NW., Suite 1020, Washington, D.C. 20036.
/s/ Ali Bahrami Acting Manager, Transport Airplane Directorate Aircraft Certification Service
v
Contents
Paragraph Page
1. BAC KGROUND1
a. Civil Air Regulations (CAR)1
b. Part 252
2. CERTIFICATION PLAN3
3. ANALYSIS4
a. Analysis of Areas and Components to be Protected4
b. The 45-Minute Hold Condition5
c. Flutter Analysis6
d. Power Sources6
e. Failure Analysis6
f. Similarity Analysis6
g. Impingement Limit Analysis7
4. DRY AIR GROUND TESTS7
5. FLIGHT TEST PLANNING7
6. COMPLIANCE TESTS (Flight/Simulation)8
a. Three Typical Stages of Icing Flight Tests8
b. Dry Air Flight Tests with Ice Protection Equipment Operating8
(1) Hot Air Thermal Anti-ice (TAI) Leading Edge Systems8
(2) Hot Air Thermal Deice Leading Edge Systems9
(3) Pneumatic Leading Edge Boots9
(4) Electrically H eated Propeller Boots9
(5) Windshield Anti-Ice10
(6) Pitot-Static and Static Pressure Sources10
(7) Compressor Bleed Air Systems10
c. Dry air Flight Tests with Predicted Artificial Ice Shapes10
d. Icing Flight Tests12
(1) Instrumentation12
(2) Simulated Icing13
(3) Natural Icing13
(4) Fluid Anti-I ce/Deice Systems14
e. Performance and Handling Qualities15
f. Pneumatic Deicer Boots15
g. Emergency and Abnormal Operating Conditions15
vi
h. Ice Detector System (Primary and Advisory)15
7. PERFORM INTENDED FUNCTION IN ICING16
a. Engines and Equipment16
b. Engine Alternate Induction Air Sources16
c. Fuel System Venting16
d. Landing Gear16
e. Stall Warning16
f. Ice Detection Cues16
g. Primary and Secondary Flight Control Surfaces16
h. Ram Air Turbine16
i. Pilot Compartment View17
8. ICE DETECTION LIGHT(S)17
9. ICE-CONTAMINATED TAILPLANE STALL (ICTS)17
10. CAUTION INFORMATION18
11. ICE SHEDDING18
12. AIRPLANE FLIGHT MANUAL19
a. Operating Limitations Section19
b. Operating Procedures Section20
c. Performance Information Section20
d. Airworthiness Directives for Severe Icing Conditions20
1. BACKGROUND.
a. Civil Air Regulations (CAR).
(1) Prior to 1953, airplanes were certificated under Part 04 of the CAR. Section
04.5814 required that if deicer boots were installed, then positive means must be provided for the deflation of all wing boots. There were no other references to an airplane ice protection system in Part 04.
(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, in § 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 provided 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.
(3) Amendment 4b-2, effective August 25, 1955, introduced icing envelopes similar to the current Appendix C to part 25. The graphs added by Amendment 4b-2 (4b-24a, 4b-24b, 4b- 24c, 4b-25a, 4b-25b, and 4b-25c) were identical in substance and format to the current Appendix C envelopes with a few exceptions. These envelopes described the liquid water content, the mean effective diameter of droplets, the temperature, and horizontal and vertical extent of the supercooled icing cloud environment. In the figures introduced by Amendment 4b-2, the units of the distances shown on the graphs were expressed in statute miles instead of nautical miles.
The Liquid Water Content (LWC), however, is identical between the Amendment 4b-2 figures and the current Appendix C envelopes, if the correction for the difference in value between nautical and statute miles is made. There are two significant differences between the Amendment 4b-2 envelopes and the current Appendix C. The minimum mean effective diameter in the intermittent maximum conditions was 20 microns versus the current 15 microns, and the LWC/distance factor was 1.5 statute miles versus the current 0.26 nautical miles (0.3 statute miles).
(4) Amendment 4b-6, effective August 12, 1957, revised the icing envelopes to the current requirements and revised § 4b.461, “Induction system de-icing and anti-icing provisions.” The preamble to the amendment states, “There are included herein changes which extend the currently effective provisions governing intermittent maximum icing conditions so as to cover conditions which might be critical insofar as the turbine engine induction system is concerned. In this regard, the data are being extended in accordance with NACA Technical Note 2738 and involve a revision of Figure 4b-25a to cover drop diameters as small as 15 microns and a revision of Figure 4b-25c to cover distances down to 0.3 mile. The icing conditions prescribed in the currently effective regulations are applicable in the main to the airframe. The changes
being made in section 4b.461 require the turbine powerplant to be subjected to the same icing conditions and require that the induction system be protected to prevent serious engine power loss. A similar requirement is incorporated with respect to certification of turbine engines by an amendment to Part 13 which is being made concurrently with this amendment.”
b. Part 25.
(1) Part 4b of the CAR was recodified into part 25, effective February 1, 1965. After recodification, with minor editorial changes the content of § 4b.640, Ice protection, became § 25.1419; § 4b.406, Propeller deicing provisions, became § 25.929; § 4b.612(a)(5), Airspeed indicating systems, became § 25.1323(e); § 4b.351(b)(1)(ii), Pilot compartment view, became § 25.773(b)(1)(ii). The § 4b.351 reference to the most severe icing conditions for which approval of the airplane is desired was changed in part 25 to reference the icing conditions specified in § 25.1419. Section 4b.461, Induction system deicing and anti-icing provisions, became § 25.1093.
(2) Amendment 25-5, effective July 29, 1965, revised § 25.1325(b) to require that the correlation between air pressure in the static pressure system and true ambient atmospheric static pressure is not changed when the airplane is exposed to the continuous and intermittent maximum icing conditions defined in Appendix C.
(3) Amendment 25-11, effective June 4, 1967, revised § 25.1585 to require that the Airplane Flight Manual include information on the use of ice protection equipment.
(4) Amendment 25-23, effective May 8, 1970, revised § 25.1419 to require that the effectiveness of the ice protection system and its components be shown by flight tests of the airplane or its components in measured natural atmospheric icing conditions. Previous to this amendment, flight tests in natural icing conditions were considered as one means of compliance but were not mandatory. Amendment 25-23 also revised § 25.1309 to include additional requirements for certificating equipment, systems, and installations. The regulation was revised to require a comprehensive systematic failure analysis, supported by appropriate test, to ensure that the safety objectives of the probability of occurrence decrease as the hazard of a failure increases.
(5) Amendment 25-38, effective February 1, 1977, added § 25.1403 to require a means for illuminating or otherwise determining the formation of ice on the parts of the wings that are critical from the standpoint of ice accumulation. The requirement is not applicable if the operating limitations prohibit operations at night in known or forecast icing conditions.
(6) Amendment 25-43, effective April 12, 1978, added § 25.1326 to require the installation of a pitot heat indication system on airplanes equipped with flight instrument pitot heating systems.
(7) Amendment 25-46, effective December 1, 1978, added § 25.1416 to require specific standards for pneumatic deicer boots.
(8) Amendment 25-72, effective July 20, 1990, transferred § 25.1416 to § 25.1419 for clarification and editorial improvement. In addition, the contents of § 25.1416(c), which required a means to indicate to the flightcrew that the pneumatic deicing boot system is receiving adequate pressure and is functioning normally, were revised to allow the use of the “dark cockpit” concept (i.e., a warning when failure occurs rather than continual pilot monitoring of a healthy system).
2. CERTIFICATION PLAN . At the start of the design and development effort, the applicant should submit a certification plan to the cognizant FAA Aircraft Certification Office (ACO) for approval. The certification plan should include the following basic information:
a. A general airplane description that includes dimensions, airworthiness limitations, and other data that may be relevant to certification of ice protection systems.
b. Ice protection systems description.
c. A compliance checklist that addresses each section of part 25 applicable to the product.
d. Identification of the certification methods for each applicable section of part 25, including a description of analyses and tests, or references to similarity of designs, that the applicant intends for certification of the ice protection system. These methods of showing compliance should be agreed upon between the applicant and the FAA early in the certification program prior to conducting certification tests.
e. A failure hazard assessment to determine the criticality of the system.
f. If the ice protection or detection systems contain software, software plans, as described in RTCA DO-178B (or another acceptable means of compliance for software).
g. Projected schedules of design, analyses, testing, and reporting.
h. A list of any anomalous 14 CFR part 33 icing certification test results, if completed, that will require special operating procedures.
i. If the ice protection or detection system s contain complex electronic hardware (such as programmable logic devices (PLD) or application specific integrated circuits (ASIC)), plans for providing a level of design assurance of these devices commensurate with their potential contribution to aircraft hazards and system failures, which could result from electronic hardware faults or malfunctions.
3. ANALYSES . The applicant should prepare analyses to substantiate decisions involving the application of selected ice protection equipment. Such analyses should clearly state the basic protection required and the assumptions made, and delineate the methods of analysis used. All analyses should be validated either by tests or have been validated by the applicant on previous programs. This substantiation should include a discussion and rationale of the assumptions made in the analyses. To utilize a previously validated methodology, the applicant should substantiate that the methodology is applicable to the new program. The following is a list of analyses typically performed during an icing certification program:
a. Analysis of Areas and Components to be Protected . The applicant should examine those areas listed below to determine the degree of protection required:
(1) Leading edges of wings, winglets, and wing struts; horizontal and vertical stabilizer; canards; and other lifting surfaces.
(2) Leading edges of control surface balance areas, if not shielded.
(3) Accessory cooling air intakes that face the airstream and/or could otherwise become restricted due to ice accretion.
(4) Antennas and masts.
(5) External tanks.
(6) External hinges, tracks, door handles, and entry steps.
(7) Instruments including pitot tube (and mast), static and dynamic ports, angle-ofattack sensor, and stall warning.
(8)) Forward fuselage nose cone and radome.
(9) Windshields.
(10) Landing gear.
(11) Retractable forward landing lights.
(12) Ram air turbines.
(13) Ice detection lights, if required for compliance with § 25.1403.
(14) Vortex generators installed on lifting surfaces and the fuselage, and stall improvement devices such as strips, vortilons, and fences.
(15) Any structure that extends into the free stream such as cameras, camera mounts, and video equipment.
Note 1: Ice protection of fuel tank vents, propellers, and engine inlet cowls is addressed by part 25, subpart E – Powerplant.
Note 2: An applicant may determine that protection is not required for one or more of these areas or components. If so, the applicant should include supporting data and rationale in the analyses for allowing those areas or components to go unprotected. The applicant should show that the lack of protection does not adversely affect the handling characteristics or performance of the airplane. Section 25.1419(b) of part 25 at amendment level 25-72 requires certain flight testing. However, flight test data from previous certification programs may be used as an integral part of a complete data package used for establishing compliance with 25.1419(b) if it can be shown that the flight test data is applicable to the airplane in question. This would generally require a similarity analysis. If a similarity analysis is used, the guidelines of paragraph 3(f) of this AC are applicable. If there is uncertainty about the effects of the lack of protection, or the similarity analysis, the manufacturer should conduct flight test demonstrations.
b. The 45-Minute Hold Condition . Ice shapes resulting from a 45-minute hold condition should be considered when developing critical ice shapes for which the overall performance and handling characteristics of the airplane are to be analyzed. The airplane's tolerance to continuous ice accumulation on the unprotected surfaces should be evaluated in accordance with the information provided in AC 20-73. The applicant should determine the effect of the 45-minute hold in continuous maximum icing conditions. It should be assumed that the airplane will remain 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. The applicant should substantiate the critical mean effective drop diameter, LWC, and temperature that result in the formation of an ice shape that is critical to the airplane’s performance and handling qualities.
The critical ice shapes derived from this 45-minute hold analysis should be compared to critical shapes derived from other analyses (i.e., for climb, cruise, and descent) to establish the most critical artificial ice shapes to be used during dry air flight tests. Not only is the thickness important, but the shape, location, and texture may contribute to adverse aerodynamic characteristics.
Note: The 45-minute hold condition will result in large ice shapes on the unprotected surfaces that, from visual assessment alone, would appear to have a drastic effect on airplane performance and handling characteristics. Operational history and wind tunnel tests have shown that the ice that forms on the protected and unprotected surfaces before activation and normal operation of the ice protection system may also have a detrimental effect on the lift and pitching moment characteristics of the wing and horizontal tail airfoils. Similarly, for airplanes with cyclic ice protection systems, the ice that accretes on the protected surfaces between cycles may have more than an insignificant contribution to the total effect of ice on the airplane performance and handling characteristics, when combined with the 45-minute holding ice accretion on the unprotected surfaces.
c. Flutter Analysis . Advisory Circular (AC) 25.629-1A, “Flutter Substantiation of Transport Category Airplanes,” provides guidance for showing compliance with § 25.629.
Section 25.629(d)(3) requires the consideration of inadvertent encounters with icing for airplanes not certificated for flight in icing conditions.
d. Power Sources . The applicant should evaluate the power sources in the ice protection system design (e.g., electrical, bleed air, and pneumatic sources). An electrical load analysis or test should be conducted on each power source to determine that it is adequate to supply the ice protection system plus all other essential electrical loads throughout the airplane flight envelope under conditions requiring operation of the ice protection system. The effect of an ice protection system component failure on power availability to other essential loads should be evaluated in accordance with § 25.1309. All power sources that affect engine or engine ice protection systems for multiengine airplanes must comply with the engine isolation requirements of § 25.903(b).
e. Failure Analysis . Advisory Circular (AC) 25.1309-1A, “System Design Analysis,” provides guidance for demonstrating compliance with the requirements of § 25.1309.
Substantiation of the hazard classification of ice protection system failure conditions is typically accomplished through analyses and/or testing. The failure analysis should include the potential contribution of complex electronic hardware faults and malfunctions to system failure conditions, and classify the hardware assurance level appropriately.
f. Similarity Analyses .
(1) In the case of certification based on similarities to other type certificated airplanes previously approved for flight in icing conditions, the applicant should specify the airplane model and the component to which the reference applies. Specific similarities should be shown in the areas of physical, functional, thermodynamic, pneumatic, aerodynamic, and environmental exposure. Analyses should be conducted to show that the component installation, operation, and the effect on the airplane’s performance and handling are equivalent to that of the previously approved configuration.
(2) Similarity requires an evaluation of both the system and installation differences that may adversely affect the system performance. An assessment of a new installation should consider differences affecting the airplane and the system. If there is uncertainty about the effects of the differences, additional tests and/or analyses should be conducted as necessary and appropriate to resolve the open issues.
(3) Engineering judgment should be used to determine if the ice protection system should be retested in measured natural icing conditions. The differences in the ice protection systems should be evaluated for their effect on the functionality of the ice protection systems and on safe flight in icing. If there is uncertainty about the effects of the differences, flight tests in measured natural icing conditions should be accomplished.