Appendix B B-2 B.1.4 Amendment 4b-6, effective August 12, 1957, revised the icing envelopes to those that exist in the current requirements. Amendment 4b-6 revised § 4b.461, Induction system deicing and anti-icing provisions. The preamble to that 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 µm and a revision of figure 4b-25c to cover distances down to 3.0 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.2 PART 25.
B.2.1 Part 4b of the CAR was codified into 14 CFR part 25, effective February 1, 1965. The recodification, with minor editorial changes, resulted in the following:
B.2.1.1 The content of § 4b.640, Ice protection, became § 25.1419.
B.2.1.2 Section 4b.406, Propeller deicing provisions, became § 25.929.
B.2.1.3 Section 4b.612(a)(5), Airspeed indicating systems, became § 25.1323(e).
B.2.1.4 Section 4b.351(b)(1)(ii), Pilot compartment view, became § 25.773(b)(1)(ii).
B.2.1.5 The § 4b.351 reference to the most severe icing conditions for which approval of the airplane is desired was changed to reference the icing conditions specified in § 25.1419.
B.2.1.6 Section 4b.461, Induction system deicing and anti-icing provisions, became § 25.1093.
B.2.2 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.
B.2.3 Amendment 25-11, effective June 4, 1967, revised § 25.1585 to require that the airplane flight manual include information on use of ice protection equipment.
B.2.4 Amendment 25-23, effective May 8, 1970, revised § 25.1419 to require that the effectiveness of the IPS and its components be shown by flight tests of the airplane or its components in measured natural atmospheric icing conditions. Before this amendment, flight tests in natural icing conditions were considered one means of
Appendix B B-3 compliance but were not mandatory. Amendment 25-23 also revised § 25.1309 to include additional requirements for certifying equipment, systems, and installations.
The regulation was revised to require a comprehensive systematic failure analysis, supported by appropriate test, to ensure that, as the hazard of a failure increases, the probability of its occurrence decreases.
B.2.5 Amendment 25-38, effective February 1, 1977, added § 25.1403. That rule requires means for illuminating or otherwise determining the formation of ice on parts of the wings for which ice accumulation is critical. This requirement does not apply if operating limitations prohibit operations at night in icing conditions.
B.2.6 Amendment 25-43, effective April 12, 1978, added § 25.1326 to require installation of a pitot heat indication system on airplanes equipped with flight instrument pitot heating systems.
B.2.7 Amendment 25-46, effective December 1, 1978, added § 25.1416 to require specific standards for pneumatic deicer boots.
B.2.8 Amendment 25-72, effective July 20, 1990, transferred the contents of § 25.1416 to § 25.1419 for clarification and editorial improvement. In addition, the content of § 25.1416(c) was revised. This rule required a means to indicate to the flightcrew that the pneumatic deicing boot system is receiving adequate pressure and is functioning normally. It was revised to allow use of the “dark cockpit” concept—a warning when failure occurs rather than continual pilot monitoring of a healthy system.
B.2.9 Amendment 25-121, effective October 9, 2007, revised the following rules, which provide performance and handling qualities requirements for safe operation in Appendix C icing conditions:
• § 25.21.
• § 25.103.
• § 25.105.
• § 25.107.
• § 25.111.
• § 25.119.
• § 25.121.
• § 25.123.
• § 25.125.
• § 25.143.
• § 25.207.
• § 25.237.
• § 25.253.
Appendix B B-4 B.2.10 In addition, Amendment 25-121 revised § 25.1419 to require compliance with § 25.1419 if certification for flight in icing is desired. Prior to that revision, compliance with § 25.1419 was only required if the airplane had ice protection provisions.
Appendix C was revised to identify airframe ice accretion that must be considered for showing compliance with subpart B. Editorial revisions were made to §§ 25.773 and
25.941 to retain consistency with the changed § 25.1419 and to revise references to changed paragraphs.
B.2.11 Amendment 25-129, effective September 2, 2009, added § 25.1419(e), (f), (g), and (h) to provide requirements for determining when to operate IPS to ensure safe operation in Appendix C icing conditions.
B.2.12 Amendment 25-140, effective January 5, 2015, introduced Appendix O and §§ 25.1324 and 25.1420, and revised the following rules:
• § 25.21.
• § 25.105.
• § 25.111.
• § 25.119.
• § 25.121.
• § 25.123.
• § 25.125.
• § 25.143.
• § 25.207.
• § 25.237.
• § 25.253.
• § 25.773.
• § 25.903.
• § 25.929.
• § 25.1093.
• § 25.1323.
• § 25.1324.
• § 25.1325.
• § 25.1521.
• § 25.1533.
• Appendix C to part 25.
Appendix B B-5 B.2.13 Amendment 25-140 expanded the certification icing environment to include freezing rain and freezing drizzle. The amendment specified airplane performance and handling qualities requirements for operating in these conditions. The amendment expanded the engine installation certification and some airplane component certification regulations to include ice crystal and mixed phase icing conditions. The amendment added a new icing regulation for angle-of-attack systems.
Appendix C C-1 Appendix C. Example of Certification to 25.1420(a)(1) Using Visual Cues The following is a hypothetical example of a certification program using the icing conditions of part 25, Appendix C, as the boundary for flight in icing. This example offers guidance on developing substantiated visual cues to identify the boundary so the flightcrew will know when that boundary has been exceeded and Appendix O conditions have been encountered so the flightcrew can exit icing conditions safely.
C.1 HYPOTHETICAL AIRPLANE.
C.1.1 The airplane is a transport category, turbofan airplane. It is a derivative of an existing type design that has already been type certificated for flight in icing, including compliance with § 25.1419 (Appendix C). The changes in the derivative airplane did not alter the original § 25.1419 compliance. The derivative airplane design is characterized by the following:
• The cruise altitude is above the Appendix C envelope.
• The wing design does not include leading edge devices.
• The airplane has ice protection over the full span of the wing and tailplane.
• The airplane has reversible flight controls on the roll and pitch axes.
C.1.2 AC 25-25A contains guidance on compliance with part 25, subpart B requirements.
AC 20-147A, Turbojet, Turboprop, and Turbofan Engine Induction System Icing and Ice Ingestion, contains guidance for engine and engine installations. All applicable certification requirements must be met. However, for this example, only some aspects will be discussed.
C.2 IMPINGEMENT AFT OF THE PROTECTED AREAS OF LIFTING SURFACES.
The airplane has ice protection over the full span of the wing and horizontal stabilizer, with minimal unprotected areas. Since this is a derivative airplane, the system has already been evaluated and certificated for Appendix C conditions. This airplane does have reversible flight controls on the roll and pitch axis. An assessment of hinge moment and lift reduction effects is required. See AC 25-25A for detailed guidance on performance and handling qualities.
C.3 IMPINGEMENT AFT OF THE ENGINE INLET PROTECTED SURFACES.
The applicant should evaluate ice accumulation aft of the engine inlet protected surfaces for the possibility of ice ingestion by the engine. For this example, the cumulative mass of this ice is less than that used for demonstrating compliance with § 33.77. Therefore, this airplane will be found compliant with § 25.1093 based on this known ingestion
Appendix C C-2 standard. See AC 20-147A for further information on engine and engine installation certification.
C.4 ICE SHAPES ON UNPROTECTED AREAS OF LIFTING SURFACES.
The airplane is designed with full span ice protection on the wing and horizontal stabilizer. The unprotected areas (wing tip, stabilizer tip) are a small percentage of the effective airfoil areas. Aerodynamic effects of Appendix O accretions on the minimal exposure areas are unlikely to be critical, but should be investigated. See AC 25-25A for details on performance and handling qualities guidance.
C.5 ICE SHAPES ON UNPROTECTED AIRFRAME REGIONS.
The applicant should examine other areas of the airframe that do not accrete ice under Appendix C conditions because of the increased impingement limits and potential additional runback in Appendix O conditions. Issues such as windshield visibility, static port location, and potential airflow disturbance from Appendix O accretions that could influence sensors (total air temperature, pitot, and angle of attack) should be examined.
A CFD analysis (icing impingement analysis) combined with a satisfactory field service history on derivative models may be sufficient to alleviate any potential concerns.
C.6 AIR DATA SENSORS.
The applicant should examine ice protection on air-data sensors to ensure adequate protection from exposure to Appendix O icing conditions.
C.7 DESIGN SUBSTANTIATION.
A review of the impingement analysis and runback ice data versus the protected area for the wing and horizontal stabilizer indicates that the wing is the critical surface for formation of a potential ridge behind the protected areas in front of the ailerons. The horizontal stabilizer has farther aft protection limits and has sufficient margin for contamination aft of the protected areas. Again, it is assumed that the horizontal stabilizer would be addressed, but that will not be presented in this example.
C.8 DETERMINATION OF EXIT CUES.
C.8.1 Although this airplane will be certified only for flight in Appendix C conditions, its operating envelope could result in exposing the airplane to any of the Appendix O icing conditions. Climbs and descents should be examined, but because of the transient nature of these encounters, they may not produce the critical accretions for Appendix O.
C.8.2 The recommended procedure for holding in icing conditions should be considered
(e.g., whether flaps or landing gear should be extended). AFM-recommended procedures would include this operating information. The case to be evaluated will consider the flaps up, gear up configuration. Since ice accretions aft of the protected
Appendix C C-3 areas on the upper wing surface are the primary concern in this example, operational conditions are analyzed to determine the critical flight conditions for such accretions.
The following conditions are determined to be critical:
C.8.2.1 Highest holding altitude within the conditions of Appendix O that maximizes watercatch (typically a function of ram air effects (effects of air exerting pressure) and liquid water content at defined static temperatures).
C.8.2.2 Highest true airspeed expected during holding conditions (typically high weight hold speed, which produces maximum water catch aft of protected areas upper surface.)
Note: Other operating conditions should be considered as required to determine the critical conditions.
C.8.3 Through a combination of impingement analysis, analytical ice shape prediction, and icing wind-tunnel testing, the applicant has determined that there is potential for an ice ridge to begin accumulating on the upper wing surface under these operating conditions when there is significant water content in drop sizes above a specific drop size and liquid water content. This specific drop size and liquid water content will be proposed as a condition for determining when icing conditions must be exited, and a visual exit cue will have to be set (see following paragraph). Note that such a drop size criterion is not a fixed boundary. At a lower altitude, and at a similar equivalent airspeed (similar angle of attack), slightly larger drop sizes are required to produce a similar accretion of ice behind the protected area. But this combination of drop size, liquid water content, and inertia in relation to the airplane is sufficient to potentially produce accumulations behind the protected areas. See the discussion on the modified inertia parameter in paragraph 12.1.1 of this AC and FAA Technical Report DOT/FAA/CT-88/8-1, Aircraft Icing Handbook, Chapter I, section 2 and Chapter IV, section 2.2.2.
C.9 DETERMINATION OF VISUAL EXIT CUE.
The target boundary will be icing conditions that exceed the specific drop size and liquid water content determined in the preceding paragraph (for high altitude, high speed hold). The target criterion for a visual exit cue is a surface on the airplane readily visible to the flightcrew that will indicate water collection in such drop sizes. A computational fluid dynamics analysis of the cockpit/fuselage area has determined that there are perceptible changes in impingement limits with increasing drop sizes along the side windows. The desired visual cue uses a specific portion of the windshield as a reference surface that can be correlated to ice accumulation on the wing. Preliminary impingement analysis indicates that this is feasible because as drop sizes increase, there is significant movement of the aft impingement limits on the windshield when the specific flight conditions of interest are considered. Based on this analysis, a specific location on the airplane side window is proposed as the place to assess a visible cue to determine when the Appendix O conditions that require exiting icing conditions have been encountered.
Appendix C C-4 C.10 DEFINITION OF CRITICAL ICE ACCRETION.
C.10.1 The applicant proposes that the exit cue be based on the first sign of accretion. It has been determined that an accretion of approximately 1/16-inch is required on the reference surface in order for the flightcrew to detect it. This reference-surface accretion is then correlated to the specific ice accretion on the monitored surface (behind the leading edge). The applicant should consider the ice accretion corresponding to the first indication on the reference surface and factor in recognition time and time to initiate escape as described in AC 25-25A, appendix A. This correlation is based on a combination of analysis and icing wind tunnel testing.
C.10.2 At the specific critical operating condition, the time it takes to observe the visual cue results in accretion of a 1/8-inch ice ridge behind the protected area. The difference in ice thickness between the ice on the reference surface and that on the wing is due to differences in the local collection efficiencies between the side window and the area aft of the leading edge protected surface. Consideration of additional accretion during the exit from the icing conditions is required to determine the full ice shape to be evaluated for aerodynamic degradation effects.
C.11 EVALUATION OF CRITICAL ICE ACCRETIONS.
Wind tunnel testing of this critical accretion has determined that there are no hinge moment situations that result in changes in roll forces. However, there is a potential increase in stall speed that must be addressed in the stall warning system. Dry air flight testing is performed using simulated ice shapes. This flight testing is performed in a manner in which the ice shapes are gradually built up with limited spanwise exposure to evaluate the effects with minimal safety risk. The stall warning margins are evaluated.
This evaluation results in a modification of the stall warning system to ensure that adequate stall margin exists both for the period before detection and during the exit from all icing conditions. See AC 25-25A for details on part 25, subpart B compliance.
C.12 VALIDATION OF THE VISUAL CUE.
C.12.1 The applicant will evaluate and validate use of the visual cue by flight testing with an icing tanker. The tanker is equipped to provide a specific median volume diameter icing condition with an associated drop distribution. Since the tanker cannot simulate the full array of Appendix O icing conditions, validation of specific conditions will be made. A fixed nozzle simulating the specific exit cue drop size will be used.
C.12.2 Icing tanker testing indicates that the side window begins to accumulate ice in the specific circumstances considered, which validates the analysis. At slower speeds, the drops do not have sufficient inertia to reach the side windows. This validates the fact that the visual cue is not overly sensitive and will not result in false indications. Tanker testing also shows good correlation with the predicted ice shapes, thus validating the expectation of a specific ice shape forming behind the protected areas. This location is
Appendix C C-5 adequately illuminated by existing cockpit lighting without glare or reflection, and is readily observable by flightcrew members.
C.13 EXITING FROM RESTRICTED APPENDIX O ICING CONDITIONS.
Flight tests are performed in accordance with § 25.21(g)(2) and (4) to show that the airplane can safely exit Appendix O icing conditions. See AC 25-25A for details on performance and handling qualities guidance.
Appendix D D-1 Appendix D. Guidance for Amended Type Certificates and Supplemental Type Certificates The guidance in this AC applies to any supplemental type certificate (STC) or amended type certificate (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, external modifications, and propeller changes could affect approval in icing and these areas should be evaluated using this AC as the method of compliance. An applicant wishing to use an alternate means of compliance should consult the appropriate FAA aircraft certification office.
D.1 EXAMPLES OF MODIFICATIONS FOR WHICH COMPLIANCE WITH ICING REGULATIONS SHOULD BE REVIEWED.
The following list is only an example and is not meant to be all inclusive:
• Engine changes.
• Propeller changes.
• Engine inlet or accessory inlet changes.
• Antennae installations or other external modifications.
• Gross weight increase.
• Center of gravity envelope increase.
• Flight envelope increase.
• Turboprop conversion.
• Modifications to lifting surfaces.
• Installation of vortex generators.
• Modifications to ice protections systems.
D.2 ENGINE CHANGES.
In addition to the guidance on similarity analysis provided in paragraph 8.7 of this AC, the applicant should address effects of increased engine power or thrust on airplane handling qualities in icing conditions, as well as on compliance with engine induction icing regulations, § 25.1093. AC 20-147A contains additional information on engine and engine installation certification.
D.2.1 Effects of Increased Engine Power or Thrust.
D.2.1.1 Stability and Control.
If increased engine power or thrust has potential for affecting airplane handling qualities in icing conditions, the applicant should conduct flight tests with ice accretions as described in AC 25-25A.
Appendix D D-2 D.2.1.2 Ice-Contaminated Tailplane Stall.
An ice-contaminated tailplane stall (ICTS) should be addressed. See AC 25-25A for compliance materials on tailplane stall.
D.2.2 Engine Induction Icing.
D.2.2.1 Similarity.
D.2.2.1.1 If similarity to a previous design is used as a method of compliance, such documentation should be supplemented with analysis or testing, or both.
In some cases, an analysis substantiating similarity, along with an installation survey conducted by the engine manufacturer, may be sufficient. The analysis should include the following:
• Heat requirements for the inlet lip ice protection, if installed.
• Location of the lip deicer thermostat, if installed.
• Inlet material to which the deicer is attached (heat sink may be different).
• Inlet geometry.
• Engine ignition system.
• Engine sensors.
D.2.2.1.2 Any differences that cannot be addressed by analysis should be addressed by flight testing. The data to be used for similarity, including data from a past certification on which the similarity analysis is based, must be available to the applicant and must be provided to the FAA upon request.
D.2.2.2 Falling/Blowing Snow and Ground Ice Fog for Turbine Engines.
If similarity to a previously certified airplane will be used as a method of compliance, the certification plan should state to which attributes of the ancestor airplane similarity will be shown. This is important because similarity may be based on a certified installation that does not have falling/blowing snow or ground ice fog in its certification basis. Service experience by itself cannot be used to show compliance. For turboprop installations, compliance is normally shown by testing. With regard to falling and blowing snow, inlets such as the oil cooler, as well as the induction system, are of concern. The certification plan should identify how compliance with these sections will be shown.
D.2.2.3 Ice Shedding.
Data showing engine compliance with § 33.77 should be compared between the currently installed engine and the proposed engine. If the ice slab used to show compliance is smaller for the proposed engine, ice shedding from the airframe should be re-addressed. Engine inlet lip ice shedding should be addressed. The applicant should compare the amount of ice mass that could be shed by the proposed installation to the
Appendix D D-3 amount that could be shed by a similar, approved installation. Alternatively, the amount of ice that could be shed should be compared to part 33 engine compliance data.
D.2.2.4 Ice Protection System Operation.
D.2.2.4.1 The applicant should compare the proposed engine to the existing, certified engine for—
• Bleed air mass flows,
• Pressures, and
• Temperatures.
D.2.2.4.2 If there is a reduction in any of these, the effectiveness of the IPS should be substantiated.
D.3 PROPELLER CHANGES.
D.3.1 Section 25.929 requires a means to prevent or remove hazardous ice accumulation on propellers or accessories where ice accumulation would jeopardize engine performance.
The typical analysis report from the deicing boot manufacturer is not sufficient by itself to show compliance with § 25.929. The typical report calculates inter-cycle ice thicknesses for various flight and icing conditions, but does not calculate the effect on propeller efficiency, which must be done to show no appreciable loss of thrust. For STCs, it would be acceptable to show that inter-cycle ice is equal to or less than accretions obtained on the same propeller on an airplane flight tested in icing conditions and shown to have no appreciable loss in thrust.
D.3.2 The typical deicing boot manufacturer report also contains a caveat that it does not address propeller runback ice. A comparison of similarity to another propeller that was flight tested in icing conditions is usually done to address runback. Points of similarity would include propeller and deicing boot aerodynamic and thermal similarity, and similarity of the deicing timer, propeller revolutions per minute (rpm), and flight conditions. Note that metal and composite propellers have different thermal masses.
D.3.3 As a final qualitative check for both inter-cycle and runback ice on new airplane certification programs, airplane performance should be checked during flight tests in icing conditions.
D.3.4 The propeller installation, including spinner and cowl geometry, should be compared to previously tested installations in icing conditions. Changes that could allow moisture to reach the brush blocks should be avoided. Water reaching the brush block area of the propeller deicing system has caused brush block/slip ring hydroplaning and loss of power to propeller deicing boots.
D.3.5 If the proposed propeller is calculated to have higher efficiency than the existing, approved propeller, follow the guidance in paragraph D.2.1, Effects of Increased Engine Power or Thrust, of this appendix.