direct penetration and exit approach is inappropriate for aircraft for which full icing clearance is requested (reference § 29.1419).
(iii) Engine induction system continuous icing protection would be necessary for aircraft for which full-icing clearance is requested (reference § 29.1419(d)). The approach is much preferred for all programs in order to reduce the scope of any eventual total aircraft icing program effort and to increase the safety level in conducting the rotorcraft natural icing tests. Sinc e some rotorcraft have been FAA/AUTHORITY certificated to operate in icing conditions, applicants may request full-icing clearance and, as a result, must demonstrate that the engine induction system will operate in a continuous icing environment.
(iv) It is noted in section 29.1419 of this AC that some natural icing tests are required to show compliance with the overall rotorcraft ice protection requirements. It is not required that the engine induction system be evaluated as a part of that natural icing test if adequate verification has been shown by tunnel testing, analysis, or other means to assure satisfactory operation in an extended continuous icing environment. If, however, subsequent rotorcraft natural icing testing shows unanticipated detrimental engine inlet effects, the inlet ice protection system should be reexamined.
(v) The regulation specifies the examination of flight idling conditions. This requirement is normally associated with a low-power letdown at the minimum practical forward airspeed. Alternatively, eval uation of the minimum power and minimum airspeed combination specified in the rotorcraft flight manual (RFM) for operation in visible moisture when below 41°F (+5°C) will accomplish the intent of the idling requirement.
(vi) An acceptable approach to a finding of compliance would be a combination of analysis of the performance of the ice protection system, which covers the range of the applicable icing flight envelope (maximum altitude, minimum temperature, etc., of the basic rotorcraft) supported and validated by tests. Ideally, these tests would be conducted in natural atmospheric ice with special instrumentation for droplet size and liquid water content. In practice, however, natural icing testing may pose unacceptably severe problems since rotorcraft may not have the range and speed to reasonably find icing clouds and may not be equipped with the airframe and rotor ice protection needed for safety during the testing.
(vii) Problems with analysis emerge if engine inlets incorporate screens, turning vanes, sideward or upward openings, and edge or lip configurations, which deviate from the airfoil shapes assumed in most of the analytical procedures described in current technical literature. The applicant should recognize that if meaningful analytical methods are not available, extensive testing with significant conservatism or possibly design changes may be required. Inlet screens in particular, if not adequately heated, fall in this category and can only be accepted if shown by very conservative ice testing to not significantly impede airflow to the engine.
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AC 29-2C, Chg 4 5/1/2014
(2) Procedures.
(i) Review section 29.1419 of this AC, ADS-4, Report No. FAA-RD-77-76, and the current version of AC 20-73. Thes e data provide extensive description and methodology for evaluation of ice protection systems, however, as noted above, these data generally apply to near straight line droplet trajectory with impingement onto conventional airfoil shaped inlets. As such, t he applicability of these data to rotorcraft engine inlet ducts is limited and may require extensive adjustment to accommodate the different inflow trajectories and shapes of rotorcraft.
(ii) An analysis, appropriate to the configuration; i.e., heated or unheated impingement surfaces, should be prepared. To be acceptable, this analysis should show the inlet to be adequately protected by heat, or if unheated, to show that the inlet with ice accretions as predicted, will provide adequate airflow to the engine throughout the flight envelope of the rotorcraft.
(A) For heated surfaces, ADS-4 and Report No. FAA-RD-77-76 provide detailed suggestions on heat transfer analysis particularly applicable to bleed air heated inlet lips formed in airfoil shapes. These data are limited in applicability and may not be useful for analyzing engine inlet water droplet trajectories to be expected at low airspeed and high engine airflow. Actual icing tests may be needed to derive the impingement patterns for these conditions.
(1) Acceptability criteria for heated inlet ducts usually require sufficient heat to evaporate the water to be expected in a “continuous maximum” icing cloud and to anti-ice the duct during flight in “intermittent maximum” icing clouds, provided the run-back and refreeze to be expected does not cause additional airflow disruption or damage to the engine. Full-scale inlet ici ng tests with the engine installed and operating should be conducted to verify the analysis. Engine power changes, which may be expected in service should be included in the testing. Wind tunnels equipped for icing tests probably are the most useful means of conducting these tests if natural icing tests are impractical. The rotor downwash effect should be considered to the extent possible by adjusting the inflow angle in the tunnel.
(2) The power loss (bleed air, generator load, etc.) attributable to the heating requirements will affect the performance of the rotorcraft. Normally, this may be accounted for by specifying a gross weight incremental deduction from the flight manual performance data for flight into visible moisture below 41°F (+5°C).
(3) Special evaluation of the possibility of ice ingestion damage to the engine should be made for heated systems, which considers the ice ingestion to be expected when the anti-ice system is actuated after a delay of 1 minute for the pilot to recognize that the rotorcraft has encountered ice. This time delay may be reduced if the crew is provided adequate distinctive cues to alert them that the rotorcraft has encountered icing conditions.
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(B) For unheated inlets, an acceptable method for showing compliance would include an extensive, detailed analysis (which shows that ice accretions on and in the inlet do not obstruct adequate airflow to the engine) and tests as necessary to validate the analysis. The analysis of ic e accretion becomes even more questionable since the unheated inlet involves ice buildups which themselves progressively change shape during icing exposure.
(1) Flight testing with an instrumented rotorcraft in natural ice to verify the analysis is desirable; however, wind tunnel tests as discussed above may be used.
Since unheated inlets typically continue to accrete ice as a function of exposure, both the analysis and the test should realistically consider the actual exposure to be expected in service. This should not be less than penetration of the continuous maximum icing cloud followed immediately by exposure to the intermittent maximum cloud for rotorcraft not certified for icing. Engine power changes which may be expected in service should be included in the testing, and a warm-up period at the conclusion of the icing exposure should be shown for some selected test points to evaluate potential ice breakaway and ingestion.
(2) For the non-icing certified rotorcraft using the limited icing exposure concept for inlet certification, some conservatism should be applied to account for the fact that inlet icing may occur without airframe icing, and that the escape procedure from this unapproved operating condition is not defined. A demonstration of 30-minute hold capability in the continuous maximum cloud would be acceptable. Alternatively, if positive cues (perhaps a carefully located ice detector) of potential inlet icing are provided to the crew, the time increment could be reduced to recognition plus 15 minutes (15-minute escape time after recognition is consistent with the single ice protection system failure recognition and escape guidance for aircraft ice protection systems in section 29.1499 of th is AC). It should not be assumed that airframe icing will always be available as a cue to potential inlet icing. The main rotor, for example, may not show icing indications above 25°F (-4°C), whereas some inlets may ice critically near 32°F (0°C) ambient. A r eduction of the acceptable 30-minute exposure should not be based on observation of ice accretions on protruding components which are likely to be changed. For example, a limited exposure inlet icing program which reduces the inlet icing exposure time based on crew recognition of icing on the windshield wipers may be invalidated at a later date if a new windscreen deletes the wipers.
(iii) Inlet capability during IGE hover in icing conditions has not generally been considered for rotorcraft not certified for icing. However, the FAA/AUTHORITY is aware that some inlets may ice at zero airspeed near 32°F (0°C) with no indications of airframe icing in the field of view of the crew. This special concern of operating within RFM limitations, and yet placing the induction system in jeopardy, may be addressed in several ways. If the induction system ice protection scheme is not dependent on airspeed for proper function, the issue may be addressed by tunnel testing with inlet airflows approximating hover with no particular attention to tunnel windspeed. For protection schemes which may be sensitive to airspeed (external screens have shown Page E - 155
AC 29-2C, Chg 4 5/1/2014 this tendency), actual hover demonstration at or near zero speed tunnel conditions may be appropriate. Icing detectors located to indicate induction system icing in hover may be an option to a hover icing protection demonstration. On an external screened configuration, the FAA/AUTHORITY has accepted a satisfactory IGE hover demonstration of 30 minutes at the critical ambient temperature (i.e., ambient consistent with no airframe icing but potential inlet icing), 0.6 grams/meter3 LWC and 40 micron droplet size as an adequate response to this concern.
(iv) For aircraft requesting full icing approval, or for those electing to show continuous induction system icing protection, the forward flight icing exposure would not be less than that time required to stabilize any ice accretions observed during repeated cycles of the continuous maximum followed by intermittent maximum cloud exposure.
Typically, any ice accretions resulting from these repeated cycles would be expected to stabilize in less than 30 minutes. The 30-minute hold capability in the continuous maximum icing environment could thus be assured without special testing by careful selection of the test points for this repeated cycle.
(v) A rotorcraft requesting full icing approval should also have hover capability in the icing environment. Intermittent maxi mum icing conditions are not likely to exist near ground level and a satisfactory demonstration could involve the ability to hover indefinitely in the continuous maximum icing environment. Alternatively, carefully worded RFM limitations to restrict hover time may be acceptable if the system is not capable of indefinite exposure. Hover c apability verification may not involve zero airspeed demonstration if the inlet protection system is insensitive to rotorcraft airspeed.
(vi) The engine(s) must be installed or protected to avoid engine damage from ice ingestion due to ice accretion in the inlet or on other parts of the rotorcraft, including the rotors, which may break away to enter the inlet. If screens or bypass arrangements are provided for these purposes, they should be included in the icing tests and shown by test or rational analysis to effectively protect the engine.
(vii) For unheated inlets, significant ice accumulations to be expected on the inlet may adversely affect the engine stall margin, acceleration characteristics, duct loss, etc. Dry air flight te sts to evaluate these aspects can be accomplished by affixing ice shapes to the inlet. These shapes should closely match the actual ice shapes defined by test or analysis.
c. Turbine Engines - Snow Protection.
Explanation. (1)
(i) Section 29.1093(b)(1)(ii) provides that the turbine engine and its air inlet system operate satisfactorily within the limitations established for the rotorcraft, in both falling and blowing snow. The section does not provide the definition of falling and blowing snow.
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(ii) Since the regulation provides for certification “within the limitations established for the rotorcraft,” the FAA/AUTHORITY can accept a restriction against snow operations in the limitations section of the RFM in lieu of demonstration of compliance to the Full Falling & Blowing Snow Conditions defined below in paragraph
c.(2)(i).
(A) If an applicant elects not to demonstrate compliance to the FULL falling and blowing snow conditions (i.e., seeks a restriction against snow operations), it can either:
(1) demonstrate that the aircraft turbine engine(s) and its inlet system(s) will operate satisfactorily in the Inadvertent Falling & Blowing Snow Conditions, defined below in paragraph c.(2)(ii), with a restriction for snow operations.
This approach will not require that a minimum operational temperature limit of 41°F (+5°C) be included in the flight manual; or
(2) include a flight manual limitation for minimum operational temperature of 41°F (+5°C).
(B) If no restriction on snow operations appears in the RFM, it is presumed that the aircraft may operate in snow at the pilot’s discretion.
(2) Guidance.
(i) Engine induction system operation in falling and blowing snow can be approved without restriction if normal operations under the following conditions are demonstrated:
FULL FALLING & BLOWING SNOW CONDITIONS Visibility: ¼-mile or less as limited by snow.
Temperature: 25°F (-4°C) to 34°F (+1° C) [28°F (-2°C) to 34°F (+1°C) desired], unless other temperatures are deemed critical.
Operations: Ground operations - 20 minutes.
IGE hover - 5 minutes.
Level flight - 1 hour.
Descent and landing.
(ii) Demonstration to the below Inadvertent Falling & Blowing Snow Conditions with a flight manual limitation that prohibits flight into falling and blowing snow is acceptable. This approach will not require that a minimum operational temperature limitation of 41°F (+5°C) be included in the flight manual.
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AC 29-2C, Chg 4 5/1/2014
INADVERTENT FALLING AND BLOWING SNOW CONDITIONS Visibility: 1 mile or less as limited by snow.
Temperature: 25°F (-4°C) to 34°F (+1° C) [28°F (-2°C) to 34°F (+1°C) desired], unless other temperatures are deemed critical.
Operations: Ground operations - 5 minutes.
IGE hover - 1 minutes.
Level flight - 10 minutes.
Descent and landing.
(iii) RFM visibility restrictions for falling and blowing snow operations are not appropriate.
(iv) Time limitations, other than possibly for ground and hover operations, are not appropriate.
(v) Artificially produced snow should not be used as the sole means of showing compliance.
(3) Guidance Rationale.
Visibility 1 ow tends to accumulate on unheated surfaces subject to impingement.
( ) Colder ambients, more conducive to dry snow conditions, may be critical for some indu 2
(i) The test conditions specified--visibility, temperature, and operations--are based on previous certification programs, previous guidance, and on research by the FAA technical center and others.
(A) . The test visibility defined, ¼-mile (Full Falling & Blowing Snow Conditions) visibility or less as limited by snow, represents a heavy snowstorm and is the maximum likely to be encountered in service. Rotorcraft, which have been certified to the ¼-mile visibility test criteria, have not shown engine inlet snow-related service difficulties. It is important to note t hat the visibility specified is a test parameter rather than an operational limitation to be imposed on the rotorcraft after the tests are completed.
(B) Temperature.
( ) The ambient temperature specified is conducive to wet snow conditions. Wet sn ction systems. Colder exterior surfaces may be bypassed, and the snow crystals may stick to partially heated interior surfaces where partial melting and refreezing may occur.
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( ) Company development testing or experience with very similar type induction systems 3 may be adequate to determine the critical ambient conditions for certification testing.
(C) Operations.
( ) Ground running, taxiing, and IGE hover operations are generally the most critical sin 1 ce the rotorcraft may be operating in recirculating snow. Twenty-five minutes under these extreme conditions would seem a reasonable maximum, both from the view of pilot stress and the maximum expected taxi time prior to takeoff in bad weather.
(2) One hour of level flight operation under ¼-mile visibility snow conditions should provide ample opportunity for hazardous accumulations to begin to build.
(3) The descent and landing will provide an engine power change, an induction system airflow change, and a variation in the external airflow pattern near the induction system entrance. The initiation of the descent and final flare for landing may also produce additional airframe vibration transmitted to the induction system. These power, airflow, and vibration changes may provide an opportunity for any level flight accumulations to be ingested into the engine. Hazardous accumulations are not acceptable during or after any test phase.
(ii) Visibility may fluctuate rapidly in snowstorms. It is affected by the presence of fog or ice crystals, is not crew measured or controlled, and is difficult to estimate. A visibility operational limitation based on snow, ther efore, is not appropriate.
(iii) Since during cruise in snow conditions the aircraft is likely to be in and out of heavy snowfall, it is not practical for the crew to account for the time spent in snow in level flight conditions. Thus, it is not appropriate to include time limitations in the RFM for level flight snow operations.
(iv) A practical ground and IGE hover time limitation of less than 25 minutes in recirculating snow may be considered. The expected action at the expiration of this specified time period would be shut down and inspection of the inlet system or transition to a safe flight condition where demonstration has shown that moisture accumulations will not intensify or shed and cause engine operational problems.
(v) Artificially produced snow is an excellent development tool and has been successfully used to indicate potential problem areas in induction systems. These devices are usually restricted to use for hover and ground evaluations, and the snow pellets produced by these machines are not sufficiently similar to natural snowflakes to justify the use of artificial snow as the sole basis of certification.
(4) Procedures.
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AC 29-2C, Chg 4 5/1/2014
(i) Satisfactory demonstration of the test conditions requires that the engine, induction system, and proximate cowling surfaces remain free of excessive snow, ice, or water accumulation. Excessive accumula tion is defined as accumulation that may cause engine instability, damage, or significant loss of engine power. If a questionable amount of snow or moisture accumulates in the inlet, the applicant may elect to demonstrate that this amount in the form of snow or water and ice, as appropriate, can be ingested by the engine without incurring surge, flameout, or damage.
(ii) The conditions specified assume actual flight demonstration in natural snow. The ground operations and IGE hover test conditions assume operation in recirculating snow. Blowing snow, resulti ng from rotor airflow recirculation, can be expected to be more severe than natural blowing snow if the rotorcraft continues to move slowly over freshly fallen snow. Thus , the blowing snow operational capability is usually demonstrated by the taxi and hover operations in recirculating snow.
(iii) For VFR rotorcraft, the airspeeds for the level flight test condition should include the maximum consistent with the visibility conditions. For IFR operations, the airspeed should be the maximum cruise speed or the maximum speed specified for snow operations in the flight manual limitations, unless other airspeeds are deemed more critical. It is recognized that many ro torcraft initially certified VFR are later IFR certified with a resulting possible increase in airspeed in snow conditions. This factor should be considered if IFR certification is anticipated.
(iv) The visibility specified assumes that visual measurements are made in falling snow in the absence of fog or recirculating snow by an observer at the test site outside the tests rotorcraft’s area of influence. An accepted equation for relating this measured visibility to snow concentration is V = 374.9/C0.7734 where C is the snow concentration (grams/meter3) and V is the visibility (meters).
(A) This equation can be reasonably applied to all snowflake type classifications and is credited to J.R. Stallabrass, National Research Council of Canada.
(B) Other equations may be applied if they are shown to be accurate for the particular snowflake types for the test program.
(v) The snow concentration corresponding to the visibility prescribed, ¼-mile or less, will be extremely difficult to locate in nature. Data from Ottawa, Canada, research indicate that fewer than 4 percent of the snowstorms encountered there meet the 0.91 grams/m3 concentration associated with the ¼-mile visibility. Furthermore, the likelihood that the desired concentration will exist for the duration of the testing is even more remote. Because of these testing realitie s, it is very likely that exact target test conditions will not be achieved. Those invo lved in certification must exercise good judgment in accepting alternate approaches.
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(vi) For some engine induction systems, it may become apparent by inspecting for moisture accumulations that ground and IGE hover operations in recirculating snow are much more severe than the level flight test. In this instance, it is reasonable to accept prolonged IGE operations in recirculating snow and to accept durations of less than 1-hour level flight in ¼-mile or less visibility. Best efforts should be made to ensure that at least some level flight time is accomplished at ¼-mile or less visibility to ensure that the spectrum is covered.
(vii) It should be determined that the visibility established at the test sight is limited by snow and not by fog or poor lighting (twilight) conditions.
(viii) The concentration of snow approaching the inlet in severe recirculation will far exceed the quantity encountered in the natural snowfall. Recirculation is necessarily a qualitative judgment by the test pilot. The snow concentration at the inlets during recirculation would vary for different rotorcraft types and would be dependent on rotor characteristics, power setting, and inlet location. For test purposes, recirculation should be the highest snow concentration attainable in the maneuver, or that corresponding to the lowest visibility at which (in the pilot’s judgment) control of the rotorcraft is possible in the IGE condition. The visibility s pecification of ¼-mile or less outside of the recirculation influence becomes inconsequential provided that fresh, loose snow is continually experienced during the ground operation and IGE hover testing phase. However, since it is in tended that the test phases be accomplished sequentially to ensure that transition to takeoff and other transients are considered, the conditions at takeoff, level flight, and descent and landing should approximate the ¼-mile visibility criteria.
d. Turbine Engines - Ground Icing.
(1) Explanation Procedure . This requirement addresses the situation where extended ground operation in icing exposes the rotorcraft and its engine inlet to icing (ground fog) conditions which may have different droplet impingement patterns and involve different or less effective means of ice protection. Note that the requirement is effective at Amendment 10 and is applicable regardless of any desire to prohibit dispatch into icing conditions.
(2) . Since this condition assumes zero airspeed, wind tunnel testing may be inappropriate unless conservative extrapolation of low speed tunnel data can be determined to be valid. For protection schemes which are dependent primarily on airspeed for proper functions (external screens have shown this tendency), it may be necessary to verify adequate ground operation protection capability by very low speed tunnels or by the use of outside facilities such as the Canadian National Research Council’s spray rig at Ottawa, Canada. For heated systems or for internal bypass schemes, tunnel speed may not be important, and adequate demonstration may be accomplished at higher tunnel speeds provided that internal inlet airflows and heat available are properly considered. Testing should proximate the regulatory test conditions and be continued for 30 minutes using engine power and control Page E – 160.1
AC 29-2C, Chg 4 5/1/2014 manipulation as normally accepted during taxiway operations, followed by an acceleration to takeoff power. The test time may be shortened if de-ice or anti-ice protection is adequate or if stabilization of ice build-up is affirmed. The induction system should be in condition for safe flight at the conclusion of the test.
AC 29.1093A. § 29.1093 (Amendment 29- 26) INDUCTION SYSTEM ICING PROTECTION.
a. Explanation Procedures . Amendment 29-26 clarifie s that the phrase, “within the limitations established for the rotorcraft” applies only to the requirement in § 29.1093(b)(1)(ii) for demonstrating flight in falling and blowing snow.
b. . All of the policy material for this section remains in effect with the update that turbine engines and turbine engine inlets should perform satisfactorily in atmospheric icing conditions defined in Appendix C of part 29. In addition to section 29.1093 of this AC, the following procedures should be followed:
(1) A “serious loss of power” in this section has been interpreted to be any power loss that requires immediate pilot action. In addition, the term “adverse effect on engine operation” in § 29.1093(b)(1)(ii) has been interpreted to be an effect that would prevent the engine from achieving rated aircraft flight manual performance (takeoff, climb, etc.).
This term also includes effects on the engine induction system characteristics to an acceptable level established by the engine manufacturer (inlet distortion, etc.).
(2) The applicant should show that rotorcraft prohibited from flight into falling and blowing snow can exit inadvertent entrance into those conditions without adverse effect upon the operating characteristics of the engine or the rotorcraft. This requires that the engine(s) and its inlet system be shown to operate satisfactorily throughout the flight power range of the engine and within the operating limitations of the rotorcraft during operation in the Inadvertent 1 mile visibility falling and blowing snow conditions defined herein.
(3) For unrestricted flight capability into Full snow conditions, both falling and blowing, the applicant should show that each engine, and its inlet system, will operate satisfactorily throughout the flight power range of the engine and within the operating limitations of the rotorcraft. The applicant should show that any build-up or accumulation of snow will not reduce or block the flow of inlet air to the engine. Any accumulations that become dislodged should not affect engine operation.
(4) If a design is not satisfactorily demonstrated to either the Full or the Inadvertent snow conditions, a limitation must be included within the flight manual prohibiting flight in temperatures below 41°F (+5°C).
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AC 29.1101. § 29.1101 CARBURETOR AIR PREHEATER DESIGN.
a. Explanation. Each carburetor air preheater must be designed and constructed to:
(1) Ensure ventilation of the preheater when the engine is operated in cold air.
(2) Allow inspection of the exhaust manifold that it surrounds.
(3) Allow inspection of critical parts of the preheater itself.
b. Procedures. Although carburetors of some design and fuel injections are free from icing difficulties, the most common remedy is to preheat the air supply entering the carburetor. In this way, sufficient heat is added to replace the heat lost due to vaporization of fuel, and the mixing chamber temperature cannot drop to the freezing point of water. The air preheater is e ssentially a tube or jacket through which the exhaust of one or more cylinders is passed with the air flowing over the heated surface raised to the required temperature before entering the carburetor. A control for adjusting the preheater valve is installed in the cockpit so that heat may be applied only when actually required to prevent ice formation.
AC 29.1103. § 29.1103 (Amendment 29- 17) INDUCTION SYSTEM DUCTS AND AIR DUCT SYSTEMS.
a. § 29.1103(a):
(1) Explanation. This paragraph is intended to require the design of induction system ducts for engines and auxiliary power units to include fuel and water drains which are effective in the ground attitude and do not discharge into any location where the fuel drainage could be ignited to cause a fire hazard.
(2) Procedures. Determine that each i nduction duct is provided with at least one drain of sufficient size to minimize clogging and located at the low point of the duct with the rotorcraft in the ground attitude. Discharge from the drai n should not create a hazard to the rotorcraft.
b. § 29.1130(b):
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