SECTION 30. INDUCTION SYSTEM
531. § 27.1091 (Amendment 27-2) AIR INDUCTION.
a. Explanation.
(1) The air induction system for each engine should be of a configuration to supply the air required under the operating conditions for which certification is required.
(2) The intake system shall be designed such that if a backfire flame occurs, it will emerge outside the engine compartment cowling.
(3) Where required in the induction system, drains must be provided which discharge clear of the rotorcraft and out of the path of exhaust flames.
(4) For rotorcraft powered by a turbine, the inlets should be located or protected to minimize foreign object ingestion as defined in the regulation. The inlets must be protected during takeoff, landing, and taxiing. There must also be means to prevent leakage of hazardous amounts of flammable fluids from entering the engine intake system.
b. Procedures.
(1) For turbine-engine installation, the induction system should supply air of suitable quality to meet the installation requirements of the engine manufacturers. The installation requirements should be met throughout the operating envelope of the rotorcraft.
(2) The inlet design should account for the prevention of hazardous fluids entering the engine. Some designs will have inlet ducts which are free from any fluid lines; however, other designs may route the engine inlet air through a compartment which has flammable fluid lines. When the condition exists, test demonstrations of critical leakage during operations have been used to substantiate the installation. The fluid leakage may not have an adverse effect on engine operation.
(3) The air induction system design should also account for and minimize the possibility of foreign matter ingestion during takeoff, landing, and taxiing.
(4) For reciprocating engine installations, the induction system should supply air of suitable quality and quantity to the carburetor inlet of the engine. The condition of the air at the entering face of the carburetor is extremely important. For proper operation, it is essential that the airflow be smooth, uniform, clean, and unrestricted throughout the very wide range of horsepower expected from the engine.
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531A. § 27.1091 (Amendment 27-23) AIR INDUCTION.
a. Explanation. Amendment 27-23 removed § 27.1091(d) since the specific test defined by this paragraph was not critical for certain rotorcraft. The turbine inlet foreignobject-ingestion protection provided by § 27.1091(d) is adequately evaluated by existing requirements in § 27.1091(e)(2).
b. Procedures. This rule change did not change the current suggested methods of compliance.
532. § 27.1093 (Amendment 27-20) INDUCTION SYSTEM ICING PROTECTION.
a. Reciprocating Engines.
(1) Explanation.
Atmospheric moisture, even in clean air and temperatures above freezing, can result in ice accumulations in induction systems to a degree which can easily cause engine failure.
(A) Impact Ice. This forms as supercooled water droplets impact on engine induction system components. Particularly heavy accumulations must be expected where bends or turns in the induction system force changes in the airflow direction thus centrifuging the droplets out of the air stream where they freeze on impact with induction system components. A serious form of impact ice is the collection of ice on fuel metering elements of the carburetor, the alternate (preheat) valve, and any screens in the system.
(B) Throttle Ice. This type of ice forms at or near the throttle in a partly closed position (up to 30° F) due to cooling effect resulting from the increase in kinetic energy (increased velocity) of the air in the restricted flow area.
(C) Refrigeration Ice. This forms as a result of the cooling effect of the fuel evaporating after the fuel is introduced into the airstream. For some float type carburetors, it is possible in rare instances to accumulate serious ice during a closed throttle glide with ambient air temperatures as high as 93° F and relative humidity of 30 percent. At low cruise power, ice can occur at outside air temperatures as high as 62° F and relative humidities as low as 60 percent. Most of the heat necessary to evaporate fuel is supplied from the air as it drops in temperature. Fuel evaporation ice can affect airflow by blocking the throat of the manifold riser, it can affect the fuel-air-ratio by interfering with the fuel flow, and it can affect mixture distribution or quantity of mixture flowing to individual cylinders by upsetting the fuel flow distribution or quantity of mixture flowing to individual cylinders by upsetting the fuel flow distribution at the fuel nozzle or airflow distribution in the manifold throat. This refrigeration phenomenon is the most serious of all factors causing carburetor ice.
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(2) Procedures. Normally, flight tests with carburetor air temperature instrumentation are required. Unless otherwise justified, conduct all tests at maximum gross weight, a median center of gravity, in level flight, and at the engine speed which, considering cooling fan effects, if any, produces the minimum heat to the carburetor muff or engine component area utilized to provide the carburetor air heat. The optimum test condition is flight at the altitude at which the measured OAT is 30° F and a power setting of 75 percent maximum continuous power can be maintained. If this combination cannot be achieved, satisfactory interpolation of data from other test conditions can be achieved using the methodology of AC 23-8A, Flight Test Guide for Certification of Part 23 Airplanes.
In addition to the preheat requirements of § 27.1093(a)(1), (a)(3), and
(a)(4), the design should consider the possibility of impact ice (supercooled droplets below freezing) on engine air inlet components opening into the airstream. However, normal practice is to provide a crew selectable, sheltered, alternate engine air intake arrangement which, by inspection can be determined to be free from impact ice accumulations. Typically, a sheltered alternate air source would be acceptable if the opening is located inside the cowling out of the free airstream. However, precautions should be taken to ensure that backfire flames to be expected do not constitute a fire or explosion hazard.
(ii) For further information review AC 20-113, Pilot Precautions and Procedures to be Taken in Preventing Aircraft Reciprocating Engine Induction System and Fuel System Icing Problems, NACA TN 1790, NTSB AAS-72-1, DOT/FAA/CT 84/44 (1982) and NACA TN 1993 (1949).
b. Turbine Engines - Ice Protection.
(1) Explanation.
This rule requires turbine engines and turbine-engine inlets to perform satisfactorily in atmospheric icing conditions defined in Appendix C of Part 25. On an equivalent safety basis, the limited icing envelopes described in AC 29-2A, Paragraph 386 may be used to show compliance with the intent of the regulation if the rotorcraft is limited to not greater than a 10,000-foot pressure altitude for all operations.
If operations are permitted above 10,000 feet, the Appendix C, Part 25, envelope must be used from 10,000 feet to the service ceiling or 22,000 feet. These possible equivalent safety approaches are not discussed herein. Compliance with the induction system icing protection rule is required regardless of flight manual limitations or restrictions against flight into atmospheric icing conditions.
(il) In showing compliance with § 27.1093(b)(1)(i), the FAA/AUTHORITY has accepted the concept of limited exposure associated with escape from inadvertent ice encounters.
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(A) Under the concept of limited exposure associated with escape from inadvertent ice encounters, it is presumed that there will be a flight manual limitation against flight into known icing, and that the engine induction system will be reevaluated if total aircraft ice protection certification is requested. Under this concept, the rotorcraft is assumed to fly directly through the icing environment; i.e., direct sequential penetration and straight line exit from both the continuous maximum and intermittent maximum icing clouds. Thus, the duration of exposure to the icing environment could be calculated by knowing the aircraft flight speed and cloud horizontal extent. A range of engine power and rotorcraft airspeeds should be evaluated to encompass the operating envelope of the rotorcraft. Note that aircraft speed has a pronounced effect (Ludlam effect) on ice accretion on small surface areas (inlet screens). A review of this phenomena may be found in National Research Council (Canada) Letter LT-92.
(B) When this limited exposure concept is used, the aircraft type certificate data sheet should clearly specify that the engine induction system must be reevaluated if certification to the general ice protection regulation, § 27.877 or § 27.1419, is requested. This direct penetration and exit approach is inappropriate for aircraft for which full icing clearance is requested (reference § 27.1419).
(iii) Engine induction system continuous icing protection would be necessary for aircraft for which full-icing clearance is requested (reference § 27.1419(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. Since at least one rotorcraft has been FAA/AUTHORITY certificated to operate in known icing conditions and others have active development programs to this end, applicants should anticipate eventual full-icing clearance and consider that the engine induction system may be required to operate routinely in a continuous icing environment.
(iv) It is noted in AC 29-2A, Paragraph 386 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 ensure 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, evaluation of the minimum power and minimum airspeed combination specified in the RFM for operation in visible moisture when below 40° F will accomplish the intent of the idling requirement.
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(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 airtrame 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.
(viii) The icing evaluation should definitely include some test points or other adequate evaluation of flight at ambient temperatures several degrees Fahrenheit above freezing and with very high water content. This condition has actually produced multiengine flameouts in in-service aircraft. The actual icing phenomena involved is not fully understood and in some instances efforts to duplicate the phenomena in icing tunnels were unsuccessful. Usually this condition does not produce rotor icing;
therefore, actual flight testing using special precautions to ensure safe autorotation landings or engine relight capability may be needed to identify this condition.
(2) Procedures.
Review AC 29-2A, Paragraph 386, ADS-4, Report No. FAA-RD-77-767; Aircraft Icing Handbook, FAA Technical Report No. DOT/FAA/CT-88/8, and Advisory Circular 20-73, Aircraft Ice Protection. (The comparative concept described under Item 34 of AC 20-73 is obsolete and should not be considered. These 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, the 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 Par 532 697
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, providing the run-back and refreeze to be expected does not cause additional airflow disruption or damage to the engine. Full-scale inlet icing 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 40° F.
(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.
(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 seriously obstruct adequate airflow to the engine) and tests as necessary to validate the analysis. The analysis of ice 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 698 Par 532
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 nonicing 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 AC 29-2A, Paragraph 386). 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, whereas some inlets may ice critically near 32° F ambient. A reduction 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.
(Ш) Inlet capability during IGE hover in icing conditions has not generally been considered for rotorcraft not certified for icing. However, the FAA/AUTHORITY is recently aware that some inlets may ice at zero airspeed near 32° F 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 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. Recently, 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/meter 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.
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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 ensured 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 maximum 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 capability 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 tests 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. In addition, it should be determined that ice shedding into the engine inlet either during continued flight into icing conditions or after emerging from the icing environment does not damage engine compressor on other inlet components.
c. Turbine Engines - Snow Protection.
(1) Explanation.
(• Section 27.1093(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.
(il) 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. 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) Acceptance Criteria.
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The FAA/AUTHORITY has accepted that engine induction system operation in falling and blowing snow can be approved without restriction if normal operations under the following conditions are demonstrated:
Visibility: one-quarter mile or less as limited by snow.
Temperature: 25° F to 34° F (28° F to 34° F desired), unless other temperatures are deemed critical.
Operations: Ground operations - 20 minutes IGE hover - 5 minutes Level flight - 1 hour Descent and landing (її) RFM visibility restrictions for falling and blowing snow operations are not appropriate.
(iii) Time limitations, other than possibly for ground and hover operations, are not appropriate.
(iv) Artificially produced snow should not be used as the sole means of showing compliance.
(3) Rationale.
The test conditions specified--visibility, temperature, and operations-- are based on previous certification programs, previous FAA/AUTHORITY guidance, and on research by the FAA technical center and others.
(A) Visibility. The test visibility defined, 1/4-mile 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 1/4-mile visibility test criteria have not shown engine inlet snow-related service difficulties. It is important to note that 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.
(1) The ambient temperature specified is conducive to wet snow conditions. Wet snow tends to accumulate on unheated surfaces subject to impingement.
(2) Colder ambients, more conducive to dry snow conditions, may be critical for some induction systems. Colder exterior surfaces may be bypassed, and Par 532 701
the snow crystals may stick to partially heated interior surfaces where partial melting and refreezing may occur.
(3) Company development testing or experience with very similar type induction systems may be adequate to determine the critical ambient conditions for certification testing.
(C) Operations.
(1) Ground running, taxiing, and IGE hover operations are generally the most critical since 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.
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, therefore, is not appropriate.
(ili) 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 702 Par 532
7/30/97 АС 27-1A 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.
(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 accumulation 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 (її) 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, resulting 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.
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 rotorcraft initially VFR certified are later IF 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/C°7734 where C is the snow concentration (grams/meter*) 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 1/4-mile or less visibility prescribed will be extremely difficult to locate in nature. Data from Ottawa, Canada, Par 532 703
research indicate that fewer than 4 percent of the snowstorms encountered there meet the 0.91 grams/m' concentration associated with the 1/4-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 realities, it is very likely that exact target test conditions will not be achieved. Those involved in certification must exercise good judgment in accepting alternate approaches.
(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 1/4-mile or less visibility. Best efforts should be made to ensure that at least some level flight time is accomplished at 1/4-mile or less visibility to ensure that the spectrum is covered.
(vil) 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 1/4-mile or less visibility specification outside of the recirculation influence becomes inconsequential provided that fresh, loose snow is continually experienced during the ground operation and GE hover testing phase. However, since it is intended 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 1/4-mile visibility criteria.
d. Turbine Engines - Ground cing.
(1) Explanation. 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 and/or less effective means of ice protection. Note that the requirement is effective in Amendment 10 and is applicable regardless of any desire to prohibit dispatch into known icing conditions.
(2) Procedure. 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 704 Par 532