AC 27-1B, Chg 4 3/31/2014 interpolation of data from other test conditions can be achieved using the methodology of the current version of AC 23-8, Flight Test Guide for Certification of part 23 Airplanes.
(i) 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 assure 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.
(i) This rule requires turbine engines and turbine-engine inlets to perform satisfactorily in atmospheric icing conditions defined in Appendix C of part 29. On an equivalent safety basis, the limited icing envelopes described in section 29.1419 of AC 29-2C 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 29, 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.
(iI) 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.
(A) It is presumed that there will be a flight manual limitation against flight into icing conditions, 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 Page E - 102
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, § 27.1419, is requested. This direct penetration and exit approach is inappropriate for aircraft for which full icing clearance is requested (reference § 27.1419).
(ili) 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 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 AC 29-2C 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, evaluation 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 Page E - 103
AC 27-1B, Chg 4 3/31/2014 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 assure safe autorotation landings or engine relight capability may be needed to identify this condition.
(2) Procedures.
Review section 29.1419 of AC 29-2C, ADS-4, Report No. FAA-RD-77-76, and the current version of AC 20-73. 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.
(il) 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 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 Page E - 104
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.
(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 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 29.1419 of AC 29-2C). 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 reduction of the acceptable 30-minute exposure Page E - 105
AC 27-1B, Chg 4 3/31/2014 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 peen considered tor rotorcratt not certitied 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 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/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.
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 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.
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(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.
(i) 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.
(ii) Since the regulation provides for certificationiwithin 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 and its inlet system will operate satistactorily 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:
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AC 27-1B, Chg 4 3/31/2014
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.
INADVERTENT FALLING & 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.
(ilI) 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.
(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) Visibility. The test visibility defined, ¼-mile (Full Falling & Blowing Snow Conditions) visibility or less as limited by snow, represents a heavy snowstorm Page E - 108
and is the maximum likely to be encountered in service. Rotorcraft, which have beer certified to the ¼-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 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 GE 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.
(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, therefore, is not appropriate.
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AC 27-1B, Chg 4 3/31/2014
(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.
(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 damage.
(il) 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.
(lI) 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 VER certified are later IFR certified with a resulting possible increase in airspeed in snow conditions. This factor should be considered if IF 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 Page E - 110
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 ¼-mile or less visibility prescribed 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/m 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 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 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.
(vill) 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 ¼-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 IGE 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 ¼-mile visibility criteria.
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AC 27-1B, Chg 4 3/31/2014
d. Turbine Engines - Ground Icing.
(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 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 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 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 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.
e. § 27.1093(c) Supercharged Reciprocating Engines.
(1) Explanation. This rule authorizes the designer to take credit for the heat-of-compression available downstream of an engine air inlet supercharger to meet the induction system heat rise requirements of § 27.1093(a)(3) or (a)(4), provided the heat rise is automatically available for the applicable altitude and operating condition.
(2) Procedures. Since a wide variety of superchargers and supercharger controls (waste-gate controls) have been devised, it is impracticable to outline specific instructions for determining compliance. However, the certification engineer can properly evaluate the arrangement by analyzing the system for trends (in heat rise available) and conducting measurements to verify these trends and quantify the actual values of heat rise available. Some factors to be considered are:
(1) Mechanically driven superchargers for rotorcraft usually operate over a very narrow speed range, thus the heat of compression (4 temperature) may remain constant over the altitude range. Conversely, turbosuperchargers usually are controlled (via waste-gate position modulation) to gradually increase the compression with increase in altitude, thus the critical (or lowest) 4 temperature may be available at very low altitudes.
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(ii) Other waste-gate controllers sense carburetor deck pressure and respond by modulating the waste gate to maintain a constant carburetor deck pressure within the capabilities of the turbo unit. Heat of compression may then vary with altitude and engine power in a complicated fashion such as to require experimental temperature measurements across a wide range of operating conditions to determine compliance.
(iii) Turbosuperchargers which are not controlled (by waste-gate modulation) but respond to an orificed exhaust generally will (at constant power) produce more heat rise at altitude than at sea level; however, size matching between engine and turbo unit may affect this. Instrumented flight tests should be used as a final compliance verification method.
AC 27.1093A. § 27.1093 (Amendment 27-23) INDUCTION SYSTEM ICING PROTECTION.
a. Explanation. Amendment 27-23 clarifies that the phrase, "within the limitations established for the rotorcraft" applies only to the requirement in § 27.1093(b)(1)(ii) for demonstrating flight in falling and blowing snow.
Procedures. 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 27.1093 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 § 27.1093(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 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.
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