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 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/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 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.
(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 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 sys tem 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:
(i) Mechanically driven superchargers for rotorcraft usually operate over a very narrow speed range, thus the heat of compression (∆ 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) ∆ 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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AC 27-1B, Chg 4 3/31/2014
(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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SUBPART E - POWERPLANT
EXHAUST SYSTEM AC 27.1121. § 27.1121 (Amendment 27-12) EXHAUST SYSTEM--GENERAL.
a. Explanation.
(1) This section addresses the arrangement of exhaust components and the protection against hazardous conditions which exist with hot exhaust gases.
(2) The objective is to allow for thermal expansion of manifolds and pipes, prevent local hot spots, and eliminate the possibility of igniting flammable fluids or vapors.
b. Procedures.
(1) Sufficient clearance of hot exhaust components must be maintained from structure, fuel cells, flammable fluid lines, and electrical components to compensate for thermal growth under normal and most extreme operating temperatures. Verify that adequate clearance exists between the exhaust system components and the surrounding structure, and that no interference occurs under the most adverse temperature excursions.
(2) Hot spots that can occur on fuselage or rotor blade skin as a result of impingement or in compartments due to an accumulation of hot gases should be eliminated with deflectors or by providing adequate flow-through ventilation.
Compliance may be shown by demonstration or analysis.
(3) It should not be possible to ingest sufficient quantities of exhaust gases which will produce engine surges, stalls, or flameouts during normal and emergency operation within the range of operating limitations of the aircraft and of the engine.
Analysis and/or flight testing may be required to demonstrate compliance. If flight testing is required, particular attention should be placed upon critical azimuths and wind conditions.
(4) Exhaust system surfaces hot enough to ignite flammable fluids or vapors must meet the isolation or shielding requirements of this section in addition to the requirements of §§ 27.1183 and 27.1185. Good design practice suggests that the isolation and shielding features incorporated would continue to be effective under the emergency landing conditions specified in § 27.561.
(5) It should be demonstrated that exhaust gases are discharged in such a manner that they do not cause distortion or glare which seriously affects pilot visibility at Page E - 113
night. One method of compliance would be a night flight evaluation at critical azimuths and variable wind conditions to verify that no degradation exists.
(6) Compliance with § 27.1121(f) can be accomplished by ensuring that the drain will discharge positively and is a minimum of 0.25 inches in diameter. No drain may discharge where it might cause a fire hazard. This can be demonstrated by discharging a colored liquid through the drain system in flight and on the ground. The dye should not impinge on any ignition source.
(7) Section 27.1121(g) is self-explanatory in specifying that a means must be provided to prevent blockage of the exhaust port after any internal heat exchanger failure. Compliance can be shown by demonstration or by analysis. In either case, it must be shown that any internal failure will not result in a significant power loss from the engine.
AC 27.1123. § 27.1123 (Amendment 27-11) EXHAUST PIPING.
a. Explanation. This section contains the following requirements that must be met for proper certification of exhaust piping on engines, auxiliary propulsion units (APU), and other similar devices.
(1) Section 27.1123(a) requires that the piping be heat and corrosion resistant so that it performs its intended function during its operational life (either the life of the rotorcraft or a specified limited life) without significant metal corrosion, metal erosion, or creation of hazardous hot spots. The piping system should be designed, have an installation design, or a combination that allows performance of its function without thermal expansion (thermal strain) induced structural failures such as ruptures caused by operating temperature excursions and overpressurization during its operational life.
(2) Section 27.1123(b) requires that the piping be supported to withstand the vibration and loading environment (including inertia loads) to which it will be subjected in service.
(3) Section 27.1123(c) requires that piping that connects to components between which relative motion exists in service must have the necessary flexibility and structural integrity to withstand the relative motion without exceeding limit load (at the maximum operating temperature) of the piping, or creating unintended loads (or load paths) on the components to which the piping connects.
b. Procedures. Exhaust piping is typically certified by analysis and installation tests conducted during the basic certification process, including flight tests, as follows:
(1) For compliance with § 27.1123(a), because of its durability in the hot exhaust environment, exhaust piping is typically made from stainless steel or alloy steel of the appropriate structurally and thermally derived wall thickness. Hot aircraft exhaust gases are very corrosive; thus, proper material selection and corrosion protective Page E - 114
design should be performed and validated during certification. Advisory Circular (AC) 43-4, “Corrosion Control for Aircraft” contains a detailed discussion of exhaust gas corrosion problems. Analysis and/or verification tests of the exhaust system should be conducted. This work is necessary to ensure thermal and structural integrity; to ensure that thermal expansion does not cause a structural overload or failure; and, to ensure that exhaust piping does not contact (or come close to) ambient temperature materials (such as structure or system components). Hot exhaust piping in contact with (or close to) ambient temperature materials can either create a fire hazard or cause an unintended strength reduction. To ensure that thermal expansion analyses and tests are properly conducted, the maximum in-service temperature excursion should be properly defined. The maximum temperature excursion should be based on the maximum temperatures of the piping and exhaust gases, as affected by the insulatory characteristics of the piping’s enclosure, and as affected by a worst case hot day. The worst case temperature environment used for analysis can be verified by a temperature survey. If run on cooler days, the survey can be adjusted for the worst case hot day environment using methods identical to those used for engine cooling tests (reference paragraph AC 27.1043, Cooling Tests). The piping should be designed to expand freely so that thermal expansion (thermal strain) induced loads on the piping and its restraint system are minimized. If thermal expansion induced loads (in conjunction with deflection induced loads and exhaust flow loads, discussed in b(4)) are significant relative to the limit load of any item in the load path, then a fatigue check on the critical design point(s) should be performed. The fatigue check should establish a safe life or an approved limited life for the critical component(s) in the system. An accurate analytical fatigue check on exhaust piping may be difficult to perform because of in-service erosion, corrosion, etc.; therefore, phased inspections should be considered to ensure the continued airworthiness of the exhaust piping.
(2) For compliance with § 27.1123(b), exhaust piping should be properly supported so that the maximum loads anticipated in-service are properly distributed and reacted, and as previously discussed, so that thermal expansion induced loading is minimized. Typically the worst case static design load conditions are either the inertia loads from an emergency impact (reference § 27.561) or the combined loading from thermal expansion, in-flight deflections and internal exhaust gas flow (see paragraph b(4)). It should be noted that several combinations of these loads should be examined to determine the critical combination. The piping should be supported and restrained such that critical frequencies are avoided and the induced vibration environment’s effect is minimized. Flight test vibration surveys may be necessary, in some cases, to properly define or validate the critical modes and environment and their effect on the exhaust piping design. Operating modes such as ground idle, flight idle, 40 percent and 80 percent of maximum continuous power, maximum continuous power, OEI power settings and other power settings should be investigated to determine their vibratory effect on the exhaust gas piping system. The strength reduction of the piping materials at operating temperature (and at worst case temperature) should be properly considered in the design and structural substantiation. MIL-HDBK-5D contains material allowables versus temperature data for a wide variety of metallic engineering materials.
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(3) For compliance with § 27.1123(c), the piping and its restraint system should be designed to minimize loading induced on the piping by the relative motion (in-service deflections) of the components to which the system attaches. Isolation of significant deflection induced loading (if required based on analysis and strain surveys) by use of flexible joints or other equivalent devices or designs should be considered. Any such in-line device used to reduce deflection loading should be fireproof and leak free when performing its intended function.
(4) For critical load case determination, the expansion induced thermal loading should be added in with mechanical relative motion induced loads and internal exhaust gas flow loads to provide total critical load for both a proper static and a proper fatigue structural substantiation. The critical combined static load should be compared with the emergency impact loads of § 29.561(paragraph b(2)) to determine the critical design load case for static strength substantiation.
(5) It should be noted that the majority of the exhaust piping verification testing required for certification can be accomplished during the rotor drive system tie down testing of § 27.923.
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SUBPART E - POWERPLANT
POWERPLANT CONTROLS AND ACCESSORIES AC 27.1141. § 27.1141 (Amendment 27-12) POWERPLANT CONTROLS:
GENERAL.
a. Explanation.
(1) Section 27.1141(a) references §§ 27.777 and 27.1555. The detailed compliance procedures for powerplant controls arrangement and markings are found in these sections.
(2) Each flexible powerplant control should be approved.
(3) In order to prevent power failure due to improper powerplant control valve positioning, § 27.1141(c) specifies acceptable open/closed positions for manual valves.
Power-assisted valves should have means to indicate to the flightcrew that the valve is either in the fully open or fully closed position or that the valve is moving between these two positions.
(4) For turbine installations, no single failure or malfunction, or probable combination thereof, of any powerplant control system should cause the failure of any powerplant function necessary for safety.
b. Procedures.
(1) Procedures for § 27.1141(a) are contained in detail in §§ 27.777 and
27.1555.
(2) Compliance with § 27.1141(b) may be accomplished by qualifying the control to Mil-C-7958, “Controls, Push-Pull, Flexible, and Rigid,” or other approved standards.
(3) Compliance with § 27.1141(c)(1) may be accomplished by installing manual valves which have positive stops in the full open and closed positions. The fuel valves, however, may have an arrangement to facilitate the capability of switching to different fuel tanks if suitable indexing is provided. Compliance with paragraph (c)(2) may be accomplished by installing a device which displays to the flightcrew one indication with valve fully open and another with the valve fully closed. Alternatively, an indication could be given when the valve is moving from fully open to fully closed with the indication ceasing when the valve position corresponds to the selected switch position (open or closed). An example would be a light that is off when the valve is fully open or closed and illuminates while the valve is transitioning.
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(4) Compliance with § 27.1141(d) can be accomplished by performing a failure mode and effects analysis (FMEA) to determine that no single failure or malfunction will cause failure of any powerplant control function necessary for safety. Included in this FMEA should be calculations showing the likelihood of any combination of failures of the powerplant control systems that would cause failure of any powerplant function necessary for safety is improbable. One acceptable procedure for documenting the analysis is contained in Society of Automotive Engineers (SAE) Fault/Failure Analysis Procedure ARP 926A, revised November 15, 1979.
AC 27.1141A. § 27.1141 (Amendment 27-23) POWERPLANT CONTROLS:
GENERAL.
a. Explanation. Amendment 27-23 changed § 27.1141(c) to extend its applicability to any powerplant valve regardless of the location of the valve control. The previous rule was only applicable for valves in the cockpit. Valves are excluded if their function is not required for safety.
b. Procedures. This rule change did not change the suggested methods of compliance.
AC 27.1143. § 27.1143 (Amendment 27-11) ENGINE CONTROLS.
a. Explanation. This regulation describes the arrangement and operation of the engine controls.
(1) Each throttle mechanism should be independent of the throttles for other engines.
(2) The arrangement of the independent throttles should allow simultaneous control of all engines with one hand.
(3) Immediate actuation at the engine control should be provided by any given input at the throttle control in the cockpit.
(4) If throttle controls incorporate a fuel shut-off feature, a means should be provided to prevent inadvertent movement to the shut-off position. This means should--
(i) Provide a positive lock or stop at the idle position. An idle detent (mechanical or electrical/mechanical such as solenoid) is an accepted arrangement.
(ii) Require a separate and distinct operation to place the control in the shut-off position. Separate action (switch or button) to displace the idle stop or distinct offsets in throttle motion to allow movement from the idle stop to shutoff are accepted arrangements.
b. Procedures. None.
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AC 27.1143A. § 27.1143 (Amendment 27-23) ENGINE CONTROLS.
a. Explanation. Amendment 27-23 revises § 27.1143 by replacing the terms “throttle control” and “thrust control” with the more general term “power control.” The changes should preclude misconceptions regarding engine control arrangements when governor-controlled turboshaft engines are employed in rotorcraft.
b. Procedures.
(1) Proper operation of the power control functions should be verified as part of the Type Inspection Authorization (TIA).
(2) Compliance with § 27.1143(d)(1) has been shown successfully in the past by using idle detentes (mechanical or electrical/mechanical, such as a solenoid).
(3) Compliance with § 27.1143(d)(2) has been achieved by using a switch or button to displace the idle stop. Distinct offsets in throttle motion to allow movement from the idle stop to shutoff have also been used to show compliance.
AC 27.1143B. § 27.1143 (Amendment 27-29) ENGINE CONTROLS.
a. Explanation. Amendment 27-29 introduced the option of using 30-second/2-minute OEI power ratings to multiengine rotorcraft. This amendment revises § 27.1143 by adding the requirement for automatic control of 30-second OEI limits in the new § 27.1143(e). Automatic control of the 30-second OEI limits are required to prevent exceeding the remaining power sections OEI limits after the precautionary shutdown of one engine. The use of 30- second OEI power must be limited to emergency use only during flight conditions where one engine has failed or has been shutdown for precautionary reasons. During this critical stage of flight, crew attention should not be focused on powerplant instruments to avoid exceeding the limit.
b. Procedures. The automatic controls used to prevent 30-second OEI limit exceedances can be installed on the airframe or the engine. The applicant should demonstrate that 30-second OEI limits that can affect the continued operation of the drive system or engine such as gas generator speed, power turbine speed, measured gas temperature, torque, etc., cannot be exceeded. It should also be shown that these devices do not restrict the ability to achieve the full 30-second OEI limits. The operation of these limit devices can be demonstrated on the aircraft or if possible by using bench tests.
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AC 27.1145. § 27.1145 (Amendment 27-12) IGNITION SWITCHES.
a. Explanation.
(1) This section addresses the arrangement and protection of ignition switches for reciprocating engines or for turbine engines which require continuous ignition.
(2) The objective is to provide a means to quickly shut off all ignition, if required, while at the same time providing protection against inadvertent ignition switch operation.
(3) Section 27.1145(a) does not specifically state that turbine engines which do not require continuous ignition are excluded from the rule, but no benefit is realized by the capability of shutting off all ignition to these engines.
b. Procedures.
(1) Section 27.1145(a) is self-explanatory in specifying that a means be available to quickly shut off all ignition by the grouping of switches or by a master ignition switch control. A “T” arrangement or split rocker switches are possible configurations. A master ignition control, if utilized, would need to be carefully evaluated if rotorcraft performance credit is given for engine isolation.
(2) Each group of ignition switches and the master ignition control should have a means to prevent inadvertent operation. “Guarded” switches are the usual means of showing compliance.
AC 27.1147. § 27.1147 MIXTURE CONTROLS .
a. Explanation. This section addresses the arrangement of fuel mixture controls for reciprocating engine installations and applies only if mixture controls are installed.
Note that this control, as used in rotorcraft, is an engine shutdown device. Adjustment of the fuel mixture in flight is not allowed to demonstrate Part 27 compliance, but may be acceptable for more efficient engine operation if suitable stops or automatic means are provided to prevent inadvertent engine shutdown with mixture movement or engine malfunction with flight condition changes.
b. Procedures.
(1) The arrangement should allow--
(i) Separate control of each engine; and
(ii) Simultaneous control of all engines.
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