(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 tunction.
(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-- 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 OEl 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-- Separate control of each engine; and
(ii) Simultaneous control of all engines.
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(2) Compliance may be accomplished by a side-by-side arrangement of the controls to allow either separate or simultaneous control.
AC 27.1151. § 27.1151 (Amendment 27-33) ROTOR BRAKE CONTROLS.
a. Explanation.
(1) Amendment 27-33 added a new § 27.1151 that establishes requirements for rotor brake controls. Paragraph a is intended to require design features which, for all practicable purposes, prevent inadvertent brake application in flight even under conditions of reasonably expected crew error or confusion.
(2) Paragraph b requires warning devices to alert the crew if the brake has not been completely released.
b. Background. Inadvertent or undetected application of the rotor brake may be expected to result in excessive heat and fire in the rotor brake area. Rotor brake components are usually located integral with, or in close proximity to, rotor drive system components and, in some cases, close to critical hydraulic main rotor control system components. Fires in these areas would be extremely hazardous.
c. Method of Compliance.
(1) For paragraph (a), literal compliance can be achieved by lock-out devices sensitive to the higher RPM range of the main rotor or other flight parameters, hydraulic bypass or lock-out devices controlled by flyweight governor systems, or engines control position, etc.
The guard required by FAR 27.921 does not, in itself, provide compliance with this requirement. However, if careful evaluation of the overall control, including location, guard mechanism, control manipulation requirements, accessibility, etc., provides a high degree of assurance that inadvertent application will not occur, compliance may be assumed. Also, if brake application does occur, annunciation appears and no immediate hazard to flight operation exists, compliance may be assumed.
(2) Alerting devices supplied to comply with this rule should provide a signal at any time the rotor brake is engaged, including partial engagement. This means to alert the crew could be:
- A warning light indicating the mechanical control position, or the position of the brake for a power assisted system, or
- An unambiguous device warning the crew that the rotor brake is engaged (or partially engaged), or Page E - 121
- A locking device preventing the engine starting when the rotor brake control is not completely released.
AC 27.1163. § 27.1163 (Amendment 27-23) POWERPLANT ACCESSORIES.
a. Explanation.
(1) This section addresses the interface requirements for powerplant accessories which are mounted on the engine or rotor drive system components.
(2) Areas which should be addressed include structural loads imposed upon the engine case and isolation between the accessory and engine oil systems. Electrical equipment isolation from flammable fluids or vapors should be addressed as well as the effect of an accessory failure on the continued operation of the engine and drive system components.
b. Procedures.
(1) Accessories installed and certified by the engine manufacturer can be mounted on the engine without additional justification.
(2) Any accessory to be mounted on the engine, which was not certificated with the engine, and does not meet the engine installation design manual requirements should have a structural analysis showing the mounting of that accessory on the engine will not induce loads into the engine case which are higher than the original design loads.
(3) When the accessory is mounted and operating on the engine, it should not be possible to contaminate either the engine or accessory oil systems. This contamination can take the form of debris following a failure, airborne dirt or water, or any other substance that would impair proper operation of the engine or accessory.
Compliance with these requirements can be accomplished by a combination of test and analysis. The design interface should be such that when the equipment is operating, there are no high/low pressure differentials between the components which would induce fluid transfer between components resulting in a low fluid level in one component and an overfill condition in the other component. Where this potential exists, an analysis and/or test should be used to demonstrate compliance.
(4) Engine mounted accessories which are subject to arcing and sparking, must be isolated from all flammable fluids or vapors to minimize the probability of fire. This can be accomplished by isolating the electrical equipment from the flammable fumes or vapors or by isolating the flammable fumes or vapors from the potential ignition source.
Compliance can be shown by analysis.
(5) A failure mode and effect analysis should be submitted which shows that a failure of any engine mounted and driven accessory will not interfere with the continued Page E - 122
operation of the engine. If a hazard is created by the continued rotation of an engine driven accessory after a failure or malfunction, provisions to stop its rotation or eliminate the hazard must be provided. The effectiveness of this device should be demonstrated
(6) The main transmission and rotor drive system should be protected from excessive torque loads and damage imposed upon them by accessory drives. One method which has been used is a torque limiting device; (i.e., shear section of main rotor driveshaft). The effectiveness of any protection device should be demonstrated by Page E - 123
SUBPART E - POWERPLANT
POWERPLANT FIRE PROTECTION AC 27.1183. § 27.1183 (Amendment 27-20) LINES, FITTINGS, AND COMPONENTS.
a. Explanation. This section requires that any line, fitting or other component of a flammable fluid, fuel or flammable gas system which carries, conveys, or contains the fluid or gas in any area subject to engine fire conditions (i.e., a severe fire) must be at least fire resistant (reference § 1.1 for definition of fire resistant and see paragraph AC 27.859 which defines a severe fire). An exception is for flammable fluid tanks and supports which are part of and attached to the engine or are in a designated fire zone. These items are required to either be fireproof (see § 1.1 for definition of fireproof and see paragraph AC 27.859 which defines a severe fire) or to be enclosed by a fireproof shield, unless fire damage to any non-fireproof part (e.g., secondary line or valve support) will not cause leakage of a flammable gas, flammable fluid or otherwise prevent continued safe flight and landing of the rotorcraft. All such components must be shielded, located, otherwise protected or a combination to safeguard against the ignition of leaking flammable fluids or gases. Integral oil sumps of less than 25 quarts capacity on a reciprocating engine need not be fireproof or enclosed by a fireproof shield; however, they should be fire resistant. Most integral sumps in this category are, by natural design and material selection, fire resistant.
Exemptions to the preceding requirements are as follows:
(1) Lines, fittings and components already approved under Part 33 as part of the engine itself.
(2) Vent and drain lines (and their fittings) whose failure will not result in or add to an operational fire hazard. In addition, all flammable fluid drains and vents must discharge clear of the induction system air inlet and other obvious ignition hazards.
b. Procedures. A detailed review of the design should be conducted to identify and quantify all lines, fittings, and other components which carry flammable fluids and/or gases and are in areas subject to engine fire conditions such as engine compartments and other fire zones. Once these items are identified the design means of fire protection should be selected and validated, as necessary, during certification. For materials and devices that cannot be qualified as fireproof or fire resistant by similarity or by known material standards, testing to severe fire conditions (see paragraph AC 27.859 definition, AC 20-135, and AC 23-2 for detailed requirements) should be conducted on full-scale specimens or representative samples to establish their fireproof or fire resistance capabilities. Exceptions to these standards (as provided in the regulatory section) should be reviewed and approved/disapproved on a case-by-case basis during certification. Also, operational fire hazards from drains, vents, and other similar sources should be identified and eliminated during certification.
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AC 27.1185. § 27.1185 (Amendment 27-11) FLAMMABLE FLUIDS.
a. Explanation. This section requires that fuel, flammable fluid, or vapor tanks, reservoirs or collectors be sufficiently isolated from engines, engine compartments, and other designated fire zones so that hazardous heat transfer from these areas to fuel, flammable fluid, and vapor tanks, reservoirs, or collectors is prevented in either normal or emergency service.
b. Definitions.
(1) Fuel or Flammable Fluid Collector. Any device such as a large valve, accumulator, or pump that contains a significant amount of flammable fluid, fuel, or vapor (e.g., the volume equal to 10 ounces or more of fluid).
(2) Flammable Fluid or Vapor Tank. Any fuel, flammable, fluid, or vapor tank, reservoir, or collector.
(3) Sufficiently Isolated. Fuel, flammable fluids, or vapors in a tank, reservoir, or collector are insulated, removed, otherwise protected or a combination such that their worst case temperatures (the worst case measured or calculated surface temperature of their containers) in either normal or emergency service is always 50° For more away from the autoignition temperature of the fuel, flammable fluid, or vapor in question.
(4) Minimum Autoignition Temperature. The temperature at a given vapor pressure at or above which liquid fuel or fuel vapor will self combust. When determining the minimum design value of autoignition temperature which will occur in either normal or emergency operations, the critical, in-service combination of vapor pressure and fuel temperature should be determined.
(5) Hazardous Heat Transfer. A total incident heat flux (a combination of conduction, convection, and radiation, as applicable) from or in an engine compartment or other designated fire zone, which would raise the temperature level of a flammable fluid or fuel, their vapors, or the surface temperature of their containers to within 50° F or less of the minimum in-service autoignition temperature. Typically, the most critical heat transfer case to be considered is emergency service where a severe fire (see definition) is assumed to occur in each engine compartment and each designated fire zone on a case-by-case basis.
(6) Severe Fire. See definition in paragraph AC 27.859.
c. Procedures.
(1) The fuel, flammable fluid, and vapor system designs should be reviewed early in certification to insure that all flammable fluid or vapor tanks are properly identified and isolated from engines, engine compartments, and other designated fire zones during both normal and emergency operations such as in-flight engine Page E - 125
AC 27-1B, Chg 4 3/31/2014 compartment or other fire zone fires. In some cases fuel or flammable fluid components must be located in an engine compartment or other designated fire zone. In these cases, an equivalent safety finding (which considers the design, construction, materials, fuel lines, fittings, and controls used in the system, or system segment, contained in the engine compartment or other designated fire zone) should be undertaken as a part of the normal certification process. If the level of safety provided is equivalent to that provided by removing the system or system segment out of the engine compartment or designated fire zone, then the design should be accepted. For fuel tanks only, isolation is required by regulation to be achieved by use of either a firewall (reference paragraph AC 27.1191 for Firewall Requirements) or by use of a shroud. A shroud if used should be fireproof (see § 1.1 for definition and the definition of a Severe Fire for further details) and should be drainable (or otherwise inspectable) to insure the fuel tank is not leaking in service. For other flammable fluid or vapor tanks, the regulations allow either the identical treatment previously described for fuel tanks (i.e., firewalls or shrouds) or, alternatively, use of an equivalent safety finding. The equivalent safety finding, if used, can be made as part of the standard certification process. Regulations require that the equivalent safety finding be based on system design, tank materials, tank supports, and flammable fluid system connectors, lines, and controls. In all cases the flammable fluids, fuels, and vapors should be sufficiently isolated from hazardous heat fluxes during both normal and emergency operations to prevent autoignition.
(2) In addition, the regulations require at least one-half inch of clear airspace between each flammable fluid or vapor tank and each firewall or shroud that isolates the system, unless equivalent means (such as fireproof insulation) are used to prevent hazardous heat transfer from each engine compartment or other fire zone to the flammable fluid or vapor mass (or its container surface) at the fluid or vapor's minimum autoignition temperature. If in-service structural deflections are significant, they must be taken into account when certifying the one-half inch minimum clear airspace requirement. For example, if a one-half inch clearance exists on the ground but in some normal and emergency flight conditions (e.g., autorotation) the one-half inch is reduced to one-fourth inch at a critical time (in-flight engine fire), then the design (static) configuration should have at least a one-half plus one-fourth equals three-fourths inch static clear airspace to insure the regulation's intent is met. Alternatively, fireproof insulation or additional stiffeners could be used to insure the regulation's intent is met
(i.e., the thermal equivalent of one-half inch clearance is maintained at all times). Any material used as insulation on or used adjacent to a flammable fluid or vapor tank, should be certified as chemically compatible with the flammable fluid or vapor and to be non-absorbent in case of fuel or vapor leaks. Otherwise, the material should either be treated for compatibility and non-absorbency or not accepted.
AC 27.1187. § 27.1187 VENTILATION AND DRAINAGE.
a. Explanation. To ensure that any component malfunction which results in fuel, flammable fluid or vapor leaks is safely drained or vented overboard and to ensure that Page E - 126
a fire hazard is not created during either normal or emergency service, there should be complete, rapid drainage and ventilation capability present for each part of the rotorcraft powerplant installation and any other designated fire zone which utilizes flammable fluid or vapor carrying components. As a minimum, the routing, drainage, and ventilation system should accomplish the following:
(1) It should be effective under normal and emergency operating conditions.
(2) It should be designed and arranged so that no discharged fluid or vapor will create a fire hazard under normal and emergency operating conditions.
(3) It should prevent accumulation of hazardous fluids and vapors in engine compartments and other designated fire zones.
b. Definitions. Drip Fence. A physical barrier that interrupts the flow of a liquid on the underside of a surface, such as a fuel tank, and allows any leaked liquid to drip from the surface away from hazardous locations to a safe external drain.
c. Procedures. The design of flammable fluid and gas systems running through engine compartments and other designated fire zones should have a thorough hazard analysis performed early during certification that is updated periodically as design changes dictate. The hazard analysis should identify and quantify all normal and emergency service failures that could result in leakage of fuel, flammable fluids and vapors. Once these potential hazards are identified and quantified, appropriate design features, such as drains, drip fences and vents, that minimize or eliminate the hazard should be provided. These means should be analyzed, tested, or a combination as necessary, to ensure that their size, flow capacity, and other design parameters are adequate to rapidly remove hazardous fluids and vapors safely away from the rotorcraft under normal and emergency flight conditions. Typically a venting or draining system should be designed to a 3-to-1 flow capacity margin over the probable worst case leak to which it could be subjected. Adverse effects such as clogging and surface tension flow reduction should be accounted for in design. Testing, including flight testing, using inert fluids or vapors may be necessary for proper design certification. In some instances it may be appropriate to include ventilation and drainage tests when the aircraft is parked.
AC 27.1189. § 27.1189 (Amendment 27-23) SHUTOFF MEANS.
a. Explanation.
(1) This section establishes the requirements for controlling hazardous quantities of flammable fluids which flow into, within, or through designated fire zones.
(2) When any shutoff valve is operated, any equipment, including a remaining engine, which is essential for continued flight, cannot be affected.
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