Explanation. This rule requires additional fireproofing of the inlet duct of auxi liary
(1)
power units (APU’s) to ensure safe disposal or containment of hot gas reverse flow from the APU from entering any other compartment of the rotorcraft in which a hazard would be created. This rule could, in some designs, require fireproof construction of the inlet duct for the APU to extend upstream beyond the confines of the firewall provided in compliance with § 29.1191(b). The extent of the fireproofing is subjective and may require malfunction testing if no applicable information can be provided by the manufacturer of the APU. For ducting upstream of the fireproof section, materials selected need not be qualified for fire impingement; however, they must be shown to be suitable for the maximum normal heat conditions to be expected.
Procedures. Normally, fireproof ducting upstream of the APU to the contour of the roto
(2) r craft is acceptable for compliance. However, if this distance is less than 36 inches, the possibility of impingement of hot gases on the contour skin of the rotorcraft is required. Fireproofing of contour skin or duct relocation should be considered if the impingement area is a nonmetallic structure or is part of or close to fuel tanks. Other system air inlets in the impingement area should also be evaluated for possible hazards due to ingestion of hot gases in event of reverse flow from the APU.
f. § 29.1103(f):
(1) Explanation. APU inlet ducts subject to reverse flow of hot gases should be constructed of materials that will not absorb fuel or other flammable liquids to avoid induction duct inlet fires which may ignite by backfires or reverse flow from an APU.
(2) Procedures. Any nonmetallic duct material should be shown by test or by previous qualification to be sealed or otherwise free of tendencies to absorb flammable liquids. Tests, if necessary, should follow the guidelines for absorption qualification set forth in TSO’s or military specifications for fuel and oil tanks.
AC 29.1105. § 29.1105 INDUCTION SYSTEM SCREENS.
a. Explanation. This paragraph concerns reciprocating engine installations. If induction system screens are used, the following considerations apply.
(1) Each screen must be upstream of the carburetor.
(2) No screen may be in any part of the induction system that is the only passage through which air can reach the engine unless it can be deiced by heated air.
(3) No screen may be deiced by alcohol alone, and it must be impossible for fuel to strike any screen.
b. . Inlet screens in the engine induction system are generally provided t Procedures o prevent the entrance of foreign objects. The induction design may Page E - 164
incorporate features which address the concerns identified above. Also, some designs incorporate an alternate air door which, with appropriate consideration, accounts for the requirements of this paragraph. The alternate air source should provide the required air to maintain flight and landing to a suitable landing site at appropriate airspeeds and gross weights.
AC 29.1107. § 29.1107 INTERCOOLERS AND AFTER-COOLERS.
a. Explanation Procedures
a. Explanation Procedures . Each intercooler and after -cooler must be able to withstand the vibration, inertia, and air pressure loads to which it would be subjected in operation.
b. . In complying with this regulation, the various vibrations, inertia, and air pressure loads should be identified. The installation may be verified by either analysis or test appropriate for the design.
AC 29.1109. § 29.1109 CARBURETOR AIR COOLING.
. It must be shown under § 29.1043 that each installation using two-stage superchargers has means to maintain the air temperature at the carburetor inlet, at or below the maximum established value.
b. . When the powerplant installation design utilizes a supercharger installation, it should be shown by testing that the air temperature at the carburetor inlet does not exceed established values.
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SUBPART E - POWERPLANT
EXHAUST SYSTEM AC 29.1121. § 29.1121 (Amendment 29-13) 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 for powerplant and auxiliary power unit installations.
(2) The objective is to ensure safe disposal of exhaust gases without fire hazard or physical impairment to any occupant.
b. Procedures.
(1) During the certification process, carbon monoxide levels should be monitored in the personnel compartments to verify that the gas levels are well within the acceptable range. The conditions under which the measurements are taken should be representative of the normal operating limitations of the rotorcraft. This paragraph is not applicable to gas turbine-engine-powered rotorcraft.
(2) 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 §§ 29.1183 and 29.1185. Good design practice suggests that the isolation and shielding features incorporated would continue to be effective under the emergency landing conditions specified in § 29.561.
(3) Compliance with the § 29.1121(c) fireproof requirements can be accomplished by demonstrating that the material or component will withstand a 2000° F ± 50° F flame for 15 minutes while still fulfilling its design purpose. This testing should accurately simulate, as near as practicable, the operating environment of the material or component in service. In addition to the fireproof requirements, the requirements of § 29.1191 must be met.
(4) Compliance with § 29.1121(d) can be accomplished by locating the vents and drains where fumes and fluids cannot interact with the hot exhaust gases. Drains should discharge positively and be a minimum of 0.25 inches in diameter. No drain may discharge where it will 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.
(5) It should be demonstrated that exhaust gases are discharged in such a manner that they do not cause distortion or glare seriously affecting the pilot’s visibility Page E - 166
at night. One method of compliance would be a night flight evaluation at critical azimuth and variable wind conditions to verify that no degradation exists.
(6) Hot spots that can occur on exhaust system components should be eliminated by providing deflectors and/or adequate ventilation. Exhaust shrouds can either be ventilated or insulated to keep the temperatures low enough so that ignition of flammable vapors or fluids cannot occur under normal operation or under the emergency landing conditions specified in § 29.561.
(7) Compliance with § 29.1121(h) can be accomplished by ensuring that the drain will not 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.
AC 29.1123. § 29.1123 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) § 29.1123(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 by overpressurization during its operational life.
(2) § 29.1123(b) requires that the piping must be supported to withstand the vibration and loading environment (including inertia loads) to which it will be subjected in service.
(3) § 29.1123(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 § 29.1123(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 design should be performed and validated during certification. Advisory Circular Page E - 167
(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 temperature 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 29.1043, 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 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 erosion, corrosion, etc., in service; therefore, phased inspections should be considered to ensure the exhaust piping’s continued airworthiness.
(2) For compliance with § 29.1123(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 § 29.561) 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 § 29.1123(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 loads 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 § 29.923.
AC 29.1125. § 29.1125 (Amendment 29-12) EXHAUST HEAT EXCHANGERS.
a. Explanation. This section applies only to rotorcraft powered by reciprocating engine(s) or equipped with reciprocating auxiliary propulsion units (APU). This regulation states the certification requirements for exhaust heat exchangers (EHE’s) which are summarized as follows:
(1) § 29.1125(a) requires that each EHE be constructed and installed to withstand vibration, inertia and other operational loads.
(2) § 29.1125(a)(1) requires that each EHE be able to operate continuously at the highest anticipation service temperature.
(3) § 29.1125(a)(1) requires that each EHE be corrosion resistant to exhaust gases and other corrosion sources.
(4) § 29.1125(a)(2) requires that each EHE have provisions for inspecting its critical parts and areas.
(5) § 29.1125(a)(3) requires that each EHE have cooling provisions where it is subjected to hot exhaust gases.
(6) § 29.1125(a)(4) requires that each EHE muff design eliminate stagnation areas or liquid traps that would contribute to ignition of leaked flammable fluids.
(7) § 29.1125(b) requires that each EHE used to heat ventilating air for occupants-- Page E - 169
(i) Either have a secondary heat exchanger between the primary EHE and the ventilating air system; or
(ii) Have other equivalent means to prevent harmful contamination of ventilating air.
b. Procedures. EHE’s and their installations are typically certified by analysis and installation tests conducted during the basic certification process, including flight tests or simulated flight tests, as follows:
(1) Because of their durability in the hot exhaust environment, EHE’s are usually constructed from stainless steel or alloy steel of the appropriate structurally and thermally derived wall thickness. The EHE and its system should be designed to expand freely to minimize thermal expansion (thermal strain) induced loads on the EHE and its restraint system. If thermal expansion induced loads (in conjunction with deflection induced loads and exhaust flow loads) are significant relative to the limit load of the EHE or its attachments, a fatigue check on 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 EHE system.
(2) EHE’s should be properly supported so that the maximum loads anticipated in service are properly distributed and reacted and so that thermal-expansion-induced loading is minimized. Typically, the worst -case static design load conditions are either the emergency impact loads acting alone (reference § 29.561), or the critical combination of loads from thermal expansion, in-flight deflections and internal exhaust gas flow. Several combinations of these loads should be examined to determine the critical combination. The EHE should be supported and restrained so that critical frequencies are avoided and the induced vibration environment is minimized. Flight tests or bench tests, such as vibration surveys conducted during rotor system endurance testing, may be necessary in some cases, to properly define or validate the vibration environment and EHE’s critical modes and their effect on EHE 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 critical power settings should be investigated to determine their vibratory effect on the EHE system. The strength reduction of EHE materials at operating temperature and at critical temperatures should be properly considered in EHE design and structural substantiation (MIL-HDBK-5D contains material allowables versus temperature data for a wide variety of metallic engineering materials). The EHE and its restraint system should be designed to minimize loads induced by the relative motion (in-service deflections) of the components to which the EHE attaches. Isolation of significant-deflection-induced loading (as required, based on analysis and strain surveys) by use of flexible joints, other equivalent flexible devices, or designs should be considered. Any such in- line device used to reduce deflection loading should meet applicable certification requirements and be leak-free.
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(3) Expansion analysis and verification tests of the EHE should be conducted to ensure its thermal (and structural) integrity and to ensure that thermal expansion does not cause the EHE to contact (or come close to) ambient temperature aircraft materials, structure or system components and either create a fire hazard or an unintended reduction in strength. To ensure that 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 EHE and exhaust gases, as affected by the insulatory characteristics of the EHE’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 which, when run on cooler days, can be adjusted to the worst-case hot day environment using methods identical to those used for engine cooling tests (reference paragraph AC 29.1043, Cooling Tests).
(4) Hot aircraft exhaust gases are very corrosive; thus, proper material selection and corrosion protection design should be performed and validated during certification. Advisory C ircular (AC) 43-4, “Corrosion Control For Aircraft” contains a detailed discussion of exhaust gas corrosion problems. The in- service corrosive environment should be identified and characterized as thoroughly as possible by chemical analysis, tests and service experience. Once defined, appropriate design techniques and materials should be selected. Certification tests may be required to ensure proper substantiation. Phased inspections and inspectability should be considered (reference (4)).
(5) The EHE’s design should be reviewed for inspectability to ensure that structural and thermal integrity is maintained over the intended life of the EHE. Also, if the design review is not conclusive relative to inspectability, a tear down inspection should be conducted.
(6) Each EHE design should be reviewed, analyzed, and tested to ensure that cooling provisions are adequate where EHE surfaces are subjected to hot exhaust gases. This is necessary to prevent hazardous hot spots or a burn through which may cause a fire and contaminate the occupied environment.
(7) Each EHE design should be reviewed, analyzed, and tested to ensure that stagnation areas and liquid traps do not exist. This can be done using bench flow tests.
These stagnant areas and traps could become ignition sources if wetted with a leaking flammable fluid. A review of potential leaking flammable fluid hazards should be conducted and appropriate preventative measures such as drains and drip fences installed to ensure they are routed away from EHE’s.
(8) Each EHE design which will be used to heat ventilating air for occupants should be reviewed to ensure that the EHE is a double walled system, (i.e., it would require failure of two EHE surfaces to allow toxic exhaust gases to intermix with cabin ventilating air). Each EHE wall should be designed with equal thermal and structural resistance since a single undetected inner wall failure would subject the outer wall to the Page E - 171
primary heat load. Also, inspectability provisions should be provided or means identified to ensure that inner wall failures can be detected in service. Any equivalent means which is applied for must clearly provide an equivalent level of safety to a double walled EHE.
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SUBPART E - POWERPLANT
POWERPLANT CONTROLS AND ACCESSORIES AC 29.1141. §. 29.1141 (Amendment 29-13) POWERPLANT CONTROLS:
GENERAL.
a. Explanation.
(1) Section 29.1141(a) References §§ 29.777 and 29.1555. The detailed compliance procedures for powerplant control arrangement and markings are found in these sections.
(2) Section 29.1141(b) requires that controls be located and/or shielded such that normal movement of cockpit personnel will not cause inadvertent control movements.
(3) Section 29.1141(c) requires that each flexible control (push-pull cables) be properly approved.
(4) Section 29.1141(d) requires that each control maintain its set position without movement from an inadvertent source such as vibration or control system loads.
This is required so that constant flightcrew attention is not necessary.
(5) Section 29.1141(e) requires that each control be able to withstand operating loads without excessive deflection. Excessive deflection is interpreted to be that deflection that would cause erratic movement, lack of crispness, or premature failure.
(6) Section 29.1141(f) specifies acceptable open/close positions for manual valves to prevent power failure due to improper control valve positioning.
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.
(7) The control system is subject to evaluation under § 29.901(c); i.e., 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. One acceptable way to determine this is by use of a failure modes and effects analysis (FMEA).
b. Procedures.
(1) For compliance with § 29.1141(a), review the procedures for paragraph AC 29.1555. Evaluation by the flight test pilot during the official flight test program is appropriate.
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(2) Compliance with § 29.1141(b) is normally evaluated during the flight test program and documented in the flight test report.
(3) Compliance with § 29.1141(c) may be accomplished by qualifying the control to MIL-C-7958, “Controls, Push-Pull, Flexible, and Rigid,” or other approved standards or by previous approval in a similar function, installation, or arrangement.
(4) Compliance with § 29.1141(d) may be shown during the flight test program by monitoring the means to prevent control creep. This device or arrangement should be effective without crew attention and should not impose undue control displacement loads or interfere with accurate settings.
(5) Compliance with § 29.1141(e) may be shown by an appropriate structural analysis and/or a witnessed static load test using the factors specified under § 29.397 unless a lower value can be shown to be applicable. Operation tests and design details described in §§ 29.683 and 29.685 should also be considered.
(6) Compliance with § 29.1141(f)(1) may be accomplished by installing manual valves which have positive stops in the fully 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 § 29.1141(f)(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 fully closed and illuminates while the valve is transitioning.
AC 29.1142. § 29.1142 (Amendment 29.17) AUXILIARY POWER UNIT CONTROLS.
a. Explanation.
(1) This section addresses control requirements for any APU installed in a rotorcraft.
(2) The requirement for starting, stopping, and emergency shutdown of the APU from the flight deck is primarily to control APU operation in the event of improper operation or malfunction which could affect the safety of the aircraft.
b. Procedure.
(1) The requirements of this section apply to all APU installations in rotorcraft without regard to whether or not the APU is to be operated on the ground only, or operated in flight and on the ground.
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(2) The APU installation must provide sufficient controls to the flight crew to enable them to control the operation of the APU under normal and emergency conditions.
(3) Compliance can be shown by both demonstration and a failure analysis.
AC 29.1143. § 29.1143 (Amendment 29-12) ENGINE CONTROLS.
a. Explanation - Procedures.
. This section prescribes safety standards applicable to arrangement and operation of the engine controls.
(1) Section 29.1143(a) requires a separate throttle for each engine.
(2) Section 29.1143(b) requires a throttle arrangement for control of all engines be achieved by:
(i) Separate control of each engine.
(ii) Simultaneous control of all engines.
(3) Section 29.1143(c) requires that immediate actuation at the engine control should be provided by any given input at the cockpit throttle control.
(4) Section 29.1143(d) requires that each fluid injection system control (e.g., water-alcohol) other than the fuel system control must reside in the throttle controls.
This does not preclude the injection system pump from having a control located separately from the throttle.
(5) Section 29.1143(e) requires that power or thrust controls (that have fuel shut-off features) provide a means to prevent inadvertent movement to the shut-off position. This means should-
(i) Provide a positive lock or stop at the idle position; and
(ii) Require a separate and distinct operation to place the control in the shut-off position.
b.
(1) Certification data submitted by the applicant should be reviewed to ensure that all the design features stated in § 29.1143 exist.
(2) Proper engine control functioning (to verify the design features of § 29.1143) should be verified as part of the type inspection authorization (TIA) for the certification project.
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