tearout and leakage. The number of drains should be minimized by design techniques such as those that avoid low points in the lines. Drain lines should be made of ductile materials or otherwise designed to provide impact tolerance. Drain line connections, fittings, and other components should be designed to meet the fatigue requirements of § 29.571 and § 29.952(d)(3). This ensures that unintended partial or full fatigue failures do not occur in normal operations that, if undetected, could compromise the CRFS’s intended level-of-safety for the mitigation of post crash fire in a survivable impact. Drain valves should be designed to have positive locking provisions in the closed position in accordance with § 29.999(b)(2).
(17) Section 29.952(f) specifies that fuel tanks, fuel lines, electrical wires, and electrical devices must be designed and constructed, as far as practicable, to be crash resistant. Typical mechanical design criteria necessary to minimize fuel spillage sources, ignition sources, and their mutual contact in a survivable impact (i.e., provide crash resistance) are stated by the following subparagraphs. These mechanical design criteria should be incorporated in each design to the maximum practicable extent.
Compliance is accomplished and assessed by a thorough design review and potential PCF hazard analysis with findings and solutions that are documented and approved.
Any additional PCF hazards that are identified should be documented, included, addressed equally, and eliminated to the maximum practicable extent. Engineering evaluation, analysis, and tests are all required to determine the maximum level of practicability.
(i) They should not initiate or contribute to a post crash fire in an otherwise survivable impact. A hazard analysis should show which components are critical in this regard and should be assessed in detail for hazard elimination purposes.
(ii) Fuel and electrical lines and components should be located away from each other, away from probable crash impact areas, and away from areas where structural deformation or large objects (such as engines or transmissions) may, by crushing or penetration, cause fuel spillage or create an electrical ignition source, or both.
(iii) Fuel and electrical lines and components should be located separately and away from areas where impact and severing by rotor blades during a survivable impact are probable.
(iv) Fuel and electrical lines and components should be in no danger of being punctured or severed during a survivable impact by locally stiff vertical understructure such as a collapsed landing gear strut.
(v) Fuel and electrical lines and components should be routed separately in areas of maximum protection, such as along heavier structural members, and away from areas where significant damage is probable.
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(vi) Fuel and electrical lines and components running through hazardous areas or directly through structure, such as a bulkhead, should be locally separated and protected from over-extension, severe abrasion and guillotine cutting by frangible panels, suitable clearance, rubber grommets, braided armor shielding (which should be nonconductive for electrical lines), or other equivalent means.
(vii) Fuel lines routed directly to instruments, transducers, or other equivalent devices should be crash resistant, in accordance with § 29.1337(a)(2), to minimize leakage in case of line rupture induced during a survivable impact.
(viii) Electrical wires routed directly into electrical boxes or instruments should be designed with sufficient local slack and locally routed in the least probable damage direction and zone, or otherwise protected to minimize the probability of damage-induced arcing.
(ix) Fuel lines routed directly into fuel tanks or other fuel system components should be locally routed in the least probable damage direction and zone, or otherwise protected, to minimize the probability of damage-induced fuel leaks.
(x) Fuel pumps mounted inside fuel tanks should be rigidly attached to the fuel tank only. If the pump is airframe mounted and has structural significance, it should have a frangible or deformable attachment (reference paragraph d(12)).
Electrical boost pumps, if used, should be installed with a minimum of 6 inches of slack wire at the pump connection. The pump wires should be shrouded to prevent cutting in a survivable impact. Nonsparking, breakaway wire disconnects or other equivalent means may be used in lieu of the 6 inches of slack wire.
(xi) Fuel filters and strainers, to the maximum practicable extent, should not be located in or adjacent to the engine intake or exhausts and should retain the smallest practicable quantity of fuel.
(xii) The number of fuel valves should be kept to a minimum. If electrically operated valves are used, they should be installed with a minimum of 6 inches of slack in the electrical lines, unless protected by equivalent means (reference 17(i)). The valves should be installed with the maximum amount of protection and separation of the electrical wires from the remainder of the valve assembly.
(xiii) Fuel quantity indicators mounted in or on fuel tanks should be selected, designed, and installed to provide the minimum puncture or tear hazard to the fuel tank in a survivable impact.
(xiv) Fuel tank and bladder enclosures should have smooth, regular shapes that avoid sharp edges and corners. Minimum concave and convex radius design criteria should be developed and adhered to. Magnesium should not be used in fuel cells, and any cadmium-plated parts should not be exposed to fuel.
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(xv) Any shielding of electrical wires from abrasion, cutting, or overextension must be nonconductive.
(xvi) All fuel line installations not containing breakaway couplings should be reviewed to insure that they will not be overtensioned in a survivable impact, that they are properly grouped and properly exit fuel tanks, firewalls, and bulkheads in the area of least probable damage, and that their number and lengths are safely minimized.
(xvii) Crash resistance guidance for other basic components is contained in related paragraphs such as AC 29.963 (§ 29.963, bladders and liners), AC 29.973 (§ 29.973, fuel tank filler connections) and AC 29.975 (§ 29.975, fuel tank vents).
(18) Section 29.952(g) requires rigid or semirigid fuel tank or bladder walls of any material construction to be both impact and tear resistant. This minimizes a PCF from impact-induced rupture and tear.
(i) A rigid tank or bladder can resist fluid pressure loads as a flat plate in bending. A semirigid tank can resist fluid pressure loads partially as a flat plate in bending and partially as a membrane in tension. Flexible liners are exempt from the requirements of § 29.952(g) since an unsupported flexible liner can resist only pure tension loads acting as a membrane (i.e., it has negligible bending strength). The rigid shell structure required by § 29.967(a)(3) that surrounds the flexible liner (membrane) carries the crash-induced impact and tear loads; whereas, the flexible liner is only significantly loaded in tension if the shell structure is penetrated by a sharp object on impact.
(ii) For metallic tanks, rigid or semirigid composite tanks (resin matrix), semirigid bladder designs (rubber matrix), metal-composite hybrid designs, and all other tank designs, impact and tear resistance should be shown by analysis and tests.
(iii) Designs using resin matrix composites should be subjected to the composite structure substantiation guidance of AC 20-107A, Composite Aircraft Structure, dated April 25, 1984, and paragraph AC 29 MG 8. Designs using rubber matrix composites are subject to the standard substantiation requirements for these devices, such as TSO-C80.
(iv) One set of crash resistance tests that constitutes an acceptable method of substantiation to the requirements of § 29.952(g) for all tank designs regardless of the materials used are those specified in paragraphs 4.6.5.1 (Constant Rate Tear); 4.6.5.2 (Impact Penetration); 4.6.5.3 (Impact Tear); 4.6.5.4 (Panel Strength Calibration); and 4.6.5.5 (Fitting Strength) of MIL-T-27422B, “Military Specification;
Tank, Fuel, Crash-Resistant Aircraft.” These test requirements, or equivalent means, should be applied for and discussed early in certification. If the MIL-T- 27422B tests are selected, severity differences between military combat requirements and the civil environment should be accounted for by reducing the MIL-T-27422B requirements, as follows:
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(A) Constant Rate Tear. The minimum energy for complete separation should be 200 foot-pounds (reference 4.6.5.1).
(B) Impact Penetration. The drop height of a 5- pound chisel should be reduced to 8.0 feet (reference 4.6.5.2).
(C) Impact Tear. The drop height of a 5 -pound chisel should be reduced to 8.0 feet and the average tear criteria should not exceed 1.0 inch (reference 4.6.5.3).
(19) Section 29.952(g) also requires that all fuel tank designs (regardless of the materials utilized and whether or not a flexible liner of any type is used) for each tank or the most critical tank be analyzed and tested to the criteria of § 29.952 d(18)(iv), or equivalent.
(20) Any type of flexible liner or bladder used in any type of fuel tank construction (integral, hard shell, etc.) must meet the strength and puncture resistance requirements of § 29.963(b). Section 29.963(b) contains the new puncture resistance requirement for flexible liners and other liner material certification requirements.
Unlined, bladderless fuel tanks are also required to meet this requirement. Most unlined, rigid fuel cell designs should readily exceed the 370-pound minimum puncture force requirement because of overriding design requirements and material characteristics, such as stiffness and ductility.
NOTE: TSO -C80, “Flexible Fuel and Oil Cell Material,” is referenced in the advisory material for § 29.963(b) and contains the detailed qualification requirements for these materials. The current puncture resistance test of TSO -C80, paragraph 16.0, states that the force required to puncture the bladder material must be greater than or equal to 15 pounds (e.g., screwdriver test). Section 29.963(b) has increased the TSO paragraph 16.0 puncture force value to be greater than or equal to 370 pounds. This is for fuel cell bladder or liner material only. Oil cell material puncture force requirements are not changed.
e. Typical Examples of Loading Modes and Test Setups for CRFS Components.
The following figures, which are referred to periodically in the advisory circular, show typical examples of test setups for CRFS components such as breakaway fuel fittings, hoses, hose end fittings, and hose assemblies.
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LOAD LOAD LOAD
STATIC TENSION TEST STATIC SHEAR TEST LOAD LOAD HOSE LOAD
STATIC BENDING STATIC SHEAR TEST (TENSION-SHEAR) TEST (TANK-TO-TANK COUPLING] FIGURE AC 29.952-1 STATIC TENSION AND SHEAR LOADING MODES Page E - 87
АС 29-2C 9/30/99
LOAD LOAD
TENSION TESTS 90-DEGREE TESTS FIGURE AC 29.952-2 HOSE ASSEMBLY TESTS Page E - 88
Tension Load (Ib) Bending Load (Ib) Hose End Fitting Minimum Minimum Minimum Minimum Fitting Type Size Average Individual Average Individual Load* Load Load* Load STRAIGHT -4 600 475 425 400 Tension = -6 700 575 425 400 -8 900 650 650 600
- 10 1450 1175 675 625 Bending = - 12 1775 1475 950 850
- 16 2125 1825 1425 1300 -20 2375 2075 1550 1425 90 ELBOW -4 600 475 425 400 Tension = -6 700 575 425 400 -8 900 650 450 400
- 10 1450 1175 475 425 Bending = - 12 1775 1475 500 450
- 16 2125 1825 775 700 -20 2375 2075 1100 1000 *Average of at least 3 tests.
FIGURE AC 29.952-3 MINIMUM AVERAGE AND INDIVIDUAL LOADS FOR
HOSE AND HOSE-END FITTING COMBINATIONS Page E - 89
AIR FRAME STRUCTURE
- TANK
HOSE END
COUPLING FLEX HOSE _ METAL TANK FITTING
BREAKAWAY VALVE
FRANGIBLE SECTION - ITEM LOWEST FAILURE LOAD (LE)* FAILURE MODE Flex Hose 3000 Tension Breakage Flex Hose 1500 Pull Out of End Fitting Tank Fitting 7500 Pull Out of Tank Hose End Coupling 1650 Break (Bending) Breakaway Valve 2500 Pull Out of Tank Fitting Breakaway Valve Not More Than Break at frangible 1500 = 750 Section 2 Not Less Than 1500 = 375 *Loads may or may not be representative; values are for explanatory purposes only.
FIGURE AC 29.952-4 TYPICAL METHOD OF BREAKAWAY FUEL FITTING
LOAD CALCULATIONS (TANK INSTALLATION USED AS EXAMPLE ONLY; BASIC TECHNIQUE
APPLICABLE TO OTHER CONFIGURATIONS) Page E - 90
AIRCRAFT STRUCTURE • TANK WALL
FRANGIBLE BOLT METAL TANK FITTING
CRITICAL FLANGE AREA A. FRANGIBLE BOLT
AIRCRAFT STRUCTURE ITEM LOWEST FAILURE LOAD (LBI* FAILURE MODE AIRCRAFT STRUCTURE 4000 SHEAR TANK FITTING 3000 PULLOUT OF TANK FLANGE 5000 SHEAR
FRANGIBLE BOLT NOT MORE THAN NOT LESS THAN I BREAK 3000 = 1500 3000 = 750 (TENSION-SHEAR) FIGURE AC 29.952-5 TYPICAL METHODS OF FRANGIBLE OR DEFORMABLE ATTACHMENT LOAD CALCULATIONS: EXAMPLE 1, FRANGIBLE BOLTS.
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- AIRFRAME RIGID BULKHEAD
GROMMET
FRANGIBLE BAFFLE
UNCUT FLEX HOSE ED
END FITTING ITEM LOWEST FAILURE LOAD (LB)* FAILURE MODE RIGID BULKHEAD 4000 BEARING FLEX HOSE 3000 TENSION BREAKAGE FLEX HOSE 1500 PULLOUT OF END FITTING END FITTING 1750 BENDING
FRANGIBLE BAFFLE NOT MORE THAN NOT LESS THAN BEARING 1500 = 750 1500 = 375 2 4 *VALUES ARE SHOWN FOR EXPLANATORY PURPOSES ONLY FIGURE AC 29.952-6 TYPICAL METHODS OF FRANGIBLE OR DEFORMABLE ATTACHMENT LOAD CALCULATIONS: EXAMPLE 2, FRANGIBLE BAFFLE.
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AC 29.953. § 29.953 FUEL SYSTEM INDEPENDENCE.
a. Explanation.
(1) Section 29.953(a)(1) stipulates that fuel systems for Category A rotorcraft must meet the requirements of § 29.903(b) engine isolation.
(2) Section 29.953(a)(2) specifies independent fuel feed systems for each engine for Category A rotorcraft unless other provisions are made to meet the § 29.903(b) engine isolation requirement.
(3) Section 29.953(b) specifies independent fuel feed systems for each engine for Category B rotorcraft, except that separate fuel tanks are not required.
b. Procedures.
(1) The purpose of § 29.953(a) is to ensure an independent fuel supply system for each engine. Multiengine Category B rotorcraft do not require separate fuel tanks, as are intended for Category A.
(2) The assessment to ensure compliance with § 29.903(b), engine isolation, should include consideration of component failure, malfunction, and damage. For multiengine Category B rotorcraft, leakage of the fuel cell could be excluded from consideration since § 29.953(b) explicitly states that separate fuel tanks are not required for this category rotorcraft.
NOTE: Of interest is that § 29.903(c), engine isolation for normal category airplanes, also excludes the fuel tank from consideration if only one tank is used.
(3) Consideration of fuel tank leakage under § 29.903(b) has dictated separate fuel tanks for Category A rotorcraft, but the regulation leaves the door open for unique designs by the expression, “Unless other provisions are made…,” in § 29.953(a)(2).
Separate tanks are intended for Category A as evidenced by the identical fuel system independence requirements for multiengine Category B rotorcraft, except that separate tanks are specifically not required.
(4) A common supply tank, with individual “collector” tanks for each engine for Category A rotorcraft, has been allowed under § 29.953 provided that the capacity of the collector tanks will allow 20 minutes of maximum allowable en route OEI power.
(5) The fuel system independence regulations are not intended to preclude single-point fueling designs.
(i) For multiengine Category B rotorcraft, the assessment of an independent fuel supply system for each engine would begin at the fuel supply pickup point within the tank and continue to the engine fuel inlet at the engine.
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(ii) For Category A rotorcraft, the assessment would begin with the tanks and continue to the engine fuel inlet.
(6) If supply line crossfeed capability is included as a feature, care must be exercised to ensure that the opening of the crossfeed does not jeopardize the continued safe operation of more than one engine. For example, if the crossfeed valve is automatically operated by a low pressure signal in the supply line for one engine, the possibility that fuel line leakage could cause opening of the crossfeed and jeopardize the continued safe operation of both engines should be considered. Similarly, opening the crossfeed valve with a suction lift system following engine or system malfunction should not allow air into the fuel supply line of the remaining engine.
(7) The independent fuel supply system requirement for each engine is for normal fuel system operations. Care should be exercised to ensure that flight manual procedures do not authorize normal usage of fuel system configurations which may violate the engine isolation principle. For example, routine fuel balance procedures should not allow usage of a common supply line if a failure can jeopardize the continued safe operation of more than one engine.
(8) Fuel system designs which allow the continued safe operation of all engines under expected fuel system component failure conditions (for example, a failed boost pump) by using common fuel flow paths under failure conditions are not prohibited.
(9) For APU’s which perform a required in-flight function, a separate, independent fuel system complying with the corresponding engine fuel system rules should be provided. Other APU’s (which do not perform a required in- flight function) may be supplied with fuel from a tee connection to a main engine fuel supply. The fuel shutoff valve for the APU should be located as close as possible to the APU system’s connection to the main engine fuel system and a checkvalve should be included in the APU fuel system to prevent reverse-flow if negative pressure exists momentarily in the main engine fuel system. Maximum fuel demand of the APU will not jeopardize compliance with § 29.955.
AC 29.954. § 29.954 (Amendment 29-26) FUEL SYSTEM LIGHTNING PROTECTION.
a. Background. During the initial development and promulgation of the standards concerning the airworthiness of rotorcraft, it was not deemed necessary to specify design features that would protect the rotorcraft from the meteorological phenomenon of lightning. This was due, in part, to the fact that rotorcraft were primarily operated in a VFR and non-icing environment. Also, a prudent pilot avoided thunderstorms where the possibility of encountering severe weather and a lightning strike was much greater. The construction, design, and operating environment of civil rotorcraft have changed markedly within the past two decades. Many rotorcraft are now authorized to fly IFR in all types of weather environment. One transport design has been approved for flight Page E - 94