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 § 27.571 and 27.952(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 § 27.999(b)(2).
(17) Section 27.952(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.
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 (Ш) 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.
Page E - 46
(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 § 27.1337(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.
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 AC 27.952d(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 mean (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.
(xili) 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 tuel cells, and any cadmium-plated parts should not be exposed to fuel.
Page E - 47
(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 AC paragraphs such as paragraphs AC 27.963 (§ 27.963, bladders and liners), AC 27.973 (§ 27.973, fuel tank filler connections) and AC 27.975 (§ 27.975, fuel tank vents).
(18) Section 27.952(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 § 27.952(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 § 27.967(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.
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.
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 27 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 § 27.952(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 Page E - 48
environment should be accounted for by reducing the MIL-T-27422B requirements, as follows:
(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 27.952(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 paragraph (18)(iv) of § 27.952, 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 § 27.963(g). Section 27.963(g) 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 § 27.963(g) 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 27.963(g) 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.
Page E - 49
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 AG 27 952-1 STATIG TENSION AND SHEAR LOADING MODES Page E - 50
9/30/99 АС 27-1B
LOAD LOAD
TENSION g0 DEGREE TESTS FIGURE AC 27.952-2 HOSE ASSEMBLY TESTS Page E - 51
Tension Load (Ib) Bending Load (Ib) Hose End Fitting Minimum Minimum Minimum Minium Fitting Type Size Average Individual Average Individual Loadi Load Load™ Load STRAIGHT -4 600 475 425 400 Tension = -6 700 575 425 400 -8 900 650 650 500 -10 1450 1175 675 625 Bending = -12 1775 1475 950 850 -16 2125 1825 1425 1300 -20 2975 2075 1550 1425 90 ELBOW -4 600 475 425 400 Tension = -6 700 575 425 400 -8 900 650 450 _400 -T0 1450 1175 475 '425 Bending = -12 1775 1475 500 450 -16 2125 1825 775 700 -20 2375 2075 1100 1000 tAverage of at least 3 tests, FIGURE AC 27,952-3 MINIMUM AVERAGE ANDINDIVIDUAL LOADS FOR
HOSE AND HOSE-END FITTING COMBINATIONS Page E - 52
AIR FRAME STRUCTURE
HOSE END TANK
COUPLING
FLEX HOSE - METAL TANK FITTING
NARE TTT BREAKAWAY VALVE
FRANGIBLE SECTION ITEM LOWEST FAILURE LOAD (LE)+ FAILURE MODE Flex Hose 3000 Tension Breakage Fles Hose 1500 Pull Out of End Fiting Tank Friting 7500 Put Out of Tank Hose End Coupling 1650 Break (Bending) Breakaway Valve 2500: Put Out of Tank Fitting Breakaway Valve Not More Than Break at frangible 1500 - 750 Section 2 Not Less Than 1500 = 375 *Loads may of may hot be representative, values are for explanatory purposes only.
FIGURE AC 27 9524 TYPICAL METHOD OF BREAKAWAY FUEL FITTING
LOAD CALCULATIONS (TANKINSTALLATION USED AS EXAMPLE ONLY; BASIC TECHNIQUE APPLICAELE
TO OTHER CONFIGURATIONS) Page E - 53
AIRCRAFT STRUCTURE ~ TANK WALL
FRANGIBLE BOLT METAL TANK FITTING
CRITICAL FLANGE AREA
A FRANGIBLE BOLT
AIRCRAFT STRUCTURE
ITEM LOWEST FAILURE LOADILBI 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 27 952-5 TYPICAL METHODS OF FRANGIBLE OR DEFORMABLE ATTACHMENT LOAD CALCULATIONS: EXAMPLE 1,
FRANGIBLE BOLTS Page E - 54
AIRFRAME RIGID BULKHEAD
GROMMET
- FRANGIBLE BAFFLE
LINCUT FLEX HOSE ED
END FITTING
ITEM LOWEST FAILURE LOAD ILBY 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 27 952-6 TYPICAL METHODS OF FRANGIBLE OR DEFORMABLE ATTACHMENT LOAD CALCULATIONS: EXAMPLE 2,
FRANGIBLE BAFFLE Page E - 55
AC 27.953. § 27.953 FUEL SYSTEM INDEPENDENCE.
a. Explanation.
(1) Section 27.953(a) specifies independent fuel feed systems for each engine of multiengine rotorcraft; however, separate fuel tanks for each engine are not required.
(2) If a single tank is used to feed more than one engine, § 27.953(b) specifies:
That independent fuel tank outlets be provided to each engine, each having a shutoff valve.
(ii) At least two vents for the tank located to minimize the probability of both vents becoming obstructed simultaneously.
(Ш) Filler caps designed to minimize the probability of incorrect installation or in-flight loss.
(iv) That fuel supply from each tank outlet to any engine be independent of fuel supply to other engines.
b. Procedures.
(1) The purpose of § 27.953(a) is to ensure an independent fuel supply system for each engine on multiengine rotorcraft. Unlike the corresponding regulation for Category A, Part 29 rotorcraft, separate fuel tanks are not required.
(2) 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.
(3) 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 should not allow air into the fuel supply line of any engine.
(4) The independent fuel supply system requirement for each engine is for normal fuel system operations. 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.
Page E - 56
(5) In § 27.953(b), the phrase "if a single fuel tank is used," is intended to mean if a single fuel tank is used to feed more than one engine. This interpretation is needed in order to preclude, for example, a tri-engine design with two fuel tanks where two engines draw fuel by independent means from one tank, but only one vent is provided for that tank. This design would clearly violate the intent of § 27.953(b)(2) to assure that two vents be supplied if fuel is drawn by more than one engine from a single tank.
(6) If a single fuel tank is used to supply fuel to more than one engine:
(i) There should be independent tank outlets for each engine, each incorporating a shutoff valve at the tank. The phrase, "at the tank," has rightfully been interpreted to allow the firewall shutoff valve, which may actually be some distance from the tank itself, to be used to show compliance with § 27.953(b)(1). Section 27.953(b)(1) specifically allows the shutoff valve, if located at the tank, to serve as the firewall shutoff valve provided the line between the valve and the engine compartment does not contain a hazardous amount of fuel that can drain into the engine compartment.
There should be at least two vents arranged to minimize the probability of both vents becoming obstructed simultaneously. Typically, the means used to prevent simultaneous obstruction is physical separation. The blockage or malfunction of any vent should not jeopardize the continued safe operation of more than one engine.
(i¡i) The filler cap(s) for the tank should be designed to minimize the probability of incorrect installation or in-flight loss. Usually, there should be only one way to install and lock a fuel cap; if more than one way is possible, either method should provide the positive sealing to avoid spillage. Minimizing the probability of in-flight fuel loss would include the ability to visually determine that the cap is properly installed and locked prior to flight.
(iv) Section 27.953(b)(4) simply clarifies that if a single tank is used to feed more than one engine, the provisions for independent fuel feed systems (reference § 27.953(a)) apply to the engines being fed from that tank.
AC 27.954. § 27.954 (Amendment 27-23) 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 nonicing 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.
Additionally, many rotorcraft now use the same advanced technologies in structures and Page E - 57