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 advis ory 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 27.952-1 STATIC TENSION AND SHEAR LOADING MODEST Page E - 50
LOAD LOAD
TENSION TESTS
LOAD LOAL 90-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 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 27.952-3 MINIMUM AVERAGE AND INDIVIDUAL LOADS FOR
HOSE AND HOSE-END FITTING COMBINATIONST Page E - 52
AIR FRAME STRUCTURE TANK
HOSE END
COUPLING FLEX HOSE — METAL TANK FITTING
T 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 4 * Loads may or may not be representative; values are for explanatory purposes only.
FIGURE AC 27.952-4 TYPICAL METHOD OF BREAKAWAY FUEL FITTING
LOAD CALCULATIONS (TANK INSTALLATION USED AS EXAMPLE ONLY; BASIC TECHNIQUE APPLICABLE
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 BREAK 3000 = 1500 3000 = 750 (TENSION-SHEAR) FIGURE AC 27.952-5 TYPICAL METHODS OF FRANGIBLE OR DEFORMABLE TTACHMENT LOAD CALCULATIONS: EXAMPLE 1.
FRANGIBLE BOLTS.
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AIRFRAME RIGID BULKHEAD
GROMMET
- FRANGIBLE BAFFLE
UNCUT FLEX HOSE
END FITTING
ITEM LOWEST FAILURE LOAD (LBIF FAILURE MODE
RIGID BULKHEAD BEARING
FLEX HOSE TENSION BREAKAGE
FLEX HOSE PULLOUT OF END FITTING
END FITTING BENDING
FRANGIBLE BAFFLE ENOT MORE THAN NOT LESS THAN BEARING 1500 = 750 1500 = 375 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.
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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:
(i) 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.
(iii) 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.
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(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.
(ii) 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.
(iii) 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
systems as do airplanes. Because of these facts the possibility of a lightning strike encounter to the rotorcraft has been greatly increased. If the fuel system of the rotorcraft has not been properly designed and constructed, a fuel vapor ignition may occur if the rotorcraft encounters a lightning strike. This occurrence generally results in a catastrophe to the rotorcraft. To prevent such a catastrophe and provide a level of safety equivalent to normal utility, acrobatic and commuter category airplanes, a specific rule for the lightning protection of normal category rotorcraft fuel systems was adopted in Amendment 27–23.
b. Explanation.
(1) This regulation requires that the rotorcraft’s fuel system be designed and constructed so that an ignition of fuel vapor will not occur when the rotorcraft is involved in a lightning strike. For the purposes of this regulation the fuel system is comprised of the fuel tank with all its associated plumbing and any other areas of the rotorcraft likely to have fuel vapor present (such as sumps and drains for the tank itself). Externally mounted fuel tanks are also considered to be part of the “fuel system.”
(2) Other associated installations such as electrical wiring in the fuel tanks which could provide a source of ignition due to an indirect or induced effect should also be considered.
c. Procedures.
(1) The current revision of Advisory Circular 20-53 provides guidance on an acceptable method and procedure to be utilized to demonstrate that the design and construction of the fuel system is compliant with § 27.954.
(2) FAA Report No. DOT/FAA/CT-89/22 contains additional information regarding the lightning environment. Also contained in this report are design and test techniques which provide for a design that will be adequately protected from fuel vapor ignition when the rotorcraft encounters the lightning environment. This report is available to the public by order from the National Technical Information Service, Springfield, VA 22161.
AC 27.955. § 27.955 FUEL FLOW.
a. Explanation.
(1) Section 27.955 is intended to ensure adequate fuel flow to the engine(s) at maximum power under the intended aircraft operating conditions and maneuvers.
(2) In showing adequate fuel flow, the rule provides--
(i) That the fuel be supplied within the appropriate engine fuel pressure range;
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(ii) That the test be conducted with minimum fuel onboard, consistent with test safety; and
(iii) That operation with both main and emergency pumps be considered.
(3) Section 27.955(b) specifies that if an engine can be supplied with fuel from more than one tank, the fuel system must feed promptly when fuel becomes low in one tank and another tank is selected.
b. Procedures.
(1) Testing (including bench tests) has been the accepted method to show compliance with § 27.955(a). Analytical techniques may be used to adjust the system test results to various fuel conditions and flows or to account for minor modifications to a system. A purely analytical approach is not generally acceptable.
(2) Methods to adjust the test data for different fuel properties and flows should be verified by limited testing.
(3) If a suction lift system is used and hot fuel verification is involved, testing is appropriate.
(4) The proper interpretation of the phrase “100 percent of the fuel flow required under the intended operating conditions and maneuvers” may include consideration of acceleration fuel flow in addition to the steady-state fuel flow requirement.
(i) For example, if on a single-engine rotorcraft on a cold-day takeoff, engine torque is the limiting parameter, the steady-state fuel flow demand corresponding to that torque may be exceeded during engine acceleration in maneuvers.
(ii) In addition to the consideration of acceleration fuel flow, good design would include some margin to account for possible inadvertent overtorque.
(5) For multiengine rotorcraft, adequate fuel flow under OEI conditions should be assured in the critical fuel system configuration.
(i) If on a multiengine rotorcraft, it is acceptable to operate following an engine failure in more than one fuel system configuration (for example, if crossfeed is an acceptable mode) then the supplying of two engines through common components may be more critical than the OEI condition.
(ii) In verifying satisfactory fuel system operation for OEI conditions, the fact that the remaining engine may go to the gas producer speed topping limit fuel flow rather than to the steady-state OEI power value should be assessed.
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(6) Adverse transient and steady-state maneuver loads should be considered since the g-loading experienced may tend to decrease the fuel inlet pressure below allowable limits.
(7) In assuring adequate fuel flow at the necessary engine inlet pressure (§ 27.955(a)(1)), both hot and cold fuel would normally be evaluated for the suction lift system, whereas cold fuel is usually more critical for the boosted pressure system.
(8) The method of specifying the fuel inlet pressure requirements varies with the engine model. Some of these include:
(i) Specification of a gage pressure as a function of altitude for suction system operation. The particular fuel and fuel temperature for demonstrating the criteria may be specified in the engine documents. Other approved fuels, fuel temperatures, and boost-pump-on operation are considered satisfactory if the demonstration with the specified fuel is successful.
(ii) Specification of a maximum allowable vapor-to-liquid ratio for hot fuel, and minimum absolute pressure as a function of altitude for cold fuels.
(iii) Specification of a fuel inlet pressure relative to the true vapor pressure of the fuel, in combination with a maximum allowable vapor-to-liquid ratio.
(iv) Specification of separate pressure limits for boost-on and suction lift operation.
(v) Specification of special limits for emergency use or emergency fuels.
(9) Because the various methods of specifying the engine inlet fuel pressure requirements are sometimes related to fuel temperature and altitude, it is often necessary to explore the extremes of the envelope to assure compliance rather than attempting to select one critical condition. Additionally, the rapid increase in fuel viscosity at colder temperatures, which tends to significantly increase system pressure drop, can more than offset a slight drop in required fuel flow such that the critical fuel inlet conditions may not be experienced at maximum engine fuel flow.
Figure AC 27.955–1 illustrates the point.
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