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
into known icing conditions. Additionally, many rotorcraft now use the same advanced technologies in structures and 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. This occurrence generally results in a catastrophe to the rotorcraft. To prevent such a catastrophe and provide a level of safety equivalent to transport category airplanes, a specific rule for the lightning protection of transport category rotorcraft fuel systems was adopted in Amendment 29–26.
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 § 29.954.
(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 29.955. § 29.955 (Amendment 29-2) FUEL FLOW.
a. Explanation.
(1) Section 29.955 is intended to ensure adequate fuel flow to the engine(s) at maximum power under the intended aircraft operating conditions and maneuvers. In ensuring adequate fuel flow, 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.
(2) In showing adequate fuel flow, the rule provides that-- Page E - 95
(i) The fuel be supplied within the appropriate engine fuel pressure range;
( ii) The test be conducted with minimum fuel onboard, consistent with test safety;
(iii) For pump systems, fuel flow requirements be satisfied with the critical airframe furnished pump inoperative; and
(iv) The f uel flowmeter, if installed, must be blocked such that fuel must flow through the meter or its bypass.
(3) Section 29.955(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 § 29.955(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.
(i) Methods to adjust the test data for different fuel properties and flows should be verified by limited testing.
(ii) If a suction lift system is used and hot fuel verification is involved (reference § 29.961) testing is appropriate.
(2) Demonstrating that the system is capable of providing “…100 percent of the fuel flow required under the intended operating conditions…” will depend on the particular system design, whether boosted or suction lift, Category A or Category B, and whether single or multiengine. Some of the factors to be evaluated are as follows:
(i) Acceleration fuel flow requirements may exceed those for steady-state operation. For example, if on a cold day, engine torque is the limiting parameter, the steady-state fuel flow demand corresponding to that torque may be exceeded during engine acceleration to that power.
(ii) For single-engine rotorcraft and for multiengine rotorcraft with all engines operating, some margin should be included to account for possible inadvertent overtorque.
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NOTE: Notice of Proposed Rulemaking (NPRM) No. 84-19 proposes to include this consideration as a firm requirement (reference 49 FR 46670; dated November 27, 1984).
(iii) For multiengine rotorcraft, adequate fuel flow under OEI conditions should be ensured.
(A) For Category A systems, evaluation of § 29.903(b) should ensure that following the failure of one engine, lack of fuel flow will not jeopardize the safe operation of the remaining engine(s). Since governor -controlled engines will automatically accelerate to some limit if power demand is high, and since immediate crew action is not presumed under § 29.903(b), compliance with § 29.955 would include adequate fuel flow to the cold day maximum OEI torque to be expected (reference § 29.927(b)(2)).
(B) A proposed revision to § 29.955 (reference NPRM No. 84-19) would require that fuel flow for multiengine Category B rotorcraft be adequate for the § 29.927(b)(2) OEI overtorque condition.
(C) Following an engine failure, the remaining engine(s) may accelerate to the gas producer speed topping limit fuel flow, rather than to the fuel flow for the steady-state OEI power value. This consideration would be most important for suction lift systems which may be critical with hot fuel at altitude.
(3) The critical fuel system configuration should be evaluated.
(i) For pump fed (boosted) systems, fuel flow requirements should be satisfied with the critical airframe furnished pump inoperative.
(ii) If on 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 multiple engines through common components may be more critical than the OEI condition.
(4) Adverse transient and steady-state maneuver loads should be considered since the g-loading experienced may tend to decrease the engine fuel inlet pressure below allowable limits.
(5) The fuel should be delivered to the engine inlet within the limits specified in the engine type certificate. The method of specifying these 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.
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(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.
(6) For those systems which specify a minimum V/L ratio, the methods provided in Aerospace Recommended Practice (ARP) 492 published by the Society of Automotive Engineers are acceptable in evaluating test results.
(7) Since the lower quantity of fuel in the tank will reduce the hydrostatic head and thus the fuel inlet pressure, § 29.955(a)(2) specifies that the quantity of fuel in the tank should be minimum.
(8) Section 29.955(a)(3) specifies that each main and emergency pump be evaluated. If it can be determined which pump and flow path is critical, only that configuration would be tested. Similarly, for suction fuel systems, the critical flow paths and flow requirements should be evaluated. If pumps are required to supply the necessary fuel, § 29.1305 would require a fuel pressure indicator and § 29.1549 would require a red radial at the minimum safe operating fuel pressure for any fuel or fuel usage condition. This pressure limit should be used to determine compliance with § 29.955(a)(1) for all operations.
(9) Section 29.955(a)(4) specifies that the fuel flowmeter, if installed, be “blocked” in showing compliance with the fuel flow requirements. Consideration of flowmeter component failure or malfunction would most often be more appropriate than blockage.
(i) If the flowmeter is completely blocked in assessing compliance, then a bypass would be dictated, and the provision for “flow through the meter” following blockage would not be a viable alternative. It is not the intent of the rule to arbitrarily preclude flowmeter installations without a bypass system.
(ii) Section 29.1337(c) clarifies that if the malfunction of a metering component severely restricts fuel flow, a bypass would be required. An example of a malfunction to be considered would be a locked rotor on a rotating element design.
(iii) NPRM No. 84-19 proposes to clarify the intent of § 29.955 by requiring that proper fuel flow be ensured with fuel flow transmitter component failure, rather than with transmitter blockage as specified in the existing rule.
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(10) Section 29.955(b) requires the fuel system to feed promptly when fuel becomes low in one tank and another tank is selected. This requirement is important because momentary fuel flow interruption must be expected to result in complete power failure and, for single engine rotorcraft, an emergency landing.
AC 29.955A. § 29.955 (Amendment 29-26) FUEL FLOW .
a. Explanation. Amendment 29- 26 adds new requirements for test conditions to ensure that adequate fuel flow is available to the engine in critical combinations of adverse conditions that may be expected during operation of the rotorcraft. The amendment also requires a correlation between fuel filter blockage and the fuel filter warning device required by § 29.1305(a)(17). Design and performance standards for auxiliary fuel tank and transfer tank fuel systems are provided. These changes were made to ensure that all parameters associated with fuel supply to the engine are adequately addressed.
b. Procedures.
(1) Section 29.955 is intended to ensure adequate fuel flow to the engine(s) during all operating conditions of the rotorcraft. This includes the fuel flows necessary to operate the engine(s) under the test conditions required by § 29.927. Testing (including bench or rig tests) has been the accepted method of showing compliance with this section although analytical techniques may be used to adjust system test results to various fuel flow conditions or to account for minor modifications to a system.
Analytical methods that are used to adjust the test results should be verified with limited testing. It should be shown during compliance testing that the fuel pressure, at the engine to airframe interface, will be within the limits specified by the engine manufacturer. The fuel pressure at this point should be maintained within limits specified by the engine manufacturer during all critical maneuvers and accelerations.
All of the following conditions should be met during compliance testing unless it can be shown that combinations of the conditions are not possible.
(i) The fuel quantity in the tank(s) in use during the test may not exceed the unusable fuel quantity established under § 29.959, plus the minimum quantity required to conduct the test.
(ii) During the compliance test, the rotorcraft should be maneuvered to create the most critical fuel pressure head between the fuel tank outlet and the engine to airframe interface (engine fuel inlet).
(iii) For boost pump fed systems, it should be determined which pump (primary or secondary) would create the most critical restriction if it failed. The critical pump should then be installed to create the critical restriction, either by actual or simulated failure.
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(iv) Various combinations of engine power demand, electrical power available, and motive flow requirements for ejector pumps, will have an effect upon the fuel flow and pressure available at the engine to airframe interface. Adequate fuel pressure should be available to the engine with the most critical combination of these parameters.
(v) Critical values of fuel properties that may adversely affect fuel flow and/or fuel pressure should be applied. This includes alternate types of fuel if certification with alternate fuels is requested. At the minimum, the fuel that will create the highest vapor to liquid ratio should be used during hot fuel tests (§ 29.961). The most viscous fuel should be used during cold fuel tests.
(vi) The fuel filter, required by § 29.997, should be partially blocked to simulate the maximum contamination allowable. The blockage should be sufficient to activate the impending bypass indicator that is required by § 29.1305(a)(17).
(2) Unique Conditions. The phrase, “…Provide the engine with at least 100 percent of the fuel required under all operating and maneuvering conditions…” (§ 29.955(a)), includes unique flight conditions within the operational envelope of the rotorcraft. Critical conditions of fuel flow to the engine(s) may exist under the following conditions (and others identified by the applicant); therefore, they should be evaluated and tested if applicable:
(i) In a single engine rotorcraft, a rapid acceleration to maximum power (torque) that will be requested for certification may be a critical condition. In this case the fuel flow required during the transient may exceed the fuel flow required for steady state at the maximum power condition.
(ii) In multiengine rotorcraft, a rapid acceleration to the maximum OEI power rating that will be requested may be a critical condition. The fuel flow during the transient may be higher than that required at the steady state OEI condition.
(3) If auxiliary fuel pumps (boost pumps) are used to supply fuel to the engines, and ejector pumps are used for cross-feed or other inter-tank fuel distribution systems, a test should be run that will place the maximum fuel demand on the auxiliary pump(s).
(4) In some multiengine rotorcraft, a single pump may be required to provide fuel flow to all engines in the event of an auxiliary pump failure. If this is the case, a test should be conducted with a simulated (or actual) failed auxiliary pump. If the functional auxiliary pump is designed to provide motive flow for cross-feed systems, the most critical condition of fuel flow demand should be tested.
(5) Transient and steady state maneuver loads (g-loading) may affect the fuel pressure at the engine to airframe interface. This effect should be considered and then tested, if appropriate.
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(6) The methods of specifying the engine inlet fuel pressure requirements are sometimes related to fuel temperature and altitude. Therefore, it is necessary to explore the extremes of the envelope to assure compliance rather than attempting to select one critical condition. For instance, the increase in fuel viscosity at cold temperatures may increase system pressure drop and offset a slight drop in required fuel flow. In this case, critical fuel inlet conditions may not be experienced at maximum engine fuel flow.
(7) A conservative demonstration would consider the maximum allowable fuel viscosity in combination with the maximum fuel flow. Otherwise, several test points may be required.
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FUEL FLOW, (Y) LIMIT
Q LIMIT
TORQUE MOT LIMIT OAT - → FIGURE AC 29.955A-1 FUEL FLOW NOTES:
(1) Point A on figure AC 29.955A-1 is the highest fuel flow within aircraft limitations, but the system pressure drop is not expected to be maximum because of the low kinematic fuel viscosity.
(2) Point B is the maximum flow at cold temperatures but as the fuel temperature is further reduced, the fuel viscosity increases very rapidly.
(3) Point C represents the maximum viscosity of the fuel, but the fuel flow is somewhat reduced from point B. The maximum system pressure drops and, therefore, minimum tuel inlet pressure may occur between points B and C depending on the specific relationship of fuel viscosity to required fuel flow.
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