AC 29-2C, Chg 4 5/1/2014 should be reduced when altitude increases) and the rotorcraft category (speed of collective input should be reduced when rotorcraft max weight increases), the duration of the maneuver should be from 1 to 3 seconds at low altitude to approximately 3 to 5 seconds at higher altitude.
(C) Some collective inputs should be made from desynchronized conditions (NR > N2) but should be carefully done as those maneuvers generally lead to larger reductions of NR and high free wheel or engine constraints.
(D) On multi-engine helicopters, collective inputs at lower speeds should be done with one engine at idle position to check the behavior of the other engines in case of an engine failure.
(E) Droop of N2/NR out of the green arc is permitted during the test, provided it is acceptable to the engine or airframe manufacturer for the purpose of the test. The goal is to check for generally satisfactory acceleration (no surge, no extreme N2/NR droop, no significant overshoot). Any N2/N R droop must not result in a condition that requires exceptional piloting skill, alertness, or strength to maintain safe flight.
(F) During those collective inputs, Nr below the minimum Nr should be avoided by lowering the collective. The reasons for poor engine acceleration should be determined.
(ii) Collective decrease inputs.
(A) Collective decrease inputs will verify:
(a) C o m b u s t i o n s t a b i l i t y (i.e. that there is no flame-out).
(b) The engine behavior in transients and in particular the maximum free turbine speed, minimum oil pressure, etc.
(c) The rotorcraft handling qualities (variation of pitch and roll and yaw, cross coupling, etc.) during the transients.
(B) Rate of collective lowering should be built up from 5 seconds down to under 3 seconds. Regarding N2 ov erspeed, a quick stop type maneuver that leads to an initial acceleration of the NR should be envisaged.
(4) Compressor stall investigation.
(i) The turbine engine installation should not suffer from compressor stalls anywhere in the flight envelope. Previous tests as described above include some of the required areas, but additional testing is necessary. In stabilized conditions, the collective pitch is moved in slow and small oscillations in:
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(A) Level flight with or without sideslip at different power settings.
(B) P u l l u p o r i n t u r n .
(C) A u t o r o t a t i o n .
(D) Descent at different rates.
(ii) The maneuvers described in paragraphs b.(1)(ii)(A)(4)(i)(A) through (D) above check the engine susceptibility to air flow distortions. The test technique involves oscillations of the collective at various rates around the identified thresholds of the compressor control bleed valves.
(B) Degraded modes: Governor failures.
(1) The specific degraded mode testing for a hydromechanically controlled engine is different than for a digitally controlled engine. However, the basic principles for both types of engine controls include:
(i) Review of the safety analysis, especially the FMEA.
(ii) Determine the failures to be checked in flight, based on prior analysis or testing.
(iii) For each failure to be checked in flight:
(A) Evaluate the behavior of the engine when the failure occurs.
(B) Determine the acceptability of the procedure of identification of the degraded engine.
(C) Assess the human factor aspects of the machine interface (cautions, warning, etc.).
(D) During the use of the degraded engine, consider the workload impact related to engine response in adjusting NR or power for approach and landing.
(E) Evaluate the adequacy of the RFM procedures.
(2) Helicopters equipped with FADECs are becoming increasingly integrated into the helicopter systems for engine control sensor inputs. For example, N2/NR values available could depend on parameters such as airspeed, altitude, or temperature as provided by the basic helicopter systems and sent to the FADEC. A Page E – 64.3
AC 29-2C, Chg 4 5/1/2014 concern is the effects of degradation, discrepancies or failures in the basic helicopter systems on engine governing laws and power available. Therefore, a review of the FMEA before beginning engine failure or degraded mode evaluations should be performed.
(3) It is recommended to have special test equipment installed on the test engine in order to simulate failures to the FADEC or digital engine control. Test risk mitigation should include the evaluation of possible malfunctions of this special test equipment.
(4) As a minimum, the following typical failures or degraded modes, including engine control reversion that is based on safety analysis and ground test results, should be evaluated in flight:
(i) Failure causing the engine to increase power.
(iii) Failure causing the engine to decrease power.
(iii) Failure fixing the engine power at a static value.
(iv) Failure introducing oscillations.
(v) Failure introducing lower acceleration or deceleration.
(vi) Failure causing the N2/NR to increase or decrease.
(2) Vibration.
(i) Explanation. Engine airflow patterns are deflected or distorted by the presence of airframe inlet hardware, cowling, fuselage panels, and, to a degree, in almost all flight regimes. Additional items su ch as airframe installed particle separators, deflectors for snow, ice, or sand protection, and obstructions forward of the engine inlet, such as a hoist kit, could affect the engine air flow patterns. The rotating elements of the engine, particularly the compressor blades, will be subjected to a cyclically varying air flow as these elements move into and out of areas of deflected airflow to the engine.
A corresponding aerodynamic load will be imposed on these engine elements. Since this loading is also cyclic, the possibility of critical frequency coupling with an engine component shall be investigated.
(ii) Procedure. Typically, this ev aluation would involve installation in the engine inlet of a special multiple probe, total pressure sensing system, and flight testing which largely follows that prescribed for evaluation of engine operating characteristics as described above. Data from these tests can be reduced to create a pressure map at the compressor inlet face which, in conjunction with compressor speeds, may be used to determine the frequencies and relative amplitudes of the cyclic air loading imposed on the engine compressor blades. The engine manufacturer either supplies the sensing Page E – 64.4
probe or specifies its design and performance. Also, the engine manufacturer may evaluate the test results or publish acceptance criteria. A wave analysis may be involved in identifying higher order excitations. Engine exhaust ducts which include bends, noise suppressors, or other obstructions may require an evaluation similar to that discussed above for the engine inlet. The engine manufacturer should be consulted for instructions or approval of this aspect. High performance engines may also require an engine inlet temperature survey. Details of instrumentation and acceptance criteria should be provided by the engine manufacturer. Engines equipped with only centrifugal compressors are less likely to encounter frequency coupling and may not require this investigation. T he engine manufacturer’s recommendations should be followed in these cases.
(3) Torsional Stability.
(i) Explanation. Governor-controlled engines installed in rotorcraft are subject to a fuel control resonant feedback condition which could be divergent if not properly designed or compensated. This condition occurs when the response frequency of the governor on the engine is coincident with or close to a low order natural torsional frequency of the rotorcraft’s rotor drive system. Typically, these frequencies appear in the 3 to 5 cycles per second (CPS) range. The manufacturer usually resolves torsional instability problems by introducing damping into the engine governor or fuel control.
Provisions for this change must be supplied by or approved by the engine manufacturer.
The final configuration may be a compromise between a lightly damped control, which will allow a positive but slow convergence of drive system torsional oscillations, and a highly damped control which exhibits excessive rotor speed droop or overspeed following rotorcraft collective control displacement.
(ii) Procedures. A ground and flight test program should be devised to evaluate the torsional response of the engine and drive system combination presented by the applicant. Instrumentation to record drive system torsionals should be applied to all major branches of the drive system. E ngine parameters such as torque, RPM fuel manifold or nozzle pressure, compressor discharge pressure, and governor lever position should be recorded simultaneously with drive system parameters. The test program should include ground tie-down operation and flight operation across a range of engine power and rotor speeds while injecting control inputs as close to the first order drive system natural frequency as possible. Mechanical methods of making these inputs are not usually necessary if the desired frequency is in the 3 to 5 CPS range and the instrumentation readout confirms that the drive system was actually excited torsionally at its natural frequency. Control inputs should include co llective, antitorque, and throttle. Also, cyclic inputs may be im portant on tandem rotor rotorcraft. The acceptance criteria may be dependent on several items. Among these are rotor and drive system fatigue loading, engine power response characteristics, limitations established by the engine manufacturer, etc. The acceptance criteria are usually stated as a percent damping (minimum). Typically, 1 percent of critical equivalent viscous damping (or greater) is acceptable. In effect, this means that the free vibration response to a control input damps to ½ amplitude in 11 cycles or less.
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SUBPART E - POWERPLANT
FUEL SYSTEM AC 29.951. § 29.951 (Amendment 29- 12) FUEL SYSTEM - GENERAL.
a. Explanation.
(1) The term “fuel system” means a system which includes all components required to deliver fuel from the tank(s) to the engine(s). This includes, but is not limited to, all components provided to contain, convey, drain, filter, shutoff, pump, jettison, meter, and distribute fuel to the engines.
(2) Paragraph (a) of this section is a general statement of the performance requirements for fuel systems and constitutes authority to require fuel systems to be adequate notwithstanding compliance with detail requirements listed in §§ 29.953 through 29.999 of this subpart.
(3) Paragraph (b) of this section requires fuel systems to be designed so that air will not enter the system under any operating conditions by either arranging the system so that no fuel pump can draw fuel from more than one tank or by other acceptable means.
(4) Paragraph (c) of this section sets forth a fuel system performance requirement intended to ensure that ice to be expected in fuel when operating in cold weather will not prevent the fuel system from supplying adequate fuel to the engines.
Although fuel system filters and strainers are the items in the fuel system most susceptible to clogging from ice particles in the fuel, this paragraph requires that the entire fuel system be shown to be capable of delivering fuel, initially contaminated with ice, to the engine(s).
b. Procedures.
(1) For paragraph (a), the applicant should show compliance with the fuel system requirements of this subpart, except that if unusual fuel system arrangements or requirements exist which are not adequately addressed by these subparts, this paragraph may be used as authority to require special tests, analysis, or system performance needed for proper engine functioning.
(2) For paragraph (b), review the fuel system design with special attention to fuel tank selector valves, crossfeed systems, and multiple tank outlet arrangements to ensure that no allowable fuel system configuration will permit air to enter the system.
For questionable situations, the applicant should conduct ground or flight tests, as necessary, to verify compliance with this section.
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(3) Paragraph (c) provides for sustained satisfactory operation of the fuel system with cold fuel initially contaminated with water. Since ice in the fuel system is not considered to be an emergency condition but, rather, is an expected service encounter, compliance would not involve the imposition of special rotorcraft limitations.
Flight manual instructions such as land as soon as practicable, reduce altitude to some value less than otherwise permitted, reduce power, turn on boost pumps, etc., are not appropriate in demonstrating compliance. Some methods of fuel system ice protection which have been used to show compliance follow.
(i) Fuel heater. Usually these devices are fuel -to-engine oil heat exchangers and are normally located to protect the fuel filter from blockage by ice in the fuel. The adequacy of these devices should be established. Usually this involves generation of a heat balance between heat gained by fuel and heat lost by oil using performance data provided by the manufacturers of the fuel-oil heater, the oil cooler, the heat rejected by the engine to the oil, etc. A minimum oil temperature associated with the adequacy of the fuel heater may need to be established, marked on the oil temperature gauge, and verified to be maintained during critical flight conditions. Other unprotected parts of the fuel system remain to be evaluated and substantiated for compliance with this requirement.
(ii) Oversized fuel filter. This method may only substantiate the fuel filter and, as with the fuel heater method, is incomplete without evaluation of the remainder of the fuel system. An icing test of the filter should be accomplished. Fuel preparation procedures and method of testing should follow the applicable portion of SAE Aerospace Recommended Practice (ARP) No. 1401. A satisfactory configuration is achieved when a filter is demonstrated to have the capacity to continue to provide the filtration function, without bypassing, when subjected to fuel contaminated by ice to the degree required by this rule. Usually, a delta pressure caution signal for the filter is needed to alert the flightcrew that progressive filter blockage is in progress. The caution device setting should be established by test which demonstrates that after illumination of the caution signal sufficient filter capacity exists to enable completion of the flight.
Fuel pressure should not fall below established limits because of ice accumulation on the filter.
(iii) Anti-ice additives. This method utilizes the properties of ethylene glycol to reduce the freezing temperature of water in the fuel. It has the advantage over other methods of protecting all components in the fuel system from ice blockage.
Compliance with the rule by this method involves the following.
(A) Eligible additives. PFA -55MB (Phillips Petroleum Co.) and additives per specification MIL-I-27868, Revision D, or earlier. Later versions of this specification do not require glycerin, which may be needed to protect fuel tank coatings.
(B) Compatibility. Both engine fuel system and aircraft fuel system should be verified to be chemically compatible with the additive at the maximum concentration to be expected in the fuel system. Usually, information on eligible system materials can Page E - 67
be obtained from the engine manufacturer for the engine fuel system and from the additive manufacturer for aircraft fuel system materials.
(C) Adding or blending the additive to the fuel. These additives do not mix well with the fuel and indiscriminate dumping of additive into the tank will not only fail to protect the system from ice accumulation but likely will damage nonmetallic components in the system. Some fuels may have additive premixed in the fuel. If other fuels are to be eligible, a method for blending additive into the fuel during refueling must be devised and demonstrated to be effective.
(D) Placards should be added near the fuel filler opening to note that fuel must contain the approved anti-ice additive within the minimum and maximum allowed concentration.
(E) The FAA/AUTHORITY-approved flight manual should contain necessary information to attain satisfactory blending of the additive and procedures to allow the operator to check the blend in the fuel tank.
(iv) Fuel system protection (other than filters). If the fuel heater method or oversize filter method (paragraphs b(3)(i) and b(3)(ii)) is proposed, the remainder of the fuel system should be shown to be free from obstruction by fuel ice. This may be shown by testing the system with ice contaminated fuel (prepared as suggested for filter tests) or, in many cases, by selecting fuel system components which by test or by previous experience are known to be free of ice collection tendencies. Tank outlet screens (or tank-mounted pump inlet screens) may be the significant fuel system feature for further evaluation. In some instances, fuel turbulence due to pump motions may be sufficient to keep the screen clear of ice. In other instances, small screen bypass openings (approximately one-fourth inch in diameter) located outside the predominant fuel flow path have been found satisfactory.
NOTE: Advisory Circular (AC) 20-29 contains information regarding compliance with the fuel ice protection requirements of Part 25. The information in this AC is largely valid except for references to the quantity of water to be expected in fuel and the amount of additive required to ensure freedom from fuel ice hazards.
AC 29.952. § 29.952 (Amendment 29-35) FUEL SYSTEM CRASH RESISTANCE.
a. Explanation.
(1) Section 29.952 provides safety standards that minimize postcrash fire (PCF) in a survivable impact. The rule contains comprehensive crash resistant fuel system (CRFS) design and test criteria that significantly minimize fuel leaks, creation of potential ignition sources, and the occurrence of PCF. Section 29.952 accomplishes this for survivable impacts by- Page E - 68
(i) Providing comprehensive criteria to minimize fuel leaks and potential ignition sources;
(ii) Requiring increased crash load factors for fuel cells in and behind occupied areas to ensure the static, ultimate strength necessary for impact energy absorption, structural integrity, fuel containment, and occupant safety;
(iii) Maintaining the load factors of § 29.561 for fuel cells in other areas (particularly underfloor cells) to ensure leak-tight fuel cell deformation in energy absorbing underfloor structure without unduly crushing or penetrating the occupiable volume; and
(iv) Requiring a 50 ft. dynamic vertical impact (drop) test to measure fuel tank structural and fuel containment integrity.
(2) Section 29.952 applies to all fuel systems (including auxiliary propulsion unit (APU) systems).
(3) Some similarities exist among the fire protection requirements of §§ 29.863,
29.1337(a)(2), and 29.952. The requirements in each standard are not mutually exclusive. Overlapping requirements should be certified simultaneously.
(4) The use of bladders is not mandated as this would unduly dictate design.
However, in the majority of cases, their use is necessary to meet the test requirements of § 29.952. If a design does not use bladders, the application should be treated as a new and unusual design feature that should be thoroughly coordinated with the Airworthiness Authority for technical policy to insure adequate safety. Experience has shown that bladders with wall thicknesses from 0.03 to 0.018 inches typically meet the § 29.952 test requirements.
b. Related Material. Documents shown below may be obtained from The Naval Publications and Forms Center, 5801 Tabor Avenue, Philadelphia, Pennsylvania 19120-5094, ATTN: Customer Service (NPODS).
(1) Military Specification, MIL-T-27422B, Amendment 1, April 13, 1971, Tank, Fuel, Crash-resistant Aircraft. (canceled 6-7- 97 without replacement)
(2) Military Standard, MIL-STD-1290 (AV), January 25, 1974, Light Fixed and Rotary Wing Aircraft Crashworthiness.
(3) Military Standard, MIL-H-83796, August 1, 1974, Hose Assembly, Rubber, Lightweight, Medium Pressure, General Specification for.
(4) Military Specification, MIL-V-27393 (USAF), July 12, 1960, Valve, Safety, Fuel Cell Fitting, Crash Resistant, General Specification, for.
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(5) Military Specification, MIL-H-25579 (USAF).
(6) Military Specification, MIL-H-38360.
(7) U.S. Army Publication USARTL-TR-79-22E, “Aircraft Crash Survival Design Guide, Volume V---Aircraft Postcrash Survival”, dated January 1989.
NOTE: Section 4, “Postcrash Fire Protection” of Volume V of the Design Guide is the modern update to MIL-STD-1290. Section 4 contains a comprehensive design guide for military CRFS designs that may be useful for civil CRFS designs.
c. Conceptual Definitions.
(1) Survivable Impact. An impact (crash) where human tolerance acceleration limits are not exceeded in any of the principal rotorcraft axes, where the structure and structural volume surrounding occupants are sufficiently intact during and after impact to constitute a livable volume and permit survival, and where an item of mass does not become unrestrained and create an occupant hazard. “Livable volume” relates to the ability of an airframe to maintain a protective shell around occupants during a crash and to minimize threats, such as accelerations, applied to the occupiable portion of the aircraft during otherwise survivable impacts. In lieu of a more rational, approved criteria, the load factors of § 29.952(b)(1) constitute the structural human survivability accelerations limits.
(2) Postcrash Fire (PCF). A fire occurring immediately after and as a direct result of an impact. The fire is either the result of fuel released from a leaking fuel system reaching an existing or a crash-induced ignition source, a crash-induced ignition source internal to an undamaged or damaged fuel system, or a combination. PCF's have an intensity range from the minimum of a small local flame to the maximum of an instantaneous massive fire or fireball (explosion).
(3) Fuel Tank or Cell. A reservoir that contains fuel and may consist of a hard shell (of a composite, metal, or hybrid construction) with either a laced-in, snapped in, or otherwise attached semirigid or flexible rubber matrix bladder (or liner), spray-on bladder, or no bladder. The hard shell may be either the airframe (integral tank) or a separate rigid tank attached to the airframe. The device has inlets and outlets for fuel transfer and internal pressure control.
(4) Ignition Source. An ignition source that when wet with fuel or in contact with fuel vapor would cause a PCF.
(5) Major Fuel System Component. A fuel system part with enough mass, installation location hazard or a combination to be structurally considered in a crash.
Structural consideration is required when crash-induced relative motion can occur between the part and its surrounding structure from inertial impact forces, airframe deformation forces, or for other reasons.
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(6) Drip Fence. A physical barrier that interrupts liquid flow on the underside of a surface, such as a fuel cell, and allows it to drip nonhazardously to an external drain.
(7) Flow Diverter. A physical barrier that interrupts or diverts the flow of a liquid.
(8) Frangible Attachment or Fitting. An attachment or fitting containing a part that is designed and constructed to fail at a predetermined location and load.
(9) Deformable Attachment or Fitting. An attachment or fitting containing a part that is designed and constructed to deform at a predetermined location and load to a predetermined final configuration.
(10) Self-Sealing Breakaway Fuel Fitting. A fuel -carrying in-line, line-to-firewall, bulkhead or line-to-tank connection that breaks in half and self-seals when subjected to forces greater than or equal to the unit’s design breakaway force.
Each half self-seals using a spring-loaded valve (e.g., trap door or equivalent means) that is normally open but is released and closed upon fitting separation. Fitting breakaway force is typically controlled by a frangible metal ring (or series of circumferential tabs) that connects the two fitting halves. Normal, fuel -tight integrity is maintained by “O” rings held under pressure by the rigid, frangible connecting ring (or tabs). When broken open, a small amount of fuel (usually less than 8 ounces) is released. This is the fuel trapped in the coupling space between the two spring -loaded valves. Once failed each coupling half may leak slightly. Typically, this leak rate should be less than 5 drops per minute per coupling half.
(11) Crash Resistant Flexible Fuel Cell Bladder. Flexible, rubberized material, usually with fibers (i.e., rubber “resin” and natural or synthetic fiber) in both the 0° (warp) and 90° (fill) directions that is used as a liner in a rigid shell or integral tank. The material acts as a membrane because, when unsupported, it can only carry pure tension loads. Therefore, it must be uniformly supported by rigid structure (reference § 29.967) so that the liner carries only compressive fluid loads and the surrounding shell structure carries the fluid-induced shear, tension, and bending loads transmitted through the liner or bladder. The material is usually secured (e.g., laced , snapped, etc.) into its surrounding structure at key locations to maintain its intended conformal shape. In many designs, lightweight spacers, such as structural foam, are used between the liner and the airframe to maintain the liners intended conformal shape and to transmit fluid loads to the airframe. The material is either qualified under TSO-C80, “Flexible Fuel and Oil Cell Material,” or qualified during certification.
Sections 29.952 and 29.963(b) have increased the minimum puncture resistance qualification requirement for liner material (See TSO-C80, paragraph 16.0) from 15 to 370 pounds.
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(12) Crash Resistant Fuel System (CRFS). A fuel system designed and approved in accordance with § 29.952 that either prevents a PCF or delays the start of a severe PCF long enough to allow escape.
(13) As Far as Practicable. “As Far as Practicable” means that within the major constraints of the applicant’s design (e.g., aerodynamic shape, space, volume, major structural relocation, etc.), this standard’s criteria should be met. The level of practicability is much higher in a new design project than in a modification project. The engineering decisions, evaluations, and trade studies that determine the maximum level of practicability should be documented and approved.
(14) Fireproof. Defined in § 1.1, “General Definitions” and in AC 20- 135, “Powerplant Installation and Propulsion System Component Fire Protection Test Methods, Standards and Criteria” dated February 6, 1990.
d. Procedures.
(1) Section 29.952 should be applied to all fuel system installations. Any major design change should be reevaluated for compliance with the CRFS requirements. It should be noted that most standard materials and processes are acceptable for crash resistant fuel system construction; however, magnesium, magnesium alloys, and cadmium plated parts (when exposed to fuel) are not recommended, because of their inherent ability to create or contribute to a post crash fire. Section 29.952(a) requires each tank, or the most critical tank (if clearly identified by rational analysis) to be drop tested. The tank is filled 80 percent with water and the remaining 20 percent is filled with air (or, in the case of a flexible fuel cell, the air may be evacuated by hand and the cell resealed). The tank openings, except for the vents, are closed with plugs (or other suitable means) so that they remain watertight. The vents are left open to simulate natural venting. Otherwise, the tank is flight configured. The test tanks are installed in their surrounding structure and dropped from a height of 50 feet on a nondeformable surface (e.g., concrete or equivalent). To be considered a valid test, the tank must impact horizontally ±10°. The 50- foot distance is measured between the nondeformable surface and the bottom of the tank. The ±10° attitude requirement can be ensured by using lightweight cord or a light sling to balance the tank assembly horizontally prior to being dropped. MIL-T- 27422B shows a typical test setup.
Tank attitude at impact should be verified by photography or equivalent means. The nondeformable floor surface should be covered by a thin plastic sheet so that any leakage is readily detected. The tank water should be tinted with dye to make leakage and seepage sources easy to identify. The tank (except for the vent openings) should be wrapped in light plastic sheet to ensure that minor leakage or seepage (and its source) is detected. Minor spillage through the open vents during the drop test is allowed. The dye should not significantly affect the water’s viscosity or other physical properties that may reduce or eliminate any leakage from the drop test. The nondeforming drop test surface should be carefully reviewed. Concrete is acceptable.
A fixed and uniformly supported steel plate (loaded only in uniform compression without any springback) is acceptable. Floors or floor coverings such as dirt, clay, wood, or Page E - 72
sand are not acceptable. Selection of the critical fuel tank is important. Factors such as size, fuel cell design and construction, and material(s) should be accounted for when selecting the critical tank. The applicant may elect to drop only a bare fuel cell, not a surrounding structural airframe segment with a fuel cell installed. If so, the applicant must show that puncture hazards to the fuel cell have been eliminated.
(i) If the applicant elects to perform the drop test with surrounding aircraft structure, the cell should be enclosed in enough surrounding structure (production or simulated) so that the airframe/fuel tank interaction during the 50-foot drop is realistically evaluated. This allows the fuel -tight integrity of the “as installed” fuel cell to be evaluated and may provide protection in some designs due to the energy absorption of the surrounding airframe when crushed by impact. This provides realistic testing of fuel cell rupture points caused by installation design features, projections, excessive deformation and local tearout of fittings, joints, or lacings. The amount of actual (or simulated) structure included in the test requires engineering evaluation, risk assessment, and detailed analysis and may require subassembly (e.g., joint) tests for proper determination. Typically, the structure surrounding and extending 1 foot forward and aft of the fuel cell is adequate. This structure has a high probability of causing crash-induced fuel cell leakage. Each application should be examined individually to include all potential structural hazards. If the surrounding structure is clearly shown not to be a contributing hazard for the drop test, and if the applicant elects to do so, the fuel cell may be conservatively dropped alone. This determination should be carefully made by a detailed engineering evaluation. The evaluation should use standard, finite element-based programs (e.g., ‘KRASH”, NASTRAN, etc.) or similar programs submitted during certification, subassembly or component tests. Elimination of the surrounding structure for the drop test configuration is not trivial. If elimination is applied for, the data should clearly and conclusively show that the surrounding structure is not an impact hazard. In any case, the drop height is a constant 50 feet. The work that determines the test article configuration should be summarized, documented, and approved.
(ii) If the drop test is used to show partial compliance with the underfloor fuel cell load factors of § 29.952(b)(3), test plans should be approved. Minor spillage from the open vents is allowed. Full compliance to these load factors should be shown by static analysis and/or tests. The intent is to provide a fuel cell that is fuel tight and does not unduly crush the occupiable volume or overly stiffen energy absorbing underfloor structure under vertical impact.
(iii) Immediately after the drop test, the tank should be placed in the same axial orientation from which it was dropped and visually examined for leakage. Minor spillage from the open vents is allowed. After 15 minutes, the tank should be reexamined and any new leakage or seepage sources noted and recorded. Any evidence of fluid on the plastic floor cover or tank wrapping sheet should be noted and recorded. Any fluid leakage or seepage constitutes a test failure. This procedure should be repeated immediately with the tank inverted and the vents plugged. The inversion procedure will identify any leak sources on the upper surfaces.
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