CHAPTER 5 - IMPACT OF SYSTEM ON AIRPLANE OPERATION AND PERFORMANCE
13. ENGINE OIL SYSTEM CAPACITY . .
14. MAIN FUEL SYSTEM CAPACITY AND FLOW REGULATION
15. RADIO FREQUENCY INTERFERENCE . .
16. AIRPLANE CENTER OF GRAVITY CONTROL CHAPTER 6 - USER INSTALLATION REQUIREMENTS
17. AIRPLANE PLACARDS AND INSTRUMENT MARKING EVALUATION
18. AIRPLANE FLIGHT MANUAL (AFM) . .
19. WEIGHT AND BALANCE MANUAL INFORMATION
20. MAINTENANCE MANUAL APPENDIX l
1. DEFINITIONS VI
CHAPTER 1 - FUEL SYSTEM INSTALLATION INTEGRITY AND CRASHWORTHINESS
1. STRUCTURAL INSPECTION.
General. Survivable accidents have occurred at vertical descent velocities greater than the 5 feet per second (f.p.s.) referenced in $ 25.561.
The energy from such descents is absorbed by the structure along the lower fuselage. As the limits of survivable accidents are approached, structure under the main cabin floor is crushed and deformed and the volume below the floor, where the auxiliary fuel tanks are frequently located, may be reduced and reshaped. For this reason the tank material chosen by the applicant should provide resilience and flexibility; or, in the absence of these characteristics, the tank installation should provide extra clearance from structure that can be crushed or be protected by primary structure not likely to be crushed. If lightweight composite structure with brittle failure characteristics is chosen, compliance with current regulations or special conditions may be required.
Design Criteria and Structural Loads (§$ 25.25, 25.365, 25.561, 25.963 and 25.965).
(1) The extent of structural substantiation required depends on the magnitude and location of the added fuel and the modifications required to accommodate the fuel tank installation. Generally, evaluation of the tank attachment hardware and local structure will be sufficient; however, as noted earlier, installations that involve changes to primary structure, aerodynamics or mass distribution may require additional extensive substantiation that is beyond the scope of this AC. Any increase in maximum weight or changes in c.g.
limits to increase the utility of the airplane with the auxiliary fuel system installed is also beyond the scope of this AC.
(2) The tank design should isolate the tank from airframe induced structural loads and from deformations induced by the wing and fuselage.
(3) The fuel tank and its attachment and support structures must be designed to withstand all design loads, including the emergency landing load specified in § 25.561(b).
(4) Fuel loads included in the structural substantiation should be based on the most critical density of the fuels approved for use in the airplane.
(5) A particular fuel tank design may require cyclic pressure testing to its service life limit using appropriate scatter factors.
In accordance with the requirements of § 25.963(d) regarding retention of the auxiliary fuel, it should be shown by a crashworthiness analysis or the equivalent that the airplane lower fuselage and auxiliary fuel Chap 1 Par I 1
tank supporting structure are capable of withstanding the crash loads found in § 25.561. Dynamic loads defined by the crashworthiness analysis should be accounted for in the stress analysis.
(7) Sufficient vehicle structural crush distance should be available to avoid auxiliary fuel tank ground contact under the loading conditions of § 25.561(b). Compliance may be shown by analysis and where necessary by test.
The analysis should identify the failure mode and define the interaction between the tank and adjacent structure and between adjacent tanks.
Structural deformation must be shown to be controllable and predictable, as required by $ 25.965.
(9) The fuselage forebody and afterbody are limited to design values of distributed loads that cannot be exceeded without extensive resubstantiation and possible modification to primary structure. Trade-offs between passengers, cargo, and tue, may be made provided the allowable floor, bulkhead and local loads are not exceeded.
(10) Airplane Flight Manual (AFM) limitations and procedures affecting fuel management may be required to ensure that structural limitations are not exceeded.
(11) Bottom and lower structure that is adequate for tank load distribution and protection against rupture in crash landing should be provided for all tanks. Consideration should be given to eccentricities introduced into the basic airframe from fuel tank attachments.
(12) The following must be considered in the evaluation of the tank and tank support structure in accordance with the applicable certification basis:
(i) Tank internal pressure developed during malfunction of the pressure shut-off means and pressure induced by engine bleed air (if applicable).
(ii) Applicable load factors, including the effects of cabin pressurization, with maximum fuel in the tank.
tank should be designed to withstand the pressure differential resulting from the pressure inside the tank at maximum cruise ambient altitude and that of the cabin at the maximum pressure relief valve setting.
(iv) When the tank is located inside the pressurized section of the fuselage, the fuselage structure must be strong enough to withstand the pressure differential loads corresponding to the maximum relief value setting by a factor of 1.33, omitting other loads. This is a limit load condition which requires a safety factor of 1.5 to obtain ultimate loads.
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(v) Except as provided in § 25.625(b), a fitting factor of at least 1.15 must be applied to all tank support fittings and their attachment to the tank.
(vi) All probable combinations of fuel distribution in multiple tanks, including slosh, should be accounted for in defining tank structural loads and airplane weight and balance.
(vii) To preclude rupture and provide durability, the face sheet thickness should be sufficient for the applicable load requirements and to prevent accidental damage. Where aluminum is used for these purposes, thicknesses are typically not less than .040 inch for the outer face sheets or .020 inch for the inner face sheets.
(viii) To facilitate inspection and to isolate primary structure from tank corrosion, the tank should generally not be designed as an integral part of the body structure.
c. Structural Modification.
(1) Where existing structure is being modified for the tank installation, adequate reinforcement should be added as necessary to maintain structural integrity equal to or better than the original structure.
(2) Moditications, such as cutouts, tank support attachments, and service openings through exterior skins, should not degrade structural load capability or lead to reduced fatigue capability. If holes must be cut through te taken to reseal the penetrations, pasticularly those affecting the pressure special care should vessel. The fuselage should be pressure tested in accordance with the requirements of § 25.843 if major modifications to the cabin pressure vessel are
(3) The effect of the modification on the existing maintenance program, including the structural supplemental inspection program, if applicable, must be considered and appropriate changes made in accordance with § 25.1529.
d. Crash Overload. Hard attachment points between the fuel tank and airframe structure restrict relative motion and, in turn, impose high concentrated loads on both the tank and the airframe. In order to limit the magnitude of these concentrated loads, crash load failure points are typically located between the tank and airframe. In addition:
(1) Attachment point loads should be evenly distributed to minimize the possibility of fuel tank rupture.
(2) In the event of an overload condition, the failure should occur at some point between the tank attach fitting and the basic airframe and floor structure to minimize potential body tank rupture. Where possible, the design Chap 1 Par 1 3
should prevent failure of the tank support from causing failure of the fuel lines, for the maximum tank displacement that could occur. It may be necessary to incorporate redundant supports or secondary constraint bulkheads to accomplish this.
the in Tie alone a fore certication basin Damage Tolerance and Fatigue Evaluation. Each structural element of amended by Amendment 25-45 or subsequent amendment, must be evaluated using damage tolerance criteria or, as provided by $ 25.571(c), by fatigue (safe-life) criteria. Airplanes having earlier certification basis may be evaluated using fail-safe criteria.
(1) For fail-safe design, it must be shown by analysis, tests, or botn, that catastrophic tallure or excessive structural deformation, that could adversely attect the tight characteristics of the airplane, are not probable after fatigue failure or obvious partial failure of a single principal structural element. After these types of failure of a single principal structural element, the remaining structure must be able to withstand certain prescribed loads considered as ultimate loads.
2) ror sate-ite design, the structure must De shown by analysis ests, or both to be able to withstand the repeated loads of variable magnitude expected in service. The evaluation must include the typical loading spectrum expected in service and identification of principal structural elements and detail design points, the fatigue failure of which could cause loss of the airplane.
(3) For damage tolerant design, the structure must be evaluated to ensure that should serious fatigue, corrosion, or accidental damage occur within the operational life, the remaining structure can withstand reasonable loads without failure or excessive deformation until the damage is detected. To achieve this design objective, it is important to draw on test results for design data. Damage-tolerant design is required unless it entails such complications that effective damage-tolerant structure is impractical.
Structures for which application of damage tolerance criteria may not be practical include single load path designs where in-service inspections are not feasible due to comparatively small critical crack sizes (tank hangers, attach fittings, single fuel containment structure, joints, splices, fuel lines, etc.).
Where damage-tolerant design is impractical, the fatigue characteristics are established to ensure that the anticipated service life can be attained. The fatigue evaluation should be based on test results. In this regard, the tank should be pressure cycled appropriate to its design life and all significant details should undergo fatigue tests including adjoining structure and supports that contribute to an accurate picture of its deflection pattern. If the tank installation results in increased loads to the airframe, and there is a required structural inspection program, the applicant must provide a damage tolerance design assessment and associated airworthiness limitations. If the airframe is safe-life, fatigue substantiation is required. In any event, more Chap 1 Par 1
stringent inspections are to be provided where they are needed. Refer to AC
25.571-1, Damage Tolerance and Fatigue Evaluation of Structure, for further guidance concerning damage tolerance.
2. TANK LOCATION CRITERIA.
Uncontained Engine Rotor and Blade Failure Considerations ($$ 25.571 and
25.903(d)).
(1) The applicant should evaluate the location of fuel tanks and other major fuel system components from the standpoint of the protection afforded against uncontained engine and auxiliary power unit (APU) rotor or blade failures. For this evaluation the applicant should review the information and recommendations given in Order 8110.11, Design Considerations for Minimizing Damage Caused by Uncontained Aircraft Turbine Engine Rotor Failures, or any superseding FAA guidance material. The applicant should also obtain information about the containment features which may be incorporated in the particular engines or APUs used on the airplane involved. The energy levels of uncontained rotor/blade fragments specified by the engine/APU manufacturer should be used.
(2) The criteria to predict the probable fragmentation scatter, shown in Figure l, can be used as a generalized guideline. Past service experience has shown that the expected overall fragmentation zone, for both large and small fragments, lies within a total spread angle of approximately + 15 degrees of the plane of any rotor. In many cases fuel tanks and critical system components can be located outside of these tragmentation zones, theredi ninimizing the hazards to the system and the airplane. Where locations withir engine fragment zones are unavoidable, the applicant should refer to Order 8110.11 for information and suggestions regarding possible design techniques to minimize the hazards to the airplane. In any event, installation of these types of systems into already certified airplanes should ensure that the basic airplane safety level is maintained and not degraded.
(3) For certification compliance, a report which addresses the above considerations should be prepared and submitted. The report should adequately consider uncontained rotor and blade failures from all engine and APU sources which may effect the integrity of the auxiliary fuel system and show that design precautions have been taken to minimize hazards to the airplane in the event of these failures and that the airplane safety level has not been degraded by the auxiliary fuel tank/system installation.
Proximity to High Temperature Ducting and Equipment ($$ 25.609,
25.613(c), 25.981 and 25.1185(a)).
(1) The applicant should evaluate the location of fuel tanks from the standpoint of exposure to heat from other adjacent systems and components. The evaluation should include an analysis of heat exposure due to failure of an adjacent component (tor example, the failure of a high temperature pneumatic duct coupling located near the tank). For this evaluation the effects oi temperature on the tank allowable stresses should be considered and it should Chap 1 Par 1 5
be shown that no hazardous loss of strength or other unsafe condition will result from the heat exposure.
(2) Consideration of the effects of external heat exposure on the fuel in the tank should also be included in this evaluation. In some cases, although the tank structure will satisfactorily withstand heat exposure, heat conduction through the tank wall could elevate the fuel temperature causing unacceptable engine performance or flammability or explosion hazard (see also Chapter 2, paragraph 6h., Maximum Surface Temperatures). For tanks which use flexible bladder cells, the effects on bladder material due to neat conducted through the supporting tank wall should also be evaluated.
(3) For the particular model of airplane under review, identification of all probable heat sources with respect to the tank system location should be provided, and it should be demonstrated that the tank installation will remain safe even after a single failure of the adjacent system. Detection systems or other means which provide adequate overheat indication are acceptable provided sufficient instructions are provided in the AFM to show the interrelationship of the "failed" system to the auxiliary tank system installation.
Proximity to ruselage Break/Separation Points. Fuselage break points (Appendix I) are typically found at areas of structural discontinuity in the fuselage shell. where possible, avoid locating the tank and its support structure at these discontinuities.
d. Installations in Cargo and Baggage Compartments ($$ 25.855(b),
25.855(a-1), (a-2) and 25.857).
(1) The various components of an auxiliary fuel system installed in cargo and baggage compartments should be protected from damage caused by shifting cargo. A cargo barrier should be used to separate the auxiliary fuel system from the cargo. The barrier should be designed to contain the maximum cargo loading for which the compartment is approved under all load conditions including the emergency landing conditions. This barrier may be either a rigid or a flexible type. Solid barriers are sometimes installed to totally separate and isolate the auxiliary fuel system from the compartment, resulting in a reduced compartment size. If the barrier is flexible, consideration should be given to deformation or displacement of the barrier when under load. If minimum tension requirements are necessary to maintain the structural integrity of a flexible barrier, the requirements should be specified and conspicuously displayed in the compartment. Finally, the barrier should prevent any type of bulk cargo, particularly slender or sharp objects, from penetrating components of the auxiliary fuel system, and be structurally capable of preventing cargo from contacting the fuel system installation under all load conditions including emergency landing inertia loads. Alternatively, a barrier would not be needed if it can be shown that the fuel tank system shroud or outer wall can offer equivalent protection to the remaining components of the system. In addition, the auxiliary fuel system installation should not adversely affect intercompartmental venting incorporated in the basic airplane.
Chap 1 6 Par 2
When the fuel system is installed in cargo and baggage compartments, ensure all material used, including cargo barriers and replacement compartment liners, meet the applicable flammability requirements. The materials in the fuel system and the cargo barrier, it installed, should protect the fuel system from fire or heat from a fire that could occur in the cargo and baggage compartment. As a minimum, a cargo barrier should meet the flammability requirements of cargo and baggage compartment liners.
(3) If the compartment is a Class B compartment, consideration should be given to the required smoke/fire detector and to the access required for a crewmember to fight a fire. If the compartment is a Class C compartment, care should be taken to avoid interfering with or reducing the effectiveness of the approved smoke or fire detector, built-in fire-extinguishing system or ventilation control system. If the compartment is a Class D compartment, consideration should be given to retaining the effectiveness of the ventilation control system and to the prolonged effect of heat from a smoldering fire on the fuel tank system. Classification of the type of cargo and baggage compartment in which the auxiliary fuel system is to be installed should be defined by the applicant and verified by the applicable FAA aircraft certification office.
(4) If holes must be cut through cargo and baggage compartment liners for auxiliary fuel system lines, care should be taken to reseal the liner.
the fuel system is installed in a cargo and baggage compartment and if positive sealing cannot be verified by inspection or localized test, the certified allowable leakage from the compartment should be obtained and evaluated with the auxiliary fuel system total leakage included to ensure that the compartment classification is not affected. This may require a flight test.
(5) When the auxiliary fuel system is designed to be quickly removed, consideration should be given to the method for resealing or plugging holes while the system is removed. Complete maintenance instructions should be provided detailing the method of removal, resealing and restoring the airplane essentially to its original configuration.
AIRPLANE FUEL SYSTEM COMPATIBILITY ($$ 25.901(C), 25.951, 25.953, 25.955,
25.957 and 25.961).
a. Configuration Criteria.
(1) the essential fuel system to ensure that no hazardous fuel transfer or feed conditions exist. The criteria used to make this evaluation will depend on the type of auxiliary fuel system selected. The requirements for the two primary types of auxiliary fuel systems, the transfer and the direct feed type, vary considerably. The transfer type system supplies fuel from the auxiliary tank to an existing main tank or tanks. The direct feed system supplies fuel, in at least one operating mode, directly to an engine.
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(2) The requirements for a direct feed auxiliary fuel system are considerably more stringent than those for a transfer auxiliary fuel system.
These requirements, in general, ensure that an uninterrupted flow of fuel at the required pressure and flow rate is provided to each engine for all operating conditions of the airplane. These requirements also address altitude performance effects and low and high temperature fuel aspects as well as providing fuel system independence in at least one configuration. Failure mode and effects analyses (FMEA) are needed to ensure that no hazardous conditions exist due to a failure of the auxiliary system. Continuous engine operation should be verified when the auxiliary tank system is depleted of fuel, to prevent engine flameout or other unacceptable operating conditions.
(3) Transfer type system requirements are not as stringent but do require basic fuel system considerations such as damage to tank and vent lines due to excessive pressure from over filling, high and low temperature fuel effects, transfer rates, drainage, altitude effects, etc. The auxiliary tank depletion characteristics, especially for bleed pressurization type systems, should also be evaluated to ensure air entrainment, overpressure, etc., does not alter main tank performance. The applicant will need to determine when transfer to an essential system tank should be established and whether transfer should be in increments or continuous. The auxiliary tank quantity, tlow rates, essential system tank levels, etc., are considerations for when to initiate transfer and should be a part of the AFM procedures.
b. Interface Considerations.
(1) The interface of the auxiliary fuel system and the existing airplane fuel system should be evaluated. Use of existing fuel system lines and manifolds can provide considerable weight savings; however, the functions of these lines should be checked to ensure that the added usage does not conflict vith other requirements. for example, auxiliary tuel tanks mignt be pressure fueled from the existing airplane fueling receptacle(s); however, in manj cases, the pressure fueling manifold is also used for emergency fuel dumping or as part of the engine crossfeed system. The use of portions of the original system may create conflicting flow requirements if not thoroughly checked, particularly when a component failure, such as a valve failing to open or close, is involved.
Another example is the use of the existing airplane vent system for venting of the auxiliary tank(s). The existing vents have been sized to maintain acceptable pressure levels during refueling and flight maneuvers.
When combining the systems, the applicant should verify that no failure conditions result in an overpressure condition, i.e., during refueling or emergency descent. Some systems may require two phase flow analysis (air venting from one tank and liquid fuel venting from another tank through a combined vent system). If the essential fuel system tanks use flexible tank liners, it may be necessary to show by analysis or test that the combined vent system does not cause any hazardous bladder collapse in the essential system tanks due to rapid descent or other operating condition.
Chap 1 8 • Par 3
(3) When the auxiliary fuel system arrangement has been determined and the interfaces with the main fuel system established, a total system failure mode analysis should be conducted to ensure that no hidden failures in the system will lead to an unsafe operating condition. Analyze the system for all modes of operation, including venting, pressure tue ing, detueling, transter, engine feed, crossfeed, and emergency fuel dumping. For example, the auxiliary tank system should be designed so that failure of the system will not result in uncontrollable transfer of fuel from the main tank to the auxiliary tank when the main tank system is in its normal operating configuration.
Other Considerations. Other systems must also be evaluated tor compatibility with the auxiTiary fuel system. Ensure that the auxiliary fuel system electrical power demands do not overtax the airplane electrical system.
Use auxiliary fuel system cockpit indicators and nomenclature which are compatible with the existing cockpit displays. Cockpit instrument compatibility is especially important for the two-man crew cockpit, where pilot workload can become a critical factor and should be evaluated for Adequate cockpit display to indicate auxiliary tank depletion or improper fuel scheduling should be provided and cockpit indication of low fuel state, c.g.
unbalance or other unsafe condition should be considered for the particular Chap 1 Par 3 9
Figure l
OVERALL FRAGMENTATION ZONES Chap 1 10
CHAPTER 2 - AUXILIARY FUEL SYSTEM ARRANGEMENT
4. GENERAL ARRANGEMENT EVALUATION.
System Layout. In addition to the criteria described in the previous section, consideration should be given to the following:
(1) Line Routing, Flexibility and Support ($ 25.993).
(i) General fuel system practices should be adhered to in installing fuel and vent lines. All flexible lines should be adequately supported along the entire line installation length. Ensure that lines will not chafe against control cables, airframe structure or other equipment items.
Avoid locating lines near high temperature sources or near electrical wiring.
Where close proximity of a fuel line to electric wiring is unavoidable, locate the fuel line so that leakage cannot drip onto the wiring. Ensure that the fuel line is adequately clamped to structure to maintain the required spacing. Do not support wire bundles from fuel or vent lines. If adequate compartment ventilation or fuel line shrouding cannot be provided, isolation of electrical wiring from fuel lines may require a vapor barrier or a conduit for the wiring.
(See Chapter 2, paragraph 4a(4).)
(11) Consider the crashworthiness characteristics of the line routing. Where possible, interconnect tanks, rigid metal lines and other major fuel system components with flexible lines. Allow sufficient flexible line length to permit some shifting of the components without breaking the lines or connections. The flexibility of the entire fuselage auxiliary fuel line routing should be sufficient to account for fuselage break points. If lines are routed near structural members, the effect of "guillotine" or slashing action due to a crash landing should be addressed. When routing fuel lines through cabin floor structural lightening holes is necessary, provide sufficient clearance to prevent line severing due to floor deformations on a crash landing. A crashworthiness evaluation report of the auxiliary fuel system installation should be submitted during certification which shows, by analysis or test, that precautions have been taken to minimize the hazards due a survivable crash environment.
(2) Fuel Tank and Component Location, Access, Mounting and Protection ($$ 25.901 (b)(ii), 25.963, 25.965, 25.993 and 25.994).
(i) Each auxiliary fuel tank or tank module design should be evaluated for the basic requirements of $$ 25.963 and 25.965. These requirements address, for example, the basic integrity of the tank, bladder cell requirements, pressurized tank requirements and the tank tests, such as, slosh and vibration, that may be required.
(ii) As a general rule, all components, such as valves, pressure transmitters or switches, filters, etc., should be directly mounted to the airplane structure or to supports which are directly attached to the structure.
If fuel or other system lines or fittings are used to support auxiliary fuel Chap 2 Par 4 11