U.S. Deportment AC 25-8 of Transportation Federal Aviation Administration DATE 5/2/86
ADVISORY CIRCULAR
AUXILIARY FUEL SYSTEM
INSTALLATIONS
Advisory U.S. Department of Transportation Circular Federal Aviation Administration Subject: AUXILIARY FUEL SYSTEM Date: 5/2/86 AC No: 25-8 INSTALLATIONS Initiated by: ANM-110 Change:
PURPOSE. This advisory circular (AC) sets forth acceptable means, but not Federal Aviation Regulations (FAR) pertaining to the installation of auxiliary SCOPE. This document provides guidance and criteria for the installation of auxiliary fuel systems, i.e., those which supplement essential fuel systems to provide additional range, in transport category airplanes. It is intended primarily for installations in which the auxiliary fuel is carried within the fuselage, such as within cargo or baggage compartments, the main deck or other similar areas. Although the material presented in this AC is generally applicable to other installations that involve changes in primary structure, aerodynamics or mass distribution, such installations may require extensive additional substantiation that is beyond the scope of this AC. Similarly, additional substantiation beyond the scope of this AC would be required for essential fuel system installations. "Auxiliary" and "essential" fuel systems are defined in more detail in Appendix 1 of this AC.
BACKGROUND. Currently, many applications for supplemental type certificates (STC) and amended type certificates are being received by a number of the FAA aircraft certification offices for incorporation of additional fuel capacity in existing certificated airplanes. The design and safety concepts for fuel storage and transfer proposed in these applications vary considerably. In addition, some auxiliary fuel system installations are being used for control of the center of gravity (c.g.), which results in fuel usage late in the flight profile. These factors, coupled with the differing requirements that each existing airplane fuel system imposes, complicate an evaluation of the safety and airplane compatibility of the proposed installation.
GENERAL. Betore determining the auxiliary tuel system contiguration and moditying the airplane, the applicant should become famillar with the existing airplane structural and systems characteristics and tunctions, and with the applicable certification requirements. To avoid structural and systems compatibility problems, a working knowiedge of the airplane is essential.
particular, the applicant should determine the effects of the addition of the auxiliary fuel system on payload, c.g., system and airplane operations and FAA Form 1320-15 (4-82) Supersedes WA Form 1320-2
structural margins. When in doubt about any certification requirement, the applicant should consult with the FAA aircraft certification office responsible for his project early in the design program to avoid possible costly changes late in theprogram.
5. CERTIFICATION BASIS.
a. New Type Certificates. For the issuance of a new type certiticate, an airplane must be shown to comply with the certification basis established in accordance with § 21.17 of the Federal Aviation Regulations (FAR). If the regulations do not provide adequate or appropriate standards because of a novel or unusual design feature, special conditions may be prescribed in accordance with § 21.16.
D. Other Design Changes. For other design changes, such as the addition of a new model to an existing type certiticate or modification of an existing model, the airplane must be shown to comply with the certification basis established in accordance with 9 21.101. Generally, the applicant may choose compliance with the regulations incorporated by reference in the type certificate (the original certification basis) or with the applicable regulations in effect on the date of the application for approval of the auxiliary fuel system (current rules). If the original certification basis does not provide adequate or appropriate safety standards because of novel or unusual design features, compliance with current rules may be prescribed in accordance with § 21.101(b). If neither the original certification basis nor current rules provide adequate or appropriate standards, special conditions may be prescribed in accordance with § 21.16. Although sections of Part 25 are referenced in this AC, the references should be interpreted to be the corresponding sections of Part 4a or 4b of the Civil Air Regulations (CAR) when Part 4a or 4b is the original certification basis.
established certification basis, 9 21.21 precludes approval if there is any feature or characteristic that makes the airplane unsafe. The applicant should Unsafe Features or Characteristics. Notwithstanding compliance with the recognize that it may be necessary, because of such a feature or characteristic, to impose special requirements which exceed the standards of the certification basis, to eliminate the unsafe condition.
PROCEDURES. In order to avoid delays and possible expensive redesign, it is strong y recommended that the following procedures be followed. The applicant should:
Submit a proposed overall certification plan that identifies the essential steps or actions and the sequence anticipated for submitting reports, orawings, process specitications, analyses, tests and other documentation to complete the installation approval. This program should include the proposed or target schedule for the FAA approval tests and inspections required.
Generate a certification test plan which describes the analytical procedures or qualification testing to be used to demonstrate the design
adequacy. Each plan should list the applicable FAR and describe how each requirement will be met. In addition, the plan should include a description of the airplane or test articles to be used, drawings, method of production simulation (if applicable), and the target date for installation and test. The certification test plan should be submitted for review and concurrence by the appropriate FAA aircraft certification office prior to initiation of tests, to prevent certification delays.
c. Obtain FAA concurrence that each certification test plan is adequate.
d. Obtain FAA conformity inspection of the test installation.
e. Schedule and conduct the ground and flight test (s) with FAA witnessing.
f. Submit a final test report describing all test results and obtain FAA approval.
Le hill LEROY A. KEITH Manager, Aircraft Certification Division III (and IV) For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402
CONTENTS Paragraph Page No.
CHAPTER 1 - FUEL SYSTEM INSTALLATION INTEGRITY AND CRASHWORTHINESS
1. STRUCTURAL INSPECTION. 1
2. TANK LOCATION CRITERIA 5
3. AIRPLANE FUEL SYSTEM COMPATIBILIT 7 CHAPTER 2 - AUXILIARY FUEL SYSTEM ARRANGEMENT
4. GENERAL ARRANGEMENT EVALUATION 11
5. FUEL SYSTEM CONTAMINATION PREVENTION ASSESSMENT 15
6. IGNITION SOURCE ISOLATION EVALUATION 17 CHAPTER 3 - COMPONENT MATERIALS
7. GENERAL 25
8. ENVIRONMENTAL PROPERTIES EVALUATION 25
9. COMPOSITES 27 CHAPTER 4 - AUXILIARY FUEL SYSTEM PERFORMANCE
10. GENERAL 29
11. NORMAL OPERATION EVALUATION. • 29
12. FAIL-SAFE AND EMERGENCY OPERATION EVALUATION • 33 V
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.
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