Advisory S. Department of Transportation CircularFederal Aviation Administration Subject: AUXILIARY FUEL SYSTEMS FOR Date: 8 / 5 / 91 AC No: 23-10 RECIPROCATING AND TURBINE Initiated by: ACE-100 Change:
POWERED PART 23 AIRPLANES
1. PURPOSE. This advisory circular (AC) provides information and guidance concerning acceptable means, but not the only means, of showing compliance with Part 23 of the Federal Aviation Regulations (FAR), Part 3 of the Civil Air Regulations (CAR), or earlier corresponding regulations applicable to auxiliary fuel system installations. Accordingly, this material is neither mandatory nor regulatory in nature and does not constitute a regulation.
2. SCOPE. This AC provides guidance and criteria for the installation of auxiliary fuel systems in Part 23 airplanes. It is intended to be used for auxiliary fuel system installations in the airplane including fuselage, wing, or external configurations.
Installations that involve changes to primary structure, aerodynamics, airspeed, mass distribution (will induce flutter changes), maximum weight, or changes in center of gravity (e.g.)
limits require additional substantiation, that is beyond the scope of this AC.
3. APPLICABLE FAR SECTIONS AND RELATED DOCUMENTS.
a. Agglicable Sections. Applicable sections of the FAR are shown in specific paragraphs, as well as listed below:
§ 23.561 § 23.843 § 23.1337 § 23.571 § 23.853 § 23.1351 § 23.601 § 23.863 § 23.1357 § 23.603 § 23.867 §§ 23.1501 through 23.1529 § 23.609 § 23.901 § 23.1541 § 23.613 § 23.903 § 23.1543 § 23.615 §§ 23.951 § 23.1553 through 23.1001 § 23.1555 § 23.619 § 23.1011 § 23.1557 § 23.625 § 23.1183 §§ 23.1581 § 23.777 § 23.1189 through 23.1589 § 23.787 § 23.1305
b. Related Documents. Related AC's and reading material are listed in appendixes 2 and 3 respectively.
4. GENERAL. Before determining the auxiliary fuel system configuration and modifying the airplane, the applicant should become familiar with the existing airplane structural and systems characteristics and functions, and with the applicable certification requirements. To avoid structural and systems compatibility probletas, a working knowledge of the airplane is essential. In particular, the applicant should determine the effects of the addition of the auxiliary fuel system on payload, e.g., mass distribution induced flutter changes, system and airplane operations, and structural margins. When in doubt about any certification requirement, the applicant should consult with the FM aircraft certification office responsible for his project early in the design program to avoid costly changes late in the program.
5. CERTIFICATION BASIS.
a. New Type Certificates. For the issuance of a new type certificate, 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 will be prescribed in accordance with § 21.16.
b. Other Design Changes. For other design changes, such as the addition of a new model to an existing type certificate or modification of an existing model, the airplane must be shown to comply with the certification basis established in accordance with § 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.lOl(b). If neither the original certification basis nor current rules provide adequate or appropriate standards, special conditions will be prescribed in accordance with § 21.16. Although sections of Part 23 are referenced in this AC, the references should be interpreted to be the corresponding sections of the earlier applicable certification basis.
c. Unsafe Features or Characteristics. Notwithstanding compliance with the established certification basis, § 21.21 precludes approval if there is any feature or characteristic that makes the airplane unsafe. The applicant should 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.
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6. PROCEDURES. In order to avoid delays and possible expensive redesign, it is strongly recommended that the following procedures be followed. After the applicant has made application for approval of an installation, the applicant should:
a. Submit a proposed overall certification program plan that identifies the essential steps or actions and the sequence anticipated for submitting reports, drawings, process specifications, analyses, tests, and other documentation to complete the installation approval.
This program plan should include the proposed or target schedule for the required FAA approval tests and inspections.
b. 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. After the FAA has reviewed and accepted the test plan, a Type Inspection Authorization (TIA) will be issued to specify the required ( official ground and flight tests.
d. Schedule FAA conformity inspection of the test installation.
e. Schedule and conduct the ground and flight test(s) with FAA witnessing.
f. Submit final test reports describing all test results and obtain FAA approval.
7. DISCUSSION.
a. General. The addition of auxiliary fuel tanks to an airplane should not compromise the basic integrity of the original fuel system.
These installations should comply with the appropriate regulations required by the certification basis of the particular airplane.
The auxiliary fuel system should be evaluated in conjunction with the main 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(s). In at least one operating mode, the direct feed system supplies fuel directly to an engine.
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( The requirements for a direct feed auxiliary fuel system are considerably more stringent than those for a transfer auxiliary fuel system . In general, these requirements 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. For turbine engine airplanes, these provisions should be automatic to meet the requirements of§ 23.955(f)(2). 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 in order to prevent engine flameout or other unacceptable operating conditions.
Transfer type system req u irements are not as stringent but do require basic fuel system considerations such as damage to tank and vent lines due to excessive pressure from overfilling, high and low temperature fuel effects, transfer rates, drainage, altitude effects, etc. The auxiliary tank depletion characteristics should also be evaluated to ensure that air entrainment, etc., do not alter main tank performance .
The applicant should determine when transfer to a main system tank should be established and whether the transfer should be in increments or continuous. The auxiliary tank quantity, flow rates, main system tank levels, etc., are considerations for when to initiate the transfer and should be a part of the AFM procedures and limitations. ( If misconfiguration of the fuel system can result in an unsafe operating condition, cockpit annunciation of the condition should be provided.
b. Structural Considerations (§ 23.561).
(1) Design Criteria and Structural Loads.
(i) 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.
Evaluation of the tank attachment hardware and local structure may be sufficient; however, installations that involve changes to primary structure, aerodynamics, maximum weight changes, or changes in e.g.
limits may require additional substantiation that is beyond the scope of this AC .
(ii) The tank installation design should isolate the tank itself from airframe induced structural loads and from deformation induced by the wing and fuselage, except for integral type tanks.
(iii) The fuel tank and its attachment and support structures should be designed to withstand all design loads, including cabin pressurization and the emergency landing load specified in § 23 . 561. The requirement for retention of fuel as required by Par 7 4
AC 23-108/5/91 § 23.967{e) should also be addressed for these loads as well as dynamic loads. The requirements of§ 23.571 should be addressed as required for the fuel tank.
(iv) Fuel loads included in the structural substantiation should be based on the most critical density of the fuels approved for use in the airplane.
(v) The fuselage is limited to design values that cannot be exceeded without resubstantiation. Tradeoffs between passengers, cargo, and fuel may be made provided the allowable floor, bulkhead, and local shell loads are not exceeded,
(vi) The following should be considered in the evaluation of the tank and tank support structure in accordance with the applicable certificatio n basis:
(A) Tank internal pressure developed during malfunction of the pressure type fueling system, if applicable. Tank pressure due to fuel head in combination with gust loads should also be considered.
(B) Except as provided in§ 23.625(a) or (b), a fitting factor of at least 1.15 should be applied to all tank support fittings and their attachment to the tank.
( (C) 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.
(D) 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 0.020 inch for the outer face sheets or 0.040 inch for the inner face sheets.
(E) The requirements of§§ 23.963, 23.965, and
23.967 are applicable.
(vii) Installation of wing auxiliary tanks should address the increased wing loads .
{2) Structural Modification.
(i) Where exis t ing 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.
(ii) Modifications such as cutouts, tank support attachments, and service openings through exterior skins, should not degrade structural load capability or lead to reduced fatigue Par 7 5
( capability. If holes are cut through the pressure vessel for auxiliary fuel system lines, etc., special care should be taken to reseal the penetrations, particularly those affecting the pressure vessel. The fuselage should be pressure tested in accordance with the requirements of§ 23.843; and if major modifications are made to the cabin pressure vessel, a fatigue evaluation should be accomplished as required by§ 23.571.
(iii) The effect of the modification on the existing maintenance program, including the structural supplemental inspection program, if applicable, should be considered and appropriate changes made in accordance with§ 23.1529.
(3) 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:
(i) Attachment point loads should be evenly distributed to minimize the possibility of fuel tank rupture.
(ii) 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 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, flexible secondary supports, or secondary constraint bulkheads to accomplish this.
c. Tank Location Criteria.
{l) Uncontained Engine Rotor and Blade Failure Considerations for Turbine Powered Airplanes(§ 23.903).
(i) The applicant should evaluate the location of fuel tanks and other fuel system components from the standpoint of 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 AC 20-128, "Design Considerations for Minimizing Hazards Caused by Uncontained Turbine Engine and Auxiliary Power Unit Rotor and Fan Blade 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 APU's used on the airplane involved. The energy levels of uncontained rotor/blade fragments specified by the engine/APU manufacturer should be used.
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(ii) For certification compl iance, a report addressing the above considerations should be prepared and submitted. The report should adequately consider uncontained rotor a nd blade failures from all engine and APU sources which may a ff e ct t he integrity of the auxiliary fuel system. The report shou l d also show that design precautions have been taken to minimize ha zar ds to the airplane in the event of these failures and that the ai r pla ne safety level has not been degraded by the auxiliary fuel tank /system installation.
(2) I nstallations in Cargo and Baggage Compartments (§ 23.787) .
(i) The various components for the auxiliary fuel system installed in cargo and baggage compartment s s hould be protected from damage caused by shifting cargo. A cargo b arrier , either rigid or flexible, should be used to separate the auxiliary fuel system from the cargo. This barrier should be designed to contain the maximum cargo loading for which the compartment is a pproved under all load conditions including the emergency landin g condition. (§ 23 . 561)
(ii) When the fuel system i s installed in cargo and baggage compartments, ensure all materi a l u sed, including cargo barriers and replacement compartment lin ers, meet the applicable flammability requirements . As a minimum, a cargo barrier should meet the flammability requirements of cargo and baggage compartment liners.
(§ 23.863)
(iii) If holes are cut thr oug h cargo and baggage compartment liners for auxiliary fuel system lines, care should be taken to reseal the l iner.
(iv) 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 b e provided detailing the method of removal, resealing, and restoring the airp l a n e essentially to its original configuration. (§ 23.1 5 29)
(v) Auxiliary fuel syst e ms are frequently installed in cargo compartments . If the tank a nd component secondary barriers are capable of withstanding a cargo comp art ment fire so that the safety of the airplane is maintained, then the secondary barrier materials are acceptable. These fire resistant cha r a cteristics should be equivalent to the liner materials of the compartme nt where they are located. The system should also be evaluated with r espect to materials from the standpoint of toxic gas release under f i re conditions. No materials should be used which act as a fue l fo r fires. Avoid the use of magnesium or flammable resins, sealant s, and coatings.
( 3 ) Design and Location. Sectio n 23.967(e) is applicable and should be addressed.
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d. Airplane Fuel System Compatibility(§§ 23.901(b)(l). 23.951. (
23.953. 23.955. 23.957. 23.961. 23.995. and 23.1189).
(1) Interface Considerations.
(i) 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 with other requirements.
NOTE: Should existing fuel lines and manifolds be used, special care should be taken to ensure that air is not introduced into the main fuel system during normal and abnormal use of an auxiliary system.
(ii) Interface with the existing airplane vent system for venting of the auxiliary tank(s) should be evaluated. The existing vents have been sized to maintain acceptable pressure levels during refueling and flight maneuvers. When combining the systems, unless it is obvious by analysis, 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 main 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 main system tanks due to rapid descent or other operating condition.
(iii) When the auxiliary fuel system arrangement has been determined and the interfaces with the main fuel system have been established, a total system analysis should be conducted to ensure that no hidden failures in the system could lead to an unsafe operating condition. Analyze the system for all modes of operation, including venting, pressure fueling, defueling, transfer, engine feed, crossfeed, and emergency fuel dumping. For example, the auxiliary tank system should be designed so that failure of the system could not result in undetected or uncontrollable transfer of fuel from the main tank to the auxiliary tank when the main tank system is in its normal operating configuration.
(2) Other Considerations. Other systems should also be evaluated for compatibility with the auxiliary 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. Adequate cockpit display to indicate auxiliary tank Par 7 8
8/5/91 ( AC 23-10 depletion or improper fuel scheduling should be provided and cockpit indication of low fuel state, e.g., unbalance or other unsafe condition should be considered for the particular installation. It should not be possible to select fuel from more than one tank at a time unless the tank vents are interconnected.
(3) control and Shutoff (§§ 23.995 and 23.1189). Sections
23.995 and 23.1189 should be complied with for valves, controls, and shutoffs.
e. General .Arrangement Evaluation.
(1) System Layout. In addition to the criteria described in the previous section, consideration should be given to the following:
(i) Line Routing. Flexibility. and support(§§ 23.863 and 23.993). General fuel system practices should be adhered to in installing fuel and vent lines (refer to AC 43.13-lA). All flexible lines should be adequately supported along the entire line installation length. Ensure that lines cannot 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.
Undrainable low spots in vent and fuel lines should be avoided.
(ii) Fuel Tank and Component Location. Access. Mounting.
and Protection(§§ 23.901, 23.963. 23.965, 23.993. and 23.994).
(A) Each auxiliary fuel tank or tank module design should be evaluated for the basic requirements of§§ 23.963 and
23.965. These requirements address, for example, the basic integrity of the tank, bladder cell requirements, and the tank tests, such as slosh and vibration, that may be required.
(B) 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. However, components may be mounted to the fuel tank to minimize relative motion.
If fuel or other system lines or fittings are used to support auxiliary fuel system "in-line" small/lightweight components, it should be shown that this practice does not result in excessive structural stresses when subjected to the vibration and other loads expected in service.
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( (C) Although function should dictate the appropriate location of components in the airplane , there is usually some latitude which can a llow select i on of a specific area which is more s uit able than others. Component location should be considered from the aspects of both access and protection . Access is especially important for components which may require routine periodic inspection and maintenance such as s t ra iners, filters, and drain valves or components with known short service life expectancy . Where possible, such components should be loca ted in areas where there are currently existing access doors and openi ngs on the airplane. When new access provisions are required, lightning and crashworthiness vulnerability as well as structural secur ity should be considered.
(D) For components which are located inside the fuel tanks, the crashworthiness aspects of the installation should be considered. Means should be provided to prevent component sharp edges from penetrating the tank surf ace due to deflection of the surface under crash load conditions should be provided, especially where flexible tank bladder cells are used.
(iii) Tank Penetr a tion Points (Access , Quantity Probes, Float Switches. and Fuel Fi ttings) (§§ 23 . 963. 23.973 , 23.975,
23.977).
(A) The location and arrangement of all tank ( penetration points should be considered. Tank penetration points are all locations where openings ex ist in the tank walls for access (inspection or repairs), for mounting fuel quantity probes, float switches, etc., and for tank venti ng, draining, fuel transfer, or fueling.
(B) Auxiliary tanks should have access openings large enough to permit completio n of required inspections. Component and line penetrations may be usable as access/inspection openings, depending on size and locatio n, and should be evaluated for acceptability.
(C) The fuel tank filler, fuel jettison, and vent openings should be designed so that they open external to the airplane and prevent spilled fuel from entering any part of the airplane .
(iv) Electrical Wiring Routing and Support. Refer to AC
43.13-lA, "Acceptable Methods, Techniques and Practices--Aircraft Inspection and Repair," for more details regarding routing and supporting of wires.
(2) Fuel Containment Seco ndary Barriers (§§ 23.967 and
23.853). For auxiliary fuel sys tems which are located in the passenger or cargo and baggage compartments (appendix 1), isolation of the fuel and fuel vapors from other areas of the compartment is of critical importance. Tanks, lines (including flexible lines), fittings, connections, and other components, such as valves, pressure Par 7 10
( transmitters, regulators, etc., should be shrouded or provided with redundant barriers so that leaks from any of these sources cannot present a hazard. Some of the important characteristics of the secondary barrier system are:
(i) The system should be capable of containing and isolating any leakage.
(ii) Secondary barrier spaces should be vented and drained in accordance with acceptable practices to prevent the accumulation of fuel or fuel vapor. The drain system also serves as a periodic visual means of detecting any leakage in the auxiliary fuel system. For this reason, shroud drains should not be connected to other types of fluid drain systems.
(iii) The secondary barrier drain system materials, construction, and sealing characteristics should be compatible with fuel and capable of long life under the altitude/cabin pressure cycling, vibration, and wear that they may be exposed to in service.
It is acceptable, with no further showing of compliance, to use the same materials, construction, and sealing as originally approved in the airplane type design, and specified in the Airplane Maintenance Manual if service experience has been acceptable.
(iv) The drain system should be reviewed from the installation location aspects to preclude the possibility of inadvertent damage by ground personnel or shifting cargo.
(3) Tank. Fuel, and Vent Line and Component Shrouds (§§ 23.853 and 23.967).
(i) Auxiliary fuel tanks installed in a passenger or cargo and baggage compartment should be completely shrouded. This means that all fittings connected to and through the tank walls should also be provided with secondary barriers.
(ii) All vent and fuel lines, fittings, and connections in a passenger or cargo compartment should also be shrouded.
(iii) Valves and other components, unless otherwise protected, can have possible leak paths through shafts and at control motor and solenoid connections or other seals in addition to their line connections, and should be completely shrouded.
(iv) Electrically operated components are of particular concern because of the possible need to route electrical leads through the secondary barrier spaces. The space should be considered an abnormal vapor zone containing flammable fluid similar to the interior of the fuel tank. Thus, an evaluation of the electrical connector and wiring temperatures should be made for both normal and fault load Par 7 11
conditions. It may be necessary from this evaluation to install the ( wiring in vapor-proof conduit, except where shown to be intrinsically safe .
(4) Fuel Tank secondary Barrier Cavity Venting(§ 23.967).
The changes in tank secondary barrier cavity pressure during all airplane maneuvers, including emergency descent, should be accounted for in the design of the auxiliary fuel tank. Bladder type tanks may be critical under emergency descent condi t ions , depending on the cavity vent line sizi ng. The vent/drai n con figuration should provide the required positive and negative pressur e relief between the outer shell and the bladder or inner wall t o prevent collapse or overexpansion of the inner tank . Depending o n the location of t he overboard vent/drain exit and the airflow c h aracteristics around the exit or exit mast, a flight test may be required to evaluate the emergency descent characteristics of the cavity vent system with the airplane in both the " clean " a nd "wheels a nd flaps down" configuration.
(5) Other Secondary Barrier Vent and Drain Provision (§§ 23.954 and 23.967}.
(i) All secondary barrier spaces should be vented and drained. The spaces in some designs are manifolded for venting and drainage or are independently vented and drained. Eventually, all cavities should vent and drain to an exit external to the airplane.
The overboard exit should be located to prevent fluid reingestion into ( such areas as the wheelwell and other critical areas of the airplane .
On many airplanes, there are existing exit drain masts which are used to vent and drain the secondary barrier cavities of the airplane's main fuel tanks. The use of these existing drain masts is recommended. The attachment of the auxiliary system drain to the existing airplane drain should ensure that backflow does not occur in either system.
(ii) For secondary barriers, the cavity vent and drain exit should always be open and vented to ambient pressure. Avoid the use of push-to - drain valves at both ends of the drain to check the cavity condition. Multiple drain outlets utilizing push-to - drain or other type valves should not inhibit venting.
(iii) If a new drain exit is req u ired, consider carefully the loc ation and configuration from the sta ndpoint of lightning vulnerabil i t y and c onductivity. Drain masts should not be located upstream of air inlets or other openings in the airplane external surface . Physical inspection by the cognizant personnel of all drain locations on the actual airplane should be accomplished to ensure all interfaces are considered. Unless it is obvious by inspection, verification should be accomplished by impingement tests conducted in flight to ensure that liquid discharge from the drain exit will not ( 12 Par 7