cause a hazard or reenter at another airplane surface. It should be ensured that freezing does not occur nor does icing buildup cause suction venting during the tests and that the test results are representative of a fuel leak.
Fuel System Contamination Prevention Assessment (§§ 23.951,
23.971, 23.977, and 23.997). The primary concerns of fuel contamination with respect to auxiliary fuel system design are water and debris contamination. The certification requirements are quite specific, and the installer should be familiar with the requirements of § 23.971, Fuel Tank Sumps, and § 23.977, Fuel Tank Outlets.
may also be a need to refer to § 23.997, Fuel Strainer or Filter.
This regulation would be applicable for auxiliary fuel systems which feed fuel to a positive displacement pump which, in turn, feeds the engine or to some alternate auxiliary fuel tank direct engine feed The existing main fuel system design should already satisfy § 23.997 requirements. The installer should, however, verify that these requirements are maintained when an auxiliary fuel system is installed. For turbine powered airplanes, an evaluation of the original and auxiliary fuel system should be done to satisty the requirements of § 23.951(c). The following features should be considered with respect to auxiliary fuel system contamination.
(1) Fuel Tank Sumps and Fuel Strainers.
(i) Sump Location and Capacity.
(A) Sumps should be installed at the lowest point in the auxiliary fuel system with the airplane in its normal static ground attitude. This should allow water in the system to migrate to the sump where it can be drained before flight. The sump may be an integral part of the tank, providing the bottom of the tank is the lowest point in the system, or may be a single separate tank specifically installed for sumping purposes. If the sump is a separate tank and located within a passenger or cargo compartment, it should be designed to the shrouding criteria set forth previously for fuel tanks including the overboard drain exit requirements.
(B) Sumps and sump tanks should have the capacity specified in § 23.971. a single sump tank is used for a number of tanks or modules that are interconnected together to function as a single tank system, the required capacity of the sump is based on the capacity of the total tank system. All tanks must be completely drainable to the sump tank.
(ii) Sump Drain Provisions. All sumps should have provisions which allow complete drainage of the sump. These drainage provisions should be carefully designed to provide high reliability in service and protected to prevent the possibility of leakage from damage as a result of a wheels-up landing. Drain valves should be the positive locking type and be reliable. Lightning aspects of the overboard access should be addressed as discussed later. Locate the Par 7 13
drain valve at or near the sump. Do not locate drain valves on the bottom surface of the fuselage or other areas where they may be inadvertently damaged or opened. In passenger cargo compartments, sump drains should be shrouded in accordance with the provisions described previously and the shrouds provided with vents per normal shroud procedures.
(iii) Fuel Strainers (§§ 23.951 and 23.977).
(A) One purpose of using a fuel tank outlet strainer is to prevent the intrusion of debris of a size sufficient to damage or jam components downstream of the tank outlet. Fuel strainers of this type should be used in the fuel tank outlets of auxiliary tank systems which transfer fuel by mechanical pump means or which have gravity fuel fillers incorporated, where there is a probability of debris inadvertently entering the tank.
(B) The mesh or size requirements are defined in § 23.977. However, because of the problem of icing, the effective flow area of the strainer should be selected considering also the requirements of § 23.951 (c). Fine mesh should not normally be used in this application.
Fuel and Vent System Low Points (§ 23.975).
creating low points in routing fuel and vent lines. It is particularly important to eliminate low points in vent and drain lines, where water may collect and freeze, blocking the lines.
in the vent system are unavoidable, drains should be installed.
Depending on the particular design, these drains may allow the fluid to flow back to the tank. Where possible, drain lines should be routed to provide continuous down slope to the drain exit so that complete drainage is accomplished with the airplane in the normal static ground or level flight attitude. If not possible, consider adding sump type drain valves at the applicable low points.
drain valves are required, use the criteria discussed previously.
g. Ignition Source Isolation Evaluation.
System Electrical Bonding (§§ 23.863 and 23.867).
auxiliary fuel system conductive components such as electrical equipment, fuel tanks, lines, etc., should be electrically bonded to airplane structure. If two or more components are grounded in series, both ends of the ground circuit should be grounded to ensure that the loss of an intermediate ground connection will not leave any component isolated from ground. The bonding jumper should be as short as practicable and installed in such a manner that the resistance of each connection does not exceed .003 ohm. Special emphasis should be placed on bonding when addressing nonmetallic or composite auxiliary fuel tank/systems. The applicable auxiliary fuel system installation drawings should identify the discrete attachment points requiring bonding. The surface treatment (s) required should also be specified by an applicant's specification(s) which should be submitted to the 14 Par 7
cognizant certification office during certification. Verification of adequate bonding should be accomplished during installation and when maintenance is performed on the auxiliary fuel system in the airplane.
(1) Wiring Isolation in Fuel or Fuel Vapor Environment.
(A) Electrical wiring in fuel or fuel vapor environment should be adequately protected to prevent damaged wire(s) from igniting fuel/fuel vapor: Installation of the electrical wiring in a sealed metallic conduit is an acceptable method to meet this criteria. The conduit seal should be sufficiently airtight that combustion cannot be maintained.
Flexible conduit conforming to specification MIL-C-6136, Conduit; Electrical, Flexible, Shielded, Aluminum Alloy tor Aircrait Installation, Types I or II, or equivalent, may be used where it is impractical to use rigid conduit, such as, areas that have motion between conduit ends or where complex bends are necessary.
(C) Conduit installation considerations include the following:
(1) Metallic conduit should have a low-resistance bond of less than .003 ohm to airplane structure.
(2) To prevent wire chafing at the conduit ends, suitable end fittings should be installed so that a smooth surface comes in contact with the wire(s) inside. when fittings are not used, flare the end of the conduit to prevent wire insulation damage.
(3) The conduit should be supported by clamps along the conduit run to prevent chafing against structure and to avoid stressing the end fittings.
(2) Lightning Vulnerability (§ 23.954).
(i) System Installation. The auxiliary fuel system installation should be evaluated from the standpoint of lightning vulnerability. Where possible, components, installation practices, and locations should be used that were originally type certificated on the airplane if service experience has been acceptable. Some items and areas that may be susceptible to fuel ignition or indirect effects of lightning include, but are not limited to, the following:
(A) Vent outlets, metal fittings and mechanical fasteners inside fuel tanks, fuel filler caps and access doors, drain plugs, tank skins, fuel transfer lines inside and outside the tanks, and electrical and electronic fuel system components and wiring.
(B) Advisory Circular 20-53A, 'Protection of Aircraft Fuel Systems Against Fuel Vapor Ignition Due to Lightning, addresses the subject in more detail.
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(C) The primary areas of concern for auxiliary systems are all fuselage penetrations, such as protruding masts, access provisions, vents, and drains which can be susceptible to a direct strike or corona effects. Details of all attachments, fittings, etc., should be reviewed to ensure there will be no arcing or sparking and that adequate electrical bonding has been provided.
Composites in the basic airplane in areas that involve a portion of the auxiliary fuel system installation should also be evaluated for indirect effects. Indirect effects are induced electromagnetic effects which occur from a lightning strike (such as induced sparking in fuel system components or wiring).
(ii) Protruding Masts.
(A) If the existing vent and drain masts were designed to reduce lightning vulnerability, it is recommended that they be used for the auxiliary fuel system also unless such use would compromise their original intended function.
(B) If additional masts or overboard drain lines are required, an evaluation of lightning protection should be conducted. However, the evaluation may be minimized by using the same design and general location as the previous type design.
(C) Access doors or caps should provide isolation of the fuel system components (drains, etc.) with respect to lightning. This isolation should prevent transfer of electrical discharge into the fuel system component (s) and channel or dissipate the energy into airplane structure.
(iii) Vent and Drain Protection.
Generally, auxiliary fuel tank vent systems are integrated with the existing main fuel tank vent system on the airplane being modified. The main tank vent outlets may have been and certified for lightning protection. Where auxiliary tank venting is provided separately, it is advisable to consult the type certificate holder to determine a satisfactory vent exit location and configuration. In general, to improve lightning protection, flush outlets are preferable instead of masts. Vent outlets should not be located in a direct (zone 1) or swept stroke area. Depending on the particular design, flame arresters may also be required in the vent system proper. Vent system exit design is a critical element of airplane safety and requires a considerable background knowledge of possible lightning strike zones of the particular airplane being modified.
(B) when fuel tanks requiring secondary barriers are adjacent to passenger areas, the vent outlet for the airspace between the barriers should be designed to the same criteria used for tank vents; i.e., assuming that the effluent is a combustible mixture 16 Par 7
of fuel and air. These cavity vents should not be combined with or directly ported into other main or auxiliary tank venting/drainage systems. Cavity venting and draining provisions can be combined as a single function and routed to a single drain mast, preferably an existing main tank cavity drain on the airplane being modified.
However, care must be taken to ensure that the pressure differential will always be such that the cavity will drain.
(C) Drain valves should be located at or in the lines near the tanks to be drained and not installed on the external surface of the airplane. If tunnels or spaces are required to obtain access to drain valves, the use of nonconductive materials to isolate the valves from possible lightning arc-over or conduction into fuel tanks or fuel carrying components should be considered: Each access configuration should be evaluated for conformance to lightning protection criteria.
(iv) Bonding for Lightning Protection (§ 23.867).
(A) A minimum of two bonding jumpers should be installed between a conductive tank and the airplane structure.
Individual bonding jumpers should not be less than no. 12 American Wiring Guide (AWG) for stranded cooper wire or no. 10 AWS for stranded aluminum wire.
(3) Explosion Considerations.
(i) Electrostatic Charge.
The auxiliary fuel system should be evaluated from the standpoint of electrostatic charge build-up. Unless there are means to progressively drain off the charge, electrostatic potential can build up to hazardous levels (sparking discharge levels) in areas of the fuel system where fuel flow velocities are high or where there is a high degree of fuel agitation. Some of the areas of concern are refueling outlets into the fuel tank and long lengths of refueling or fuel transfer hoses where the hose material is highly nonconductive to electrical currents. (For example, teflon hose was a problem in this respect until the teflon was formulated with a graphite or other materials to make it more conductive.) High cells.
Electrostatic potential build-up can be reduced in tanks by locating fuel outlets near the bottom of the tanks and directing outlets away from direct impingement on tank structure (to reduce splashing and sloshing) and expanding the outlet fitting area to reduce the outflow velocity. Electrically bonding the tank fittings to structure may also help by progressively draining off the Par 7 17
AC 23-10 8/5/91/ charge. Sizing hoses and other components to reduce flow velocities may allow more time for charge relaxation. A long run of electrically nonconductive hose can be broken up using metal fittings or lengths of metal tubing, each grounded to structure, to provide additional area for charge relaxation.
(C) The internal tank coatings can influence the degree of fuel electrostatic charge relaxation. The use of dielectric primer, corrosion protective coatings, and painting on internal tank walls should be consistent with the desired overall fuel tank wall conductivity.
(D) Tank foam may be used.
(ii) Component Isolation. Components located in a fuel or a fuel vapor environment should be designed such that there is no fire or explosion hazard during normal operation or under a failure condition. This should be accomplished by ensuring that there is no sparking that could cause ignition.
(iii) Spark Isolation (Explosion Proofing).
Generally, electrical component manufacturers have demonstrated spark isolation or explosion proofing by test similar to MIL STD 810 where a high intensity spark is intentionally set-off inside the component case which is filled with a flammable fuel air mixture and with the component immersed in a flammable fuelair mixture environment. The external environment is then ignited to prove its flammability. A test of this type does not depend on the failure mode since the cause of the spark is not important in this situation. However, a failure mode analysis should be conducted to ensure that the test is adequate and simulates all the actual conditions anticipated in service. For the particular auxiliary fuel system application intended, service life characteristics (such as the degradation of seals with time, etc.) should be considered to ensure adequate explosion proofing. Auto ignition of fuel vapors due to hot component surfaces resulting from failures such as dragging fuel pump rotors or shorting of electrical motor stator wires to the case should also be considered.
For some components, such as fuel quantity probes, explosion proofing can be shown by demonstrating that the circuit energy is less than 0.2 millijoule under normal or any failed condition. Such components are then considered as intrinsically safe with no further substantiation.
h. Environmental Properties Evaluation.
(1) Normal Environment Properties.
(i) Fuel Resistance. Fuel resistance deficiencies have produced problems in the past.
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An aspect which should be evaluated is the prolonged effects of fuel exposure. Some materials, certain plastics in particular, which exhibit short-term fuel resistance have, over a prolonged period, deteriorated under the influence of fuel or rue.
New innovative material applications, such as composites and new bonding adhesives, should be thoroughly tested to determine the long-term effects of fuel and fuel vapor exposure.
Surface treatments, coatings, and sealants intended to reduce weathering and corrosion and seal structural areas should also be substantiated to ensure fuel and fuel vapor resistance, where applicable.
If a component is life limited, the limits due to fuel environmental conditions should be defined, particularly for nonmetallic items.
(ii) Fuel Additive Compatibility. An evaluation should be made of the effects of additives, approved for the airplane, on the components of the auxiliary fuel system. The applicant should substantiate, by suitable methods, that the use of approved additives does not deteriorate these components or restrict the use of specific additives by suitable warning placards on the airplane and notices in the limitations section of the AFM.
(iii) Corrosion and Micro-Organism Resistance. Corrosion in auxiliary fuel systems is primarily due to entrained water in the fuel and the acids produced by the associated microbial contamination.
Thus, corrosion protection is especially important for tanks, sumps, and equipment located in sump areas. Metal tanks and components should be made of materials resistant to corrosion or otherwise suitably protected. Metal combinations (dissimilar materials) which are subject to electrolytic corrosion problems should be avoided.
Magnesium, copper, cadmium, and brass should not be used in auxiliary fuel systems as these metals are very active chemically.
combination of graphite layers of composite construction attached directly to aluminum may result in intergranular corrosion and should be avoided. Materials used for bladder cells, seals, and composite tanks and fittings should be resistant to microbial contamination.
Bladder cells should conform to Technical Standard Order (TsO) C80 or otherwise be shown suitable for the intended application.
Temperature Range Suitability. Materials and components used in the auxiliary fuel system should have suitable properties and should perform their intended function throughout the approved airplane operating envelope. In some instances, they have been exposed to temperatures as low as -65° F or lower and as high as 250° F, or even higher. Some rubber and plastic materials, for example, have excellent flexure properties at room temperature, but become unacceptably brittle at low temperatures.
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Extreme Environment Properties. Extreme environment properties are those properties a material or combination of materials should have under certain conditions which are not encountered during the routine operating life of the materials. Examples are conditions which may be imposed by component failure or crash environments.
evaluation of the extreme environment properties should account for the following:
(i) Flexibility and Resilience. The properties of flexibility and resilience should, to a certain degree, be considered as a part of the normal environmental condition for flexible liners.
Materials and parts for rubber fuel lines should provide resilience and flexibility and conform to TSO-C53 or otherwise be shown suitable for the intended application.
Heat-Strength Characteristics. The high temperature strength properties of materials used in the auxiliary fuel system should be considered for those components which may be subjected to sources of heat due to the failure of some component in another adjacent compartment or adjacent system. figurations mould Capable sustaining the critical flight and landing loads and thus the integrity of the system under these conditions.
i. Nonmetallic Material (§§ 23.601, 23.603, 23.609, 23.613,
23.615, 23.619, and 23.867). The suitability and durability of materials used for nonmetallic auxiliary fuel tanks should be established by tests. Advisory Circular 20-107A, 'Composite Aircraft Structure,' should be reviewed for applicability when composite materials are used. The following elements should be considered in establishing material properties and substantiating the nonmetallic auxiliary fuel tank structure by tests:
(1) Aging of the laminate in the operating environment, including wear, due to temperature, pressure, cavitation, moisture content changes, etc.
(2) Chemical reactions with fuel vapor, cleaning liquids, solvents, salt water vapor, and any other contaminates such as fungus in the tank.
(3) Static electrical charge, bonding, or lightning strike.
(4) Any other elements characteristic of or unique to the type of nonmetallic material, method of processing, and design applications.
(5) A manufacturing process to ensure repeatability of material properties.
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(6) Inspection techniques for manufacturing and for continued airworthiness.
(7) Statistically based material strength properties for critical tank structure. Strength, detail design, and fabrication should minimize the probability of fatigue failure. Chapter 9 of Structures," and MII-HDBK-17, "plastics for Flight Vehicles," contrin #Metallic Materials and Elements for Aerospace Vehicle procedures for establishing such properties.
Composite repair procedures defined by the applicant and approved by the FAA.
Fuel System Performance. The applicant should evaluate the auxiliary fuel system performance throughout the certified operating envelope of the system. This should include an FAA witnessed functional ground and flight test program. As noted earlier, installations that involve changes in primary structure, airspeed,
c.g. limits, aerodynamics, weight limits, or mass distribution require additional substantiation that is beyond the scope of this AC.
k. Normal Operation Evaluation.
(1) Refueling and Transfer Performance.
(i) Refueling and Transfer Flow Rates and Pressures (§§ 23.951, 23.955, 23.957, 23.961, 23.979, and 23.991).
The auxiliary refueling system should be analyzed to determine that the flow rates and pressures are acceptable. Fueling flow rates should also be demonstrated and verified by ground tests. The tank vent system capacity should be verified for the required operations and refueling system malfunctions to ensure that overpressurization of the tanks, including the existing airplane main tanks when applicable, does not occur. Verification is also required that the pressure fueling manual or automatic shutoff means does not produce unacceptable surge pressures which may damage the system or fueling equipment or rupture fueling lines. This verification can be accomplished by ground or laboratory refueling tests using fast response pressure transducers and recorders or submittal of previously accepted similar tests and service history Transfer rates should be determined to show compliance with § 23.955(d) for all flight conditions in which transfer will be permitted to ensure that the receiving tank will be neither overfilled nor depleted before transfer is completed.
should be shown that transfer does not present any hazard, such as unwanted fuel migration, during such flight conditions. Any restrictions on transfer, such as duration or flight operating condition, should be outlined in the limitations section of the AFM.
It should be assumed that the transfer system might be left on Par 7 21
AC 23-10 8/5/91 inadvertently; therefore, it should also be shown that transfer under conditions not permitted by the AFM would not present any hazard. It should be shown that the fuel system complies with § 23.955(d).
performance requirements of § 23.961 should be shown for auxiliary feed systems that feed directly to the engine. This is normally accomplished by conducting a hot fuel test (see AC 23.961-1); however, it may, in some instances, be sufficient to show that the system is similar to a previously approved system or to submit an analysis that is supplemented with component test data.
(ii) Fuel Tank Capacities (Usable Fuel, Sump Capacity, Undrainable Fuel, and Expansion Space) (§§ 23.959, 23.969, and
23.9711.
(A) For direct feed auxiliary fuel systems, the unusable fuel requirements of § 23.959, apply (see AC 23.959-1 for method for determination). For transfer type auxiliary fuel systems, the unusable fuel is the quantity of fuel remaining after transfer under the most critical steady state airplane flight attitude and altitude conditions permitted by the AFM.
(B) Sump capacity is a part of the unusable fuel and as previously noted, is defined by § 23.971(a).
The undrainable fuel quantity should be measured, usually during the initial filling of the auxiliary tanks as a part of FAA witnessed ground tests. The undrainable fuel quantity is the fuel remaining in the system after all fuel possible has been sump drained from the system. It is not usually necessary to completely fill the tanks to determine the undrainable fuel, as the undrainable fuel is fuel trapped between stringers, at low points in fuel fittings, etc. Determination of undrainable fuel is typically done by adding a known quantity of fuel to the tank sufficient to cover all potential trapping cavities and then draining the tank.
difference between the amount of fuel added and that which is drained is the undrainable fuel. It is recommended that the undrainable fuel not exceed the capacity of the fuel tank sump.
(D) The maximum tank fueling capacity should take into consideration the expansion space requirements of § 23.969, Fuel Tank Expansion Space. The expansion space volume is the space available for fuel thermal expansion within the tank itself and does not include vent line volume. Basically, the expansion space is the volume from the tank full level to the level where fuel will just begin to enter the vent line.
(E) Figure 1 shows how the expansion space would be defined for three different vent configurations. The expansion space volume should be subtracted from the total volume of fuel at the level at which fuel will just begin to enter the vent line, and does not 22 Par 7
include the "compression" space above the vent opening. The expansion space should be derived for the airplane in its normal ground attitude. Expansion space capacity is verified during the airplane ground fueling tests conducted as a part of the test program.
Ground tests of the installed system should be conducted in accordance with § 23.979 with the airplane in the correct ground attitude to verify that the expansion space requirements are maintained for pressure fueling to the maximum automatic fuel shutoff level, verify tank(s) pressures during failure of the auto shutoff and also provide the correct values for usable and unusable system fuel.
Expansion Space Expansion Space Expansion Space Vent Fuel Level _ Tank (A) (B) (c) Figure 1 - EXAMPLES OF EXPANSION SPACE DEFINITION
(iii) Pressure Relief (Water Hammer and Thermal Expansion) (88 23.995). The designer should evaluate the pressure fueling and transfer or direct feed system operation to ensure that no damaging fuel line pressures will occur during flow shutoff or due to thermal expansion effects. Normally, sufficient flex line length and the use of "slow" shutoff valves or valves with thermal relief features will negate this problem. In some cases, relief valves or other means may need to be incorporated in the system.
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(iv) Vent System Anti-Siphoning (§ 23.975). The auxiliary tank vent system should be arranged so that no hazardous quantities of fuel can migrate from one tank to another tank or be discharged overboard during any normal flight attitudes. However, the tank vent design is to ensure that fuel will drain clear of the airplane during fuel discharge conditions. This should also be addressed for a ground attitude with the airplane parked on a ramp with a one percent slope.
Operating Limits (§§ 23.1501 through 23.1529). Assuming that the airspeed, c.g., maximum takeoff weight and maximum landing weight limitations will not be changed as a result of the auxiliary fuel system installation, the applicant's alternatives may be tradeoffs between payload weight and auxiliary fuel system weight at maximum capacity loading. Using these criteria, the applicant should review the requirements of Part 23, subpart G, Operating Limitations and Information, to ensure that the auxiliary fuel system design does not degrade the airplane performance and other requirements stated in this subpart.
Operating Airplane Flight Envelope. The installation of the auxiliary fuel system should not restrict the operating flight envelope of the airplane, or a major recertification effort may be required. This does not mean that the auxiliary fuel system must function (transfer fuel) at all existing extremes of the airplane flight envelope. Envelope limitations on the transfer of auxiliary tank fuel should, however, be stated in the AFM. Using the above criteria, the applicant should ensure that the auxiliary fuel system will not be adversely affected by exposure to the temperatures, pressures, altitude variation, and flight loads encountered in all regions of the airplane flight envelope.
(ii) System Electrical Power Requirements (§§ 23.1351 and
23.13571.
(A) If additional equipment which consumes electrical power is installed in an airplane, the revised total electrical loads should not exceed the generator or alternator output ratings and limits prescribed for the airplane, or the ratings of any airplane bus(s). To ensure that the design meets the criteria, the applicant should provide the following:
(1) Power requirements for each of the equipment items which will be installed.
(2) Wiring diagrams for the equipment installation.
(3) An updated, electrical load analysis for the airplane including the auxiliary fuel system.
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