c. Protruding Masts.
Because existing vent and drain masts have been 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.
(2) If additional masts or overboard drain lines are required, an evaluation of lightning protection should be conducted.
(3) Access Protection. 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.
d. Vent and Drain Protection.
(1) 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 on transport category airplanes have been designed and certified for lightning protection. Where auxiliary tank venting must be provided separately, it is advisable to consult the type certificate holder to determine a satisfactory vent exit location and configuration. In general, flush outlets are preferable instead of masts. Vent outlets should not be located in a direct (Zone 1) or swept stroke (Zone 2) area of the airplane.
The entire fuselage is considered within the Zone 2, 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 the possible lightning strike zones of the particular airplane being modified.
(2) If secondary barriers are required in passenger/cargo areas, the cavity vents should be designed to the same criteria used for tank vents; i.e., assuming that the effluent is a combustible mixture of fuel and air. Cavity vents should not be combined with or directly ported into any tank vent system.
wherever possible, the cavity venting and draining provisions should 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.
(3) 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.
Chap 2 Par 6 19
e. Bonding for Lightning Protection ($ 25.581).
(1) A minimum of two bonding jumpers should be installed between a conductive tank and the airplane structure.
(2) Individual bonding jumpers should not be less than No. 12 American Wiring Guide (AWG) for stranded copper wire or No. 10 AWG for stranded aluminum
f. Electrostatic Considerations.
(1) The auxiliary fuel system should be evaluated from the standpoint of electrostatic charge build-up. Unless there are means to progressively irain 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 electrostatic potential can persist for long periods after build-up in some cases. Explosions have occurred many hours after refueling when personnel introduced objects at ground potential into the bladder cells.
Electrostatic potential build-up can be reduced in tanks by locating fuel outlets near the bottom of the tanks (to reduce splashing and sloshing) and expanding the outlet fitting to reduce the outflow velocity.
Electrically bonding the tank fittings to structure will also help by progressively draining off the charge. Sizing hoses and other components to reduce flow velocities will 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.
(3) 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.
g. Component Isolation.
(1) Components located in a fuel or a fuel vapor environment must 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 the component surfaces will never exceed the minimum surface ignition temperature of the fuel used and that there will be no sparking that could cause ignition.
(2) The designer should refer to AC 25.981-lA, Guidelines for Substantiating Compliance with the Fuel Tank Temperature Requirements.
Chap 2 20 Par 6
Advisory Circular 25.981-1A discusses the requirements and some of the possible failure situations which can lead to unsafe temperature conditions.
h. Maximum Surface Temperatures.
(1) There is a general industry/FAA practice that a temperature providing a safe margin under all normal or failure conditions is at least 50° F. below the lowest expected auto ignition temperature of the fuel.
auto ignition temperature of fuels will vary because of a variety of factors (ambient pressure, dwell time, fuel type, etc) but the value generally accepted without further substantiation for kerosene type fuels, under static sea level conditions, is 450° F. This results in a maximum surface temperature of approximately 400° F. for an affected component.
(2) In general, component manufacturers have conducted qualification tests to certify that their products will meet the above criteria for known conditions of operation or failure. The auxiliary fuel system installer should ensure that the surface temperature requirements are not exceeded by any component used in the system for any conditions including extended life service and maintenance required on the component. New components should be tested to the above criteria unless such tests have already been conducted by the component manufacturer. In assessing the failure mode conditions of the component, the designer should assess any single failure in combination with an undetectable failure. Failure mode analysis is discussed in Chapter 4, paragraph 12a(1) of this AC.
i. Spark Isolation (Explosion Proofing).
Generally, electrical component manufactures have demonstrated spark isolation or explosion proofing by test where a high intensity spark is intentionally set-off inside the component case with the component immersed in a flammable fuel-air mixture environment. This environment is then ignited to prove its flammability. A test of this type does not depend on the failure node 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 will simulate 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 also be considered to ensure adequate explosion proofing.
2) For some components, such as fuel quantity probes, sate surtace emperatures and 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.
Chap 2 Par 6 21
AC 25-8 5/2/86 Figure 2
AN EXAMPLE OF SHROUDING OF EQUIPMENT ITEMS _ Secondary Seal . Tank Cavity Warning Conduit May Be Required — Primary seal Switch Tube Float Switch Drainage Path 22 Chap 2
Figure 3
AN EXAMPLE OF INSTALLATION OF ITEMS ON TANK WALLS Shroud Tube Coupling Hose Secondary Seals Vent Line Fitting Tank Cavity Drainage Path - Primary Seals Chap 2 23
Figure 4
AN EXAMPLE OF SHROUDING OF FUEL OR VENT LINE
FITTINGS IN PASSENGER OR CARGO COMPARTMENTS Shroud Hose Chap 2 24
CHAPTER 3 - COMPONENT MATERIALS GENERAL. The applicant is responsible for ensuring the integrity of all materials used in the system. This Chapter describes the material characteristics which have been found important in auxiliary fuel systems.
Information and documents concerning material properties and environmental testing requirements can be obtained by contacting the American Society for Testing and Materials (AST).
8. ENVIRONMENTAL PROPERTIES EVALUATION.
a. Normal Environment Properties.
(1) Fuel Resistance. Fuel resistance deficiencies have produced problems in the past.
(i) One problem is the presence of fuel or fuel vapor in air pressurizing systems for auxiliary fuel tanks. Pressurizing components which normally are exposed to air will also, during normal static conditions, be exposed to fuel vapor, especially when the airplane is exposed to solar heating while parked for some period of time. Expansion and vaporization of fuel in the tanks can cause vapor migration through a part of the pressurizing system.
A failure of a pressurizing system component can also inadvertently cause exposure to fuel. Seals and diaphragms in components which make up this system should be fuel resistant.
(ii) Another 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 fuel vapor.
(iii) 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. The use of composite materials is discussed in more detail in Chapter 3, paragraph 9 of this AC. Surface treatments, coatings and sealants intended to reduce weathering and corrosion and seal structural areas should also be substantiated to ensure ruel and tuel vapor resistance, where applicable.
(iv) If a component is life limited, the limits due to fuel environmental conditions should be defined, particularly for nonmetallic items.
(2) Fuel Additive Compatibility. An evaluation should De made of the ettects 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 will 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.
Chap 3 Par 7 25
Ozone Resistance. High concentrations of ozone may be encountered by airplanes at cruise altitudes. Some materials, particularly certain rubber and plastic compounds, are susceptible to ozone deterioration and should not be used in auxiliary fuel systems. In some cases the combined ozone and fuel environment can produce rapid weathering and disintegration of the material.
The eftects of ozone should be evaluated with respect to the particular auxiliary fuel system design in question. Ozone compatibility should be substantiated using approved methods and specifications.
(4) 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 are very active chemically. The combination of graphite layers of composite construction attached directly to aluminum will 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 must conform to Technical Standard Order (TSO) C80 or otherwise be shown suitable for the intended application.
(5) Temperature Range Suitability. Materials and components used in the auxiliary fuel system must have suitable properties and must perfor their intended function throughout the approved airplane operating envelope. In some instances, they may be exposed to temperatures as low as -65 F. or lower and as high as 2500 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.
b. 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. An evaluation of the extreme environment properties should account for the following:
(1) Flexibility and Resilience. The properties of flexibility and resilience must to a certain degree be considered as a part of the normal environmental condition for flexible liners, flex connections and other components; however, these properties become of critical importance under crash conditions. Flexure and resilience must be considered to ensure fuel containment under these conditions. It may be necessary to demonstrate by test that certain fuel lines and shrouds are sufficiently resilient or ductile to withstand survivable crash load impact without fuel leakage. 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.
Chap 3 26 Par 8
(2) 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.
in addition, these configurations should be capable of sustaining the critical flight and landing loads and thus the integrity of the system under these conditions.
(3) Fire Resistance and Crash Fire Propagation Characteristics.
Auxiliary fuel systems are frequently installed in cargo compartments.
definition, if the tank and component secondary barriers are capable of withstanding a cargo compartment fire such that the safety of the airplane is maintained then the secondary barrier materials are acceptable. These fire resistant characteristics should be equivalent to the liner materials of the compartment in which they are located. For Class B through Class E cargo or baggage compartments the materials must, at least, meet the requirements of $$ 25.853(b) and 25.855(a-1). The system should also be evaluated with respect to materials from the standpoint of toxic gas release under fire conditions. No materials should be used which act as a fuel for fires. Avoid the use of magnesium or flammable resins, sealants and coatings.
9. COMPOSITES ($$ 25.601, 25.603, 25.609, 25.613, 25.615, 25.619, 25.5/1,
5.581). The suitability and durability of materials used for nonmetallic auxiliary fuel tanks must be established by tests. Advisory Circular 20-107, 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:
a. Aging of the laminate in the operating environment, including wear, due to temperature, pressure, cavitation, moisture content changes, etc.
b. Chemical reactions with fuel vapor, cleaning liquids, solvents, salt water vapor and any other contaminates such as fungus in the tank.
c. Static electrical charge, bonding or lightning strike.
Any other elements characteristic of or unique to the type of nonmetallic material, method of processing and design applications.
e. A manufacturing process to ensure repeatability of material properties.
f. Inspection techniques for manufacturing and for continued airworthiness.
g. Statistically based material strength properties for critical tank structure. Strength, detail design and fabrication must minimize the Chap 3 Par 8 27
probability of fatigue failure. Chapter 9 of MIL-HDBK-5D, Metallic Materials and Elements for Aerospace Vehicle Structures, and MIL-HDBK-17, Plastics for light Vehicles, contain procedures for establishing such properties.
h. Composite repair procedures defined by the applicant and approved by the FAA.
Chap 3 28 Par 9
CHAPTER 4 - AUXILIARY FUEL SYSTEM PERFORMANCE GENERAL. The designer should evaluate the auxiliary fuel system performance for all normal operating conditions of the airplane. This will, as a minimum, require a FAA witnessed functional ground and flight test program.
Some of the performance criteria discussed in this section can be sufficiently evaluated by analytical methods, and substantiation reports submitted to the applicable FAA aircraft certification office for approval. As noted earlier, installations that involve changes in primary structure, aerodynamics, or mass distribution may require additional extensive substantiation that is beyond the scope of this AC. The following criteria should be considered by the applicant in generating test plans and substantiation reports for certification of the system.
11. NORMAL OPERATION EVALUATION.
a - Refueling and Transfer Performance.
(1) Refueling and Transfer Flow Rates and Pressures ($$ 25.951,
25.955, 25.957, 25.961, 25.991 and 25.979).
(i) 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 to ensure that over pressurization of the tanks, including the existing airplane main tanks when applicable, does not occur. Verification is also required that the pressure fueling automatic shutoff means does not produce unacceptable surge pressures which may damage the system or fueling equipment or rupture fueling lines.
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 data.
(ii) Transfer rates should be determined 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, must be outlined in the limitations section of the AFM. It should be assumed that the transfer system might be left on inadvertently; therefore, it should also be shown that transter under conditions not permitted by the AFM would not present any hazard.
(iii) Compliance with the hot weather operational performance requirements of § 25.961 must be shown for auxiliary feed systems that feed directly to the engine. This is normally accomplished by conducting a hot fuel test; 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.
Chap 4 Par 10 29
For auxiliary fuel systems that depend on bleed air for fuel transfer the airplane engine is, in effect, a transter pump, and tail-sate requirements for pump transfer apply. This means that there must be an alternate means of providing fuel transfer in event of an engine inflight shutdown in accordance with § 25.901(c). This is usually provided by extracting bleed air for auxiliary tank transfer downstream of the pneumatic cross-over point. It is suggested that the applicant consult the airplane type certificate holder concerning the proper location of bleed air extraction points.
(2) Fuel Tank Capacities (Usable Fuel, Sump Capacity, Undrainable Fuel, and Expansion Space) ($$ 25.959, 25.969, and 25.971).
(1) For direct feed auxiliary tuel systems, the unusable fuel requirements of $ 25.959, apply. For transfer type auxiliary fuel systems, the 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.
Sump capacity is a part of the unusable fuel and as previously noted, is defined by § 25.971 (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: Usually measuring the quantity of fuel necessary to fill all trapping cavities in the system is all that is necessary.
(iv) The maximum tank fueling capacity must take into consideration the expansion space requirements of $ 25.969, 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.
(v) Figure 5 shows how the expansion space would be defined for three different vent configurations. The expansion space volume must 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 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.
(vi) Ground and flight tests of the installed system must be conducted in accordance with § 25.979 with the airplane in the correct ground and flight attitudes to verify that the expansion space requirements are Chap 4 30 Par 11