There may also be a need to refer to § 25.997, 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 system. The existing main fuel system design should already satisfy $ 25.997 requirements. The installer should, however, verity that these requirements are maintained when an auxiliary fuel system is installed. The following features should be considered with respect to auxiliary fuel system contamination.
a. Fuel Tank Sumps and Fuel Strainers.
(1) Sump Location and Capacity.
(i) Sumps should be installed at the lowest point in the auxiliary fuel system with the airplane in its normal ground static attitude. This will 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 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 in the previous section for fuel tanks including the overboard drain exit requirements. Separate sump tanks should not be located below the cargo compartment floor if they will be vulnerable to damage in the event of a wheels-up landing.
(ii) Sumps and sump tanks must have the capacity specified in § 25.9/1. It a separate 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.
(2) 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 a high degree of crashworthiness. Drain valves should be positive locking and reliable. Drain valve installations should provide double seals to prevent overboard leakage from a single seal failure. Lightning aspects of the overboard access should be addressed as discussed in the next section. Locate the 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 in the previous section and the shrouds provided with vents per normal shroud procedures. The shrouded fitting between the sump drain and the overboard penetration should provide a "fuse" point or other means to ensure that upward penetration of the tank does not occur during a crash landing.
(3) Fuel Strainers ($$ 25.951 (c) and 25.977).
(i) 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 Chap 2 16 Par 5
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.
The mesh or size requirements are defined in § 25.977.
However, because of the problem of icing, the effective flow area of the strainer should be selected considering also the requirements of § 25.951 (c).
Fine mesh screen should not normally be used in this application.
Fuel and Vent System Low Points ($ 25.975). Avoid creating low points in routing fuel and vent lines. This condition is usually a problem where long fuselage runs of flexible fuel lines/shrouds are installed. It is particularly important to eliminate low spots in vent and drain lines, where water may collect and freeze, blocking the lines. Unavoidable shallow low areas may be acceptable without drains if fluid can not collect to any appreciable degree.
Where traps 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. Auxiliary tank vent systems should ensure that fuel sloshing in the main tank vent line does not enter the auxiliary vent system in unacceptable quantities. 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 ground attitude. If not possible, consider adding sump type drain valves at the applicable low points. Where drain valves are required use the criteria discussed in the previous sections.
6. IGNITION SOURCE ISOLATION EVALUATION.
System Electrical Bonding (8 25.863, MIL-B-508/B). All 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 ponding when addressing nonmetallic or composite auxiliary tuel 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 cognizant certification office during certitication.
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.
(i) Electrical power wiring in fuel or fuel vapor environment should be adequately protected to prevent damaged wire(s) from igniting Chap 2 Par 5 17
fuel/fuel vapor. Installation of the electrical power wiring in a metallic conduit is an acceptable method to meet this criteria. Electrical power wiring is the wiring that is connected to a power source.
(ii) Flexible conduit conforming to Specification MIL-C-6136, Installation, Types i or il, or equivalent, may be used where it is impractica Installation, lypes to use rigid conduit, such as, areas that have motion between conduit ends or where complex bends are necessary.
(iii) Conduit installation considerations include the following:
(A) Metallic conduit should have a low-resistance bond of less than .003 ohm to airplane structure.
(B) 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.
(C) The conduit should be supported by clamps along the conduit run to prevent chafing against structure and to avoid stressing the end fittings.
Lightning Vulnerability ($ 25.954). The auxiliary fuel system installation should be evaluated from the standpoint of lightning vulnerability. Some items and areas that may be susceptible to fuel ignition or indirect effects of lightning include, but are not limited to, the following:
(1) Vent outlets, metal fittings and mechanical fasteners inside fuel anks, tuel tiller caps and access doors, drain plugs, tank skins, tue transfer lines inside and outside of ine tanks, and electrical and electroni fuel system components and wiring.
(2) Advisory Circular 20-53A, Protection of Aircraft Fuel Systems Against Fuel Vapor Ignition Due to Lightning, addresses the subject in more detail.
(3) 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.
ot all attachments, fittings, etc., must 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).
Chap 2 18 Par 6
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