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.
Although function will dictate the appropriate location of components in the airplane, there is usually some latitude which will allow selection of a specific area which is more suitable than others. Component location should be considered from the aspects of both access and protection.
Access is especially important for components which will require routine periodic inspection and maintenance such as strainers, filters and drain valves or components with known short service life expectancy. Where possible, such components should be located in areas where there are currently existing access doors and openings on the airplane. When new access provisions are required, lightning and crashworthiness vulnerability as well as structural security should be considered.
(iv) Locating components in areas where there is a high probability that they can be stepped on or tripped over by personnel during the routine servicing or maintenance of the airplane should be avoided. The crashworthiness of the location should also be considered. Components should not be installed below the fuselage cargo floor if they may be crushed, scraped oft, or cause penetration into the auxiliary fuel tank which can result in leakage during a wheels-up landing. Protection from damage due to shifting baggage and other objects which may not be tied down in the cargo area should be provided. See Chapter 1, paragraph 2d for cargo barrier criteria.
(v) For components which must be located inside the fuel tanks, the crashworthiness aspects of the installation should be considered. Means to prevent component sharp edges from penetrating the tank surface due to deflection of the surface under crash load conditions should be provided, especially where flexible tank bladder cells are used.
(3) Tank Penetration Points (Access, Quantity Probes, Float Switches and Fuel Fittings) ($§ 25.963, 25.973; 25.975, 25.977).
(!) The location and arrangement of all tank penetration points should be considered. Tank penetration points are all locations where openings exist in the tank walls for access (inspection or repairs), for mounting fuel quantity probes, float switches, etc., and for tank venting, drainage and tuel transfer or fueling.
(ii) Auxiliary tanks should have access openings large enough to permit completion of required inspections. Component and line penetrations may be usable as access/inspection openings, dependent on size and location, and should be evaluated for acceptability.
(iii) All tank fuel line to airplane structure attachments should be evaluated tor the flight, flight vibration and crash loads which may be ransmitted to the tank walls. From the crashworthiness standpoint, to prevent fuel tank fittings from being torn out of the tank wall, it may be advisable to Chap 2 12 Par 4
consider the need for frangible disconnect valves or fittings, mounted on the external surface of the tank, which separate and shut off any hazardous fuel flow from the tank in event of a crash. However, a tallure analysis must show that inadvertent closure of these frangible fittings will not interfere with continued safe flight.
(4) Electrical Wiring Routing and Support ($$ 25.1301 and 43.13). The following practices should be adhered to for wire routing and support; however, in all cases, clearance between wires and fuel or oxygen lines and control cables should be adequate to prevent damage due to deflections of structure or support which may occur during a survivable crash.
(i) A minimum clearance of 3 inches between wires and any control cable should be maintained unless a shield is provided.
(ii) Wires should be located above (preferably), or on a level with, metallic fluid lines. A minimum clearance of 6 inches between wires and combustible fluid or oxygen lines should be maintained whenever possible.
a 6-inch clearance cannot be maintained, install additional clamps to provide separation. The fluid lines should not be used as a means of supporting wire bundles. Additional clamps should be installed to support the wire bundle and these clamps fastened to the same structure used to support the fluid or oxygen lines to prevent relative motion.
(iii) A minimum separation of at least 1/2-inch should be maintained between noncombustible plumbing lines and any wire.
(iv) Wires and wire bundles should be supported with clamps meeting Specification MS-21919, Clamp, Looptype, Cushion Support, or equivalent.
(v) Where wires or wire bundles pass through cutouts in bulkheads or other structural members, a grommet and suitable clamp should be provided to prevent abrasion if the clearance is less than 1/4-inch.
(vi) Refer to AC 43.13-1A, Acceptable Methods, Techniques, and Practices--Aircraft Inspection and Repair, for more details regarding routing and supporting of wires, acceptable methods, techniques and practices-aircraft inspection and repair.
b. Fuel Containment Secondary Barriers ($$ 25.967, 25.863). For auxiliary fuel systems which are located in the passenger or cargo and baggage compartments (Appendix 1), isolation of the fuel and tue, vapors trom other areas of the compartment is of critical importance. lanks, line tittings, connections and other components, such as valves, pressure transmitters, regulators etc., must de shrouded or provided with redundant barriers such that eaks from any of these sources will not present a fire hazard. Some of the important characteristics of the secondary barrier system are:
(1) The system should be capable of containing and isolating any leakage. To contain any leakage, integrity at the maximum system operating pressure should be demonstrated.
Chap 2 Par 4 13
(2) Secondary barrier spaces should be vented and drained in accordance The drain system also serves as a periodic visual means of detecting any be connected to other types of fluid drain systems.
(3) The secondary barrier drain system materials, construction and sealing characteristics must be compatible with fuel and capable of long life under the altitude/cabin pressure cycling, vibration and wear to which they will be exposed in service.
(4) The drain system should be reviewed from the installation location aspects to preclude the possibility of inadvertant damage by ground personnel or shifting cargo.
c. Tank, Fuel and Vent Line and Component Shrouds ($ 25.967).
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. Figures 2 and 3 show some acceptable designs for shrouding equipment items and fittings installed on or through the tank walls. Each tank penetration design should be reviewed to ensure a single failure (such as a seal failure) does not result in fuel or fuel vapors entering the compartment.
A primary seal with a secondary shroud/seal provides the required protection if indication of a primary seal failure is also provided and the secondary seal is pressure tested periodically.
(2) All vent and fuel fittings and connections in a passenger or cargo compartment should also be shrouded. An example of this is shown in Figure 4.
3) valves and other components, unless otherwise protected, can nave possible leak paths through shatts and at control motor or solenoid connections or other seals in addition to their line connections, and should be completely shrouded.
(4) 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 conditions. It may be necessary from this evaluation to install the wiring in vapor-proof conduit, except where shown to be intrinsically safe.
d. Fuel Tank Secondary Barrier Cavity Venting ($ 25.967). The changes in tank secondary barrier cavity pressure during all airplane maneuvers, including emergency descent, must be accounted for in the design of the auxiliary fuel tank. Bladder type tanks may be critical under emergency descent conditions, Chap 2 14 Par 4
depending on the cavity vent line sizing. The vent/drain configuration must provide the required positive and negative pressure reliet between the outer shell and the bladder or inner wall to prevent collapse or over expansion of the inner tank. Depending on the location of the overboard vent/drain exit and the airtlow characteristics 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" and "wheels and flaps down" configuration.
e. Other Secondary Barrier Vent and Drain Provisions ($$ 25.954 and
25.967).
(1) All secondary barrier spaces should be vented and drained. The spaces in some designs are manifolded for venting and drainage as shown in the figures in the previous section or are independently vented and drained.
Figures 2 and 3 the tank fitting cavities are ported into the tank shroud cavity proper. In Figure 4 the line fitting shroud vents to the tank fitting cavity; and this cavity, in turn, is vented to the tank shroud cavity.
Eventually all cavities must 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 main fuel tanks or shrouds. 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.
(2) For secondary barriers located in passenger or cargo areas, 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.
(3) If a new exit mast is required, consider carefully the location and configuration from the standpoint of lightning vulnerability and conductivity (see Chapter 2, paragraph 6b). Drain masts should not be located upstream of air inlets or other openings in the airplane external surface. Physical inspection by the cognizant FAA personnel of all drain locations on the actual airplane should be accomplished to ensure all interfaces are considered.
Verification that liquid discharge from the mast will flow clear of the airplane and not impinge or reenter at another airplane surface should be accomplished by impingement tests conducted in flight. Ensure that freezing does not occur during the tests and that the test results will be representative of a fuel leak.
5. FUEL SYSTEM CONTAMINATION PREVENTION ASSESSMENT (99 25.971, 25.977 and
25.997). 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 § 25.971, Fuel Tank Sumps, and § 25.977, Fuel Tank Outlets.
Chap 2 Par 4 15
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