21 Figure 1. Examples of Fuel Tank Ignition Source Considerations
9.2.2 Another example would be installation of a flame arrestor in the inlet line to a fuel pump. The compliance demonstration for the fuel pump may be limited to showing that the arrestor was effective at precluding propagation of the flame from the pump back down the inlet line into the tank, and showing that any anticipated failures or events could not violate the explosion-proof features of the pump assembly. A CDCCL may be necessary to maintain the flame arrestor design feature. If the flame arrestor cannot be shown to be effective for the life of the installation, an airworthiness limitation limiting the life of the flame arrestor would be necessary. In addition, revalidation of the fuel system with other regulations (e.g., icing and reduced flow due to contamination) would be required if modifications were incorporated into the fuel feed system. The SSA criteria, process, analysis methods, validation, and documentation should be consistent with the guidance material provided in SAE ARP4761, using the unique guidance specific to the fuel tank system as defined in this AC.
9.3 Assumptions and Considerations for Fuel Tank System Analysis.
The applicant should conduct the fuel tank system analysis based on the following assumptions:
9.3.1 Fuel Tank Flammability.
The analysis should assume that the environment inside the fuel tank is always flammable. The conditions required to ignite fuel vapors from ignition sources vary with pressures and temperatures within the fuel tank and can be affected by sloshing or
22 spraying of fuel in the tank. Due to the difficulty in predicting fuel tank flammability, the FAA has always assumed that a flammable fuel/air mixture exists in airplane fuel tanks and has required that no ignition sources be present. The SSA should be prepared considering all in-flight, ground, service, and maintenance conditions for the airplane, assuming that an explosive fuel/air mixture is present in the vapor space of fuel tanks and vent systems at all times9, unless the fuel tank has features that mitigate the effects of tank ignition (e.g., polyurethane foam).
9.3.2 Failure Condition Classification.
Unless design features are incorporated that mitigate the hazards resulting from a fuel tank ignition event (e.g., polyurethane foam, adequate structural margin), the SSA should assume that the presence of an ignition source is a catastrophic failure condition.
9.3.3 Latent Failures.
9.3.3.1 In order to eliminate any ambiguity as to the restrictions on latent failures, § 25.981(a)(3) explicitly requires that any anticipated latent failure condition not leave the airplane one failure away from a catastrophic fuel tank ignition. In addition to this limitation on latency, § 25.1309(c) limits latent failure conditions to those that do not create an “unsafe system operating condition.” Consequently, if a latent failure condition is not extremely remote (i.e., it is anticipated to occur) and it creates an “unsafe system operating condition,” then “warning information must be provided to alert the crew…and to enable them to take appropriate corrective action.” Notwithstanding these applicable regulatory restrictions on latency, there are practical limitations on the available means of compliance. For example, detecting a failure condition requires a finite period of time, and there are not always “appropriate corrective actions” that can be taken during the flight. Consequently, for the purpose of complying with § 25.981(a)(3), the period of latency for any anticipated significant latent failure condition should be minimized and not allowed to exceed one flight cycle. For the purpose of complying with § 25.1309(c), any time the airplane is operating one failure away from a catastrophic fuel tank ignition should be considered an “unsafe system operating condition,” recognizing that sometimes the only “appropriate corrective action when problem detection is available is to continue on to your destination but not to initiate another flight without making appropriate repairs.”
9.3.3.2 Another practical limitation on the available means of compliance is the technological feasibility of providing inherent failure detection within the design for all significant failures. Sometimes periodic inspection is the
9 The preamble to amendment 25-102 states “The FAA intends that those failure conditions identified earlier in this document, and any other foreseeable failures, should be assumed when performing the safety review needed to substantiate that the fuel tank system design is fail-safe. The safety review should be prepared considering all airplane inflight, ground, service, and maintenance conditions, assuming that an explosive fuel air mixture is present in the fuel tanks at all times, unless the fuel tank has been purged of fuel vapor for maintenance .” (66 FR 23094)
23 only practicable means of reliably detecting a failure condition.
Consequently, when such inspections are identified within the analysis as the means of detection, the inspection method and frequency must be sufficient to conclude that the occurrence of the significant latent failure condition is extremely remote.
9.3.3.3 Any mandatory replacement time, inspection interval, related inspection procedure, and all CDCCLs identified as required to prevent development of ignition sources within the fuel tank system for § 25.981(a) must be identified in the Airworthiness Limitations section of the ICA as a fuel system Airworthiness Limitation. The Airworthiness Limitations section should include the following:
9.3.3.3.1 A designation of the maintenance actions and alterations that must be inspected (critical inspections), including at least those that could result in a failure, malfunction, or defect endangering the safe operation of the aircraft, if not performed properly or if improper parts or materials are used.
Note: A validation inspection should be conducted to reaffirm all or a portion of the initial inspection requirements for those critical inspections that, if not performed properly or if improper parts or material are used, could result in a failure, malfunction, or defect endangering the safe operation of the airplane. For those air carriers that use a mechanic for the initial inspection, an inspector should be used to conduct the validation inspection. For those air carriers that use an inspector for the initial inspection, another qualified inspector should be used to conduct the validation inspection.
9.3.3.3.2 Procedures, standards, and limits necessary for critical inspections and acceptance or rejection of the items required to be inspected, and for periodic inspection and calibration of precision tools, measuring devices, and test equipment.
9.3.4 Failure Conditions.
When showing compliance with § 25.981(a)(3), the analysis must consider the effects of manufacturing variability, aging, wear, corrosion, and likely damage. For the purpose of compliance with § 25.981, “extremely remote” failure conditions are those not anticipated to occur to each airplane during its total life, but which may occur a few times when considering the total operational life of all airplanes of one type. This definition is consistent with that developed by the Aviation Rulemaking Advisory Committee (ARAC) for a revision to AC 25.1309-1A, System Design and Analysis.
“Extremely improbable” failure conditions are those so unlikely that they are not anticipated to occur during the entire operational life of all airplanes of one type. This definition is consistent with the definition provided in AC 25.1309-1A. Likely damage is damage that, using engineering judgment or past experience, would lead one to conclude that an occurrence is foreseeable. Examples of likely damage are a wire
24 bundle located where a mechanic could use it as a handhold; an instrument located where, if someone dropped a wrench, damage would result; or a fuel probe located where a mechanic could use it as a step in the tank; etc.
9.3.4.1 The analysis should be conducted considering the deficiencies and anomalies listed in paragraph 6.3 of this AC, failure modes identified by the review of service information (including review of supplier service data), and any other failure modes identified by the functional hazard assessment of the fuel tank system. For example, the applicant should assume the presence of conductive debris such as lockwire, steel wool, nuts, bolts, rivets, etc. Section 25.981 requires that the effects of manufacturing variability, aging, wear, corrosion, and likely damage must be considered when showing compliance, which is needed to show compliance with § 25.901(c).10 Credit for fail-safe features must be substantiated.
9.3.4.2 The level of manufacturing variability, aging, wear, corrosion, and likely damage that must be considered should be determined based upon evaluation of the detectability of degraded or out-of-specification configurations, and established and documented within the analysis.
In-service and production functional tests, component acceptance tests, and maintenance checks may be used to substantiate the degree to which these states must be considered. For example, inspection of fuel tank system bonding on production airplanes has shown that some bonds were inadequate. Functional testing of all bonding was incorporated to address this deficiency. In some cases (e.g., component bonding or ground paths), a degraded state will not be detectable without periodic functional test of the feature. For these features, inspection/test intervals should be established based on previous service experience of equipment installed in the same environment. If previous experience on similar or identical components is not available, conservative initial inspection/test intervals should be established until design maturity can be assured.
9.3.5 External Environment.
The severity of the external environmental conditions that should be considered when showing compliance with § 25.981 are those established by certification regulations.
9.3.6 External Sources of Tank Autoignition.
The possibility of fuel tank ignition due to surface-ignition sources created by external tank heating should be considered. This includes heating of the tank due to operation or failure of systems outside the tank within both the pressurized and unpressurized areas of the airplane, such as overloaded electric motors or transformers, failures in the
10 See Docket No. FAA-1999-6411 (66 FR 23097, May 7, 2001), Transport Airplane Fuel Tank System Design Review, Flammability Reduction, and Maintenance and Inspection Requirements , amendment 25-102, https://www.gpo.gov/fdsys/pkg/FR-2001-05-07/pdf/01-10129.pdf.
25 pneumatic system, and/or ducting that could cause localized heating of tank surfaces. In addition, the possibility of localized heating due to external fires should be considered.
9.3.6.1 Section 25.967(e) requires that, “Each fuel tank must be isolated from personnel compartments by a fumeproof and fuelproof enclosure.”
9.3.6.1.1 Leakage of fuel or vapor into spaces adjacent to the fuel tank, where a secondary fuelproof and fumeproof barrier is not provided, has typically been assumed for areas such as:
The wing leading (including any adjacent compartment such as the strut) and trailing edges, Fairings located below the fuel tanks, Fuel pump enclosures, and Unpressurized areas of the fuselage surrounding fuel tanks located in the empennage.
9.3.6.1.2 Components located in these areas have been required to meet explosion-proof requirements. These components or systems should be included in the analysis. Examples of equipment include, but are not limited to, environmental control system (ECS) air conditioning packs, motors, power assisted valves, fuel pumps, hydraulic pumps/motors, certain flight control actuators, ECS controls, and wiring and valves.
9.3.6.2 A safety review of flammable fluid leakage zones adjacent to fuel tanks should be conducted to determine that the design complies with the requirements of §§ 25.863(a) and 25.981. In general, the fire protection philosophy for any area considered a flammable fluid leakage zone is to assume that flammable vapors may be present in the zone and to minimize the probability of ignition of vapors (§ 25.863(a)). This has typically been accomplished by using combinations of the following design considerations:
Grounding and bonding of electrical equipment, Qualification of electrical equipment as explosion proof, Sealing of electrical connectors, Proper support, protection, and separation of wiring, Drainage provisions in the leakage zone, Ventilation of the leakage zone in flight and of areas around the auxiliary tanks, and Immediate maintenance action to correct leaks in these areas.
9.3.6.3 Surface Temperatures in Areas Adjacent to Fuel Tanks.
The FAA has approved installations where surfaces adjacent to the tank experience temperatures in excess of the internal fuel tank surface temperature limit. Manufacturers have substantiated that the conditions (ambient pressure, dwell time, fuel type, etc.) within these areas are such that a higher value may be used. For example, applicants have successfully substantiated, for certain pneumatic system installations, a maximum allowable surface temperature of 400 °F with a transient excursion up to 500 °F for a maximum duration of two minutes. The excursion above 400 °F occurs only during failure conditions such as failure of the engine pneumatic system to regulate temperature, or duct rupture. Approval of these elevated temperatures has been based on specific design features, such as an over-temperature shutoff of the pneumatic system so that the surface temperatures adjacent to the tank cannot exceed the surface ignition temperature justified for the fluid type including the effect of local airflow and ventilation conditions within the zone. Internal tank surface temperatures resulting from the failure should not exceed the surface temperature limit for the fuel type used as described in paragraph 7.5 of this AC.
9.3.7 Electrical Ignition Sources.
The applicant should perform a failure analysis of all fuel systems and subsystems with wiring routed into fuel tanks. Systems that should be considered include fuel pump power and control and indication, fuel quantity indication, fuel temperature indication, fuel level sensors, and any other wiring routed into or adjacent to fuel tanks. The analysis should consider system level failures, failures within LRUs, and component level failures discussed below. The analysis should include existence of latent failures and subsequent failures that may lead to an ignition source within the fuel tank.
Examples include undetected failures of tank components or wiring, the undetected presence of conductive debris, damage to FQIS or level sensor probes, or corrosion, in combination with external failures such as hot shorts or electromagnetic effects. In addition, the applicant should provide a description of the protective means employed in the fuel system wiring. This should include a description of features such as separation/segregation, transient suppression devices, shielding of wiring, and methods employed to maintain configuration control of critical wiring throughout the life of the airplane.
9.3.8 Electrical Short-Circuits.
9.3.8.1 One method that may provide protection of circuits that enter fuel tanks is the incorporation of a transient suppression device (TSD) on the circuit close to the point where those wires enter fuel tanks. Consideration should also be given to protection of wiring between the TSDs and the tank if the protection devices are not located at the tank entrance, and also to the possibility of transients being induced in the wiring between the TSDs and the electrical devices in the fuel tanks. Caution should be exercised when
27 using a TSD to ensure that the TSD addresses both voltage and current suppression in order to limit the energy and current below the limits provided in section 7.2 of this AC.
9.3.8.2 Another method of protection that has been used to provide a fail-safe design with respect to electrical shorts is separation of wiring to electrical devices in the fuel tanks from other electrical power wires and circuits, combined with shielding between wiring that enters fuel tanks and any other electrical power-carrying wires in the aircraft installation. The effects of electrical short circuits, including hot shorts, on equipment and wiring that enters the fuel tanks should be considered, particularly for the FQIS wiring, fuel level sensors, and probes. Latent failures from factors such as contamination, damage/pinching of wires during installation, or corrosion on the probes, connectors, or wiring should be considered when evaluating the effects of short circuits. The wire routing, shielding, and segregation outside the fuel tanks, including within the FQIS components
(e.g., gauging units), should also be considered when evaluating the effects of short circuits. The evaluation should consider both electrical arcing and localized heating that may result from short circuits on equipment, FQIS probes, and wiring. The evaluation of electrical short circuits should include consideration of shorts within electrical equipment, and wiring from the equipment into the fuel tank. Prevention of fuel ignition from electrical shorts to wiring that enters fuel tanks may require specific wire and circuit separation and wire bundle shielding.
9.3.9 LRU Design Evaluation.
The design review should include evaluation of the separation and protective features incorporated into any fuel system LRU whose failure could result in high-level electrical power (i.e., above intrinsically safe levels) entering the fuel tank. For example, circuit board failures could cause the LRU power supply circuits for the fuel quantity gauging system to come into contact with circuits that lead into the fuel tank, resulting in a possible ignition source. Failures that can lead to violating separation within the LRU can be external and internal events. External failures include over voltage or over current, high humidity, temperature, vibration, shock, and contamination. Internal failures include manufacturing defects or flaws in the conductor, substrate, or coating. To address these failures, the design should either provide isolation and physical separation between critical circuits, such as circuits that enter a fuel tank, or adequate protective features, such as transient suppression devices as discussed earlier, to protect the circuits that enter the fuel tank. Any LRU that meets the design requirements identified in Underwriters Laboratories Inc., UL 913, Intrinsically Safe Apparatus and Associated Apparatus for use in Class I, II, III, Division 1, Hazardous (Classified) Locations, is considered acceptable, provided the following issues are addressed. Ideally, higher power circuits within the LRU should not be located on the same circuit board or in a wire harness or electrical connector with intrinsically safe circuits or wiring. There should be a physical barrier between circuit boards to isolate the intrinsically safe circuits from the effects of broken components or
28 fire within the LRU. If limiting devices are installed on the same circuit board in series with system circuitry to limit the amount of power or current transmitted to the fuel tank, there should be three inches between the traces, unless the manufacturer can justify a smaller separation on the basis that the effects of fire on the circuit board will not compromise the intrinsically safe circuit(s).
9.3.10 Electromagnetic Effects including HIRF.
See AC 25.954-1 for guidance in establishing compliance with the requirements for fuel system protection from lightning effects.
9.3.10.1 The evaluation should consider electromagnetic effects due to HIRF, electrical transients, and RF emissions on fuel system conductors (e.g., fuel tank plumbing, structure, fuel, equipment and wiring) within the fuel tanks, particularly for the FQIS wiring and probes. The applicant should also consider latent failures from factors such as contamination, damage, or corrosion on the probes or wiring when evaluating the effects of electrical transients. The wire routing, shielding, and segregation of conductors (e.g., plumbing, component casings, wiring, etc.) outside the fuel tanks should also be considered when evaluating the effects of electrical transients because the transient generation and coupling to conductors may occur outside the fuel tanks. The evaluation should consider both electrical sparks and arcs, and localized heating, that may result from electromagnetic effects on the fuel tank system, FQIS probes, and wiring.
9.3.10.2 The evaluation should consider latent failure of electromagnetic protection features, such as shielding termination corrosion, shield damage, and transient limiting device failure, and the applicant should establish appropriate indication or inspection intervals to prevent the existence of latent failure conditions. The failure of other system components may also affect protection against electromagnetic effects. Consequently, the evaluation should consider the effect of any anticipated failure on the continued environmental protection.
9.3.10.3 The evaluation of electromagnetic effects should be based on the specific electromagnetic environment of a particular airplane model. Standardized tests, such as those in RTCA DO-160G, sections 19 and 20, are not sufficient alone to show that the appropriate standardized test categories, procedures, and test levels of RTCA DO-160G are selected, without an evaluation of the characteristics of the specific electromagnetic environment and induced transient levels assigned to systems installed within a particular airplane model. Simulation of various latent failures of fuel system components within the tanks may be needed to show the effectiveness of the transient protection. Effectiveness of these features should be verified using the appropriate test procedures and test levels of RTCA DO-160G, determined above.
9.3.10.4 Prevention of fuel ignition due to electromagnetic effects may require specific wire segregation and separation, wire bundle shielding, or transient suppression for wires entering fuel tanks. Effectiveness of the transient protection features should be verified using the appropriate test procedures and test levels of RTCA DO-160G, determined above.
9.3.10.5 Redundancy of Bond Paths.
Failure of bonding jumpers is generally considered a latent failure, since there is no annunciation or indication of the bonding failure. The airplane fleet fuel tank inspections that occurred as a result of the TWA 800 investigation11 showed that failure of bonding jumpers, due to damage, wear, or manufacturing errors, was not unusual. Based on this, it would be difficult to show that failure of a single bonding jumper is extremely remote or extremely improbable. Therefore, the electrical bonding jumper or other bonding provisions would need to consider the consequences of these latent failures. This may result in designs that incorporate electrical bonding redundancy, if the failure of a single electrical bonding feature could create a fuel tank ignition source. Additionally, manufacturers would need to consider the use of appropriate maintenance to detect failed bonding jumpers. An example of such maintenance might include periodic inspections to limit latency.
9.3.10.6 Self-Bonding Couplers.
Early generation, self-bonding, flexible fuel couplers did not have multiple bonding paths. Thus, these bonding couplers exhibited single-point failures that caused loss of function. These self-bonding flexible couplers have failed because of missing bonding springs, anodizing on bonding surfaces, and incorrect installation. The safety assessment of newer designs incorporating multiple bonding paths must consider these failure modes, and qualification testing should show no ignition sources are present in the full-up (non-degraded condition) and possible degraded condition with failure modes present within the couplings. For example, failure assessments of clamshell-type, self-bonding metallic couplings in composite fuel tanks have shown arcing could occur if a coupling was improperly latched, or became unlatched and fell to the bottom of the fuel tank. The design of the coupling would need to address these failure modes. Improper latching could be addressed through positive latching features with tactile and visual indication that the coupling is properly latched. Redundant fail-safe features, such as redundant hinge and latching features, redundant bonding features, etc., may be needed to address other possible failure modes.
11 National Transportation Safety Board Aircraft Accident Report NTSB/AAR-00/03, “In-flight Breakup Over the Atlantic Ocean Trans World Airlines Flight 800, Boeing 747-131, N93119, Near East Moriches, New York,” dated July 17, 1996.
9.3.10.7 Resistance or Impedance Limits of Airplane Electrical Bonding Provisions.
9.3.10.7.1 There is no specific FAA guidance on maximum resistance or impedance of airplane electrical bonding provisions because electrical bonding within a fuel system should be tailored to the performance requirements of a particular airplane design. The electrical bonding should consider the electrical sources, electrical faults, and electrostatic charges. The electrical bonding should also consider the fuel system design of the specific airplane, which would include the structure material used (aluminum, carbon fiber composites, fiberglass composites, etc.), the configuration of the fuel system (routing of fuel tubes, wires, and hydraulic tubes), and the electrical bonding concept (intentional isolation, self-bonding fittings, separate bonding jumpers, etc.). Given the large variation in design approaches and the close relationship between the design approach and the electrical bonding requirements, it is not practical for the FAA to provide specific guidance on maximum bonding resistance or impedance.
9.3.10.7.2 Some type certificate (TC) holders have performed tests on their airplanes to determine the specific requirements for electrical bonding. Others, in the absence of specific airplane test data, have chosen conservative electrical bonding approaches. The approach is a decision each TC holder should make based on the specific situation for that TC holder’s airplane models.
9.3.10.8 Bonding Integrity Checks.
Past experience has shown measurement of bond resistance is the desired method of ensuring bond path integrity. During bonding resistance measurements, the protective finish of components might be damaged in order to penetrate the insulating anodized surface layer, which may lead to subsequent corrosion damage. This concern has resulted in some TC holders defining non-intrusive inspections for electrical bonding. These inspections may include detailed visual inspections provided the quality of the electrical bonding feature can be adequately assessed by visual cues, such as visible corrosion, breakage, tightness, or missing bonding provisions. For critical bonds, this method would not by itself be adequate.
Other inspection methods include inductively-coupled loop resistance measurements that eliminate the need to disconnect bonding jumpers, or penetrate corrosion-prevention coatings. The need for bonding inspections, frequency of the inspections, and determination as to whether the inspections must be an airworthiness limitation should be established under the fuel tank SSA.
9.3.10.9 Bond Corrosion and Integrity.
9.3.10.9.1 Degradation of electrical bonding provisions, such as bonding jumpers, has occurred on in-service airplanes. Results from airplane fuel tank
31 inspections conducted on a sample of airplanes by manufacturers and operators showed discoloration, corrosion, and damage to bonding jumpers. It is not clear if the discoloration indicates corrosion that will become more severe with time, or if it is simply a surface color change.
The applicant should define the bonding feature characteristics—such as visible corrosion, discoloration, jumper strand separation, and jumper strand breakage—that will be used to distinguish discrepant bonding provisions.
9.3.10.9.2 The level of corrosion observed on bonding features, specifically on bonding jumpers, varies greatly across airplane fleets. While some airplanes within a fleet and certain locations within the fuel tanks showed no evidence of corrosion, other airplanes and locations exhibited higher levels of corrosion. Inspection results reported to the FAA indicate materials used in certain bonding jumpers (tin-plated copper) may be more prone to corrosion. Nickel-plated copper wire does not experience similar corrosion. Corrosion programs for airplane structure have long recognized the variability of corrosion within the fleet. Factors that influence the level of corrosion of bonding jumpers include fuel type (sulfur content, etc.), presence of water in the fuel tank, installation effects such as cracking of the tin plating when the jumper is installed, temperature, humidity, and chemicals used for preparation of the fuel tanks prior to airplane storage, etc. While certain levels of corrosion or discoloration may be acceptable between inspection intervals, compliance should include substantiation that the materials used in the bonding jumpers are appropriate for use in the fuel tanks in consideration of the proposed inspection intervals. This substantiation should consider the variability in corrosive environments and factors noted above that may exist on in-service and stored airplanes in the fleet.
9.3.10.10 Section 25.981 states: “(a) No ignition source may be present at each point in the fuel tank or fuel tank system where catastrophic failure could occur due to ignition of fuel or vapors.” Fuel tube flexible couplings and components as small as nuts, bolts, and washers may develop sufficient charge to cause arcing due to electrostatic conditions if not properly accounted for in the design. Electrical bonding would need to be considered if these couplings are identified as ignition sources during the ignition source evaluation and assessment.
9.3.11 Friction Sparks.
The failure modes and effects analysis (FMEA) should include evaluation of the effects of debris entering the fuel pumps, including any debris that could be generated internally, such as any components upstream of the pump inlet. Industry practices for fuel tank cleanliness, and design features intended to preclude debris entering the fuel pumps, have not been effective at eliminating debris. Service experience has shown that pump inlet check valves, inducers, nuts, bolts, rivets, fasteners, sealant, lockwire, and so forth have been drawn into fuel pumps and contacted the impeller. This condition could
32 result in creation of friction sparks, and it should be an assumed failure condition when conducting the SSA. Fail-safe features should be incorporated into the fuel pump design to address this condition. Examples of means that may be incorporated into the fuel pump design to address this concern include:
Installation of inlet flame arrestors, Use of reticulated foam, Use/installation of ejector fuel pumps without impellers to scavenge fuel, or Maintaining fuel over the pump inlet throughout the airplane flight attitude envelope.
10 COMPONENT FAILURE MODE CONSIDERATIONS.
10.1 Component Qualification Review.
Qualification of components, such as fuel pumps, has not always accounted for unforeseen failures, wear, or inappropriate overhaul or maintenance. Failure to account for these failure modes and testing the pump, using the procedures defined in Military Standard MIL-STD-810E, Method 511.3, Explosive Atmosphere, has led to some fuel pumps entering airline service having never been tested to demonstrate explosion-proof capabilities. This combined experience suggests that more needs to be done to establish the capabilities of fuel pumps and other fuel system components to operate safely in an explosive environment. Such capability should be substantiated considering these factors in addition to the conditions noted in paragraph 7.3 of this AC. The amount of qualification review can be significantly reduced if the fail-safe features noted earlier in this AC are followed (e.g., not operating pumps in vapor spaces of the tank, incorporating arc fault or ground fault protection on the electrical circuit, etc.).
Therefore, an extensive evaluation of the qualification of components may be required if a qualitative assessment of the component and installation features does not eliminate the component as a potential ignition source.
10.2 Maximum Component Temperature for Qualification of Fuel System Components.
Maximum component temperatures may be determined experimentally. Tests should be conducted long enough for the component to reach the maximum temperature. All foreseeable failures and malfunctions of the fuel tank components (including those failures and malfunctions that could be undetected by the flightcrew and maintenance personnel) should be considered when determining maximum temperatures.
10.2.1 Components mounted adjacent to the exterior surface of the fuel tank can create a high localized temperature at the inner surface of the tank. This can be investigated by laboratory tests that duplicate the installation, or by a validated heat transfer analysis using the maximum potential temperature of the component.
10.2.2 When airplane equipment or system components such as engine bleed air ducting or ECS are located near fuel tanks, an FMEA should be done to determine failures of