This advisory circular (AC) provides information and guidance concerning compliance with Federal Aviation Administration (FAA) requirements in Title 14, Code of Regulations (14 CFR) 25.975 and related regulations for preventing fuel tank explosions caused by ignition of vapors outside the fuel tank vents.
ote: The guidance provided in this document is directed to airplane manufacturers, modifiers, foreign regulatory authorities, and FAA transport airplane type certification engineers and their designees.
ote: The material in this AC is neither mandatory nor regulatory in nature and does not constitute a regulation. While these guidelines are not mandatory, they are derived from extensive FAA and industry experience in determining compliance with the relevant regulations. These means are issued, in the interest of standardization, for guidance purposes and to outline a method that has been found acceptable in showing compliance with the standards set forth in the rule. If, however, we become aware of circumstances that convince us that following this AC would not result in compliance with the applicable regulations, we will not be bound by the terms of this AC, and we may require additional substantiation or design changes as a basis for finding compliance.
ote: The material in this AC does not change or create any additional regulatory requirements, nor does it authorize changes in, or permit deviations from, existing regulatory requirements.
ote: Except in the explanations of what the regulations require, the term “must” is used in this AC only in the sense of ensuring applicability of this particular method of compliance when the acceptable method of compliance described in this AC is used.
ote: Related Documents.
ote: Sections of 14 CFR.
§ 25.863, Flammable fluid fire protection.
§ 25.867, Fire protection: other components.
§ 25.901, Installation [paragraphs (b)(2) and (c)].
§ 25.954, Fuel system lightning protection.
§ 25.963, Fuel tanks: general [paragraphs (e)(2) and (d)].
§ 25.1181, Designated fire zones; regions included.
§ 25.1182, Nacelle areas behind firewalls, and engine pod attaching structures containing flammable fluid lines.
§ 25.1187, Drainage and ventilation fire zones.
ote: FAA ACs.
AC 25-8, Auxiliary Fuel System Installations, dated May 2, 1986.
AC 20-53B, Protection of Airplane Fuel Systems Against Fuel Vapor Ignition Caused by Lightning, dated June 5, 2006.
AC 20-135, Powerplant Installation and Propulsion System Component Fire Protection Test Methods, Standards, and Criteria, dated February 6, 1990.
AC 25.981-1C, Fuel Tank Ignition Source Prevention Guidelines, dated September 19, 2008.
Draft AC 25.8631, Flammable Fluid Fire Protection. Available on page 17 of the final report of the FAA Aviation Rulemaking Advisory Committee (ARAC), Transport Airplane and Engine Issue Area, Powerplant Installation Harmonization Working Group: Task 5 – Powerplant Fire Mitigation Requirements. Available at: http://www.faa.gov.
ote: Technical Publications.
Hill, Richard and George R. Johnson, Investigation of Aircraft Fuel Tank Explosions and Nitrogen Inerting Requirements During Ground Fires, FAA Technical Report No. FAA-RD-75-119. Washington, D.C.: U.S. Department of Transportation, 1975.
FAA Technical Report ADS-18, National Technical Information Service (NTIS), Lightning Protection Measures for Aircraft Fuel Systems. Springfield, VA: U.S. Department of Commerce, 1964.
Military Standard, Environmental Test Methods, MIL-SPEC-810C, Method 511.1 Procedure II. Philadelphia, PA: U.S. Department of Defense, 1975.
RTCA, Inc., Environmental Conditions and Test Procedures for Airborne Equipment, RTCA/DO160G. Washington DC: RTCA, Inc., 2010.
Coordinating Research Council, Inc., Handbook of Aviation Fuel Properties. Atlanta, GA: CRC, Inc., 2004.
Kuchta, Joseph M., Summary of Ignition Properties of Jet Fuels and Other Aircraft Combustible Fluids, Technical Report AFAPLTR-75-70. Springfield, VA: U.S. Department of Commerce, 1975.
Autogenous Ignition (Auto-Ignition) Temperature (AIT). The minimum temperature at which an optimized flammable vapor and air mixture will spontaneously ignite.
Flammability Limit. The highest and lowest concentration of fuel-in-air-by-volume percent that will sustain combustion. A fueltoair mixture below the lower limit is too lean to burn, while a mixture above the upper limit is too rich to burn. The flammability limit varies with altitude and temperature and is typically presented on a temperature-versus-altitude plot.
Flash Point. The minimum temperature at which a flammable liquid will produce flammable vapor at sea level ambient pressure.
Ignition Source. A heat source of sufficient temperature and energy to initiate combustion of flammable fluid. Surfaces that can exceed the autogenous ignition temperature of the fluid under consideration are considered to be ignition sources. Electrical arcs and friction sparks are also common ignition sources.
Stoichiometric Ratio. The ratio of fuel to air corresponding to the condition in which the available amounts of fuel and oxygen completely react with each other thereby resulting in combustion products containing neither fuel nor oxygen.
In its final report published in 1980, the Special Aviation Fire and Explosion Reduction (SAFER) Advisory Committee (“the committee”) established the value of installing flame arrestors in fuel tank outlets and vent lines as a result of a review of transport category airplane accident history. The committee reviewed worldwide transport airplane accidents involving postcrash fuel tank explosions that had occurred since 1964 and concluded that with existing technology, the potential for postcrash explosion hazards could be reduced. The committee evaluated methods to address fuel tank explosions such as fuel tank inerting, fuel vent flame arrestors, and/or surge tank explosion suppression systems. The committee determined fuel vent flame arrestors were the most practical method available at that time. This method is used in many current airplanes to delay propagation of ground fires and the subsequent explosions, which would provide additional time for safe evacuation of passengers and crew. Surge tank fire suppression systems have been installed in several airplane models to prevent propagation of fire resulting from lightning. Advancements in technology since the SAFER committee recommendations have resulted in installation of inerting systems in currently-produced transport airplanes.
ote: Acceptable means of compliance.
Acceptable means of compliance to § 25.975 include:
Preventing flame propagation into the fuel tank using fuel tank vent flame arrestors;
Demonstrating flame cannot pass into the tank through use of a closed vent system1; and
Preventing fuel tank explosions using a fuel tank inerting system exceeding the basic requirements of § 25.9812.
ote: Flame Arrestors.
ote: The guidance contained in this AC for flame arrestors addresses performance standards for post-crash ground fire and normal operating condition protection of the fuel tank vents.
ote: Flame arrestors meeting the standards defined in this AC are not effective at preventing propagation of fires that may occur following lightning strikes near the fuel tank vent outlet. Ignition of fuel vapors near the vent outlet caused by lightning results in a high speed pressure wave that can travel through the flame arrestor without sufficient time for the heat transfer necessary for the flame arrestor to quench the flame front. Instead, fuel tank vent lightning protection may be addressed as discussed in AC 2053, which is based on locating vents outside strike areas of the airplane. While flame arrestors have been used to address lightning protection in several instances, dedicated testing and unique design features were needed to demonstrate the effectiveness of the installation. The guidance in this AC is intended to address compliance with § 25.975 and is not an acceptable means for showing compliance to § 25.954.
ote: Installation of flame arrestors in the airplane fuel vent system will impact fuel tank vent system performance. Factors such as added pressure loss during refueling system failure conditions, as well as the impact of environmental conditions such as icing and lightning, must also be addressed when requesting approval of the fuel tank installation. Compliance means for these considerations are not addressed in this AC. General fuel system guidance is provided in AC 25-8, Auxiliary Fuel Systems Installations.
ote: To minimize the possibility of propagation of external ground fires through the fuel tank vents, it is beneficial to design a flame arrestor or flame suppression system to be effective for a finite period of time. The FAA previously established a performance standard that, under specified conditions, the airplane must be capable of being evacuated within 90 seconds (§ 25.803, Emergency evacuation). However, this time period did not consider the effects of a fuel tank explosion on the ability of passengers to leave the crash site or on the safety of emergency crews. In light of these considerations, the FAA proposed that the flame arrestor performance requirements be based on the capability of current technology flame arrestors that have previously been shown to provide acceptable performance on airplanes in the transport airplane fleet. Based on these criteria, the FAA has established a minimum standard of 2 minutes and 30 seconds. This time is consistent with the evacuation time noted above and allows additional time for passengers and crew to exit the crash scene.
ote: The guidance in this AC as a means of meeting the 2 minute and 30 second time requirement is based on the worstcase conditions regarding fuel vapor emissions from the fuel tank vents and is, therefore, conservative. This condition would not likely be present at the fuel tank outlet for long periods of time; therefore, the actual effectiveness of the arrestor should exceed the time that would be required for the external ground fire to breech the fuel tank structural boundary formed by the wing surface. The worstcase condition would likely be a low vapor emission rate. However, typically, if fire is present near the vent outlet following an accident, large areas of the fuel tank would also be exposed to the fire causing high vapor flow out of the vent. High vapor flow rates typically extend the flame arrestor effectiveness time. Test data from FAA Technical Report ADS-18, Lightning Protection Measures for Aircraft Fuel Systems (reference of this AC), show that steel flame arrestors will typically reach thermal equilibrium during the highflow condition, and flame will not propagate past the arrestor.
ote: Test results documented in Lightning Protection Measures for Aircraft Fuel Systems indicate that the lowflow case is critical, because during lowflow conditions, the flame front contacts the surface of the flame arrestor, which results in heating of the flame arrestor. As the flame arrestor is heated, the ability of the arrestor to absorb energy may be reduced resulting in the inability to quench the flame. The flame will then pass through the arrestor resulting in flashback. It is important to realize that flashback through heated flame arrestor channels, which are normally quenching, should not be confused with the concepts of auto-ignition or hot surface ignition. Flashback will occur when the rate of heat loss to the channel wall is insufficient to quench the flame. In this case, the wall acts as an inadequate heat sink and not as an ignition source. The flame then passes to the upstream side of the arrestor.
ote: It is possible for the effectiveness of the flame arrestor assembly, including the line and housing, to be affected by the time required for the flame arrestor assembly surfaces to be heated above the AIT of the flammable mixture on the internal side of the flame arrestor. The ignition of combustible mixtures by hot surfaces (auto-ignition) involves different phenomena than for flashback as noted above. For auto-ignition to be a factor, a portion of the combustible gas must dwell near a hot surface for a time, such that chemical heat evolution is produced in a volume in excess of heat dissipation to the surroundings. The dwell time (commonly termed “ignition lag”) is a function of the heat transfer characteristics of the gas and heat source, as well as the kinetics of the combustion process. For this reason, the geometry of the heat source and the flow field around the heat source are critical factors in determining whether ignition will occur.
ote: The test conditions defined in this AC are intended to evaluate flame arrestor effectiveness for addressing two conditions. The first condition is one where flammable vapors are present at the vent outlet and are ignited by an external source. The flame arrestor should be effective at stopping the initial propagation of flame. The second condition is one where a continuous flow of vapor is exiting the fuel vent, and the flame arrestor should hold the flame without passing the flame to the upstream portion of the vent system. The critical test conditions should be determined following review and analysis of the flame arrestor installation to determine the characteristics of a particular installation.
ote: The conditions under which the flame arrestor should be effective would include those where flammable fluid vapors are exiting the fuel tank at flow rates varying from no flow, typically occurring during normal ground operations, to highflow conditions, typically occurring during refueling or when the fuel tank is heated due to ground fire following an accident.
ote: When determining whether the flame arrestor assembly meets the flame propagation prevention time of 2 minutes and 30 seconds, hot surface ignition should also be considered. Duct sidewall temperatures and the flame arrestor surface temperatures should be monitored. It is important to consider the velocity of the flammable fluid vapor on the surface of the flame arrestor and the duct sidewall upstream (tank) side of the flame arrestor. Provided that a uniform vapor velocity is present (no stagnation areas), a heat source whose temperature exceeds the AITs quoted for static conditions, typically 450 °F, will not cause ignition in the flame arrestor installation. Data in the Handbook of Aviation Fuels Properties (reference of this AC), show relationships of velocity versus AITs. Test results from developmental testing of flame arrestors installed in fuel vent lines have shown that ignition will not occur provided the center of the flame arrestor remains below 700 °F. The applicant should run an adequate number of test conditions to cover a range of flow conditions and establish the conditions that result in the highest surface temperatures.
ote: DEMONSTRATING COMPLIANCE Using Flame Arrestors.
ote: Flame arrestor performance is influenced by installation effects that may cause variation in critical parameters such as flame front speed and surface temperatures. Installation effects must be accounted for in the compliance demonstration. Applicants may choose to show compliance with § 25.975 by testing a complete, conformed production installation of the flame arrestor (including the upstream and downstream ducting). Alternatively, the applicant may request FAA approval to use separate tests and analysis of the flame arrestor and the installation as a means of compliance.
ote: In many cases the flame arrestor is vendorfurnished and, therefore, qualified to meet the flame propagation requirements by the vendor without consideration of the airplane flame arrestor installation. A separate test is then conducted by the airplane manufacturer to show that the flame arrestor installation, including considerations such as flame front speeds and duct sidewall temperatures, have been accounted for in meeting the requirement. Fuel types that may be used for each of these tests differ and should be established as discussed below prior to conducting any testing.
ote: Flame Arrestor Installation Test.
A schematic of the test setup is shown in figure A . The test setup involves mounting the arrestor element in a tube of approximately the same diameter. The speed of the flame front that travels down the fuel vent system tubing is a critical factor in the performance of the flame arrestor. The flame front will accelerate down the tubing so higher velocities will occur as the arrestor is located farther away from the fuel tank vent outlet. Therefore the tubing and length from the fuel tank vent inlet to the flame arrestor should be representative of the production configuration. In addition, the orientation of the flame arrestor in the fixture is a critical parameter for the compliance demonstration. For instance, flame arrestor installations that face downward so a ground fire impinges on its face have significantly shorter effectiveness than an arrestor that is mounted vertically. Design the flame arrestor test fixture so the element is oriented to simulate the actual airplane installation. Mix air that is at a temperature higher than the boiling point of the fuel being used (see paragraph i.1 of this AC) with fuel and introduce it at the inlet of the tube. Vary fuel/air ratios by adjusting the respective fuel vapor and air supply rates. Include a viewing window in the pipe upstream of the element to provide visual access to the flame arrestor element. Cut a viewing section into the pipe upstream of the element, and cover it with transparent plastic. Locate igniters upstream and downstream of the element. Locate thermocouples in the duct to measure incoming flammable mixture temperature and vapor temperatures downstream of the arrestor element. Also install thermocouples on the surface of the center of the arrestor element upstream face and on the surface of the upstream side of the duct. Incorporate a pressure relief feature in the upstream portion of the system to relieve explosive pressures when ignition of the upstream flammable fluid vapor occurs.
ote: Test article, including flame arrestor and the downstream section of the vent system assembly that meets production specifications.
ote: A section of ducting representative of the production flame arrestor installation.
ote: A means of generating a supply of fuel vapor at preselected fueltovapor air ratios and various flow rates.
ote: A window for observing upstream and downstream conditions during the test. This means should allow determination of the location of the flame front relative to the flame arrestor.
ote: A means to measure temperatures on the upstream duct surfaces and flame arrestor.
ote: A means to measure fuel vapor mixture temperatures both upstream and downstream of the arrestor.
ote: A means to relieve explosive pressure upstream of the flame arrestor.
ote: Ignition sources for igniting the explosive mixture upstream and downstream of the flame arrestor.
i.1.aFuels used in the test should have the same characteristics as the critical fuel approved for use in the airplane. Typically JP-4 is the critical fuel, so JP-4 or hexane has been acceptable provided the correct fueltoair ratio is established for the fuel type being used. Hexane (C6 H14) is readily available and easily manipulated in the gaseous state, and so it is typically a fuel of choice. The AIT of 433 °F closely simulates that of JP-4 vapor, which is 445 °F.
ote: Fuels with higher AITs, such as propane, should not be used for the flame arrestor element test because ignition on the back side of the arrestor would not be adequately evaluated.
i.1.aTable A summarizes the properties of hexane and provides an example of the method for calculating the stoichiometric relationship of hexane needed for the test.
i.1.bPropane may be used for flame arrestor installation testing where AIT is not a critical parameter for the test. For example, testing of a simulated production flame arrestor installation to validate that temperatures of portions of the installation fuel tank remain below the maximum permitted fuel tank surface temperature (typically 390 °F) would be acceptable, provided the flame arrestor element had been previously qualified to meet the flame propagation prevention requirements.
i.1.cTable A summarizes the properties of propane as provided in the Lightning Protection Measures for Aircraft Fuel Systems report and provides an example of the method for calculating the stoichiometric ratio of propane.
The thermocouples to be used should be bare junction 1/16 to 1/8inch metalsheathed, ceramicpacked, chromelalumel thermocouples with nominal 22 to 30 AWG (American wire gage) size conductors or equivalent. An airaspirated, shielded thermocouple should not be used. Experience has shown that 1/16inch thermocouples may provide more accurate calibration than 1/8inch thermocouples; the 1/16-inch thermocouples are therefore recommended.
The test specimen should be a production component that conforms to the type design intended for certification.
Two tests are run—one for flame propagation prevention in a static vent vapor flow condition, and one for flame propagation prevention in a dynamic vapor flow condition. These conditions provide a conservative demonstration of fuel tank vent fire protection capability with respect to delaying flame front propagation through the fuel vent flame arrestor during ground fire conditions.
ii.1Flame Propagation Tests.
ii.1.aThese are tests of the element’s flame arresting performance in a static condition with six different fuel/air ratios (lean, between lean and stoichiometric, stoichiometric, 1.15 stoichiometric, between stoichiometric and rich, and rich). FAAsponsored tests done by Atlantic Research, documented in the Lightning Protection Measures for Aircraft Fuel Systems report, show curves of flame arrestor equilibrium temperature for various air-flow ratios as a function of percent stoichiometric fuel/air ratio (Figure A ). These curves maximize at about 1.1 to 1.15 stoichiometric. The curves indicate higher temperatures occur at lower flow rates.
ii.1.bEstablish the mixed flow. Close fuel and air valves. Ignite the mixture downstream of the element. Verify that flame did not propagate through the arrestor by observation through the viewing window. Verify the upstream mixture is combustible by energizing the upstream igniter and observing ignition of the upstream mixture.
ii.2Flame Propagation Prevention Tests.
ii.2.aThe test conditions for this test are based on test results documented in the Lightning Protection Measures for Aircraft Fuel Systems report that resulted in the highest flame arrestor temperature. Run this test at a 1.15 stoichiometric fuel/air ratio. Adjust the flow to achieve a velocity of 0.75 feet per second (ft/s) (+.25, 0 ft/s) across the arrestor and ignite it downstream of the arrestor. Determine and establish the location of the flame front by viewing through the viewing window. Monitor the temperature at the upstream center of the arrestor; it is required to stay below 700 °F for 2 minutes and 30 seconds. Record the outer tube temperature just downstream of the arrestor and limit it to a temperature of 390 °F to 450 °F depending on the location of the arrestor in the wing. AC 25.9811C provides guidance that establishes a maximum allowable surface temperature within the fuel tank (the tank walls, baffles, or any components) that provides a safe margin under all normal or failure conditions that is at least 50 °F (10 °C) below the lowest expected AIT of the approved fuels. The AIT of fuels will vary because of a variety of factors (ambient pressure, dwell time, fuel type, etc.). As stated in AC 25.9811C, the AIT accepted by the FAA without further substantiation for kerosene fuels, such as Jet A, under static sea level conditions, is 450 °F (232.2 °C). This results in a maximum allowable surface temperature of 400 °F (204.4 °C) for an affected component surface of a fuel tank. The ARAC draft AC 25.8631, Flammable Fluid Fire Protection, (reference of this AC) provides similar guidance that limits surface temperatures in flammable fluid leakage zones to AIT-50 °F. The ARAC draft AC also provides guidance for allowing somewhat higher surface temperature limits in certain cases where substantiated.
ii.2.bThe flow rate that achieves a velocity of 0.75 to 1.0 ft/s across the arrestor is the range where flame arrestor failure occurred in the shortest time in development testing. Care should be taken to ensure that the fuel flow rate is maintained at a constant value throughout the test so that the correct fueltoair ratio is maintained. The position of the flame front should be determined and the vapor flow rate adjusted such that the flame contacts the downstream arrestor face, resulting in the greatest rate of heating of the arrestor surface.
ii.2.cData from developmental testing show that the temperature of the center of the upstream arrestor face at which failure (propagation of the flame) occurred was typically above 700 °F, which is well above the AIT of JP-4 fuel vapor of 445 °F as established during no flow conditions. The upstream flame arrestor temperature can go well above the AIT without causing upstream ignition because of the high local velocity. For this reason hexane, with an AIT of 433 °F, is used for the flame arrestor element test. FAA Technical Report No. FAA-RD-75-119, Investigation of Aircraft Fuel Tank Explosions and Nitrogen Inerting Requirements During Ground Fires (see section of this AC) showed hot surface ignition in the fuel tank to occur at about 520 °F, when it occurred at all.
iii.1The flame arrestor assembly should meet the performance criteria noted above.
iii.2Following passing of the flame arrestor tests noted above, careful examination of the arrestor structural integrity should be conducted. Flame arrestors have been constructed of one flat and one corrugated stainless steel sheet that is rolled up and placed into a flanged casing. This construction produces a series of small passages. Structural integrity of the coiled sheet metal is maintained by either rods that cross at the front and rear face of the coil or by brazing or welding of the coiled sheet metal at various points around the surface. Flame arrestors have failed the test when structural integrity is lost due to weld or brazed joint failures.
ivRelated Qualification and Installation Considerations.
The flame arrestor should be qualified for the vibration environment of the installation.
Installation of a flame arrestor will likely introduce a point in the vent system where icing is likely. This effect should be accounted for in the vent system design by installation of pressure relief provisions that protect the tank from excessive pressure differentials or by showing that icing or clogging of the arrestor is not possible. In many cases, fuel tank refueling rates are established based on fuel tank bottom pressures during overflow of the tank through the vent system. Installation of a flame arrestor or modifications to the vent system may result in increased tank bottom pressures. Therefore, if a flame arrestor is added to a fuel vent or the existing flame arrestor is modified, the effects on tank bottom pressure should be evaluated and refueling rates adjusted accordingly.
iv.3Flame Front Velocity.
The effectiveness of the flame arrestor is dependent on the velocity of the flame front. The vent line length, diameter, and flow losses between the ignition source and arrestor influence the velocity. The flame arrestor installation may have a different vent line length and diameter, with associated flow losses, than what is used for the element test. These installation differences should be accounted for in the compliance demonstration. A separate test may be required to demonstrate that the installed flame arrestor is effective.
ote: DEMONSTRATING COMPLIANCE Using Fuel Tank Inerting and pressurization Systems.
Use of fuel tank inerting and pressurization systems to show compliance to § 25.975 requires demonstrating fuel tank explosions are prevented during normal operating and post-crash fire scenarios. Inerting systems designed to comply with § 25.981 are not required to inert the fuel tanks during all operating conditions. For example, during refueling operations and when the inerting system is inoperative, the fuel tanks may become flammable. Fuel tank pressurization systems or features of the system that result in a “closed” vent system may become inoperative during an accident or the subsequent post-crash fire scenario. If fuel tank inerting or pressurization is proposed as the means of compliance to § 25.975, the effectiveness of these means to prevent a fuel tank explosion during all operating and post-crash fire conditions is required.
If you have any suggestions for improvements or changes, you may use the template provided at the end of this AC.
Appendix A. Example of Calculation for Fuel-to-Air Ratio
Table A. Combustion Properties of Hexane
Heat of Combustion, BTU/lb.
Limits of inflammability in air (% by volume) percent:
Vapor pressure at 70 °F (psia)
ote: The combustion of hexane and oxygen is written as:
2 C6 H14 + 19 O2 = 14 H2O + 12 CO2
For every 2 mole of hexane consumed, 19 moles of oxygen are required for complete combustion with no residual oxygen. Thus, 172.34 gm of hexane require 19 x 32.00 = 608 gm of oxygen or 2627.48 gm of air, which is 23.14 percent by weight oxygen. Hence, the weight of air to weight of hexane required for stoichiometric burning (i.e., complete combustion of hexane with no excess oxygen) is 15.24. A 1.15 fraction of stoichiometric mixture of air and hexane has a fuel-to-air weight ratio of:
Table A. Fuel/Air Mixtures for Flame Arrestors Tests
*Applicable to 1.1 stoichiometric propane-air at STP.
ote: The combustion of propane and oxygen is written as:
C3 H8 + 5 O2 = 4 H2O + 3 CO2
For every mole of propane consumed, 5 moles of oxygen are required for complete combustion with no residual oxygen. Thus, 44.09 gm of propane require 5 x 32.00 = 160 gm of oxygen or 691.44 gm of air, which is 23.14 percent by weight oxygen. Hence, the weight of air to weight of propane required for stoichiometric burning (i.e., complete combustion of propane with no excess oxygen) is 15.7. A 1.15 fraction of stoichiometric mixture of air and propane has an air-to-fuel weight ratio of:
Figure A. Fuel Tank Vent Flame Arrestor Test Schematic
This test schematic is not intended to define the airplane installation. The test installation, including the orientation of the flame arrestor may have a significant impact on the effectiveness of the unit. The actual test configuration should simulate the airplane configuration.
Figure A. Flame Arrestor Surface Temperature at Various Flow Rates and Stoichiometric Mixture Ratios3
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1 Some airplane models use pressurized fuel tanks such that the fuel system vents are closed during airplane operation. Demonstrating compliance using this means would require showing the vents remain closed and prevent flame propagation into the fuel tanks during normal operating conditions, all foreseeable ground fire conditions (e.g., refueling, fuel tank leakage, refueling overflow, etc.), and post-crash ground fire conditions.
2 Fuel tank inerting systems meeting § 25.981 would not necessarily be adequate for demonstrating compliance to § 25.975 because § 25.981 does not require the fuel tank ullage to be fully inert at all times. If inerting is used as the means of compliance to § 25.975, the effectiveness of the inerting system would need to be shown during normal operating conditions, all foreseeable ground fire conditions (e.g., refueling, fuel tank leakage, refueling overflow, etc.), and post-crash ground fire conditions.
3 FAA Technical Report ADS-18, Lightning Protection Measures for Aircraft Fuel Systems (see paragraph of this AC).