U.S. Department

of Transportation

Federal Aviation

Administration

Advisory 

Circular

Subject: Fuel Vent Fire Protection Date: XX/XX/XX

Initiated By: ANM-112

AC No: 25.975-X

1 PURPOSE.

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.

2 APPLICABILITY.

2.1 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.

2.2 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.

2.3 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.

2.4 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.
mm/dd/yy D R A F T AC 25.975-X

2

3 RELATED DOCUMENTS.

3.1 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.

3.2 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 25.863-1, 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.

3.3 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.
mm/dd/yy D R A F T AC 25.975-X

3

 RTCA, Inc., Environmental Conditions and Test Procedures for Airborne 

Equipment, RTCA/DO-160G. 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 AFAPL-TR-75-70. Springfield, VA: U.S. 

Department of Commerce, 1975.

4 DEFINITIONS.

 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-byvolume percent that will sustain combustion. A fuel-to-air 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.

5 BACKGROUND.

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 post-crash fuel tank explosions that had occurred since 

1964 and concluded that with existing technology, the potential for post-crash 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 
mm/dd/yy D R A F T AC 25.975-X

4

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.

6 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.

7 FLAME ARRESTORS.

7.1 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.

7.2 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 20-53, 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.

7.3 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 

 

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.
mm/dd/yy D R A F T AC 25.975-X

5

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.

7.4 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.

7.5 The guidance in this AC as a means of meeting the 2 minute and 30 second time 

requirement is based on the worst-case 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 

worst-case 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 3.3 of this AC), show that steel flame arrestors will typically reach thermal 

equilibrium during the high-flow condition, and flame will not propagate past the 

arrestor.

7.6 Test results documented in Lightning Protection Measures for Aircraft Fuel Systems

indicate that the low-flow case is critical, because during low-flow 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.
mm/dd/yy D R A F T AC 25.975-X

6

7.7 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.

7.8 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.

7.9 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 high-flow conditions,

typically occurring during refueling or when the fuel tank is heated due to ground fire 

following an accident.

7.10 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 3.3 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.
mm/dd/yy D R A F T AC 25.975-X

7

8 DEMONSTRATING COMPLIANCE USING FLAME ARRESTORS.

8.1 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.

8.2 In many cases the flame arrestor is vendor-furnished 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.

8.3 Flame Arrestor Installation Test.

8.3.1 Test Setup.

A schematic of the test setup is shown in figure A-1. 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 8.3.1.2 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.
mm/dd/yy D R A F T AC 25.975-X

8

8.3.1.1 Test Equipment.

1. Test article, including flame arrestor and the downstream section of 

the vent system assembly that meets production specifications.

2. A section of ducting representative of the production flame arrestor 

installation.

3. A means of generating a supply of fuel vapor at preselected 

fuel-to-vapor air ratios and various flow rates.

4. 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.

5. A means to measure temperatures on the upstream duct surfaces and 

flame arrestor.

6. A means to measure fuel vapor mixture temperatures both upstream 

and downstream of the arrestor.

7. A means to relieve explosive pressure upstream of the flame arrestor.

8. Ignition sources for igniting the explosive mixture upstream and 

downstream of the flame arrestor.

8.3.1.2 Fuel Type.

8.3.1.2.1 Fuels 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 fuel-to-air 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.

Note: 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.

8.3.1.2.2 Table A-1 summarizes the properties of hexane and provides an example 

of the method for calculating the stoichiometric relationship of hexane 

needed for the test.

8.3.1.2.3 Propane 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.
mm/dd/yy D R A F T AC 25.975-X

9

8.3.1.2.4 Table A-3 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.

8.3.1.3 Thermocouples.

The thermocouples to be used should be bare junction 1/16- to 1/8-inch 

metal-sheathed, ceramic-packed, chromel-alumel thermocouples with 

nominal 22 to 30 AWG (American wire gage) size conductors or 

equivalent. An air-aspirated, shielded thermocouple should not be used. 

Experience has shown that 1/16-inch thermocouples may provide more 

accurate calibration than 1/8-inch thermocouples; the 1/16-inch 

thermocouples are therefore recommended.

8.3.1.4 Test Specimen.

The test specimen should be a production component that conforms to the 

type design intended for certification.

8.3.2 Test Conditions.

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.

8.3.2.1 Flame Propagation Tests.

8.3.2.1.1 These 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). FAA-sponsored 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-2). These curves maximize at about 1.1 to 1.15 stoichiometric. 

The curves indicate higher temperatures occur at lower flow rates.

8.3.2.1.2 Establish 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.
mm/dd/yy D R A F T AC 25.975-X

10

8.3.2.2 Flame Propagation Prevention Tests.

8.3.2.2.1 The 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

25.981-1C 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 25.981-1C, 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 25.863-1, Flammable Fluid 

Fire Protection, (reference 3.2 of this AC) provides similar guidance that 

limits surface temperatures in flammable fluid leakage zones to AIT50 °F. The ARAC draft AC also provides guidance for allowing somewhat 

higher surface temperature limits in certain cases where substantiated.

8.3.2.2.2 The 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 

fuel-to-air 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.

8.3.2.2.3 Data 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 
mm/dd/yy D R A F T AC 25.975-X

11

During Ground Fires (see section 3.3 of this AC) showed hot surface 

ignition in the fuel tank to occur at about 520 °F, when it occurred at all.

8.3.3 Pass/Fail Criteria.

8.3.3.1 The flame arrestor assembly should meet the performance criteria noted 

above.

8.3.3.2 Following 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.

8.3.4 Related Qualification and Installation Considerations.

8.3.4.1 Vibration.

The flame arrestor should be qualified for the vibration environment of the 

installation.

8.3.4.2 Icing.

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.

8.3.4.3 Flame 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.
mm/dd/yy D R A F T AC 25.975-X

12

9 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.

END
mm/dd/yy D R A F T AC XX.X-X

Appendix A

A-1

Appendix A. Example of Calculation for Fuel-to-Air Ratio

Table A-1. Combustion Properties of Hexane

Heat of Combustion, BTU/lb. 19,200

Molecular weight 86.17

Limits of inflammability in air (% by volume) percent:

Lower

Upper

1.2

7.4

Flash point -7 °F

Boiling point 156 °F

Auto-ignition temperature 433 °F

Vapor pressure at 70 °F (psia) 2.5

Note: 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:

2627.48

1.15 × 172.34 = 13.2

Table A-2. Fuel/Air Mixtures for Flame Arrestors Tests

Condition JP-4 Percent 

by Volume

JP-4 Fuel/Air 

Mass Ratio

Hexane 

Percent by 

Volume

Hexane 

Fuel/Air 

Mass Ratio

Lean limit 0.90 0.035 1.0 0.04

Between lean limit and 

stoichiometric 1.10 0.045 1.5 0.05

Stoichiometric 1.58 0.065 2.0 0.0658

1.15 Stoichiometric 1.82 0.074 2.3 0.07567

Between stoichiometric 

and rich limit 3.0 0.15 5.0 0.2

Rich limit 6.16 0.23 7.0 0.26
mm/dd/yy D R A F T AC XX.X-X

Appendix A

A-2

Table A-3. Combustion Properties of Propane

*Applicable to 1.1 stoichiometric propane-air at STP.

Note: 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:

691.44

1.15 × 44.09 = 13.7

Heat of combustion (298K), kcal/gm-mole 530.6

Flammability limits in air (% by volume), percent

Lower

Upper

2.2

9.5

Flame temperature (stoichiometric in air, STP), C 1925

Quenching diameter*, inches 0.11

Minimum spark ignition energy*, millijoules .027

Critical velocity gradient for flashback*, sec-1 600

Laminar flame speed*, cm-sec 40
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Appendix A

A-3

Figure A-1. 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.
mm/dd/yy D R A F T AC XX.X-X

Appendix A

A-4

Figure A-2. Flame Arrestor Surface Temperature at Various Flow Rates and 

Stoichiometric Mixture Ratios3

 

3

FAA Technical Report ADS-18, Lightning Protection Measures for Aircraft Fuel Systems (see paragraph 3.3 of 

this AC).
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