U.S. Department

of Transportation **Federal A**

**viation Ad**  **ministration** 

# Advisory

**Circular**

**Subject:** Fuel Vent Fire Protection **Date:** XX/ XX/XX **Initiated By:** A

NM - 1 12 **AC No:** 25 . 975 -X

1 **PURPOSE.** This advisory c ircular (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 vent s . 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 doe s not constitute a regulation. While these guidelines are not mandatory , they are derived from extensiv e 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 s t andards 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 E xcept 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 DOCUMENT**  **S.** 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 s

ystem lightning p rotection .  § 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 AC**  **s .**   AC 25 - 8, Auxiliary Fuel System Installations , dated May 2, 1986 .  AC 20 - 53 B , 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 Protectio n 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 N o . FAA - RD - 75 - 119 . Washington, D.C.: U.S. Department of Transportation , 1975 .  FAA Technical Report ADS - 18 , National Technical Information Service (NTI S) , Lightning Protection Measures for Aircraft Fu el Systems . Springfield, VA: U . S. Department of Commerce , 1964.  Military

S tandard , Environmental Test Methods , MIL - SPEC - 810C, Method 511.1 Procedure II . Philadelphia, PA : U.S. Department o f 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 I**  **gnition ( A** **uto -**  **I gnition)**  **T emperature**  **(AIT) .**  The minimum temperature at which an optimized flammable vapor and air mixture will spontaneously

ignite.  **Flammability L**  **im it.**  The highest and lowest concentration of fuel - in - air- byvolume percent that will sustain combustion. A fuel - to - air mixture below th e 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 ty pically presented on a temperature - versus - altitude plot.  **Flash P**  **oint.** The minimum temperature at which a flammable liquid will produce flammable vapor at sea level ambient pressure.  **Ignition S**  **ource.** 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 R**  **atio.** The ratio of fuel to air corresponding to the condition in whic h the available amounts of fuel and oxygen completely react with each other thereb y resulting in combustion products containing neither fuel nor oxyge n. 5 **BACKGROUND .**  In its f inal r eport published in 1980, the Special Aviation Fire and Explosion Reduction (S AFER) 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 c ommittee 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 explo sion hazards could be reduced. The c ommittee evaluated methods to address fuel tank explosions s uch as fuel tank inerting, fuel vent flame arrestors , and / or surge tank explosion suppression sy stems . The committee determined fuel vent flame arrestors were the most practical method available at that time. This method is used in many current airplanes t o 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 sy stems have been installed in several airplane models to preventmm/dd/yy D R A F T AC 25.975 -X

4 propagation of f

ire resulting from lightning. Advancements in technology since the SAFER committee recommendations have resulted in installation of inerting s ys tems in currentl y- produced transport airplanes. 6 **AC CEPTABLE MEANS OF CO**  **MPLIANCE.** 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 sy stem 1 ; and  Preventing fuel tank explosions using a fuel tank inerting s ys tem exceeding the basic requirements of § 25.981 2

. 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 b y lightning result s in a high speed pressure wave that can travel through the flame arrestor without sufficient time for the heat transfer necessary for th e flame arrestor to quench the flame front. In stead, 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 f lame arrestors have been used to address lightning protection in several instances, dedicated testing and unique design features were needed to demonstrate the effec tiveness 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 sy stem will impact fuel tank vent sy stem performance. Factors such as added pressure loss during refueling sy stem 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 condition s (e.g., refueling, fuel tank leakage, refueling overflow, etc.) , and post - crash ground fire conditions. 2 Fuel tank inerting systems meeting § 2 5.981 would not necessarily be adequate for demonstrating compliance to § 2 5.975 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 install ation. Compliance means for these considerations are not addressed in this AC. General fuel sy stem 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 t ank vents , it is beneficial to design a flame arrestor or flame suppression sy stem 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 ev acuated 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 c rash 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 vent s an d 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, ty picall y , 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 hi gh vapor flow out of the vent. High vapor flow rates typicall y extend the flame arrestor effectiveness time. Test data from FAA Technical Report ADS - 18, Lightning Protection Measures for Aircraft Fuel Systems (reference 3.3 o f this AC ) , show that steel flame arrestors will ty pically reach thermal equilibrium during the high - flow condition , and flame will n ot 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 h eating 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 ar

restor resulting in flashback. It is important to realize that flashback through heated flame arrestor channels, which are normally quenching, should n ot 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 ins ufficient to quench the flame. In this case, the wall acts as an inadequate heat sink and not as an ignitio n source . The flame then passes to th e 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 AI T of the flammable mixture on the intern al side of the flame arrestor. The ignition of combustible mixtures by hot surfaces (auto - ignition) involves diff erent 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 di ssipation to the s urroundings. 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 fie ld 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 flammabl e 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 upstre am portion of the vent sy stem. The critical test conditions should be determined following review and anal ys is of the flame arrestor installation to determine the characteristic s 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, ty pically occurring during normal ground operations, to high - flow conditions , ty picall y 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 igniti on should also be considered. Duct sidewall temperatures and the flame arrestor surface temperature s should be monitored. Itis 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 AI T s quoted for static conditions, ty picall y 450 ° F, will not cause ignition in th e flame arrestor installation. Data in the Handbook of Aviation Fuels Properties (reference 3.3 of this AC ) , show relationships of velocity ver sus AI T s. 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 condition s 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 COMPLI ANCE**  **USING FLAME ARRESTOR S**  **.** 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 , conform ed production installation of the flame arrestor (including the upstream and downstream ducting). Alternativel y, the applicant may request FAA approval to use separate tests and anal ys is 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 with out consideration of the airplane flame arrestor installation. A separate test is then conducted b y the airplane manufacturer to show that the flame arrestor installation, including considerations such as fl ame 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 f igure 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 s ys tem 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 fu el 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 inst allations that face downward so a ground fire impinges on its face have significantl y shorter effectiveness than an arrestor that is mounted verticall y. 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. In clude 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. L ocate 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. In corporate a pressure relief feature in the upstream portion of th e sy stem 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 sy stem assembly that meets production specifications. 2. A sectio n 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 tes t. 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 T ype. 8.3.1.2.1 Fuels used in the test should have the same c haracteristics as the critical

fuel approved for use in the airplane. Ty picall y JP - 4 is the critical fuel , so JP - 4 or hexane has been accep table provided the correct fuel - to - air ratio is established for the fuel type being used. Hexane (C 6 H 14 ) is readily available and easil y manipulated in the gaseous state , and so it is ty picall y a fuel of choice. The AI T of 433 ° F closel y simulates that of JP - 4 vapor , which is 445 °F. **Note:** Fuels with higher AI T s, such as propane, should not be used for the flame arrestor elem ent 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 AI T 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 (typicall y 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 ther mocouple 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 ty pe 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 dy namic vapor flow conditio n. 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 te sts of the element ’ s flame arresting performance in a static

co ndition 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 b y 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 abo ut 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 f uel and air valves. I gnite the mixture downstream of the element. Verify that flame did not propagate through the arrestor b y observati on 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 condi tions for this test are based on test results documented in the

Lightning Protection Measures for Aircraft Fuel Systems report that resulted in the highe st flame arrestor temperature. R un this test at a 1.15 stoichiometric fuel/air ratio. A djust the flow to achieve a velocity of 0.75 feet per second ( ft/s ) (+.25, - 0 f t/ s) across the arrestor and ignite it downstream of the arrestor. Determine and esta blish the location of the flame front b y 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 i s at least 50 ° F (10 ° C) below the lowest expected AI T of the approved fuels. The AI T of fuels will vary because of a variety

of factors (ambient pressure, dwell time, fuel ty pe, etc.). As stated in AC 25.981 - 1C, the AI T accepted b y 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 AIT 50 ° F. The ARAC draft AC also provides guidance for allowing somewhat higher surface temperature limits in cert ain cases where substantiated. 8.3.2.2.2 The flow rate that achieves a velocity of 0.75 to 1.0 f t/ s across the arrestor is the range where flame arrestor failure occurred in the shortes t 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 maintaine d. The position of the flame front should be determined and the vapor flow rate adjusted such that the flame contact s 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 wa s typically above 700 ° F , which is well above the AI T of JP - 4 fuel vapor of 445 ° F as establish ed during no flow conditions. The upstream flame arrestor temperature can go well above the AI T without causing upstream ignition becau se of the high local veloc ity . For this reason hexane, with an AI T of 433 ° F, is used for the flame arrestor element t est. FAA Technical Report No. FAA - RD - 75 - 119, Investigation of Aircraft Fuel Tank Explosions and Nitrogen Inerting Requirementsmm/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 arres tor 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 on e corrugated stainless steel sheet that is rolled up and placed into a flanged casing. This construction produc es 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 sy stem where icing is likely . This effect should be accounted fo r in the vent sy stem 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 establi shed based on fuel tank bottom pressures during overflow of th e tank through the vent sy stem. Installation of a flame arrestor or modifications to the vent sy stem may result in increased tank bottom pressures. Therefore, if a flame arrestor is added to a f uel vent or the existing flame arrestor is modified, the effects on tank bottom pressure should be evaluated and refueling rates adjusted accordingl y. 8.3.4.3 Flame F ront V elocity .

The effectiveness of the flame arrestor is dependent on the velocity of the flame f ront. 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 th e 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 COMPLI ANCE**  **USING FUEL TANK INER TING**  **AND PRESSURIZATION**  **SYSTEMS .**  Use of fuel tank inerting and pressurization sy stems to show compliance to § 25.975 requires demonstrating fuel tank explosions are prevented during normal operating and post - crash fire scenarios. Inerting s ys tems designed to co mply with § 25.981 are not required to inert the fuel tanks during all operating conditions. For example, during refueling operations and when the inerting s ys tem is inoperative, the fuel tanks may become flammable. Fuel tank pressurization sy stems or features of the s ys tem that result in a “closed” vent sy stem 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 the se 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.

ENDmm/dd/yy D R A F T AC XX.X -X Appendix A

A - 1 **Appendix A. Example of Ca**  **lculation for Fuel -**  **to -** **Air Ratio Table**  **A -**  **1 . Combustion Properties of Hexane**  Heat of Combustion, BTU/lb. 19,200 Molecular w eight 86.17 Limits of inflamma

bility 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 ox ygen is written as:

2	C 6 H 14 +	19 O 2 =	14	H 2 O	+	12	CO 2 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 ox ygen or 2627.48 gm of air, which is 23.14 percent b y 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: 26 27 . 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 b y Volume Hexane

Fuel/Air Mass Ratio Lean l imit 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 l imit 3.0 0.15 5.0 0.2 Rich l imit 6.16 0.23 7.0 0.26mm/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 combu stion of propane and ox ygen is written as:

C 3 H 8 + 5 O 2 = 4 H 2 O + 3 CO 2 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 ox ygen or 691.44 gm of air , which is 23.14 percent b y weight ox ygen. Hence, the weight of air to weight of propane required for stoichiometric burning (i.e., complete combustion of propane w ith 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 c ombustion (298K), kcal/gm - mole 530.6 Flammability l imits in air (% b y volume), percent Lower Upper 2.2 9.5 Flame t emperature (stoichiometric in air, STP), C 1925 Quenching diameter * , inches 0.11 Minimum s park i gnition energy * , millijoules .027 Critical v elocity gradient for f lashback * , sec - 1 600 Laminar f lame s peed * , cm - sec 40mm/dd/yy D R A F T AC XX.X -X Appendix A

A - 3 **F igure**  **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 o rientation 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 Ratios 3** 

3 FAA Technical Report ADS - 18, Lightning Protection Measures for Aircraft Fuel Systems (see paragraph 3.3 of this AC) .**Advisory Circular Feedback**

If you find an error in this advisory circular (AC), have recommendations for improving it, or have suggestions for new items/subjects to be added, you may let us know by

(1) emailing this form to 9 - AWA - AVS - AI R500 - Coord@faa.gov or (2) faxing it to the attention of the Aircraft Certification Service Directives Management Officer at (202) 267 - 3983. Subject: (insert AC title/number here) Date: Click here to enter text. Please check all appropriate line items: ☐ An error (procedural or ty pographical) has been noted in paragraph Click here to enter text. on page Click here to enter text. . ☐ Recommend paragraph Click here to enter text. on page Click here to enter text. be changed as follows: Click here to enter text. ☐ In a future change to this AC, please cover the following subject: (Briefly describe what you want added.) Click here to enter text. ☐ Other comments: Click here to enter text. ☐ I would like to discuss the above. Please contact me. Submitted by : Date: