AMC 25.975(a)(7) Fuel tank vent fire protection

• ED Decision 2018/005/R found in: CS-25 Amdt 27 - Large Aeroplanes (Jan 2023)

AMC 25.975(a)(7) Fuel tank vent fire protection ED Decision 2018/005/R 1. Purpose This AMC provides guidance and acceptable means of compliance with CS 25.975(a)(7) and the related specifications for the prevention of fuel tank explosions caused by the ignition of vapours outside fuel tank vents. 2. References 2.1. Related certification specifications: — [CS 25.863](#_DxCrossRefBm1709821171) Flammable fluid fire protection — [CS 25.867](#_DxCrossRefBm1709821364) Fire protection: other components — [CS 25.901](#_DxCrossRefBm1709820973) Installation (paragraphs (b)(2) and (c)) — [CS 25.954](#_DxCrossRefBm1709821378) Fuel system lightning protection — [CS 25.963](#_DxCrossRefBm1709821436) Fuel tanks: general (paragraphs (d) and (e)(2)) — [CS 25.981](#_DxCrossRefBm1709821428) Fuel tank ignition prevention. 2.2. 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 Engineering Considerations and Laboratory Test Methods, MIL‑STD-810G w/Change1, Method 511.6 Procedure II. Philadelphia, PA: U.S. Department of Defense, 2014 — 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 3. Definitions — Autogenous Ignition (Auto-Ignition) Temperature (AIT). The minimum temperature at which an optimised flammable vapour and air mixture will spontaneously ignite when heated to a uniform temperature in a normal atmosphere without an external source of ignition, such as a flame or spark. — Flammability Limit. The highest and lowest concentration of fuel-in-air-by-volume per cent 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 vapour at sea level ambient pressure. — Flame Holding. The ability of a flame arrestor to halt the propagation of a flame front through a passage. — Ignition Source. A source of sufficient energy to initiate combustion of a fuel-air mixture. Hot surfaces that can exceed the auto-ignition temperature of the flammable vapour under consideration are considered to be ignition sources. Electrical arcs, electrical sparks, and friction sparks are also considered to be ignition sources if sufficient energy is released to initiate combustion. — 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 that contain neither fuel nor oxygen. 4. Acceptable means of compliance Acceptable means of compliance with [CS 25.975(a)(7)](#_DxCrossRefBm1709821429) include: — flame arrestors in the fuel tank vents that prevent flame propagation into the fuel tank (see paragraph 5 of this AMC); — fuel tank inerting systems that exceed the basic requirements of CS 25.981 and prevent fuel tank explosions\* (see paragraph 7.1 of this AMC); — fuel tank pressurisation systems or features of the system that result in a closed vent system and that are effective in preventing a fuel tank explosion during all operating conditions (e.g. taxiing, take-off, landing, refuelling, etc.) and post-crash fire conditions (see paragraph 7.2 of this AMC); and — fuel tank or vent system fire suppression systems that prevent a fuel tank explosion with a fire present at the fuel tank vent outlet for the required 2 minutes and 30 seconds (see paragraph 7.3 of this AMC). \* Fuel tank inerting systems that meet [CS 25.981](#_DxCrossRefBm1709821428) would not necessarily be adequate for demonstrating compliance with [CS 25.975](#_DxCrossRefBm1709821429) because [CS 25.981](#_DxCrossRefBm1709821428) does not require the fuel tank ullage to be fully inert at all times. If inerting is used as the means of compliance with [CS 25.975](#_DxCrossRefBm1709821429), the inerting system must be effective in preventing flame that is present at the vent outlet from propagating to the fuel tank. The applicant should show this during normal operating conditions, all foreseeable ground fire conditions (e.g. from refuelling, refuelling overflow, etc.), and post-crash ground fire conditions. 5. Flame arrestors 5.1. This paragraph describes the use of flame arrestors as a means of meeting the 2-minute and 30-second time requirements defined in [CS 25.975(a)(7)](#_DxCrossRefBm1709821429). The guidance is based on evaluating the flame arrestor performance during critical case conditions anticipated to occur when fire is adjacent to the fuel tank vent outlet. The flame arrestor should meet the performance described in this AMC during post‑crash ground fires or other fire scenarios such as those resulting from fuel leakage due to fuel tank damage or fuel spilled during refuelling mishaps. 5.2. Flame arrestors that meet the standards defined in this AMC may not be effective in preventing the propagation of fires that may occur following lightning strikes near the fuel tank vent outlet. The ignition of fuel vapours 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 [AMC 25.954](#_DxCrossRefBm1709821426) ‘Fuel System Lightning Protection’, which is based on locating vents outside the lightning strike zones of the aeroplane. While aeroplane manufacturers have used flame arrestors to address lightning protection in several instances, they needed dedicated testing that addressed the unique design features to demonstrate the effectiveness of the installation. The guidance in this AMC is intended to address compliance with [CS 25.975(a)(7)](#_DxCrossRefBm1709821429) and is not intended to be used as guidance for showing compliance with the lightning protection requirements in [CS 25.954](#_DxCrossRefBm1709821378). 5.3. The installation of flame arrestors in the aeroplane fuel vent system will affect the performance of the fuel tank vent system. The applicant should account for factors such as the introduction of a flow restriction and the associated increase in the pressure drop during refuelling system failure conditions, as well as the impact of environmental conditions such as icing and lightning, when requesting approval of the fuel tank installation. Means of compliance for these considerations are not addressed in this AMC. General fuel system guidance is provided in [AMC 25.963](#_DxCrossRefBm1709821439) and [AMC 25.981](#_DxCrossRefBm1709821459). 5.4. Previous results from flame arrestor performance tests indicated that the critical condition for evaluating the effectiveness of the flame arrestor occurs when 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 flame arrestor to absorb energy may be reduced, resulting in its inability to quench the flame. Once this occurs, the flame will then pass through the flame arrestor, resulting in flashback. It is important to realise that flashback through heated flame arrestor channels, which normally quench flames, should not be confused with 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 retains sufficient heat energy to pass to the upstream side of the flame arrestor. 5.5. Flame propagation past the flame arrestor may also occur due to the ignition of flammable vapours by hot surfaces. The time it takes for the assembly surfaces on the internal side of the flame arrestor, including the line and housing, to be heated to a temperature higher than the AIT of the flammable vapour mixture could be the limiting factor in establishing the effectiveness of the flame arrestor assembly. The ignition of combustible mixtures by hot surfaces (auto-ignition) involves different phenomena from the phenomena involved in flashback as discussed in paragraph 5.4 of this AMC. For auto-ignition to occur, a portion of the combustible gas must dwell near a hot surface long enough for the amount of chemical heat produced to become greater than the heat dissipated to the surroundings. The maximum dwell time (commonly termed the ‘ignition lag’) is a function of the heat transfer characteristics of the gas and the heat source, as well as the kinetics of the combustion process. For this reason, the surface area and the shape of the hot surface, and the flow field around the heat source, are critical factors in determining whether ignition will occur. 5.6. The test conditions defined in this AMC are intended to evaluate the effectiveness of flame arrestors during two conditions. The first condition is the ignition, by an external source, of flammable vapours at the fuel tank vent outlet. The flame arrestor should be effective in stopping the initial propagation of flames. The second condition is a continuous flow of vapour exiting the fuel vent. The flame arrestor should hold the flames without passing the flames to the upstream portion of the vent system. The applicant should determine the critical test conditions following a review and analysis of the particular flame arrestor installation and its characteristics. 5.7. The conditions under which the flame arrestor should be effective include those where flammable fluid vapours are exiting the fuel tank at flow rates that vary from no flow, which typically occurs during normal ground operations, to high-flow conditions, which typically occur during refuelling or when the fuel tank is heated due to a ground fire following an accident. 5.8. The applicant should conduct an analysis to determine the pass/fail criteria for the aeroplane-specific flame arrestor installation. The analysis should include consideration of hot surface ignition when determining whether the flame arrestor assembly meets the explosion prevention requirement of 2 minutes and 30 seconds. The maximum surface temperatures of the flame arrestor installation and the flame arrestor should be established when meeting the requirement. The applicant should consider the velocity of the flammable fluid vapour on the surface of the flame arrestor and the duct sidewall upstream (tank) side of the flame arrestor. Provided that a uniform vapour velocity is present (i.e. there are no areas of stagnation), a heat source whose temperature exceeds the AITs quoted for static conditions (typically 230 °C/450 °F) will not cause ignition in the flame arrestor installation. Data in the Handbook of Aviation Fuels Properties (see Chapter 2.2 of this AMC) show the relationships between vapour velocities and AITs. Test results from developmental testing of flame arrestors installed in fuel vent lines have shown that ignition will not occur if the temperature of the centre of the flame arrestor remains below 370 °C/700 °F. However, this temperature limit may not be appropriate for other surfaces in the flame arrestor installation where a uniform flammable vapour flow is not present. The applicant should analyse the flame arrestor design to determine the critical locations and fuel vapour flow conditions that result in the highest surface temperatures, and run an adequate number of test conditions to validate the analysis. 6. Demonstrating compliance using flame arrestors 6.1. The performance of a flame arrestor is influenced by installation effects that may cause variations in critical parameters such as the speed of the flame front and the temperatures of the surfaces. The applicant should account for such installation effects in demonstrating compliance. The applicant may choose to show compliance with [CS 25.975(a)(7)](#_DxCrossRefBm1709821429) by testing a complete, conformed production installation of the flame arrestor (including the upstream and downstream ducting). Alternatively, the applicant may request EASA approval to use other tests and analysis of the flame arrestor and the installation as a means of compliance. 6.2. The applicant may propose to use flame arrestor elements from a supplier. The supplier may have previously qualified an element to flame propagation requirements without consideration of the design of the aeroplane into which the flame arrestor will be installed. The applicant should conduct tests to show that they have accounted for any effects of the installation, including flame front speeds and duct sidewall temperatures. The fuel types for these tests differ, and should be established as discussed in paragraph 6.3.1.3 of this AMC prior to conducting any testing. 6.3. Flame arrestor installation test. 6.3.1. Test Set-up. Figure A-1 shows a schematic of the test set-up. The test set-up involves mounting the flame arrestor element in a tube configuration that is representative of the aeroplane installation. The speed of the flame front that travels down the fuel vent system tubing toward the flame arrestor is a critical factor in the performance of the flame arrestor in preventing flame propagation. The flame front will accelerate down the tubing, so higher velocities will occur if the flame arrestor is located farther away from the fuel tank vent outlet. Therefore, the shape and diameter of the tubing and its length from the fuel tank vent inlet to the flame arrestor should be representative of the production configuration, unless the flame arrestor element was previously found to comply in an installation in which the speed of the flame reaching the flame arrestor was higher. In addition, the orientation of the flame arrestor in the fixture is a critical parameter for the compliance demonstration. For instance, a flame arrestor installation that faces downward, so a ground fire impinges on its face, will have a shorter duration flame-holding capability than a flame arrestor that is mounted horizontally. 6.3.1.1. Test fixture features. The applicant should consider the following features in designing the flame arrestor test fixture: 1. Orient the element to simulate the actual aeroplane installation. 2. Cut viewing sections into the pipe upstream and downstream of the flame arrestor element and cover them with transparent material to provide visual access to the element. 3. Locate igniters upstream and downstream of the element. 4. Locate thermocouples in the duct to measure the incoming flammable mixture temperature and the vapour temperatures downstream of the flame arrestor element. 5. Install thermocouples on the surface of the centre of the flame arrestor element’s upstream face and on the surface of the upstream side of the duct. 6. Incorporate a pressure-relief feature in the upstream portion of the system to relieve explosive pressures when ignition of the upstream flammable fluid vapour occurs. 7. Mix air that is at a temperature higher than the boiling point of the fuel being used (see paragraph 6.3.1.3 of this AMC) with fuel, and introduce it at the inlet of the tube. 8. Vary fuel–air ratios by adjusting the respective fuel-vapour and air-supply rates. 6.3.1.2. Test equipment. The test equipment should include: 1. The test article, including the flame arrestor and the downstream section of the vent system assembly that meets production specifications. 2. A section of ducting that is representative of the production flame arrestor installation. 3. A means of generating a supply of fuel vapour at preselected fuel-to-vapour air ratios and various flow rates. 4. A window for observing upstream and downstream conditions during the test. This should allow to determine the location of the flame front relative to the flame arrestor. 5. A means to measure temperatures on the upstream duct surfaces and the flame arrestor. 6. A means to measure fuel vapour mixture temperatures both upstream and downstream of the flame 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. 6.3.1.3. Fuel type. 6.3.1.3.1. The applicant should establish the critical fuel type for the test based on a review of the approved fuels for the aeroplane model. The applicant should use fuels in the test that have representative characteristics of the critical fuel approved for use in the aeroplane. The use of hexane as a representative fuel for kerosene fuels such as Jet A and TS-1 has been found to be acceptable. Hexane (C6H14) is readily available and easily manipulated in the gaseous state, so it is typically a fuel of choice. The AIT for hexane of 223°C/433°F closely simulates that of Jet A kerosene fuel, which has an AIT of 224°C/ 435°F, and JP-4 which has an AIT of 229°C/445°F. Note: The applicant should not use fuels with higher AITs than these, such as propane, for the flame arrestor element test because ignition on the back side of the flame arrestor would not be adequately evaluated. 6.3.1.3.2. Table A-1 summarises the properties of hexane and provides an example of the method for calculating the stoichiometric relationship of hexane needed for the test. 6.3.1.3.3. The applicant may use propane for testing of a flame arrestor installation if the 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 within the fuel tank remain below the maximum permitted fuel tank surface temperature (typically 200 °C/400 °F) would be acceptable, provided that the applicant or supplier has previously shown that the flame arrestor element meets the flame-holding requirements. 6.3.1.3.4. Table A-3 summarises the properties of propane as provided in FAA Technical Report ADS‑18, Lightning Protection Measures for Aircraft Fuel Systems (see Chapter 2.2 of this AMC), and provides an example of the method for calculating the stoichiometric ratio of propane. 6.3.1.4. Thermocouples. The applicant should use 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. The applicant should not use air-aspirated, shielded thermocouples. 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. 6.3.1.5. Test specimen. The test specimen should be a production component that conforms to the type design intended for certification. 6.3.2. Test conditions. Two types of tests are typically needed to demonstrate compliance: one for flame propagation prevention in a static vent vapour flow condition, and one for flame holding in a continuous vapour 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 installation during ground fire conditions. 6.3.2.1. Flame propagation test (static). This test demonstrates the element’s flame-arresting performance in a static condition at the critical fuel mixture condition of 1.15 ± 0.05 stoichiometric. This mixture is based on FAA-sponsored tests done by Atlantic Research, documented in the Lightning Protection Measures for Aircraft Fuel Systems report. The report shows curves of the flame arrestor equilibrium temperature for various air–flow ratios as a function of the per cent stoichiometric fuel–air ratio (see Figure A-2 in this AMC). These curves maximise at about 1.10 to 1.20 stoichiometric. The curves indicate that higher temperatures occur at lower flow rates. 6.3.2.1.1. Establish the mixed flow. Close the fuel and air valves. Ignite the mixture downstream of the element. Verify that flames did not propagate through the flame arrestor by observing it through the viewing window. Verify that the upstream mixture is combustible by energising the upstream igniter and observing the ignition of the upstream mixture. The applicant should repeat this test a minimum of 5 times at this mixture, as is done with explosion proof testing. 6.3.2.1.2. Flame front velocity. The velocity of the flame front as it reaches the flame arrestor can significantly influence the effectiveness of the flame arrestor in preventing flame propagation. The flame front velocity increases as the flame travels down a vent line containing flammable vapours. The velocity of the flame front is installation-dependent and influenced by the length and diameter of the vent line, and by flow losses between the ignition source and the flame arrestor. The test configuration should include consideration of these critical features. If an applicant proposes to use a previously approved flame arrestor element in a new installation with a different length or diameter of the vent line than previously tested, the applicant should account for these installation differences in the compliance demonstration. The applicant may need to conduct a separate test to demonstrate that the flame arrestor is effective in the installed configuration. 6.3.2.2. Flame-holding test. The purpose of this test is to show that a flame present at the fuel tank vent outlet, when a continuous flow of flammable vapour is exiting the vent, will not propagate into the fuel tank. 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. The flammable vapour flow rate that achieves a velocity of 0.75 to 1.0 feet per second (ft/s) across the flame arrestor is the range where flame arrestor failure occurred in the shortest time during development testing. Adjust the flow to achieve a velocity of 0.75 ft/s (+ 0.25, – 0 ft/s) across the flame arrestor and ignite it downstream of the flame arrestor. Determine and establish the location of the flame front by viewing it through the viewing window. Determine the position of the flame front and adjust the vapour flow rate such that the flame front contacts the downstream flame arrestor face, resulting in the greatest rate of heating of the flame arrestor surface. Take care to maintain the flammable vapour flow rate at a constant value throughout the test so as to maintain the correct fuel‑to-air ratio. 6.3.2.2.1. Flame arrestor element maximum surface temperatures. Monitor the temperature at the upstream centre of the flame arrestor during the flame-holding test; it is required to stay below 370 °C/700 °F for the first 2 minutes and 30 seconds after the ignition. Data from developmental testing show that the temperature of the centre of the upstream flame arrestor face at which failure (i.e. propagation of the flame) occurred was typically above 370 °C/700 °F, which is well above the AIT of JP‑4 fuel vapour of 229 °C/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 of the vapour. For this reason, hexane, with an AIT of 223 °C/433 °F, should be used for the test of the flame arrestor element. 6.3.2.2.2. Flame arrestor installation and vent system maximum surface temperatures. The compliance demonstration must show that flames present at the vent outlet do not propagate into the fuel tank during the first 2 minutes and 30 seconds after ignition. If the flame arrestor installation or any vent system components that are exposed to the flame are installed in locations where the ignition of flammable vapours could result in the propagation of the fire into the fuel tank, the applicant must show that ignition of the fuel vapours does not occur. This may require the installation of additional surface temperature instrumentation as part of the compliance demonstration test. The applicant should establish temperature limits for any components of the vent or flame arrestor assembly that are located in spaces where flammable vapours may be present, based on the location of the components in relation to the fuel tank. [AMC 25.981](#_DxCrossRefBm1709821459) provides guidance for establishing 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 30 °C/50 °F below the lowest expected AIT of the approved fuels. The AIT of fuels will vary because of a variety of factors (e.g. ambient pressure, dwell time, fuel type, etc.). The AIT accepted by EASA without further substantiation for kerosene fuels, such as Jet A, under static sea level conditions, is 232 °C/450 °F. This results in a maximum allowable surface temperature of 200 °C/400 °F for an affected surface of a fuel tank component. Higher surface temperature limits in flammable fluid leakage zones may be allowed in certain cases where the applicant can substantiate that the higher temperature limits are acceptable. The applicant should monitor and record surface temperatures for any components where the analysis-established limits were required, and should show that the surface temperatures remain below the established limits. 6.3.3. Pass/fail criteria. 6.3.3.1. The flame arrestor installation should meet the following performance criteria, as described in paragraph 6.3.2 of this AMC: It should pass the static propagation test; It should have a minimum flame-holding time of 2 minutes and 30 seconds; Installation-dependent maximum surface temperature limits should be established for any flame arrestor and vent system components located in fuel tanks or flammable fluid leakage zones that are determined to be potential sources that could propagate the flame from the external vent to the fuel tank. 6.3.3.2. After completing the flame arrestor tests noted above, the applicant should carefully examine the integrity of the structure of the flame arrestor. Suppliers have constructed flame arrestors from one flat and one corrugated stainless steel sheet that are 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 faces 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 the flame passed across the flame arrestor because structural integrity was lost during the test due to failures of welds or brazed joints. Damage to components of the flame arrestor assembly is acceptable if the flame arrestor installation prevents flame propagation during the test, and the maintenance requirements specify that the flame arrestor must be repaired or replaced following an event where the flame arrestor was exposed to flame. 6.3.4. Related qualification and installation considerations. This paragraph does not contain an all-inclusive list of applicable qualification considerations. The tests should show that each component performs its intended function within the environment where it is installed. The applicant should establish design-specific qualification requirements in addition to the items listed in this paragraph. 6.3.4.1. Vibration. Test the flame arrestor in a vibration environment representative of the installation. 6.3.4.2. Icing. Installation of a flame arrestor will probably introduce a point in the vent system where icing is likely. The applicant should account for this effect in the vent system design by either installing pressure‑relief provisions that protect the tank from excessive pressure differentials, or by showing that icing or clogging of the flame arrestor with ice is not possible. 6.3.4.3. Fuel tank bottom pressures. In many cases, applicants have established the size of fuel tank vent systems, and the associated fuel tank refuelling rates, based on the bottom pressure of the fuel tank after failure of the refuelling system shut-off system and the resulting fuel overflow of the tank through the vent system. However, installation of a flame arrestor or modifications to the vent system may result in increased tank bottom pressures. Therefore, if an applicant adds a flame arrestor to a fuel vent, or modifies an existing flame arrestor, the applicant should evaluate the effects of these changes on the tank bottom pressure, and adjust the refuelling rates to maintain the fuel tank bottom pressures within the limits that were established by the fuel tank structural analysis. 6.3.4.4. Lightning. The applicant must show that the fuel tank vent system installation complies with [CS 25.954](#_DxCrossRefBm1709821378). [AMC 25.954](#_DxCrossRefBm1709821426) provides guidance in meeting those requirements. FAA Technical Report ADS-18 (see paragraph 2.2 of this AMC) provides factors that the applicant should consider when developing features to protect fuel tank vents from lightning. 7. Demonstrating compliance using fuel tank inerting, fuel tank pressurisation, and fire suppression systems 7.1. Fuel tank inerting. An applicant’s use of fuel tank inerting systems to show compliance with [CS 25.975(a)(7)](#_DxCrossRefBm1709821429) requires them to demonstrate that the design prevents fuel tank explosions during all operating conditions (e.g. taxiing, take-off, landing, refuelling, etc.) and post-crash fire scenarios. To comply with [CS 25.981](#_DxCrossRefBm1709821428), inerting systems are not required to inert the fuel tanks during all operating conditions. Therefore, if an applicant proposes an inerting system as the means of compliance with [CS 25.975(a)(7)](#_DxCrossRefBm1709821429), the system would need to have additional capability to prevent fuel tank explosions during all operating conditions. For example, inerting systems found compliant with [CS 25.981](#_DxCrossRefBm1709821428)typically allow the fuel tanks to become flammable during refuelling operations, and when the inerting system is inoperative. The applicant would need to address these conditions in order to ensure that the system continues to meet the requirements of [CS 25.975(a)(7)](#_DxCrossRefBm1709821429). 7.2. Fuel tank pressurisation systems. Fuel tank pressurisation 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 the applicant proposes fuel tank inerting or pressurisation as the means of compliance with [CS 25.975(a)(7)](#_DxCrossRefBm1709821429), the applicant must show that these means are effective in preventing a fuel tank explosion during all operating conditions (e.g. taxiing, take-off, landing, refuelling, etc.) and post-crash fire conditions. 7.3. Fire suppression systems. Fuel tank or vent system fire suppression systems are typically activated by a light sensor, and they discharge a fire-suppressant agent that is only effective for a short time. Demonstrating compliance using this technology would require the applicant to show its effectiveness in preventing a fuel tank explosion with a fire present at the fuel tank vent outlet for a minimum of 2 minutes and 30 seconds. [Amdt 25/21]