11 ignition of flammable vapors could result in the propagation of the fire into the fuel tank, the applicant must show ignition of the fuel vapors will not occur. This may require 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 vapors may be present based on the location of the components in relation to the fuel tank. AC 25.981-1C 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 50 °F
(27.7 °C) 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.). 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, (see paragraph 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 flammable fluid leakage zones in certain cases where the applicant can substantiate the higher temperature limits. The applicant should monitor and record surface temperatures for any components where the analysis-established limits were required and show the surface temperatures remain below the established limits.
8.3.3 Pass/Fail Criteria.
8.3.3.1 The flame arrestor installation should meet the following performance criteria, as described in paragraph 8.3.2 of this AC:
• Static propagation test.
• Minimum flame holding time of 2 minutes and 30 seconds.
• Installation-dependent maximum surface temperature limits established for any flame arrestor and vent system components located in fuel tanks or flammable fluid leakage zones that are determined to be a potential source of propagating the external vent flame to the fuel tank.
8.3.3.2 After completing the flame arrestor tests noted above, the applicant should carefully examine the integrity of the flame arrestor structure. 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
12 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 the flame passed across the flame arrestor because structural integrity was lost during the test due to weld or brazed joint failures.
Damage of flame arrestor assembly components is acceptable if the flame arrestor installation prevents flame propagation during the test, and the maintenance requirements specify the flame arrestor must be repaired or replaced following an event where the flame arrestor was exposed to flame.
8.3.4 Related Qualification and Installation Considerations.
This section 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 section.
8.3.4.1 Vibration.
Test the flame arrestor 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. 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.
8.3.4.3 Fuel Tank Bottom Pressures.
In many cases, applicants have established the size of fuel tank vent systems, and associated fuel tank refueling rates, based on the bottom pressure of the fuel tank after failure of the refueling system shutoff 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 tank bottom pressure, and adjust refueling rates to maintain fuel tank bottom pressures within the limits that were established by the fuel tank structural analysis.
8.3.4.4 Lightning.
The applicant must show the fuel tank vent system installation complies with § 25.954. AC 20-53B provides guidance in meeting those requirements. FAA Technical Report ADS-18 (see paragraph 3.3 of this AC) provides factors the applicant should consider when developing fuel tank vent lightning protection features.
13 9 DEMONSTRATING COMPLIANCE USING FUEL TANK INERTING, FUEL TANK PRESSURIZATION, AND FIRE SUPPRESSION SYSTEMS.
9.1 Fuel Tank Inerting.
An applicant’s use of fuel tank inerting systems to show compliance with § 25.975(a)(7) requires demonstrating that the design prevents fuel tank explosions during all operating
(e.g., taxi, takeoff, landing, refueling, etc.) and post-crash fire scenarios. To comply with § 25.981, 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 to § 25.975(a)(7), the system would need to have additional capability to prevent fuel tank explosions during all operating conditions. For example, inerting systems found compliant with § 25.981 typically allow the fuel tanks to become flammable during refueling 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 § 25.975(a)(7).
9.2 Fuel Tank Pressurization Systems.
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 the applicant proposes fuel tank inerting or pressurization as the means of compliance with § 25.975(a)(7), the applicant must show these means are effective in preventing a fuel tank explosion during all operating (e.g., taxi, takeoff, landing, refueling, etc.) and post-crash fire conditions.
9.3 Fire Suppression Systems.
Fuel tank or vent system fire suppression systems typically are activated by a light sensor and discharge a fire suppressant agent that is only effective for a short period of time. Demonstrating compliance using this technology would require showing effectiveness at preventing a fuel tank explosion with a fire present at the fuel tank vent outlet for a minimum of 2 minutes and 30 seconds.
Appendix A A-1 Appendix A. Example of Calculation for Fuel-to-Air Ratio Table A-1. Combustion Properties of Hexane Property Value 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 moles 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 an air-to-fuel weight ratio of:
2627.48
1.15 × 172.34 = 13.2 Table A-2. Fuel-to-Air Mixtures for Flame Arrestor 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 1.3 0.04 Between lean limit and stoichiometric 1.10 0.045 1.7 0.05 Stoichiometric 1.58 0.065 2.2 0.0658
1.15 Stoichiometric 1.82 0.074 2.5 0.07567 Between stoichiometric and rich limit 3.0 0.15 6.3 0.2 Rich limit 6.16 0.23 8.0 0.26
Appendix A A-2 Table A-3. Combustion Properties of Propane Property Value 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 0.027 Critical velocity gradient for flashback,* sec-1 600 Laminar flame speed,* cm-sec 40 *Applicable to 1.1 stoichiometric propane-to-air at standard temperature and pressure (STP).
Note: The combustion of propane and oxygen is written as:
𝐶3𝐻8 + 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
Appendix A A-3 Figure A-1. Fuel Tank Vent Flame Arrestor Test Schematic
Not e: This test schematic is not intended to represent an acceptable test configuration for project-specific compliance testing. The test installation, including the length and diameter of downstream vent line and the orientation of the flame arrestor will have a significant impact on the effectiveness of the flame arrestor. Therefore, the test configuration should be representative of the actual airplane installation. Additional surface temperature measurements may be needed if the flame arrestor is installed in a location where ignition of flammable vapors could lead to propagation of the external vent fire into the fuel tank. For example, a vent flame arrestor could be installed adjacent to a fuel surge tank, such that hot surfaces of the flame arrestor or vent system could cause ignition of flammable vapors and propagation of the fire into the fuel tank.
Additional surface temperature data would be needed to demonstrate that surface temperature limits are not exceeded.
Appendix A Figure A-2. Flame Arrestor Surface Temperature at Various Flow Rates and Stoichiometric Mixture Ratios2 500 480 Relative Flow Rate 460
0.550 BOTTOM SURFACE TEMPERATURE (°F) 440 420
• 0.648 400
0.774 380
0.915 360 1 Inch X 0.050 Inch Stainless Steel Arrestor 1.000 340
0.9 1.0 1.1 1.2 1.3 1.4 1.5
FRACTION OF STOICHIOMETRIC 2 FAA Technical Report ADS-18, Lightning Protection Measures for Aircraft Fuel Systems (see paragraph 3.3 of this AC).
A-4
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