The Airport and Aircraft Safety Research and Development Group Fire Safety Team performed tests at the FAAWilliam J. Hughes Technical Center to examine the variation in flammability exposure of fuel tanks comprised of a composite material skin and a traditional aluminum skin. The variation in topcoat color of the aluminum material was analyzed, as was the variation in thickness of the composite material.
Tests examining the effects of topcoat color of the aluminum fuel tank were consistent. These tests showed that while the bare composite material transmits radiant heat into the fuel tank much more readily than the bare aluminum material, once aviation grade primer and a topcoat (regardless of color) are applied, the aluminum skin behaves in a similar manner to the composite. The application of both white and black topcoat colors to the aluminum panels resulted in the aluminum tank temperatures and total hydrocarbon concentration (THC) measurements being consistent with the composite tank test results, which is evidence that a difference in material properties is not what leads to differences in temperatures and THC measurements. Instead, it was thought that the reflective behavior of the bare aluminum material, causing much of the radiant heat to be reflected off the tank, resulted in lower fuel tank temperatures and, therefore, lower THC measurements. However, additional testing with the composite material, with a reflective aluminum epoxy applied to it, did not exhibit the anticipated impact to the internal tank temperatures and flammability measurements. As a result, the testing conducted was inconclusive as to the cause of the fuel tank behavior. Panel heat tests with the composite materials of varying thicknesses showed a correlation between panel thickness and temperature on the bottom surface of the panel. However, tests showed that when these panels are installed on a fuel tank, the difference in thickness provides little variance in resulting tank temperatures and THC measurements. These tests, however, did once again confirm the strong correlation between ullage temperature and THC within a fuel tank when heated from above.
The effectiveness of aircraft depressurization (reduced pressure) on the burning behavior of stacked cargo, batteries, fuel, and materials was measured in a 381-cubic-foot (10.8-cubic-meter) pressure vessel, modified to conduct fire tests at a specific reduced pressure or programmed to vary the pressure to simulate aircraft depressurization to control a cargo fire and subsequent emergency descent to sea level. It was determined that depressurization did not prevent flashover during cargo fires consisting of stacked cargo boxes filled with shredded paper, although the burning behavior of individual fuels and materials was reduced at lower pressures. The discharge of Halon 1301 prevented flashover during the cargo fires and also significantly reduced the air temperature. In addition, thermal runaway of lithium batteries overheated under controlled fire-exposure conditions was not prevented over a range of pressures from sea level to an elevation of 26,000 ft (7.9 km).
Lithium-ion (rechargeable) and lithium-metal (non-rechargeable) battery cells put aircraft at risk of igniting and fueling fires. Lithium batteries can be packed in bulk and shipped in the cargo holds of freighter aircraft; currently lithium batteries are banned from bulk shipment on passenger aircraft .
The federally regulated Class C cargo compartment extinguishing system’s utilization of a 5 %vol Halon 1301 knockdown concentration and a sustained 3 %vol Halon 1301 may not be sufficient at inerting lithium-ion battery vent gas and air mixtures . At 5 %vol Halon 1301 the flammability limits of lithium-ion premixed battery vent gas (Li-Ion pBVG) in air range from 13.80 %vol to 26.07 %vol Li-Ion pBVG. Testing suggests that 8.59 %vol Halon 1301 is required to render all ratios of the Li-Ion pBVG in air inert.
The lower flammability limit (LFL) and upper flammability limit (UFL) of hydrogen and air mixtures are 4.95 %vol and 76.52 %vol hydrogen, respectively. With the addition of 10 %vol and 20 %vol Halon 1301, the LFL is 9.02 %vol and 11.55 %vol hydrogen, respectively, and the UFL is 45.70 %vol and 28.39 %vol hydrogen, respectively. The minimum inerting concentration (MIC) of Halon 1301 in hydrogen and air mixtures is 26.72 %vol Halon 1301 at 16.2 %vol hydrogen.
The LFL and UFL of Li-Ion pBVG and air mixtures are 7.88 %vol and 37.14 %vol Li-Ion pBVG, respectively. With the addition of 5 %vol, 7 %vol, and 8 %vol Halon 1301, the LFLs are 13.80 %vol, 16.15 %vol, and 17.62 % vol Li-Ion pBVG, respectively; the UFLs are 26.07 %vol, 23.31 %vol, and 21.84 %vol Li-Ion pBVG, respectively. The MIC of Halon 1301 in Li-Ion pBVG and air mixtures is 8.59 %vol Halon 1301 at 19.52 %vol Li-Ion pBVG.
Le Chatelier’s mixing rule has been shown to be an effective measure for estimating the flammability limits of Li-Ion pBVGes. The LFL has a 1.79 % difference while the UFL has a 4.53 % difference. The state of charge (SOC) affects the flammability limits in an apparent parabolic manner, where the widest flammability limits are at or near 100 % SOC.