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.
A series of tests was conducted to determine the effect that concentrations of hydrogen below its lower flammability limit can have on the burning of other materials. The vertical Bunsen burner test cabinet was set up to run tests with hydrogen concentrations varying between 0% and 4% by volume. Three different materials were tested: a 1/16″ thick woven carbon fiber, a fabric aircraft seat cover, and an 8-ply unidirectional carbon fiber. All three materials showed significantly increased after-flame times and burn lengths as the concentration of hydrogen increased. The burn rate of both carbon-fiber materials also significantly increased with increased hydrogen concentrations, whereas the burn rate of the seat-cover fabric remained relatively constant for all concentrations.