The flammability of the fuel vapors existing in an aircraft fuel tank ullage depends on numerous parameters and is difficult to measure. ASTM D1655 sets forth the specifications for aviation turbine fuels, but states only that these fuels shall consist of refined hydrocarbons derived from crude petroleum, natural gasoline, or blends thereof with synthetic hydrocarbons. Thus, jet fuels typically are mixtures of hundreds of compounds, controlled only by specifications such as the boiling point range, flashpoint, etc., rather than properties like the flammability limits, ignition energy and quenching distance, which would lend themselves more to determining fuel flammability characteristics. At the Fire Safety Section's Pressure Fire Modeling Facility, experiments are conducted to determine the effects of several parameters, such as ambient temperature and pressure, oxygen concentration and mass loading on the fuel vapor concentrations residing in a fuel tank ullage. It is hoped that these tests will aid in further determining jet fuel flammability characteristics-knowledge that can be applied to promote the innovation of new fuel tank protection technologies.

The Pressure Fire Modeling Facility is currently being used to conducttests to determine the oxygen concentration limit at which ignition is no longer supported, for a given fuel-air mixture, in a fuel tank under varying conditions. These tests are conducted within a 9 ft3 vented tank, which is instrumented with 13 K-type thermocouples, 2 pressure transducers and sample lines for hydrocarbon and oxygen concentration measurement. The thermocouples are placed on each of the tank walls, including the ceiling and floor, inside the liquid fuel, and in the remaining ullage space. A bank of nitrogen bottles is used to control the oxygen concentration while heaters placed underneath the tank control the liquid fuel's temperature. In addition, a vacuum pump is used to evacuate the pressure vessel to simulate altitude. Spark initiation for the ignition tests is achieved through a 10,000-volt transformer connected to two tungsten electrodes with controllable gap separation and alignment. For each test run, fuel is placed within the test article, the pressure vessel is depressurized corresponding to the desired altitude, nitrogen is injected to obtain the desired oxygen concentration and the liquid fuel is heated to a predetermined temperature. The fuel is maintained at this temperature until the desired hydrocarbon concentration is achieved. At this point, the ignition spark is initiated. For each altitude, fuel loading and hydrocarbon concentration, experiments are conducted to determine the effect of the oxygen concentration on the reactivity of the fuel-air mixture. The aim is to determine the oxygen concentrations, ranging from the level that prevents ignition of the fuel vapor to the level that has no effect on the ignition, for each set of conditions.

Previous projects at the FAA Technical Center regarding fuel flammability have included experiments to determine the effects of the fuel mass loading and cold ambient temperatures on the fuel vapor concentrations formed within a fuel tank. Reports and presentations of the findings of these projects can be found below, as well as on the FAA Fire Safety Section's main website.