Limiting Oxygen Concentration of Aviation Fuels

Experiments were performed at the Federal Aviation Administration (FAA) William J. Hughes Technical Center to determine the reduction in oxygen concentration required to prevent an accidental fuel tank explosion. It was determined that the limiting oxygen concentration (LOC) at sea level through 10,000 ft is approximately 12% O₂, with a linear increase from 12% at 10,000 ft to approximately 14.5% at 40,000 ft. Tests with various sparks/arcs as ignition sources at sea level showed little variation in results with the LOC ranging from 11.9% to 12.8%. The only ignition event falling below 12% O₂ was attributed to an inherent error in the oxygen measurement system, whose sensitivity is stated to be ±1% of the full-scale value (25% O₂). Peak pressures resulting from ignition at oxygen concentrations 1% to 1.5% above LOC values decreased as the altitude was increased to 30,000 ft while the duration to reach the peak pressure increased.

Although fuel tank inerting systems have been designed to values as low as 9% O₂, a review of previously published research found that virtually all sea level data related to aircraft fuel tank inerting was in excellent agreement with the current dataset. Little data is available at flight altitudes; however, the data that is available verifies the finding of decreasing inerting requirements (increasing LOC values) as altitude is increased. Discrepancies across datasets were attributed to two effects: (1) a difference in ignition criteria used in the experimental process and (2) safety factors added on to the experimentally derived LOC values.