A methodology to determine pre-crash fuel quantity from post-crash fire thermal damage to an aircraft structure

Abstract

The cause of engine failure that resulted in a catastrophic crash of a twin turbo-jet commuter aircraft is sought. The US National Transportation Safety Board and aircraft manufacturer contend that the engines failed because the aircraft ran out of jet fuel. The aircraft operators are confident that there was adequate fuel and that some other process caused fuel starvation. There was a post-crash fire that caused substantial thermal destruction of aircraft structure. If the majority of fire damage could be attributed to the burning of jet fuel, then it would be likely that the fuel inventory, just prior to impact with the ground, was adequate for continued flight. Thus investigators would have to conclude that some other mechanism caused the fuel starvation. In this work, a methodology is presented to determine the amount of fuel necessary to cause the observed thermal damage on the aircraft structure. The methodology is based on the determination of the heat flux and burning rates from the associated jet fuel pool fire and a heat transfer analysis to obtain the time required to melt structural parts subjected to the resulting heat fluxes. For an Aluminum alloy member, 70 mm thick, the time to reach the melting temperature of the alloy (∼500° C) is almost 4 min. Fully developed jet fuel pool fires burn at a rate of ∼0.06 kg/m²s, thus, for a pool fire of 10 m², the fuel burned would be ∼130 kg, which is equivalent to about 151.4 L (40 US liquid gallons) of fuel. This fuel quantity would have been more than sufficient for the aircraft to reach its destination. Moreover, the actual quantity of fuel burnt would have been substantially more because of the conservative assumptions made in the analysis.