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• January 29, 2017 Create Date
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National Advisory Committee for Aeronautics, Technical Notes - Origin and Prevention of Crash Fires in Turbojet Aircraft

The tendency for the Jet engine to continue to rotate after crash
presents the probability that crash—spilled combustibles suspended in
the air or puddled on the ground at the engine inlet may be sucked into
the engine. Studies with Jet engines operating on a test stand and full-
scale crashes of turboJet-powered airplanes showed that combustibles
drawn into the engine in this way ignite explosively within the engine.
Flames that could set the major fire appear at the engine tailpipe and
often at the inlet when this explosive ignition occurs. This ignition
may occur on the hot metal of the engine interior even after the combus-
tor flame is extinguished and the engine is coasting to rest.

Experiment showed that the gas flow through the engine is too rapid
to permit the ignition of ingested combustibles on the hot metal in con-
tact with the main gas stream. Ignition will occur on those hot surfaces
not in the main gas stream. A portion of the engine airflow is diverted
for cooling and ventilation to these zones where the gas moves slowly
enough for ignition to occur.

The limited extent of the hot-metal zones that may start a fire per-
mitted an approach to inerting the engine that involved the simultaneous
initiation of the following actions immediately upon crash impact: (1)
Shut off fuel flow to engine; (2) Spray coolant (water) on those hot sur-
faces found to be ignition sources; (5) disconnect airplane electrical
system at the battery and generator.

The effectiveness of this approach was evaluated by crashing air-
planes powered by jet engines. Pylon-mounted engines attached to the
wings were used to simulate airplanes with exposed pod nacelles. Fight-
ers represented airplane types whose engines are contained within the
main airplane structure. The quantity of water required as a coolant
ranged from 9 to 12 gallons for each engine, depending on the engine and
the length of the attached tailpipe. No fires occurred in six crashes
in which the inerting system was used. The two airplanes crashed with-
out protection caught fire.