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A flashover is the near simultaneous ignition of all combustible material within an enclosed area. When many materials are heated, they release flammable gases. Flashover occurs when the majority of surfaces within the enclosed area are heated to the auto-ignition temperature of those flammable gases.
Fires in an aircraft are particularly hazardous because of the small size of the compartment(s) and the difficulty (on the ground) or impossibility (in flight) of escape. Although cabin furnishings are made from fire resistant materials, an uncontrollable in-flight cargo compartment fire, a wheel well fire or an out of control lavatory fire can become life threatening when the cabin materials become involved. Similarly, during a post-crash cabin fire ignited from spilled jet fuel, the fire can spread through the cabin generating heat, smoke, and toxic decomposition products. Hot combustion products rise from the fire to form a distinct, hot, smoky layer just below the ceiling which deepens as the fire continues to burn. Burning side and ceiling panels fall and ignite seats resulting in the total conflagration of the cabin interior. This in turn raises the temperature of the trapped smoke and gasses towards their auto-ignition point. On reaching the auto-ignition temperature, Flashover occurs.
FAA testing, using full-scale aircraft cabin measurements, of fire hazards such as temperature, smoke, oxygen depletion, carbon dioxide, carbon monoxide and the presence of irritant and poisonous gases indicate that all of these hazards increase markedly at flashover. At that time, the human tolerance limits to these individual or combined hazards are exceeded. Consequently the time that is required to reach flashover is a measure of the maximum time available for escape from an aircraft cabin fire.
If a fire occurs while the aircraft is in flight, it must be fought aggressively and the aircraft landed as soon as possible.
On the ground, evacuation must be initiated without delay and completed as efficiently and as quickly as possible.