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Aircraft Safety - Fires

            In general, aircraft have a relatively small risk of an in flight fire hazard.  It is, however, a real possibility, so flight crews must be trained to deal with it.  As far as aircraft accident investigation is concerned, the problem is with the post-accident fire that destroys the remains.  There are several chemistry rules that must be understood before we go any further..

            Let’s define a few things first.  A fire is nothing more than a rapid oxidation of a fuel.  A deflagration is a subsonic, low-level shockwave combustion, also known as a fireball.  A diffusion flame is an uncontained flame.  Detonation is a supersonic combustion fire.  An explosion is a confined supersonic combustion fire that ruptures its container.  Flash point is the temperature where an applied ignition source will cause a mixture to flash at its lowest limit of flammability, which is on the lean (high air/fuel ratio) side.  Auto-ignition temperature is the temperature which a fuel/air parcel will ignite itself.  An example of something that uses this law is the diesel engine.  Flammability limits are the temperature ranges for a specific fuel to have an explosive fuel/air ratio.  Flammability limits, or flammability ranges, vary directly with pressure. 

            In order for a fire to sustain itself, there are a few prerequisites.  All you need is a fuel, an oxidizer (oxygen), ignition source (some kind of extreme heat), and the required time and energy to sustain the reaction.  The fuel may be a solid, liquid, or gas that is waiting to be oxidized.  To be a little more technical, a fuel is either a solid or a gas as there as no such thing as a liquid fire.  The physical properties of liquids make them incombustible, so the only property of a liquid that we take into account is volatility.  This is the vapor pressure properties of a liquid.  Put another way, it is how readily a liquid will evaporate for a given pressure and temperature.  We are concerned about this because it is the vapor of the liquid that oxidizes.  The gas must be combined with a sufficient amount of oxygen in order for the chain reaction to create enough energy to have fire.  If the gas has too much oxygen mixed in, there insufficient chemical energy (potential energy) to have a fire.  So there you have it, a cookbook formula for fire.  Folks, please don’t try this at home.