both sustain itself and grow.  This process of fire is now called the "fire tetrahedron." See figure 12-3. The    most    common    method    of    controlling    or extinguishing a fire is to eliminate one or more of sides of  the  tetrahedron.    This  can  be  accomplished  by  the following methods. 1. Smothering—removing the oxygen 2. Cooling—removing the heat 3. Starving—removing  the  fuel  or  combustible matter There are two terms you need to understand about fires.   These are the  fire point  and the  flash point. The fire    point of    a    substance    is    the    lowest temperature at which its vapors can be ignited and will continue  to  burn.    At  this  temperature,  the  vapor  will ignite spontaneously in the air.   Also, substances don't have     to     be     heated     to     this     ignition     temperature throughout in order to ignite. The  flash point  of a substance is the temperature at which the substance gives off enough vapors to form an ignitable   mixture   with   the   air   near   the   substance's surface.    An ignitable mixture is a mixture within the explosive range.  The mixture is capable of spreading a flame away from the source of ignition when ignited. For  example,  fuel  will  spontaneously  ignite  when  a portion of it (or its vapors) is exposed to temperatures around 500°F (ignition temperature).    It is capable of being touched off by a match or spark at temperatures down to -5°F (fire point).   It will also flash across the surface at temperatures from -5°F down to -45°F (flash point).   From these examples, you can readily see that fuel has a low flash point and is easily ignited.  Fuel is a constant  fire  hazard  around  aircraft. A  spark,  heat caused   by   friction,   or   an   electrical   discharge   could supply enough heat to cause fuel to flash. CLASSES OF FIRE Different types of fires are combated by different means.   It is important that you know how to identify the various types of fires and understand why each type must be combated in a specific way. Class A Class A fires occur in combustible materials, such as  bedding,  mattresses,  books,  cloth,  and  any  matter that   produces   an   ash. All   fires   of   this   class   leave embers,  which  are  likely  to  rekindle  if  air  comes  in contact with them.  Class A fires must not be considered extinguished   until   the   entire   mass   has   been   cooled below its ignition temperature.   Smothering (removing the oxygen) is not effective for class A fires because it does   not   lower   the   temperature   of   the   smoldering embers  below  the  surface.  The  extinguishing  agents most effective for class A fires are solid water stream, both   high-   and   low-velocity   fog,   CO₂,   and   water immersion. 12-2 FLAMING COMBUSTION AND SURFACE GLOWING COMBUSTIONN TEMPERATURE OXYGEN FUEL FUEL NO CHAIN REACTION FUEL IS IN FORM OF INCANDESCENT SOLID OXYGEN IS AT INTERFACE OF GLOWING FUEL UNINHIBITED CHAIN REACTION OF COMBUSTION PROCESS DIFFUSION & CONTINUOUS REIGNITION  & AUTOMATICALLY OBTAINED AT FLAME TEMPERATURE LEVELS FUEL IS IN FORM OF VAPOR AND GAS Anf1203 3 Figure 12-3.—Tetrahedron and fire triangle.