Smoke is a combination of the airborne solid and liquid particulates and gases generated when a material undergoes combustion and the air that they are mixed with.
Particulate smoke is a product of incomplete combustion and is generated during both smoldering and flaming combustion, although the nature of particles and their mode of formation are very different.
Smoke from Smoldering Combustion
Smoke from smoldering is similar to that obtained when any carbon-based material is heated to temperatures at which there is chemical degradation and evolution of volatiles. The volatiles mix with cool ambient air to create a mist consisting of minute droplets of tar and liquids with high boiling points.
Smoke from Flaming Combustion
Smoke from flaming combustion consists entirely of solid particles, most of which are formed in the gas phase as a result of either incomplete combustion or high temperature combustion in a low temperature environment.
Smoke may originate within the cabin in either open or hidden areas, from elsewhere in the pressure hull (holds, equipment bays), from an external ground fire source which either takes advantage of a structural breach of the pressure hull or a similar source which creates such a breach itself.
Smoke, but more usually oil-based fumes, can also enter the cabin via the air conditioning system. However, this system is normally an essential element of smoke removal until either smoke generation reduces or ceases altogether or evacuation becomes possible. Switching it off during a cabin fire without introducing ram air has aggravated cabin fire smoke build up in past accidents and incidents; see for example the NTSB report on the Air Canada DC9 fire: DC93, en-route, Cincinnati OH USA, 1983
The heat generated by any fire causes gases in the vicinity to increase in temperature and therefore to also expand. The forces which this creates are responsible for what is termed "buoyancy" in which the hot gases rise and flow away from the fire. Buoyancy is why hot gases in hidden areas are sometimes expelled through ventilation openings or any similar ‘leakage’ path. As hot gases rise up from a fire due to buoyancy, they begin to mix with ‘local’ air and cool down. This reduces buoyancy and therefore the upward mobility of the gases which tend to become layered or stratified. The pattern of smoke buoyancy and stratification in any particular fire can be affected by many variables, for example the ‘fuel’ which is feeding the fire, the burn rate of the fire, local environmental conditions, the geometry of the space containing the smoke and the extent of opportunities for smoke leakage.
On-Aircraft Smoke Sources
Smoke within an aircraft fuselage during normal operations may arise from a number of causes. These include:
- failure of electrical equipment and short circuits in electrical wiring;
- overheating of equipment due to malfunction of thermostats or other controlling devices;
- leakage of very hot air from pneumatic ducts;
- spillage of combustible fluid (hydraulic oil, glycol) on a hot surface;
- overheating of galley ovens with subsequent spillages of hot food oils, fats and greases.
Smoke from fires which are the result of ground impact are usually fuel fed. Since the supply of fuel may be large, consequential smoke build up in the cabin can be rapid.