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|Category:||Fire Smoke and Fumes|
The most critical product of aircraft fuselage fires is the smoke gases which are produced by combustion in the cabin environment.
The increased use of flame retardant materials for cabin furnishings has reduced smoke gas production; however, once a fire takes hold, the materials used for cabin trim and fittings still generate substantial visible smoke. Some of the gas constituents of this smoke are especially, and very rapidly, disabling.
Inhalation of toxic gases in smoke is the main direct cause of fatalities in most aircraft fires. To survive, significant inhalation of these gases must be avoided and some oxygen intake maintained until evacuation is possible. Minimising exposure to abnormal gases, both the directly toxic and otherwise, will also limit the irritant effects on the eyes.
It was the recognition of the importance of avoiding inhalation of the hazardous elements of smoke gases, rather than loss of visibility in the ground evacuation case, which led to the development of and regulatory mandate for Floor Path Lighting Systems. These provide exit route guidance for circumstances where the standard high level exit signs become invisible because of smoke. The smoke, because of convection and relative density factors, tends to begin at ceiling level in the early stages of a major fire and then work downwards as its volume increases.
The Main Smoke Gases
Since the products of combustion depend on many factors, including the fuel being consumed, the availability of oxygen, the rate of combustion and the fire suppression agent(s) used, the toxicity of the smoke encountered in a fire at any particular moment will vary considerably.
Smoke gases fall into two categories - the irritant and the immediately hazardous.
The irritant category includes hydrogen chloride and acrolein which are generated from burning electrical insulation and some other cabin materials. Any burning materials which contain Carbon will not only produce immediately hazardous Carbon Monoxide but also Carbon Dioxide which replaces the oxygen being used to sustain the fire. This process alters (reduces) the partial pressure of Oxygen in the Cabin atmosphere and thus increases respiration rates. As a result, people involuntarily take in a relatively greater quantity of other gases - including the unwanted ones. This raised respiration level is in addition to that which occurs, even in normal flight, when the cabin altitude is greater than ground level. However, since the partial pressure of Oxygen at aircraft altitudes above 8000 feet will be greater in a pressurised cabin than in the external environment, it is wise to keep the cabin pressurised to retain some oxygen availability (since passenger oxygen masks can clearly not be deployed in a cabin fire scenario).
The main immediately hazardous gas occurring in cabin fires, in addition to Carbon Monoxide noted above, is Hydrogen Cyanide. This is produced during combustion of wool, silk and many nitrogen-containing synthetics, so is almost guaranteed to occur. Irritant smoke gases can induce tears, pain and disorientation, which adds to the disorientation resulting from poor visibility in the smoke. However, the more subtle effect of the two main toxic gases produced in aircraft cabin fires, Carbon Monoxide and Hydrogen Cyanide, is physical incapacitation; this has often been shown to have prevented successful evacuation from post-crash fires.
Key Points on Minimising Smoke Gas Inhalation
The resistance to inhalation of toxic gases is known to vary widely between individuals. However, as well as prompt ground evacuation (if available), the principal defences against succumbing to the effects are the same for everyone and are:
- Avoid inhalation of smoke as much as possible - keep head below any obvious (visible) accumulation of smoke gases;
- Hold a piece of wet material over nose and mouth to filter out water soluble gases during inhalation. As well as some irritant gases, this will also be effective against one of the two immediately dangerous gases, hydrogen cyanide;
- Minimise unnecessary activity so as to keep respiration levels as low as possible.
- Boeing article Reducing Smoke and Burning Odor Events
Several Accident Reports discuss the generation, propagation and avoidance of smoke gases during aircraft cabin fires at length, in association with the evacuation issues which may apply. Although it took place as long ago as 1985, and many of its 31 Safety Recommendations have since been implemented, the UK AAIB Report into the Boeing 737-200 fire which occurred during and after a rejected takeoff at Manchester, UK is still one of the most comprehensive documentations of an actual cabin fire in which the primary hazard was smoke. See: B732, Manchester UK, 1985 (AW FIRE)