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.
Accidents and Incidents
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.
Other events where cabin air contamination was experienced:
On 15 December 2019, an Airbus A330-200 turned back to Sydney shortly after departure when a major hydraulic system leak was annunciated. The return was uneventful until engine shutdown after clearing the runway following which APU use for air conditioning was followed by a gradual build up of hydraulic haze and fumes which eventually prompted an emergency evacuation. The Investigation found that fluid leaking from ruptured rudder servo hose had entered the APU air intake. The resulting evacuation was found to have been somewhat disorganised with this being attributed mainly to a combination of inadequate cabin crew procedures and training.
On 5 August 2019, an Airbus A321 crew declared a MAYDAY immediately after clearing the landing runway at Valencia when a hold smoke warning was annunciated. An emergency evacuation was completed without injuries. This warning followed “white smoke” from the air conditioning system entering both the passenger cabin and flight deck in the four minutes before landing which had prompted the pilots to don oxygen masks. The Investigation found the white smoke was the direct consequence of an oil leak from the right engine as a result of the misalignment and breakage of a bearing and its associated hydraulic seal.
On 6 March 2018, smoke was detected coming from flight deck and passenger cabin air conditioning vents of an en-route Bombardier DHC8-400. A MAYDAY was declared to ATC but the prescribed response effectively cleared the smoke and no emergency evacuation on landing was deemed necessary. The Investigation found that the smoke was caused by oil leaking into the air conditioning system due to a failed right hand engine seal. The operator subsequently began to implement a recommended engine modification and adopt a system provided by the engine manufacturer to proactively detect such oil leaks before air conditioning systems are contaminated.
On 28 February 2019, an Embraer E195 abandoned takeoff from Exeter when fight deck fumes/smoke accompanied thrust applied against the brakes. When informed of similar conditions in the cabin, the Captain ordered an emergency evacuation. Some passengers using the overwing exits re-entered the cabin after becoming confused as to how to leave the wing. The Investigation attributed the fumes to an incorrectly-performed engine compressor wash arising in a context of poorly-managed maintenance and concluded that guidance on overwing exit use had been inadequate and that the 1.8 metre certification height limit for exits without evacuation slides should be reduced.
On 3 August 2018, smoke appeared and began to intensify in the passenger cabin but not the flight deck of an Airbus A319 taxiing for departure at Helsinki. Cabin crew notified the Captain who stopped the aircraft and sanctioned an emergency evacuation. This then commenced whilst the engines were still running and inadequate instructions to passengers resulted in a completely disorderly evacuation. The Investigation attributed this to inadequate crew procedures which only envisaged an evacuation ordered by the Captain for reasons they were directly aware of and not a situation where the evacuation need was only obvious in the cabin.