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Emergency Depressurisation
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| Category: | General | 
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| Content source: | SKYbrary | 
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| Content control: | EUROCONTROL | 
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Definition
Sudden and rapid depressurisation of the aircraft cabin as a result of structural failure, pressurisation system failure, or deliberate act of the crew.
Description
Air pressure reduces with increase in altitude and therefore the amount of oxygen in any given volume of air also reduces with increase in altitude. Furthermore, the reduction in pressure impedes the ability of oxygen to pass across lung tissues and into the human bloodstream. The condition whereby the concentration of useful oxygen in the bloodstream is reduced because of a decrease in atmospheric pressure is known as Hypoxia.
The degree to which an individual’s performance is affected by lack of oxygen varies depending on the altitude of the aircraft, and on personal factors such as the general health of the person and whether he/she is a smoker. Below 10,000 ft, the reduced levels of oxygen have little effect on most crew and passengers but the higher the aircraft is, the greater the impact of lack of oxygen. Above 20,000 ft, lack of oxygen leads to loss of intellectual ability followed by unconsciousness and eventually respiratory and heart failure. Importantly, the Time of Useful Consciousness reduces with altitude – at 35,000 ft the time of useful consciousness is less than one minute. See the separate article on Hypoxia for more detailed information.
The cabins of modern passenger aircraft are pressurised in order to create an environment which is physiologically suitable for humans (Aircraft Pressurisation Systems). Maintaining a pressure difference between the outside and the inside of the aircraft places stress on the structure of the aircraft. The higher the aircraft flies, the higher the pressure differential that needs to be maintained and the higher the stress on the aircraft structure. A compromise between structural design and physiological need is achieved on most aircraft by maintaining a maximum cabin altitude of 8,000 ft.
Loss of pressurisation is a serious emergency in an aircraft flying at the normal cruising altitude for most passenger aircraft.
Note that some military flights may involve deliberate depressurisation at high altitude for the purpose of dropping troops or equipment by parachute. Such flights are conducted in accordance with specific procedures and will usually be notified in advance.
Types
- Structural Failure. Failure of a window, door, or pressure bulkhead for example, or in-flight explosion. An in-flight explosion may be due to a system failure, dangerous cargo, or a malicious act such as an explosive device carried on board by a terrorist.
- Pressurisation system failure. Failure of some part of the pressurisation system such as an outflow valve perhaps.
- Deliberate Act A drastic measure but one which an aircraft captain might consider, for example, as a way of clearing the cabin of smoke.
Effects
- Crew Incapacitation. Depending on the altitude of the aircraft when depressurisation takes place, loss of pressurisation can very quickly lead to the incapacitation of the crew and passengers unless they receive supplementary oxygen.
Solutions
- Oxygen. In the event of loss of pressurisation, it is essential that the crew don Oxygen equipment as soon as possible. In the case of a deliberate depressurisation, the crew should be on oxygen before the depressurisation commences.
- Emergency Descent. In the case of an emergency depressurisation, the crew will want to descend immediately to an altitude at which they and the passengers can breathe without supplementary oxygen – conventionally 10,000 ft.
see the articles Emergency Depressurisation: Guidance for Flight Crews and Emergency Depressurisation: Guidance for Controllers.
This, and related articles, do not consider the aspects of aircraft design which reduce the risk of pressurisation failure, just the general principles and issues related to the safety of a flight once an explosive or rapid depressurisation has occurred.
Related Articles
Accidents and Incidents Involving Loss of Pressurisation
Further Reading
Airbus
ATSB