Loss of pressurisation is a potentially serious emergency in an aircraft flying at the normal cruising altitude for most jet passenger aircraft. Loss of cabin pressure, or depressurisation, is normally classified as explosive, rapid, or gradual based on the time interval over which cabin pressure is lost.
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.
Types of depressurization
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 licious act consequential on such as an explosive device on board.
Pressurisation system failure: Malfunction of some part of the pressurisation system such as an outflow valve.
Inadvertent system control input(s): Accidental or incorrect activation of a critical pressurisation control.
Deliberate Act: A drastic measure but one which an aircraft captain might consider, for example, as a way of clearing the cabin of smoke.
Published by: Flight Safety & Quality Assurance Division (OE)
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