Depressurization of the aircraft cabin as a result of structural failure, pressurization system malfunction, an inadvertent crew action or a deliberate crew intervention.
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.
The composition of atmospheric air remains constant as air pressure reduces with increasing in altitude and since the partial pressure of oxygen also reduces, the absolute amount of oxygen available also reduces. The reduction in air pressure reduces the flow of oxygen across lung tissue and into the human bloodstream. A significant reduction in the normal concentration of oxygen in the bloodstream is called 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 are considered to have little effect on aircrew and healthy passengers but above that, the effect becomes progressively more pronounced. Above 20,000 ft, lack of oxygen leads to loss of intellectual ability followed by unconsciousness and eventually respiratory and heart failure. When suddenly deprived of normal levels of oxygen, estimates of the Time of Useful Consciousness are a pertinent guide – at 35,000 ft it is less than one minute. See the separate article on Hypoxia for more detailed information.
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 normally conducted in accordance with specific special procedures.
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 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.
Effects
Crew Incapacitation. Depending on the altitude of the aircraft when depressurization takes place, loss of pressurization 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 pressurization, it is essential that the flight crew don oxygen equipment as soon as possible. In the case of a deliberate depressurization, the crew should be on oxygen before the depressurization commences.
Emergency Descent. In the case of an uncontrolled depressurization, the crew will want to descend immediately to an altitude at which they and the passengers can breathe without supplementary oxygen – usually given as 10,000 feet amsl subject to adequate terrain clearance.
For further information see the articles Pressurization Problems: Guidance for Flight Crews and Emergency Depressurisation: Guidance for Controllers.
Accidents and Incidents Involving Loss of Pressurisation
B733, en-route, northwest of Athens Greece, 2005 (On 14 August 2005, a B737-300 aircraft belonging to Helios Airways, crashed near Grammatiko, Greece following the incapacitation of the crew due to Hypoxia)
LJ35, Aberdeen SD USA, 1999 (On October 25, 1999, about 1213 central daylight time (CDT), a Learjet Model 35, N47BA, operated by Sunjet Aviation, Inc., of Sanford, Florida, crashed near Aberdeen, South Dakota. The airplane departed Orlando, Florida, for Dallas, Texas, about 0920 eastern daylight time (EDT). Radio contact with the flight was lost north of Gainesville, Florida, after air traffic control (ATC) cleared the airplane to flight level (FL) 390. The airplane was intercepted by several U.S. Air Force (USAF) and Air National Guard (ANG) aircraft as it proceeded northwestbound. The military pilots in a position to observe the accident airplane at close range stated (in interviews or via radio transmissions) that the forward windshields of the Learjet seemed to be frosted or covered with condensation. The military pilots could not see into the cabin. They did not observe any structural anomaly or other unusual condition. The military pilots observed the airplane depart controlled flight and spiral to the ground, impacting an open field. All occupants on board the airplane (the captain, first officer, and four passengers) were killed, and the airplane was destroyed)
A320, en route, north of Marseilles France, 2013 (On 12 September 2013, pressurisation control failed in an A320 after a bleed air fault occurred following dispatch with one of the two pneumatic systems deactivated under MEL provisions. The Investigation found that the cause of the in-flight failure was addressed by an optional SB not yet incorporated. Also, relevant crew response SOPs lacking clarity and a delay in provision of a revised MEL procedure meant that use of the single system had not been optimal and after a necessary progressive descent to FL100 was delayed by inadequate ATC response, and ATC failure to respond to a PAN call required it to be upgraded to MAYDAY. )
B738, en-route, near Lugano Switzerland, 2012 (On 4 April 2012, the cabin pressurisation controller (CPC) on a Boeing 737-800 failed during the climb passing FL305 and automatic transfer to the alternate CPC was followed by a loss of cabin pressure control and rapid depressurisation because it had been inadvertently installed with the shipping plug fitted. An emergency descent and diversion followed. The subsequent Investigation attributed the failure to remove the shipping plug to procedural human error and the poor visibility of the installed plug. It was also found that “the pressurisation system ground test after CPC installation was not suitable to detect the error”.)
B744, en-route, South China Sea, 2008 (On 25 July 2008, a Boeing 747 suffered a rapid depressurisation of the cabin following the sudden failure of an oxygen cylinder, which had ruptured the aircraft’s pressure hull. The incident occurred 475 km north-west of Manila, Philippines.)
Source – https://www.skybrary.aero/