Generally uncontrolled decompression results from human error, material fatigue, engineering failure or impact, that causes a pressure vessel either not to pressurize, or to vent into lower-pressure surroundings.
Decompression can occur due to structural failure of the pressure vessel, or failure of the compression system itself. The speed and violence of the decompression is affected by the size of the pressure vessel, the differential pressure between the inside and outside of the vessel and the size of the leak hole.
The Federal Aviation Administration recognises three distinct types of decompression events in aircraft:
Explosive decompression occurs at a rate faster than that at which air can escape from the lungs, typically in less than 0.1 to 0.5 seconds. The risk of lung trauma is very high, as is the danger from any unsecured objects which can become projectiles due to the explosive force.
Paul Withey, an aviation expert, described an explosive decompression inside an aircraft cabin as similar to the explosion of a 500 pound (225 kilogram) bomb inside the cabin.
Seals in high-pressure vessels are also susceptible to explosive decompression; the O-rings or rubber gaskets used to seal pressurised pipelines tend to become saturated with high-pressure gases. If the pressure inside the vessel is suddenly released, then the gases within the rubber gasket may expand violently, causing blistering or explosion of the material. For this reason, it is common for military and industrial equipment to be subjected to an explosive decompression test before it is certified as safe for use.
Some believe that if a bullet is shot through the hull of an airplane, it will explosively decompress outwards, sucking chairs, baggage and people out of the hole. Using a high-pressure airplane and several scale tests, the television program, Mythbusters, demonstrated that fuselage design does not allow this to happen.
Cause BOAC Flight 781
de Havilland Comet
Metal fatigue South African Airways Flight 201
de Havilland Comet
Metal fatigue Soyuz 11 re-entry
Damaged cabin ventilation valve American Airlines Flight 96
Cargo door failure Turkish Airlines Flight 981
Cargo door failure Byford Dolphin accident
Human error, no fail-safe in the design Korean Air Lines Flight 007
Intentionally fired air-to-air missile after aircraft strayed into prohibited airspace Japan Airlines Flight 123
Structural failure of rear pressure bulkhead Aloha Airlines Flight 243
Metal fatigue United Airlines Flight 811
Cargo door failure British Airways Flight 5390
Windscreen failure Lionair Flight LN 602
Probable MANPAD shootdown South Dakota Learjet
Gradual or rapid decompression
(Undetermined) China Airlines Flight 611
Metal fatigue Helios Airways Flight 522
Automatic pressurization system disabled (suspected) Qantas Flight 30
Oxygen cylinder explosion
The FAA imposes a regulation on aircraft operators, stating that cabin pressure altitude may not exceed 25,000 feet for more than 2 minutes after a decompression event, typically as a result of an uncontained engine failure. Furthermore, a second ruling states that cabin pressure may at no time following a decompression event exceed 40,000 feet. In effect, these rulings have historically limited civilian aircraft to a maximum operating altitude of 40,000 feet. In 2004, Airbus successfully petitioned the FAA to allow cabin pressure of the A380 to reach 43,000 feet in the event of a decompression incident, and to exceed 40,000 feet for one minute. This special exemption allows the new aircraft to operate at a higher altitude than other civilian aircraft.
Other national and international standards for explosive decompression testing include:
Taking the pressure off: high-pressure air has long been a thorn in the side of engineers, but a new air valve design aims to get rid of the woes associated with 40 bar and higher specs. (In focus: valves & pumps).
Sep 01, 2002; Controlling high-pressure compressed air--40 bar and higher--has always been a nemesis for machine builders and valve...