Uncontrolled decompression is an unplanned drop in the pressure of a sealed system, such as an aircraft cabin or hyperbaric chamber, and typically results from human error, material fatigue, engineering failure, or impact, causing a pressure vessel to vent into its lower-pressure surroundings or fail to pressurize at all.
Such decompression may be classed as Explosive, Rapid, or Slow:
The term uncontrolled decompression here refers to the unplanned depressurisation of vessels that are occupied by people; for example, a pressurised aircraft cabin at high altitude, a spacecraft, or a hyperbaric chamber. For the catastrophic failure of other pressure vessels used to contain gas, liquids, or reactants under pressure, the term explosion is more commonly used, or other specialised terms such as BLEVE may apply to particular situations.
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.
Explosive decompression occurs at a rate swifter 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 that can become projectiles because of the explosive force, which may be likened to a bomb detonation.
After an explosive decompression within an aircraft, a heavy fog may immediately fill the interior as the relative humidity of cabin air rapidly changes as the air cools and condenses. Military pilots with oxygen masks have to pressure-breathe, whereby the lungs fill with air when relaxed, and effort has to be exerted to expel the air again.
Rapid decompression typically takes more than 0.1 to 0.5 seconds, allowing the lungs to decompress more quickly than the cabin. The risk of lung damage is still present, but significantly reduced compared with explosive decompression.
Slow, or gradual, decompression occurs slowly enough to go unnoticed and might only be detected by instruments. This type of decompression may also come about from a failure to pressurize as an aircraft climbs to altitude. An example of this is the 2005 Helios Airways Flight 522 crash, in which the pilots failed to check the aircraft was pressurising automatically and then to react to the warnings that the aircraft was depressurising, eventually losing consciousness (along with most of the passengers and crew) from hypoxia.
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.
This persistent myth is based on a failure to distinguish between two types of decompression and their exaggerated portrayal in some fictional works. The first type of decompression deals with changing from normal atmospheric pressure (one atmosphere) to a vacuum (zero atmosphere) which is usually centered around space exploration. The second type of decompression changes from exceptionally high pressure (many atmospheres) to normal atmospheric pressure (one atmosphere) as what would be found from deep-sea diving.
The first type is more common as pressure reduction from normal atmospheric pressure to a vacuum can be found in both space exploration and high-altitudes aviation. Research and experience have shown that while exposure to a vacuum causes swelling, human skin is tough enough to withstand the drop of one atmosphere. One of the major issues of vacuum exposure is hypoxia, in which the body is starved of oxygen that leads to unconsciousness within a few seconds. Rapid uncontrolled decompression can be much more dangerous than vacuum exposure itself. Even if the victim does not hold their breath, venting through the windpipe may be too slow to prevent the fatal rupture of the delicate alveoli of the lungs.Eardrums and sinuses may be ruptured by rapid decompression, soft tissues may bruise and seep blood, and the stress of shock accelerates oxygen consumption, leading to hypoxia. At the extreme low pressures encountered at altitudes above about 63,000 feet (19,000 m), the boiling point of water becomes less than normal body temperature. This measure of altitude is known as the Armstrong limit, which is the practical limit to survivable altitude without pressurization. Fictional accounts of bodies exploding due to exposure from a vacuum include among others a character's death in the movie Total Recall, when he is exposed to the atmosphere of Mars.
The second type is rare since it involves a pressure drop over several atmospheres, which would require the person to have been placed in a pressure vessel. The only likely situation in which this might occur is during decompression after deep-sea diving. A pressure drop as small as 100 Torr (13 kPa), which produces no symptoms if it is gradual, may be fatal if it occurs suddenly. Such an incident occurred in 1983 in the North Sea, where violent explosive decompression from nine atmospheres to one caused four divers to die instantly from massive and lethal barotrauma. Fictional accounts of this include a scene from the film Licence to Kill, when a character's head explodes after his hyperbaric chamber is rapidly depressurized, and another in the film DeepStar Six, wherein rapid depressurization causes a character to hemorrhage profusely before exploding in a similar fashion.
In 2004, the TV show MythBusters examined if explosive decompression occurs when a bullet is fired through the fuselage of an airplane by informally using a pressurised aircraft and several scale tests. The results of these tests suggested that the fuselage design does not allow people to be blown out. Professional pilot David Lombardo states that a bullet hole would have no perceived effect on cabin pressure as the hole would be smaller than the opening of the aircraft's outflow valve.NASA scientist Geoffrey A. Landis points out though that the impact depends on the size of the hole, which can be expanded by debris that is blown through it. Landis went on to say that "it would take about 100 seconds for pressure to equalise through a roughly 30.0 cm (11.8 in) hole in the fuselage of a Boeing 747." He then stated that anyone sitting next to the hole would have half a ton of force pulling them in the direction of it. On April 17, 2018 a seat-belted woman on Southwest Airlines Flight 1380 was partially blown through an airplane window that had been broken due to debris from an engine failure. Although the other passengers were able to pull her back inside, she later died from her injuries.
The following physical injuries may be associated with decompression incidents:
Modern aircraft are specifically designed with longitudinal and circumferential reinforcing ribs in order to prevent localised damage from tearing the whole fuselage open during a decompression incident. However, decompression events have nevertheless proved fatal for aircraft in other ways. In 1974, explosive decompression onboard Turkish Airlines Flight 981 caused the floor to collapse, severing vital flight control cables in the process. The FAA issued an Airworthiness Directive the following year requiring manufacturers of wide-body aircraft to strengthen floors so that they could withstand the effects of in-flight decompression caused by an opening of up to 20 square feet (1.9 m2) in the lower deck cargo compartment. Manufacturers were able to comply with the Directive either by strengthening the floors and/or installing relief vents called "dado panels" between the passenger cabin and the cargo compartment.
Cabin doors are designed to make it nearly impossible to lose pressurization through opening a cabin door in flight, either accidentally or intentionally. The plug door design ensures that when the pressure inside the cabin exceeds the pressure outside the doors are forced shut and will not open until the pressure is equalised. Cabin doors, including the emergency exits, but not all cargo doors, open inwards, or must first be pulled inwards and then rotated before they can be pushed out through the door frame because at least one dimension of the door is larger than the door frame. Pressurization prevented the doors of Saudia Flight 163 from being opened on the ground after the aircraft made a successful emergency landing, resulting in the deaths of all 287 passengers and 14 crew members from fire and smoke.
Prior to 1996, approximately 6,000 large commercial transport airplanes were type certified to fly up to 45,000 feet (14,000 m), without being required to meet special conditions related to flight at high altitude. In 1996, the FAA adopted Amendment 25-87, which imposed additional high-altitude cabin-pressure specifications, for new designs of aircraft types. For aircraft certified to operate above 25,000 feet (FL 250; 7,600 m), it "must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of 15,000 feet (4,600 m) after any probable failure condition in the pressurization system." In the event of a decompression which results from "any failure condition not shown to be extremely improbable," the aircraft must be designed so that occupants will not be exposed to a cabin altitude exceeding 25,000 feet (7,600 m) for more than 2 minutes, nor exceeding an altitude of 40,000 feet (12,000 m) at any time. In practice, that new FAR amendment imposes an operational ceiling of 40,000 feet on the majority of newly designed commercial aircraft.[Note 1]
In 2004, Airbus successfully petitioned the FAA to allow cabin pressure of the A380 to reach 43,000 feet (13,000 m) in the event of a decompression incident and to exceed 40,000 feet (12,000 m) for one minute. This special exemption allows the A380 to operate at a higher altitude than other newly designed civilian aircraft, which have not yet been granted a similar exemption.
The Depressurization Exposure Integral (DEI) is a quantitative model that is used by the FAA to enforce compliance with decompression-related design directives. The model relies on the fact that the pressure that the subject is exposed to and the duration of that exposure are the two most important variables at play in a decompression event.
Other national and international standards for explosive decompression testing include:
Decompression incidents are not uncommon on military and civilian aircraft, with approximately 40-50 rapid decompression events occurring worldwide annually. However, in most cases the problem is manageable, injuries or structural damage rare and the incident not considered notable.  One notable, recent case was Southwest Airlines Flight 1380 in 2018, where an uncontained engine failure ruptured a window, causing a passenger to be partially blown out.
Decompression incidents do not occur solely in aircraft; the Byford Dolphin accident is an example of violent explosive decompression of a saturation diving system on an oil rig. A decompression event is an effect of a failure caused by another problem (such as an explosion or mid-air collision), but the decompression event may worsen the initial issue.
|Event||Date||Pressure vessel||Event type||Fatalities/number on board||Decompression type||Cause|
|BOAC Flight 781||1954||de Havilland Comet 1||Accident||35/35||Explosive decompression||Metal fatigue|
|South African Airways Flight 201||1954||de Havilland Comet 1||Accident||21/21||Explosive decompression||Metal fatigue|
|TWA Flight 2||1956||Lockheed L-1049 Super Constellation||Accident||70/70||Explosive decompression||Mid-air collision|
|1961 Yuba City B-52 crash||1961||Boeing B-52 Stratofortress||Accident||0/8||Gradual or rapid decompression||(Undetermined)|
|Continental Airlines Flight 11||1962||Boeing 707-100||Terrorist bombing||45/45||Explosive decompression||Bomb explosion in passenger cabin|
|Volsk parachute jump accident||1962||Pressure suit||Accident||1/1||Rapid decompression||Collision with gondola upon jumping from balloon|
|Strato Jump III||1966||Pressure suit||Accident||1/1||Rapid decompression||Pressure suit failure|
|Apollo program spacesuit testing accident||1966||Apollo A7L spacesuit (or possibly a prototype of it)||Accident||0/1||Rapid decompression||Oxygen line coupling failure|
|Hughes Airwest Flight 706||1971||McDonnell Douglas DC-9-31||Accident||49/49||Explosive decompression||Mid-air collision|
|Soyuz 11 re-entry||1971||Soyuz spacecraft||Accident||3/3||Rapid decompression||Pressure equalisation valve damaged by faulty pyrotechnic separation charges|
|BEA Flight 706||1971||Vickers Vanguard||Accident||63/63||Explosive decompression||Structural failure of rear pressure bulkhead due to corrosion|
|LANSA Flight 508||1971||Lockheed L-188A Electra||Accident||91/92||Explosive decompression||Lightning strike and fuel vapour fire, leading to separation of right wing|
|JAT Flight 367||1972||McDonnell Douglas DC-9-32||Terrorist bombing||27/28||Explosive decompression||Bomb explosion in cargo hold|
|American Airlines Flight 96||1972||Douglas DC-10-10||Accident||0/67||Rapid decompression||Cargo door failure|
|National Airlines Flight 27||1973||Douglas DC-10-10||Accident||1/116||Explosive decompression||Uncontained engine failure|
|Turkish Airlines Flight 981||1974||Douglas DC-10-10||Accident||346/346||Explosive decompression||Cargo door failure|
|TWA Flight 841||1974||Boeing 707-331B||Terrorist bombing||88/88||Explosive decompression||Bomb explosion in cargo hold|
|1975 Tân S?n Nh?t C-5 accident||1975||Lockheed C-5 Galaxy||Accident||155/330||Explosive decompression||Improper maintenance of rear doors, cargo door failure|
|British Airways Flight 476||1976||Hawker Siddeley Trident 3B||Accident||63/63||Explosive decompression||Mid-air collision|
|Korean Air Lines Flight 902||1978||Boeing 707-320B||Shootdown||2/109||Explosive decompression||Shootdown after straying into prohibited airspace over the Soviet Union|
|Itavia Flight 870||1980||McDonnell Douglas DC-9-15||(Disputed)||81/81||Explosive decompression||In-flight breakup due to explosion (cause of explosion still disputed)|
|Saudia Flight 162||1980||Lockheed L-1011 TriStar||Accident||2/292||Explosive decompression||Tyre blowout|
|Far Eastern Air Transport Flight 103||1981||Boeing 737-222||Accident||110/110||Explosive decompression||Severe corrosion and metal fatigue|
|British Airways Flight 9||1982||Boeing 747-200||Accident||0/263||Gradual decompression||Engine flameout due to volcanic ash ingestion|
|Reeve Aleutian Airways Flight 8||1983||Lockheed L-188 Electra||Accident||0/15||Rapid decompression||Propeller failure and collision with fuselage|
|Korean Air Lines Flight 007||1983||Boeing 747-200B||Shootdown||269/269||Initially rapid decompression, then explosive||Intentionally fired air-to-air missile after aircraft strayed into prohibited airspace over the Soviet Union|
|Gulf Air Flight 771||1983||Boeing 737-200||Terrorist bombing||112/112||Explosive decompression||Bomb explosion in cargo hold|
|Byford Dolphin accident||1983||Diving bell||Accident||5/6||Explosive decompression||Human error, no fail-safe in the design|
|Japan Airlines Flight 123||1985||Boeing 747SR||Accident||520/524||Explosive decompression||Structural failure of rear pressure bulkhead due to metal fatigue|
|Air India Flight 182||1985||Boeing 747-200B||Terrorist bombing||329/329||Explosive decompression||Bomb explosion in cargo hold|
|Space Shuttle Challenger disaster||1986||Space Shuttle Challenger||Accident||7/7||Gradual or rapid decompression||Breach in solid rocket booster O-ring, leading to damage from escaping superheated gas and eventual disintegration of launch vehicle|
|Pan Am Flight 125||1987||Boeing 747-121||Incident||0/245||Rapid decompression||Cargo door malfunction|
|LOT Polish Airlines Flight 5055||1987||Ilyushin Il-62M||Accident||183/183||Rapid decompression||Uncontained engine failure|
|South African Airways Flight 295||1987||Boeing 747-200M||Accident||159/159||Explosive decompression||In-flight fire|
|Aloha Airlines Flight 243||1988||Boeing 737-200||Accident||1/95||Explosive decompression||Metal fatigue|
|Iran Air Flight 655||1988||Airbus A300B2-203||Shootdown||290/290||Explosive decompression||Intentionally fired surface-to-air missiles from the USS Vincennes|
|Partnair Flight 394||1988||Convair CV-580||Accident||55/55||Explosive decompression||Rudder failure leading to loss of control and mid-air breakup|
|Pan Am Flight 103||1988||Boeing 747-100||Terrorist bombing||259/259||Explosive decompression||Bomb explosion in cargo hold|
|United Airlines Flight 811||1989||Boeing 747-122||Accident||9/355||Explosive decompression||Cargo door failure|
|UTA Flight 772||1989||Douglas DC-10-30||Terrorist bombing||170/170||Explosive decompression||Bomb explosion in cargo hold|
|Avianca Flight 203||1989||Boeing 727-21||Terrorist bombing||107/107||Explosive decompression||Bomb explosion igniting vapours in an empty fuel tank|
|British Airways Flight 5390||1990||BAC One-Eleven||Incident||0/87||Rapid decompression||Cockpit windscreen failure|
|Lauda Air Flight 004||1991||Boeing 767-300ER||Accident||223/223||Explosive decompression||Uncommanded thrust reverser deployment leading to loss of control and mid-air breakup|
|Copa Airlines Flight 201||1992||Boeing 737-200||Accident||47/47||Explosive decompression||Mid-air breakup|
|Libyan Arab Airlines Flight 1103||1992||Boeing 727-200||(Disputed)||157/157||Explosive decompression||Mid air collision (cause of collision disputed)|
|China Northwest Airlines Flight 2303||1994||Tupolev TU-154M||Accident||160/160||Explosive decompression||Improper maintenance|
|TWA Flight 800||1996||Boeing 747-100||Accident||230/230||Explosive decompression||Vapour explosion in fuel tank|
|Saudia Flight 763||1996||Boeing 747-100B||Accident||312/312||Explosive decompression||Mid-air collision|
|Progress M-34 docking test||1997||Spektr space station module||Accident||0/3||Rapid decompression||Collision while in orbit|
|SilkAir Flight 185||1997||Boeing 737-300||(Disputed)||104/104||Explosive decompression||Steep dive and mid-air breakup (cause of crash unknown and disputed)|
|Lionair Flight 602||1998||Antonov An-24RV||Shootdown||55/55||Rapid decompression||Probable MANPAD shootdown|
|China Southwest Airlines Flight 4509||1999||Tupolev TU-154M||Accident||61/61||Explosive decompression||Improper maintenance|
|1999 South Dakota Learjet crash||1999||Learjet 35||Accident||6/6||Gradual or rapid decompression||(Undetermined)|
|EgyptAir Flight 990||1999||Boeing 767-300ER||(Disputed)||217/217||Explosive decompression||Mid-air breakup (cause of crash unknown and disputed)|
|Australia "Ghost Flight"||2000||Beechcraft Super King Air||Accident||8/8||Gradual decompression||Inconclusive; likely pilot error or mechanical failure|
|Hainan Island incident||2001||Lockheed EP-3||Accident||0/24||Rapid decompression||Mid-air collision|
|Air Transat Flight 236||2001||Airbus A330-243||Accident||0/306||Gradual decompression||Engine flameout due to fuel exhaustion resulting from ruptured fuel line|
|TAM Airlines Flight 9755||2001||Fokker 100||Accident||1/82||Rapid decompression||Uncontained engine failure|
|China Airlines Flight 611||2002||Boeing 747-200B||Accident||225/225||Explosive decompression||Metal fatigue|
|Bashkirian Airlines Flight 2937||2002||Tupolev Tu-154M||Accident||69/69||Explosive decompression||Mid-air collision|
|Space Shuttle Columbia disaster||2003||Space Shuttle Columbia||Accident||7/7||Rapid decompression||Damage to orbiter thermal protection system at liftoff, leading to disintegration during reentry|
|Helios Airways Flight 522||2005||Boeing 737-300||Accident||121/121||Gradual decompression||Pressurization system set to manual for the entire flight|
|Alaska Airlines Flight 536||2005||McDonnell Douglas MD-80||Incident||0/142||Rapid decompression||Failure of operator to report collision involving a baggage loading cart at the departure gate|
|Gol Transportes Aéreos Flight 1907||2006||Boeing 737-800||Accident||154/154||Explosive decompression||Mid-air collision|
|Adam Air Flight 574||2007||Boeing 737-400||Accident||102/102||Explosive decompression||Mid-air breakup|
|Qantas Flight 30||2008||Boeing 747-400||Incident||0/365||Rapid decompression||Fuselage ruptured by oxygen cylinder explosion|
|Southwest Airlines Flight 2294||2009||Boeing 737-300||Incident||0/131||Rapid decompression||Metal fatigue|
|Southwest Airlines Flight 812||2011||Boeing 737-300||Incident||0/123||Rapid decompression||Metal fatigue|
|Malaysia Airlines Flight 17||2014||Boeing 777-200ER||Shootdown||298/298||Explosive decompression||Shot down by a Buk surface-to-air missile launcher; under criminal investigation|
|Metrojet Flight 9268||2015||Airbus A321-231||Terrorist bombing||224/224||Explosive decompression||Bomb explosion caused disintegration of aircraft at flight altitude; under criminal investigation|
|Daallo Airlines Flight 159||2016||Airbus A321||Terrorist bombing||1/81||Explosive decompression||Bomb explosion in passenger cabin|
|Southwest Airlines Flight 1380||2018||Boeing 737-700||Accident||1/148||Rapid decompression||Uncontained engine failure; under investigation|
The 51-L Challenger accident investigation showed that the Challenger CM remained intact and the crew was able to take some immediate actions after vehicle breakup, although the loads experienced were much higher as a result of the aerodynamic loads (estimated at 16 G to 21 G).5 The Challenger crew became incapacitated quickly and could not complete activation of all breathing air systems, leading to the conclusion that an incapacitating cabin depressurization occurred. By comparison, the Columbia crew experienced lower loads (~3.5 G) at the CE. The fact that none of the crew members lowered their visors strongly suggests that the crew was incapacitated after the CE by a rapid depressurization. Although no quantitative conclusion can be made regarding the cabin depressurization rate, it is probable that the cabin depressurization rate was high enough to incapacitate the crew in a matter of seconds. Conclusion L1-5. The depressurization incapacitated the crew members so rapidly that they were not able to lower their helmet visors.