Uncontrolled decompression

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Uncontrolled decompression is an unplanned drop in the pressure of a sealed system, such as an aircraft cabin, 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:

  • Explosive decompression (ED) is violent, the decompression being too fast for air to safely escape from the lungs.
  • Rapid decompression, while still fast, is slow enough to allow the lungs to vent.
  • Slow or gradual decompression occurs so slowly that it may not be sensed before hypoxia sets in.

Description[edit]

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.[1][2] 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 recognizes three distinct types of decompression events in aircraft:[1][2]

  • Explosive decompression
  • Rapid decompression
  • Gradual decompression

Explosive decompression[edit]

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.[1][3] 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.[4]

Rapid decompression[edit]

Rapid decompression typically takes more than 0.1 to 0.5 seconds, allowing the lungs to decompress more quickly than the cabin.[1][5] The risk of lung damage is still present, but significantly reduced compared with explosive decompression.

Gradual decompression[edit]

Slow, or gradual, decompression occurs slowly enough to go unnoticed and might only be detected by instruments.[1] 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.

Pressure vessel seals and testing[edit]

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.

Myths[edit]

Exposure to a vacuum causes the body to explode[edit]

This persistent myth is based on a failure to distinguish between two types of decompression: the first, from normal atmospheric pressure (one atmosphere) to a vacuum (zero atmospheres); the second, from an exceptionally high pressure (many atmospheres) to normal atmospheric pressure.

The first type, a sudden change from normal atmospheric pressure to a vacuum, is the more common. Research and experience in space exploration and high-altitude aviation have shown that while exposure to vacuum causes swelling, human skin is tough enough to withstand the drop of one atmosphere, although the resulting hypoxia will cause unconsciousness after a few seconds.[6][7] It is also possible that pulmonary barotrauma (lung rupture) will occur if the breath is forcibly held.

The second type is rare, since the only normal situation in which it can occur is during decompression after deep-sea diving. In fact, there is only a single well-documented occurrence: the 1983 Byford Dolphin incident in the North Sea, in which a catastrophic pressure drop of eight atmospheres, from nine atmospheres to one atmosphere instantaneously, caused massive and lethal barotrauma, including the actual explosion of one diver. A similar but fictional death was shown in the James Bond film Licence to Kill, when a character's head explodes after his hyperbaric chamber is rapidly depressurized. Neither of these incidents would have been possible if the pressure drop had been only from normal atmosphere to a vacuum.

Bullets cause explosive decompression[edit]

Aircraft fuselages are designed with ribs to prevent tearing; the size of the hole is one of the factors that determines the speed of decompression, and a bullet hole is too small to cause rapid or explosive decompression.

A small hole will blow people out of a fuselage[edit]

The television program Mythbusters examined this belief informally using a pressurised aircraft and several scale tests. The Mythbusters approximations suggested that fuselage design does not allow this to happen.

Flight Attendant C.B. Lansing was blown from Aloha Airlines Flight 243 in 1988 when a large section of cabin roof (about 18 by 25 feet (5.5 m × 7.6 m)) detached; the report states she was swept overboard rather than blown through the resulting hole. The Air Crash Investigation documentary report on Flight 243 (season 3, 2005) notes that the 'tear line' construction of the aircraft was supposed to prevent such a large slab failure. Working from passenger accounts (including one report of the hostess' legs disappearing through the roof), forensic evidence including NTSB photographs, and stress calculations,[8] experts speculated that the air hostess was blown against the foot-square hole initially permitted by the tear strips, blocking it: this would have caused a 10 atmosphere pressure spike, hence the much greater material failure.[9] One corrosion engineer takes the view that the tear straps could also have been defeated by the airstream impact through Lansing's body.[10]

Decompression injuries[edit]

NASA candidate Astronauts being monitored for signs of hypoxia during training in an altitude chamber.

The following physical injuries may be associated with decompression incidents:

Implications for aircraft design[edit]

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.[17] 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.[18] 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.[19]

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 apparently 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 287 passengers from fire.

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.[20] In 1996, the FAA adopted Amendment 25-87, which imposed additional high-altitude cabin-pressure specifications, for new designs of aircraft types.[21] 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."[22] 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.[22] In practice, that new FAR amendment imposes an operational ceiling of 40,000 feet on the majority of newly designed commercial aircraft.[23][24][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 that new aircraft to operate at a higher altitude than other newly designed civilian aircraft, which have not yet been granted a similar exemption.[23]

International standards[edit]

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.[25]

Other national and international standards for explosive decompression testing include:

Notable decompression accidents and incidents[edit]

Decompression incidents are not uncommon on military and civilian aircraft, with approximately 40–50 rapid decompression events occurring worldwide annually.[26] In the majority of cases the problem is relatively manageable for aircrew.[11] Consequently where passengers and the aircraft do not suffer any ill-effects, the incidents tend not to be considered notable.[11] Injuries resulting from decompression incidents are rare.[11]

Decompression incidents do not occur solely in aircraft—the Byford Dolphin incident is an example of violent explosive decompression 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[27] 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 B-52 Stratofortress Accident 0/8 Gradual or rapid decompression (Undetermined)
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[28]
Apollo program spacesuit testing accident 1966 Apollo A7L spacesuit (or possibly a prototype of it) Accident 0/1 Rapid decompression Oxygen line coupling failure[29]
Soyuz 11 re-entry 1971 Soyuz spacecraft Accident 3/3 Rapid decompression Pressure equalisation valve damaged by faulty pyrotechnic separation charges[30]
BEA Flight 706 1971 Vickers Vanguard Accident 63/63 Explosive decompression Structural failure of rear pressure bulkhead, leading to separation of horizontal stabilizer
American Airlines Flight 96 1972 Douglas DC-10-10 Accident 0/67 Rapid decompression[31] Cargo door failure
National Airlines Flight 27 1973 Douglas DC-10-10 Accident 1/116 Explosive decompression[32] Uncontained engine failure
Turkish Airlines Flight 981 1974 Douglas DC-10-10 Accident 346/346 Explosive decompression[33] Cargo door failure
Tan Son Nhut C-5 accident 1975 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 Shootdown 2/109 Explosive decompression Shootdown after straying into prohibited airspace over the Soviet Union
Saudia Flight 162 1980 Lockheed L-1011 TriStar Accident 2/292 Explosive decompression Tire blowout
Far Eastern Air Transport Flight 103 1981 Boeing 737 Accident 110/110 Explosive decompression Corrosion
Byford Dolphin accident 1983 Diving bell Accident 5/6 Explosive decompression Human error, no fail-safe in the design
Korean Air Lines Flight 007 1983 Boeing 747-230B Shootdown 269/269 Rapid decompression[34][35] Intentionally fired air-to-air missile after aircraft strayed into prohibited airspace over the Soviet Union[36]
Japan Airlines Flight 123 1985 Boeing 747-146SR Accident 520/524 Explosive decompression Structural failure of rear pressure bulkhead
Air India Flight 182 1985 Boeing 747-237B Terrorist bombing 329/329 Explosive decompression Bomb explosion in cargo hold
1985 Alia incident 1985 Lockheed L-1011 TriStar Incident 0/? Rapid decompression In-flight fire which burned though the rear pressure bulkhead[37]
LOT Flight 5055 1987 Ilyushin Il-62M Accident 183/183 Rapid decompression Engine turbine failure
Aloha Airlines Flight 243 1988 Boeing 737-297 Accident 1/95 Explosive decompression[38] Metal fatigue
Iran Air Flight 655 1988 Airbus A300B2 Shootdown 290/290 Explosive decompression Intentionally fired surface-to-air missiles from the USS Vincennes
Pan Am Flight 103 1988 Boeing 747-121 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
British Airways Flight 5390 1990 BAC One-Eleven Incident 0/87 Rapid decompression[39] Cockpit windscreen failure
TWA Flight 800 1996 Boeing 747-131 Accident 230/230 Explosive decompression Vapour explosion in fuel tank
Progress M-34 docking test 1997 Spektr space station module Accident 0/3 Rapid decompression Collision while in orbit
Lionair Flight LN 602 1998 Antonov An-24RV Shootdown 55/55 Rapid decompression Probable MANPAD shootdown
1999 South Dakota Learjet crash 1999 Learjet 35 Accident 6/6 Gradual or rapid decompression (Undetermined)
Australia “Ghost Flight” 2000 Beechcraft Super King Air Accident 8/8 Decompression suspected (Undetermined)
Hainan Island incident 2001 Lockheed EP-3 Accident 0/24 Rapid decompression Mid-air collision
TAM Airlines Flight 9755 2001 Fokker 100 Accident 1/82 Rapid decompression Window ruptured by shrapnel after engine failure[40]
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
Helios Airways Flight 522 2005 Boeing 737-31S Accident 121/121 Gradual decompression Pressurization system set to manual for the entire flight[41]
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[42]
Qantas Flight 30 2008 Boeing 747-438 Incident 0/365 Rapid decompression[43] Fuselage ruptured by explosion of an oxygen cylinder
Southwest Airlines Flight 2294 2009 Boeing 737-300 Incident 0/131 Rapid decompression Metal fatigue[44]
Southwest Airlines Flight 812 2011 Boeing 737-300 Incident 0/123 Rapid decompression Metal fatigue[45]
Allegiant Air Flight 683 2014 McDonnell Douglas MD-83 Incident 0/159 Rapid decompression Broken seal (under investigation)[46]
2014 SOCATA TBM crash 2014 SOCATA TBM-900 Accident 2/2 Decompression suspected
Malaysia Airlines Flight 17 2014 Boeing 777 Shootdown 298/298 Explosive decompression Hit by "high-energy objects"

See also[edit]

Notes[edit]

  1. ^ Notable exceptions include the Airbus A380, Boeing 787, and Concorde

References[edit]

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  10. ^ "Revisiting Aloha Airline Flight 243" (PDF). School of chemical and materials engineering, National University of Science and technology, Pakistan. pp. 26 et seq. 
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  42. ^ http://investigativereportingworkshop.org/flying-cheap/incident/20051229X02026/1/
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External links[edit]