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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.
The term uncontrolled decompression here refers to the unplanned depressurisation of vessels that are occupied by people. For example, an 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
- Rapid decompression
- Gradual decompression
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 Helios Airways Flight 522 crash, in which the pilots failed to check the aircraft was pressurising automatically and then react to the warnings that the aircraft was depressurising, eventually both losing consciousness (as well as most of the passengers and crew) from hypoxia.
Pressure vessel seals and testing
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.
Exposure to a vacuum causes the body to explode
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. 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 Byford Dolphin incident, in which a catastrophic pressure drop of eight atmospheres caused massive, lethal, barotrauma, including the actual explosion of one diver. A similar but fictional death was shown in the 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
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
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 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 hole. The Air Crash Investigation documentary report on Flight 243 (season 3, 2005) notes that the 'tear line' construction 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, 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. One corrosion engineer takes the view that the tear straps could also have been defeated by the airstream impact through Lansing's body.
The following physical injuries may be associated with decompression incidents:
- Hypoxia is the most serious risk associated with decompression, especially as it may go undetected or incapacitate the aircrew.
- Barotrauma: an inability to equalize pressure in internal air spaces such as the middle ear or gastrointestinal tract, or more serious injury such as a burst lung.
- Decompression sickness.
- Physical trauma caused by the violence of explosive decompression, which can turn people and loose objects into projectiles.
- Altitude sickness
- Frostbite or hypothermia from exposure to freezing cold air at high altitude.