An uncontrolled decompression is an undesired drop in the pressure of a sealed system, such as a pressurised aircraft cabin or hyperbaric chamber, that typically results from human error, structural failure, or impact, causing the pressurised vessel to vent into its surroundings or fail to pressurize at all. Such decompression may be classed as explosive, rapid, or slow:
Explosive decompression (ED) is violent and too fast for air to escape safely from the lungs and other air-filled cavities in the body such as the sinuses and eustachian tubes, typically resulting in severe to fatal barotrauma. Rapid decompression may be slow enough to allow cavities to vent but may still cause serious barotrauma or discomfort. Slow or gradual decompression occurs so slowly that it may not be sensed before hypoxia sets in.
Description
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. The US Federal Aviation Administration recognizes three distinct types of decompression events in aircraft: explosive, rapid, and gradual decompression.
Explosive decompression Explosive decompression occurs typically in less than 0.1 to 0.5 seconds, a change in cabin pressure faster than the lungs can decompress. Normally, the time required to release air from the lungs without restrictions, such as masks, is 0.2 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. Immediately after an explosive decompression, a heavy fog may fill the aircraft cabin as the air cools, raising the relative humidity and causing sudden condensation. Military pilots with oxygen masks must pressure-breathe, whereby the lungs fill with air when relaxed, and effort has to be exerted to expel the air again.
Rapid decompression 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.
Gradual 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 the cabin as an aircraft climbs to altitude. An example of this is the 2005 Helios Airways Flight 522 crash, in which the maintenance service left the pressurization system in manual mode and the pilots did not check the mode selected. As a result, they (as well as most of the passengers and crew) suffered a loss of consciousness due to hypoxia (lack of oxygen). The plane continued to fly on autopilot and eventually crashed due to fuel exhaustion.
Decompression injuries
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 Altitude sickness Frostbite or hypothermia from exposure to freezing cold air at high altitude Physical trauma caused by the violence of explosive decompression, which can turn people and loose objects into projectiles At least two confirmed cases have been documented of a person being sucked through an airplane passenger window. The first occurred in 1973 on National Airlines Flight 27 when debris from an engine failure struck a window roughly midway in the fuselage. Despite efforts to pull the passenger back into the airplane, the occupant was forced entirely through the cabin window. The second incident occurred on April 17, 2018, when a woman on Southwest Airlines Flight 1380 was partially sucked through an airplane passenger window that had broken from a similar engine failure. Although other passengers were able to pull her back inside, she later died from her injuries. In both incidents, the plane landed safely with the sole fatality being the person seated next to the window involved.
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