Uncontrolled decompression


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.
The US Federal Aviation Administration recognizes three distinct types of decompression events in aircraft:
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

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 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 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.

Myths

Exposure to a vacuum causes the body to explode

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 to a vacuum which is usually centered around space exploration. The second type of decompression changes from exceptionally high pressure to normal atmospheric pressure 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. The most serious risk from 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 also be ruptured by rapid decompression, and soft tissues may be impacted by bruises seeping blood. If the victim somehow survives, the stress and shock would accelerate oxygen consumption leading to hypoxia at a rapid rate. At the extreme low pressures encountered at altitudes above about, 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, 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. Dramatized 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.

Small hole will blow people out of a fuselage

In 2004, the TV show MythBusters examined if explosive decompression occurs when a bullet is fired through the fuselage of an airplane informally by way of several tests using a decommissioned pressurised DC-9. A single shot through the side or the window did not have any effect – it took actual explosives to cause explosive decompression – suggesting that the fuselage is designed to prevent people from being 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 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.
At least two confirmed cases have been documented of a person being blown through an airplane passenger window. The first occurred in 1973 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 passenger's skeletal remains were eventually found by a construction crew, and were positively identified two years later. The second incident occurred on April 17, 2018 when a woman on Southwest Airlines Flight 1380 was partially blown through an airplane passenger window that had broken from a similar engine failure. Although the 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. Fictional accounts of this include a scene in Goldfinger, when James Bond kills the eponymous villain by blowing him out a passenger window.

Decompression 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 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, 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, it "must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of 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 for more than 2 minutes, nor exceeding an altitude of 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.
In 2004, Airbus successfully petitioned the FAA to allow cabin pressure of the A380 to reach in the event of a decompression incident and to exceed 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.

International standards

The Depressurization Exposure Integral 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, but the decompression event may worsen the initial issue.
EventDatePressure vesselEvent typeFatalities/number on boardDecompression typeCause
BOAC Flight 7811954de Havilland Comet 1Accident35/35Explosive decompressionMetal fatigue
South African Airways Flight 2011954de Havilland Comet 1Accident21/21Explosive decompressionMetal fatigue
TWA Flight 21956Lockheed L-1049 Super ConstellationAccident70/70Explosive decompressionMid-air collision
Continental Airlines Flight 111962Boeing 707-100Terrorist bombing45/45Explosive decompressionBomb explosion in passenger cabin
Volsk parachute jump accident1962Pressure suitAccident1/1Rapid decompressionCollision with gondola upon jumping from balloon
Strato Jump III1966Pressure suitAccident1/1Rapid decompressionPressure suit failure
Apollo program spacesuit testing accident1966Apollo A7L spacesuit Accident0/1Rapid decompressionOxygen line coupling failure
Soyuz 11 re-entry1971Soyuz spacecraftAccident3/3Rapid decompressionPressure equalisation valve damaged by faulty pyrotechnic separation charges
BEA Flight 7061971Vickers VanguardAccident63/63Explosive decompressionStructural failure of rear pressure bulkhead due to corrosion
JAT Flight 3671972McDonnell Douglas DC-9-32Terrorist bombing27/28Explosive decompressionBomb explosion in cargo hold
American Airlines Flight 961972Douglas DC-10-10Accident0/67Rapid decompressionCargo door failure
National Airlines Flight 271973Douglas DC-10-10Accident1/128Explosive decompressionUncontained engine failure
Turkish Airlines Flight 9811974Douglas DC-10-10Accident346/346Explosive decompressionCargo door failure
TWA Flight 8411974Boeing 707-331BTerrorist bombing88/88Explosive decompressionBomb explosion in cargo hold
1975 Tân Sơn Nhứt C-5 accident1975Lockheed C-5 GalaxyAccident155/330Explosive decompressionImproper maintenance of rear doors, cargo door failure
British Airways Flight 4761976Hawker Siddeley Trident 3BAccident63/63Explosive decompressionMid-air collision
Korean Air Lines Flight 9021978Boeing 707-320BShootdown2/109Explosive decompressionShootdown after straying into prohibited airspace over the Soviet Union
Saudia Flight 1621980Lockheed L-1011 TriStarAccident2/292Explosive decompressionTyre blowout
Far Eastern Air Transport Flight 1031981Boeing 737-222Accident110/110Explosive decompressionSevere corrosion and metal fatigue
British Airways Flight 91982Boeing 747-200Accident0/263Gradual decompressionEngine flameout due to volcanic ash ingestion
Reeve Aleutian Airways Flight 81983Lockheed L-188 ElectraAccident0/15Rapid decompressionPropeller failure and collision with fuselage
Korean Air Lines Flight 0071983Boeing 747-200BShootdown269/269Rapid decompressionIntentionally fired air-to-air missile after aircraft strayed into prohibited airspace over the Soviet Union
Gulf Air Flight 7711983Boeing 737-200Terrorist bombing112/112Explosive decompressionBomb explosion in cargo hold
Byford Dolphin accident1983Diving bellAccident5/6Explosive decompressionHuman error, no fail-safe in the design
Air India Flight 1821985Boeing 747-200BTerrorist bombing329/329Explosive decompressionBomb explosion in cargo hold
Japan Airlines Flight 1231985Boeing 747SRAccident520/524Explosive decompressionDelayed structural failure of the rear pressure bulkhead following improper repairs
Space Shuttle Challenger disaster1986Space Shuttle ChallengerAccident7/7Gradual or rapid decompressionBreach in solid rocket booster O-ring, leading to damage from escaping superheated gas and eventual disintegration of launch vehicle
Pan Am Flight 1251987Boeing 747-121Incident0/245Rapid decompressionCargo door malfunction
LOT Polish Airlines Flight 50551987Ilyushin Il-62MAccident183/183Rapid decompressionUncontained engine failure
Aloha Airlines Flight 2431988Boeing 737-200Accident1/95Explosive decompressionMetal fatigue
Iran Air Flight 6551988Airbus A300B2-203Shootdown290/290Explosive decompressionIntentionally fired surface-to-air missiles from the USS Vincennes
Pan Am Flight 1031988Boeing 747-100Terrorist bombing259/259Explosive decompressionBomb explosion in cargo hold
United Airlines Flight 8111989Boeing 747-122Accident9/355Explosive decompressionCargo door failure
UTA Flight 7721989Douglas DC-10-30Terrorist bombing170/170Explosive decompressionBomb explosion in cargo hold
Avianca Flight 2031989Boeing 727-21Terrorist bombing107/107Explosive decompressionBomb explosion igniting vapours in an empty fuel tank
British Airways Flight 53901990BAC One-ElevenIncident0/87Rapid decompressionCockpit windscreen failure
China Northwest Airlines Flight 23031994Tupolev TU-154MAccident160/160Explosive decompressionImproper maintenance
Delta Air Lines Flight 1571995Lockheed L-1011 TriStarAccident0/236Rapid decompressionStructural failure of the bulkhead following inadequate inspection of the airframe
TWA Flight 8001996Boeing 747-100Accident230/230Explosive decompressionVapour explosion in fuel tank
Progress M-34 docking test1997Spektr space station moduleAccident0/3Rapid decompressionCollision while in orbit
TAM Airlines Flight 2831997Fokker 100Bombing1/60Explosive decompressionBomb explosion
SilkAir Flight 1851997Boeing 737-300104/104Explosive decompressionSteep dive and mid-air breakup
Lionair Flight 6021998Antonov An-24RVShootdown55/55Rapid decompressionProbable MANPAD shootdown
1999 South Dakota Learjet crash1999Learjet 35Accident6/6Gradual or rapid decompression
Australia “Ghost Flight”2000Beechcraft Super King AirAccident8/8Gradual decompressionInconclusive; likely pilot error or mechanical failure
Hainan Island incident2001Lockheed EP-3Accident0/24Rapid decompressionMid-air collision
TAM Flight 97552001Fokker 100Accident1/82Rapid decompressionUncontained engine failure
China Airlines Flight 6112002Boeing 747-200BAccident225/225Explosive decompressionMetal fatigue
Space Shuttle Columbia disaster2003Space Shuttle ColumbiaAccident7/7Rapid decompressionDamage to orbiter thermal protection system at liftoff, leading to disintegration during reentry
Pinnacle Airlines Flight 37012004Bombardier CRJ-200Accident2/2Gradual decompressionEngine flameout
Helios Airways Flight 5222005Boeing 737-300Accident121/121Gradual decompressionPressurization system set to manual for the entire flight
Alaska Airlines Flight 5362005McDonnell Douglas MD-80Incident0/142Rapid decompressionFailure of operator to report collision involving a baggage loading cart at the departure gate
Adam Air Flight 5742007Boeing 737-400Accident102/102Explosive decompressionMid-air breakup
Qantas Flight 302008Boeing 747-400Incident0/365Rapid decompressionFuselage ruptured by oxygen cylinder explosion
Southwest Airlines Flight 22942009Boeing 737-300Incident0/131Rapid decompressionMetal fatigue
Southwest Airlines Flight 8122011Boeing 737-300Incident0/123Rapid decompressionMetal fatigue
Daallo Airlines Flight 1592016Airbus A321Terrorist bombing1/81Explosive decompressionBomb explosion in passenger cabin
Southwest Airlines Flight 13802018Boeing 737-700Accident1/148Rapid decompressionUncontained engine failure caused by metal fatigue
Sichuan Airlines Flight 86332018Airbus A319-100Accident0/128Explosive decompressionCockpit windscreen failure