Uncontrolled decompression

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

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

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

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

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

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: 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

Aircraft fuselages are designed with ribs to prevent tearing; the size of the hole is one of the factors that determine 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 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

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:

• Hypoxia is the most serious risk associated with decompression, especially as it may go undetected or incapacitate the aircrew.^{[11][12][13][14]}
• 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.^[11]
• Decompression sickness.^{[11][12][15][16]}
• 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.

Implications for aircraft design

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 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.^[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

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:

• MIL-STD-810, 202
• RTCA/DO-160
• NORSOK M710
• API 17K and 17J
• NACE TM0192 and TM0297
• TOTALELFFINA SP TCS 142 Appendix H

Notable decompression accidents and incidents

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.

See also

• Cabin pressurization
• Decompression (altitude)
• Decompression (physics)
• Space exposure
• Time of useful consciousness

Notes

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

References

External links

• Human Exposure to Vacuum
• Will an astronaut explode if he takes off his helmet?

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