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Uncontrolled decompression refers to an unexpected drop in the pressure of a sealed system, such as an aircraft cabin. Where the speed of the decompression occurs faster than air can escape from the lungs, this is known as explosive decompression (ED), and is associated with explosive violence. Where decompression is still rapid, but not faster than the lungs can decompress, this is known as rapid decompression. Lastly, slow decompression or gradual decompression occurs so slowly that humans may not detect it before hypoxia sets in.

Generally uncontrolled decompression results from human error, material fatigue, engineering failure or impact, that causes a pressure vessel either not to pressurize, or to vent into lower-pressure surroundings.

Description

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.^[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

Aloha Airlines Flight 243

Explosive decompression occurs at a rate faster 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 which can become projectiles due to the explosive force.

After an explosive decompression within an aircraft, a heavy fog may immediately fill the interior. 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]

Paul Withey, an aviation expert, described an explosive decompression inside an aircraft cabin as similar to the explosion of a 500 pound (225 kilogram) bomb inside the cabin.^[5]

Rapid decompression

Rapid decompression typically takes more than 0.1 to 0.5 seconds, allowing the lungs to decompress faster than the cabin.^[1][6] The risk of lung damage is still present, but significantly reduced compared to explosive decompression.

Slow 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. This happened on a Ryanair, Boeing 737 flight in 2001 where the pressurization system was not activated by flight crew during pre-flight checks.^[7]

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.

Fallacies

Misunderstandings of "explosive decompression" are quite likely to be a fueling factor for a persistent myth that humans would explode if exposed to the non-pressure of outer space. Extravagant depictions in media such as the film Licence to Kill, where one character's head detonates after the hyperbaric chamber he is in is rapidly depressurized, have helped to fuel the myth. This is possible with the pressures experienced in diving chambers but not with the far smaller pressure changes involved in space exploration. Accidents in space exploration research and high-altitude aviation have shown that while vacuum exposure causes swelling, human skin is tough enough to withstand a drop of one atmosphere. This assumes that the person does not attempt to hold their breath (which is likely to cause acute lung trauma), the limiting factor on consciousness then being hypoxia after a few seconds.^[8][9] A sudden drop of eight atm in the Byford Dolphin accident had immediately fatal results. ^[10]

The term explosive decompression refers to a rapid drop in air pressure itself and does not imply a cabin or fuselage exploding due to a rapid pressure change. Using a high-pressure airplane and several scale tests, the television program Mythbusters examined the belief that a bullet shot through the hull of an airplane will cause it to "explosively decompress" outwards, sucking chairs, baggage and people out of the hole. The program's tests indicated that fuselage design does not allow this to happen. Rigorous study would still be necessary for full scientific validation of the results.

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.^{[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.

Notable decompression accidents and incidents

Decompression incidents are not uncommon on military and civilian aircraft (approximately 40–50 rapid decompression events worldwide annually^[17]), however 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 notable.^[11] Injuries resulting from decompression incidents are rare.^[11]

Decompression incidents do not only occur 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, even though the event may worsen the initial issue.

Implications for aircraft design

Modern aircraft are specifically designed with longitudinal and circumferential re-enforcing ribs in order to prevent localised damage from tearing the whole fuselage open during a decompression incident.^[30] 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.^[31] Manufacturers were able to comply with the directive either by strengthening the floors and/or installing relief vents between the passenger cabin and aft cargo compartment.

Prior to 1996, approximately 6,000 large commercial transport airplanes were type certificated to fly up to 45,000 feet, without being required to meet high altitude special conditions.^[32] In 1996, the FAA adopted Amendment 25-87, which imposed additional high altitude cabin pressure specifications, for new type aircraft designs.^[33] For aircraft certified to operate above 25,000 feet (FL 250), it "must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of 15,000 feet after any probable failure condition in the pressurization system."^[34] In the event of a decompression which results from "any failure condition not shown to be extremely improbable," the plane must be designed so that occupants will not be exposed to a cabin altitude exceeding 25,000 feet for more than 2 minutes, nor exceeding an altitude of 40,000 feet at any time. ^[35] In practice, that new FAR amendment imposes an operational ceiling of 40,000 feet on the majority of newly designed commercial aircraft.^{[36][37][Note 1]}

In 2004, Airbus successfully petitioned the FAA to allow cabin pressure of the A380 to reach 43,000 feet in the event of a decompression incident, and to exceed 40,000 feet for one minute. This special exemption allows that new aircraft to operate at a higher altitude than other newly design civilian aircraft, which have not yet been granted a similar exemption.^[38]

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.^[39]

Other national and international standards for explosive decompression testing include:

• MIL-STD-810, 202
• RTCA/D0-160
• NORSOK M710
• API 17K and 17J
• NACE TM0192 and TM0297
• TOTALELFFINA SP TCS 142 Appendix H

See also

• Cabin pressurization
• Time of Useful Consciousness

Notes

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

References

External links

• Human Exposure to Vacuum