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Uncontrolled decompression

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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 recognises three distinct types of decompression events in aircraft:[1][2]

  • Explosive decompression
  • Rapid decompression
  • Gradual decompression

Explosive decompression

File:Alohaairlinesdisaster.jpg
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, a heavy fog may immediately fill the aircraft. 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 fuelling 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's 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 atm. This assumes that the person doesn't 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]

Many[who?] conflate the term explosive decompresion with the act of a cabin or fuselage exploding due to a rapid change in air pressure. This is not the proper usage of the term. Semantics aside, some[who?] believe that if a bullet is shot through the hull of an airplane, it will "explosively decompress" outwards, sucking chairs, baggage and people out of the hole. Using a high-pressure airplane and several scale tests, the television program Mythbusters demonstrated 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:

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], but where they do occur, especially when there are fatalities, then the events tend to be notable.

Decompression incidents do not only occur in aircraft. The Byford Dolphin incident is an example of violent explosive decompression on an oil rig.


Event Date Pressure vessel Event Type Fatalities/of # on board Decompression Type Cause
BOAC Flight 781 1954 de Havilland Comet Accident 35/35 Explosive decompression Metal fatigue
South African Airways Flight 201 1954 de Havilland Comet Accident 21/21 Explosive decompression[18] Metal fatigue
1961 Yuba City B-52 crash 1961 B-52 Stratofortress Accident 0/8 Slow/rapid decompression Fuel exhaustion following increased fuel consumption caused by having to fly below 10,000ft after depressurisation event. Two nuclear bombs did not detonate on impact.
Soyuz 11 re-entry 1971 Soyuz spacecraft Accident 3/3 Gradual decompression Damaged cabin ventilation valve
American Airlines Flight 96 1972 Douglas DC-10-10 Accident 0/67 Rapid decompression[19] Cargo door failure
Turkish Airlines Flight 981 1974 Douglas DC-10-10 Accident 346/346 Explosive decompression[20] Cargo door failure
Far Eastern Air Transport Flight 103 1981 Boeing 737 Accident 110/110 Explosive decompression Corrosion
Byford Dolphin accident 1983 Diving bell Accident 5/6 Explosive decompression Human error, no fail-safe in the design
Korean Air Lines Flight 007 1983 Boeing 747-230B Shootdown 269/269 Rapid decompression[21][22] Intentionally fired air-to-air missile after aircraft strayed into prohibited airspace
Japan Airlines Flight 123 1985 Boeing 747-SR46 Accident 520/524 Explosive decompression Structural failure of rear pressure bulkhead
Aloha Airlines Flight 243 1988 Boeing 737-297 Accident 1/95 Explosive decompression[23] Metal fatigue
United Airlines Flight 811 1989 Boeing 747-122 Accident 9/355 Explosive decompression Cargo door failure
British Airways Flight 5390 1990 BAC One-Eleven Incident 0/87 Rapid decompression[24] Windscreen failure
Lionair Flight LN 602 1998 Antonov An-24RV Shootdown 55/55 Rapid decompression Probable MANPAD shootdown
South Dakota Learjet 1999 Learjet 35 Accident 6/6 Gradual or rapid decompression (Undetermined)
China Airlines Flight 611 2002 Boeing 747-200B Accident 225/225 Explosive decompression Metal fatigue
Helios Airways Flight 522 2005 Boeing 737-31S Accident 121/121 Gradual decompression Automatic pressurization system disabled (suspected)
Alaska Airlines Flight 536 2005 McDonnell Douglas MD-80 Incident 0/140 + crew Rapid decompression Failure of operator to report collision involving a baggage loading cart at the departure gate. Decompressed at 26,000 feet
Qantas Flight 30 2008 Boeing 747-438 Incident 0/365 Rapid decompression[25] Oxygen cylinder explosion


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.[26] 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.[27] 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.

The FAA imposes a regulation[28] on aircraft operators, stating that cabin pressure altitude may not exceed 25,000 feet for more than 2 minutes after a decompression event, typically as a result of an uncontained engine failure. Furthermore, a second ruling[28] states that cabin pressure may at no time following a decompression event exceed 40,000 feet. In effect, these rulings have historically limited civilian aircraft to a maximum operating altitude of 40,000 feet. 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 the new aircraft to operate at a higher altitude than other civilian aircraft.[29]

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.[30]

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

References

  1. ^ a b c d e "AC 61-107A - Operations of aircraft at altitudes above 25,000 feet msl and/or mach numbers (MMO) greater than .75" (PDF). Federal Aviation Administration. 2007-07-15. Retrieved 2008-07-29.
  2. ^ a b Dehart, R. L. (2002). Fundamentals Of Aerospace Medicine: Translating Research Into Clinical Applications, 3rd Rev Ed. United States: Lippincott Williams And Wilkins. p. 720. ISBN 9780781728980. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  3. ^ Flight Training Handbook. U.S. Dept. of Transportation, Federal Aviation Administration, Flight Standards Service. 1980. p. 250. Retrieved 2007-07-28.
  4. ^ Robert V. Brulle (2008-09-11). "Engineering the Space Age: A Rocket Scientist Remembers" (PDF). AU Press. Retrieved 2008-10-13.
  5. ^ "Comet Air Crash" ("Crash of the Comet"). Seconds From Disaster.
  6. ^ Kenneth Gabriel Williams (1959). The New Frontier: Man's Survival in the Sky. Thomas. Retrieved 2008-07-28.
  7. ^ "AAIU Report No. 2001/0018" (PDF). Air Accident Investigation Unit. 2001-11-30. Retrieved 2008-09-01.
  8. ^ "Advisory Circular 61-107" (PDF). FAA. pp. table 1.1.
  9. ^ "Flight Surgeon's Guide". USAF. {{cite web}}: |chapter= ignored (help)
  10. ^ Giertsen JC, Sandstad E, Morild I, Bang G, Bjersand AJ, Eidsvik S (1988). "An explosive decompression accident". Am J Forensic Med Pathol. 9 (2): 94–101. PMID 3381801. {{cite journal}}: |access-date= requires |url= (help); Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  11. ^ a b c d e f Martin B. Hocking, Diana Hocking (2005). Air Quality in Airplane Cabins and Similar Enclosed Spaces. Springer Science & Business. ISBN 3540250190. Retrieved 2008-09-01.
  12. ^ a b Bason R, Yacavone DW (1992). "Loss of cabin pressurization in U.S. Naval aircraft: 1969-90". Aviat Space Environ Med. 63 (5): 341–5. PMID 1599378. {{cite journal}}: Unknown parameter |month= ignored (help)
  13. ^ Brooks CJ (1987). "Loss of cabin pressure in Canadian Forces transport aircraft, 1963-1984". Aviat Space Environ Med. 58 (3): 268–75. PMID 3579812. {{cite journal}}: Unknown parameter |month= ignored (help)
  14. ^ Mark Wolff (2006-01-06). "Cabin Decompression and Hypoxia". theairlinepilots.com. Retrieved 2008-09-01. {{cite web}}: External link in |publisher= (help)
  15. ^ Robinson, RR; Dervay, JP; Conkin, J. "An Evidenced-Based Approach for Estimating Decompression Sickness Risk in Aircraft Operations" (PDF). NASA STI Report Series. NASA/TM—1999–209374. Retrieved 2008-09-01.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  16. ^ Powell, MR (2002). "Decompression limits in commercial aircraft cabins with forced descent". Undersea Hyperb Med. Supplement (abstract). Retrieved 2008-09-01.
  17. ^ "Rapid Decompression In Air Transport Aircraft" (PDF). Aviation Medical Society of Australia and New Zealand. 2000-11-13. Retrieved 2008-09-01.
  18. ^ Neil Schlager (1994). When technology fails: Significant technological disasters, accidents, and failures of the twentieth century. Gail Research. ISBN 0810389088. Retrieved 2008-07-28.
  19. ^ "Aircraft accident report: American Airlines, Inc. McDonnell Douglas DC-10-10, N103AA. Near Windsor, Ontario, Canada. June 12, 1972" (PDF). National Transportation Safety Board. 1973-02-28. Retrieved 2009-03-22.
  20. ^ "FAA historical chronology, 1926-1996" (PDF). Federal Aviation Administration. 2005-02-18. Retrieved 2008-07-29.
  21. ^ Brnes Warnock McCormick, M. P. Papadakis, Joseph J. Asselta (2003). Aircraft Accident Reconstruction and Litigation. Lawyers & Judges Publishing Company. ISBN 1930056613. Retrieved 2008-09-05.{{cite book}}: CS1 maint: multiple names: authors list (link)
  22. ^ Alexander Dallin (1985). Black Box. University of California Press. ISBN 0520055152. Retrieved 2008-09-06.
  23. ^ "Aging airplane safety". Federal Aviation Administration. 2002-12-02. Retrieved 2008-07-29.
  24. ^ "Human factors in aircraft maintenance and inspection" (PDF). Civil Aviation Authority. 2005-12-01. Retrieved 2008-07-29.
  25. ^ "Qantas Boeing 747-400 depressurisation and diversion to Manila on 25 July 2008" (Press release). Australian Transport Safety Bureau. 2008-07-28. Retrieved 2008-07-28.
  26. ^ George Bibel (2007). Beyond the Black Box. JHU Press. pp. 141–142. ISBN 0801886317. Retrieved 2008-09-01.
  27. ^ "FAA HISTORICAL CHRONOLOGY, 1926-1996" (PDF). Federal Aviation Authority. 2005-02-18. Retrieved 2008-09-01.
  28. ^ a b "Section 25.841: Airworthiness Standards: Transport Category Airplanes". Federal Aviation Administration. 1996-05-07. Retrieved 2008-10-02.
  29. ^ "Exemption No. 8695". Renton, Washington: Federal Aviation Authority. 2006-03-24. Retrieved 2008-10-02.
  30. ^ "Amendment 25-87". Federal Aviation Authority. Retrieved 2008-09-01.
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