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Cabin pressurization is the active pumping of compressed air into an aircraft cabin when flying at altitude to maintain a safe and comfortable environment for crew and passengers in the low outside atmospheric pressure.

Pressurization is essential over 3,000 metres (9,800 ft) to protect crew and passengers from the risk of hypoxia and a number of other physiological problems (see below) in the thin air above that altitude and increases passenger comfort generally. "The outflow valve is constantly being positioned to maintain cabin pressure as close to sea level as practical, without exceeding a cabin-to-outside pressure differential of 8.60 psi." At a cruising altitude of 39,000 feet (FL 390), a Boeing 767's cabin will be pressurized to an altitude of 6,900 feet.[1]


The need for cabin pressurization

Flights above 3,000 metres (9,800 ft) in unpressurized aircraft put crew and passengers at risk from four separate sources, hypoxia, altitude sickness, decompression sickness and barotrauma as follows:

Hypoxia. The low partial pressure of oxygen at altitude reduces the alveolar oxygen tension in the lungs and subsequently in the brain leading to sluggish thinking, dimmed vision, loss of consciousness and ultimately death. In some individuals, particularly those with heart or lung disease, symptoms may begin as low as 1,500 metres (4,900 ft) above sea level although most passengers can tolerate altitudes of 2,500 metres (8,200 ft) without ill effect. At this altitude, there is about 25% less oxygen than there is at sea level.[2] Hypoxia may be addressed by the administration of supplemental oxygen, usually through an oxygen mask sometimes through a nasal cannula.

Altitude sickness. The low local partial pressure of carbon dioxide (CO2) causes CO2 to out-gas from the blood raising the blood pH and inducing alkalosis. Passengers may experience fatigue, nausea, headaches, sleeplessness and on extended flights even pulmonary oedema. These are the same symptoms that mountain climbers experience but the limited duration of powered flight makes the development of pulmonary oedema unlikely. Altitude sickness may be controlled by a full pressure suit with helmet and faceplate, which completely envelopes the body in a pressurized environment; this is clearly impractical for commercial passengers.

Decompression sickness. The low local partial pressure of gases, principally nitrogen (N2) but including all other gases, may cause dissolved gases in the bloodstream to precipitate out resulting in gas embolism or bubbles in the bloodstream. The mechanism is the same as for compressed air divers on ascent from depth. Symptoms may include the early symptoms of the diver's bends: tiredness, forgetfulness, headache, stroke, thrombosis subcutaneous itching but rarely the full symptoms of the bends. Decompression sickness may also be controlled by a full pressure suit as for altitude sickness.

Barotrauma. As the aircraft climbs or descends passengers may experience discomfort or acute pain as gases trapped within their bodies expand or contract. The most common problems occur with air trapped in the middle ear (aerotitus) or paranasal sinuses by a blocked Eustachian tube or sinuses. Pain may also be experienced in the gastrointestinal tract or even the teeth (barodontalgia). Usually these are not severe enough to cause actual trauma but can result in soreness in the ear that persists after the flight and can exacerbate or precipitate pre-existing medical conditions such as pneumothorax (collapsed lung).

Pressurized flight

An empty water bottle which was closed during a commercial transatlantic flight with a cabin pressure equivalent to an altitude in the range of 6,000 to 8,000 ft, photographed when back on the ground, showing that the higher pressure compressed it during the descent (Cabin pressure decreases as the aircraft climbs, and cabin pressure increases as the aircraft descends).

Maintaining the cabin pressure altitude to below 3,000 metres (9,800 ft) generally avoids significant hypoxia, altitude sickness, decompression sickness and barotrauma. Emergency oxygen systems are installed, both for passengers and cockpit crew, to prevent loss of consciousness in the event that cabin pressure rapidly rises above 10,000 feet MSL. Those systems contain more than enough oxygen for all on board, to give the pilot adequate time to descend the plane to a safe altitude, where supplemental oxygen is not needed. FAA regulations mandate that the cabin altitude may not exceed 8,000 feet at the maximum operating altitude of the airplane under normal operating conditions.

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.[3] In 1996, the FAA adopted Amendment 25-87, which imposed additional high altitude cabin pressure specifications, for new type aircraft designs. 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."[4] 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. [5] In practice, that new FAR amendment imposes an operational ceiling of 40,000 feet on the majority of newly designed commercial aircraft.[6][7]

However, companies that build the newer designed aircraft can apply for exemption from that more restrictive rule. 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 A380 to operate at a higher altitude than other newly designed civilian aircraft, which have not yet been granted a similar exemption.[8]

The pressure maintained within the cabin is referred to as the equivalent effective cabin altitude or more normally, the ‘cabin altitude’. Cabin altitude is not normally maintained at average mean sea level (MSL) pressure (1013.25 mbar, or 29.921 inches of mercury) throughout the flight, because doing so would cause the designed differential pressure limits of the fuselage to be exceeded. An aircraft planning to cruise at 40,000 ft (12,000 m) is programmed to rise gradually from take-off to around 8,000 ft (2,400 m) in cabin pressure altitude, and to then reduce gently to match the ambient air pressure of the destination. That destination may be significantly above sea level and this needs to be taken into account; for example, El Alto International Airport in La Paz, Bolivia is 4,061 metres (13,320 ft) above sea level.

Pressurization is achieved by the design of an airtight fuselage engineered to be pressurized, a source of compressed air and an environmental control system (ECS). The most common source of compressed air for pressurization is bleed air extracted from the compressor stage of a gas turbine or turboprop propulsion engine, usually the second or third last compressor ring. By the time the cold outside air has reached this part of the compressor it has been adiabatically heated to around 200 °C (392 °F) and is at a very high pressure. It is then expanded and cooled to a suitable temperature by passing it through a heat exchanger and air cycle machine ('the packs system'). There is no need to further heat or refrigerate the air. Typically, compressed air is bled from at least two propulsion engines each system being fully redundant. Compressed air is also obtained from the Auxiliary Power Unit (APU), if fitted, in the event of an emergency and for cabin air supply on the ground before the main engines are started. Most modern commercial aircraft today have a fully redundant, duplicated electronic controller for maintaining pressurization along with a manual back-up system.

All exhaust air is dumped to atmosphere via a valve, usually at the rear of the fuselage. This valve controls the cabin pressure and also acts as a safety relief. In the event that the automatic pressure controllers fail, the pilot can manually control the cabin pressure valve, according to the backup emergency procedure checklist. The automatic controller normally maintains the proper cabin pressure altitude by constantly adjusting the outflow valve position, so that the cabin pressure is as near to sea level pressure as practical, without exceeding the maximum differential limit of 8.60 psi. At FL 390, the cabin pressure would be automatically maintained at about 6,900 feet (450 feet lower than Mexico City), which is about 11.5 psi (76 kPa).[9]

Bleed air extraction from the engines reduces engine efficiency only slightly but introduces a danger of oils and other chemicals from the engine being supplied to the cabin. The BAe 146 has achieved some notoriety with this problem, with some pilots refusing to fly it.[10] A vivid description of this happening appeared in a 2008 Telegraph article.[11]

Some aircraft, such as the Boeing 787 have re-introduced the use of electric compressors previously used on piston-engined airliners to provide pressurization. The use of electric compressors increases the electrical generation load on the engines and introduces a number of stages of energy transfer, therefore it is unclear whether this increases the overall efficiency of the aircraft air handling system. It does, however, remove the danger of chemical contamination of the cabin, simplifies engine design, avoids the need to run high pressure pipework around the aircraft and provides greater design flexibility.

Cabin altitudes are maintained at up to 2,500 metres (8,200 ft), so pressurization does not eliminate all physiological problems. Passengers with conditions such as a pneumothorax are advised not to fly until fully healed; pain may still be experienced in the ears and sinuses by people suffering from a cold or other infection; SCUBA divers flying within the 'no fly' period after a dive may risk decompression sickness because their dive tables are calibrated to sea level.


The aircraft that pioneered pressurized cabin systems include:

  • Junkers Ju 49 (1931 - a German experimental aircraft purpose built to test the concept of cabin pressurization)
  • Lockheed XC-35 (1937 - the first American pressurized aircraft also purpose built to test the concept)
  • Boeing 307 (1938 - the first pressurized piston airliner)
  • Lockheed Constellation (1943 - the first pressurized airliner in wide service)
  • Avro Tudor (1946 - first British pressurized airliner)
  • de Havilland Comet (British, Comet 1 1949 - the first jetliner, Comet 4 1958 - resolving the Comet 1 problems)
  • Tupolev Tu-144 and Concorde (1968 USSR and 1969 Anglo-French respectively - first to operate at very high altitude)

The first airliner with a pressurized cabin was the Boeing 307 Stratoliner, built 1938, prior to World War II, though only ten were produced. The 307's "pressure compartment was from the nose of the aircraft to a pressure bulkhead in the aft just forward of the horizontal stabilizer."[12]

World War II was a catalyst for aircraft development. Initially the piston aircraft of World War II, though they often flew at very high altitudes were not pressurized and relied on oxygen masks. This became impractical with the development of larger bombers where crew were required to move about the cabin and this led to the first bomber with cabin pressurization (though restricted to crew areas), the B-29 Superfortress. The control system for this was designed by Garrett AiResearch Manufacturing Company, drawing in part on licensing of patents held by Boeing for the Stratoliner.[13]

Post-war piston airliners such as the Lockheed Constellation (1943) extended the technology to civilian service. The piston engined airliners generally relied on electrical compressors to provide air and operated below 20,000 ft where the piston engine is more efficient. Designing a pressurized fuselage to cope with this altitude was within the engineering and metallurgical knowledge of the time. The introduction of jet airliners required a large increase in cruise altitude to 30,000 ft where the jet engine is more efficient. This increase in altitude required far more rigorous engineering of the fuselage and in the beginning not all the engineering problems were understood.

The world’s first commercial jet airliner was the British de Havilland Comet (1949) designed with a service ceiling of 36,000 ft (11,000 m). It was the first time that a large diameter, pressurized fuselage with windows had been built and flown at this altitude. Initially the design was very successful but two catastrophic airframe failures in 1954 resulting in the total loss of the aircraft, passengers and crew grounded what was then the entire world jet airliner fleet. Extensive investigation and groundbreaking engineering analysis of the wreckage led to a number of very significant engineering advances that solved the basic problems of pressurized fuselage design at altitude. The critical problem proved to be a combination of an inadequate understanding of the effect of progressive metal fatigue as the fuselage undergoes repeated stress cycles coupled with a misunderstanding of how aircraft skin stresses are redistributed around openings in the fuselage such as windows and rivet holes.

The critical engineering principles learned from the Comet 1 program were applied directly to the design of the Boeing 707 (1957) and all subsequent jet airliners. One immediately noticeable legacy of the Comet disasters is the oval windows on every jet airliner; the metal fatigue cracks that destroyed the Comets were initiated by the small radius corners on the Comet 1’s almost square windows. The Comet fuselage was redesigned and the Comet 4 (1958) went on to become a successful airliner, pioneering the first transatlantic jet service, but the program never really recovered from these disasters and was overtaken by the Boeing 707.

Concorde had to deal with unusually high pressure differentials, as of necessity it flew at unusually high altitude (up to 60,000 ft) while the cabin altitude was maintained at 6000 ft.[14] This made the vehicle significantly heavier and contributed to the high cost of a flight. Concorde also had to have smaller than normal cabin windows to limit decompression speed in the event of window failure.

The designed operating cabin altitude for proposed aircraft now in development is falling and this is expected to reduce any remaining physiological problems. The Boeing 787 will feature a standard cabin altitude of 1,800 metres (5,900 ft).

Loss of pressurization

Rapid decompression of commercial aircraft is a rare, but dangerous event with American Airlines Flight 96 being an example. People seated close to a very large hole may be forced out by explosive decompression or injured by exiting debris and unsecured cabin objects that may become projectiles. However, contrary to Hollywood myth, as in the James Bond film Goldfinger, people just a few feet from the hole are more at risk from hypoxia or hypothermia than from being forced out. Floors and internal panels have deformed in previous incidents. Consequently all modern commercial jets now have blow-out panels or vents between pressurized compartments of the plane, such as between the passenger and cargo spaces, to equalize destructive internal pressure differentials.

Gradual or slow decompression, sometimes caused by a failure to pressurize the cabin with an increase in altitude, is dangerous because it may not be detected. The Helios Airways 2005 accident is a good example.[15] Warning systems may be ignored, misinterpreted or fail and self-recognition of the subtle effects of hypoxia really depends upon previous experience and hypoxia familiarization training. Unfortunately, in most countries this has been largely restricted to military hypobaric chamber training with its risk of decompression sickness and barotrauma. Newer reduced oxygen breathing systems [16] are more accessible, safer and provide valuable practical experience. [17] Adding such practical training to knowledge required by regulatory authorities is likely to increase hypoxia awareness and aviation safety.

Hypoxia may result in loss of consciousness without emergency oxygen. The Time of Useful Consciousness varies depending on the altitude. Additionally, the air temperature will plummet to the ambient outside temperature with a danger of hypothermia or frostbite.

Failure of cabin pressurization above 3,000 metres (9,800 ft) for whatever reason requires an emergency descent to below 3,000 metres (9,800 ft) and the deployment of an oxygen mask above each seat. In almost all pressurized jet airliners passenger oxygen masks are automatically deployed when the cabin altitude exceeds 14,000 feet.[18] The Boeing 737 emergency equipment is typical.

It is generally 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.

Notable decompression incidents

A list of notable aircraft and other decompression incidents, as well as links to further detailed information is given in the table below from the main article uncontrolled decompression

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[19] 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[20] Cargo door failure
Turkish Airlines Flight 981 1974 Douglas DC-10-10 Accident 346/346 Explosive decompression[21] 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[22][23] 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[24] 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[25] 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 The pressurization system was set to manual for the entire flight, resulting in a failure to pressurize the cabin.[26]
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[27] Fuselage ruptured by explosion of an oxygen cylinder
Southwest Airlines Flight 2294 2009 Boeing 737-300 Incident 0/126 + 5 crew Rapid decompression 1 square foot (0.093 m2) hole blown in fuselage during flight.[28] Under investigation

Popular culture

  • In the 1996 movie Eraser, U.S. Marshall John Kruger, played by Arnold Schwarzenegger, causes decompression in order to escape the plane.
  • In the 2006 movie Snakes on a Plane, decompression forces helped to remove the offending snakes from the aircraft through "open" windows.
  • In "The Langoliers", a short story in the book Four Past Midnight by Stephen King, the characters devise a way to knock themselves out by decreasing the cabin pressure of the plane so they can safely pass through a time rift.

See also



  1. ^ "Commercial Airliner Environmental Control System: Engineering Aspects of Cabin Air Quality".  
  2. ^ K. Baillie and A. Simpson. "Altitude oxygen calculator". Retrieved 2006-08-13.   - Online interactive altitude oxygen calculator
  3. ^ "Final Policy FAR Part 25 Sec. 25.841 07/05/1996".  
  4. ^ "FARs, 14 CFR, Part 25, Section 841".  
  5. ^ "FARs, 14 CFR, Part 25, Section 841".  
  6. ^ "Exemption No. 8695". Renton, Washington: Federal Aviation Authority. 2006-03-24.$FILE/8695.doc. Retrieved 2008-10-02.  
  7. ^ Steve Happenny (2006-03-24). "PS-ANM-03-112-16". Federal Aviation Authority. Retrieved 2009-09-23.  
  8. ^ "Exemption No. 8695". Renton, Washington: Federal Aviation Authority. 2006-03-24.$FILE/8695.doc. Retrieved 2008-10-02.  
  9. ^ Airliner Environmental Control System "Engineering Aspects of Cabin Air". Airliner Environmental Control System.  
  10. ^ "Pilots refuse to fly BAe 146 in polluted cabin air row".  
  11. ^ "Is cabin air making us sick?".  
  12. ^ William A. Schoneberger and Robert R. H. Scholl, Out of Thin Air: Garrett's First 50 Years, Phoenix: Garrett Corporation, 1985 (ISBN 0-9617029-0-7), p. 275.
  13. ^ Seymour L. Chapin (August 1966). "Garrett and Pressurized Flight: A Business Built on Thin Air". Pacific Historical Review 35: 329–43.  
  14. ^ Hepburn, A.N. "Human Factors in the Concord". Occupational Medicine, 17: 1967, pp. 47–51.
  15. ^ J. Laming. "Helios out of oxygen. Flight Safety Australia magazine - November-December 2005, pp 27-33" (PDF).  
  16. ^ R. Westerman (2004). "Hypoxia familiarization training by the reduced oxygen breathing method." (PDF). ADF Health 5 (1): 11–5.  
  17. ^ Smith AM (January 2008). "Hypoxia symptoms in military aircrew: long-term recall vs. acute experience in training". Aviat Space Environ Med 79 (1): 54–7. doi:10.3357/ASEM.2013.2008. PMID 18225780.  
  18. ^ - When oxygen masks mysteriously appear
  19. ^ Neil Schlager (1994). When technology fails: Significant technological disasters, accidents, and failures of the twentieth century. Gail Research. ISBN 0810389088. Retrieved 2008-07-28.  
  20. ^ "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.  
  21. ^ "FAA historical chronology, 1926-1996" (PDF). Federal Aviation Administration. 2005-02-18. Retrieved 2008-07-29.  
  22. ^ 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.  
  23. ^ Alexander Dallin (1985). Black Box. University of California Press. ISBN 0520055152. Retrieved 2008-09-06.  
  24. ^ "Aging airplane safety". Federal Aviation Administration. 2002-12-02.!OpenDocument. Retrieved 2008-07-29.  
  25. ^ "Human factors in aircraft maintenance and inspection" (PDF). Civil Aviation Authority. 2005-12-01. Retrieved 2008-07-29.  
  26. ^ "Aircraft Accident Report - Helios Airways Flight HCY522 Boeing 737-31S at Grammatike, Hellas on 14 August 2005". Hellenic Republic Ministry Of Transport & Communications: Air Accident Investigation & Aviation Safety Board. Nov 2006.$file/FINAL%20REPORT%205B-DBY.pdf. Retrieved 2009-07-14.  
  27. ^ Australian Transport Safety Bureau (2008-07-28). "Qantas Boeing 747-400 depressurisation and diversion to Manila on 25 July 2008". Press release. Retrieved 2008-07-28.  
  28. ^ "Hole in US plane forces landing". BBC News. 2009-07-14. Retrieved 2009-07-15.  

General references



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