Cabin pressurization
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Cabin pressurization is the active pumping of air into the cabin of an aircraft to increase the air pressure within the cabin. It is required when an aircraft reaches high altitudes, because the natural atmospheric pressure is too low to allow people to absorb sufficient oxygen, leading to altitude sickness and ultimately hypoxia.
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[edit] Unpressurized flight
A lack of sufficient oxygen will bring on hypoxia by reducing the alveolar oxygen tension. In some individuals, particularly those with heart or lung disease, symptoms may begin as low as 1500 metres (5000 feet) above MSL, although most passengers can tolerate altitudes of 2500m (around 8,000 feet) without ill effect. At this altitude, there is about 25% less oxygen than there is at sea level.[1]
Passengers may also develop fatigue or headache as the plane flies higher. As the operational altitude increases, reactions become sluggish and unconsciousness will eventually result. Sustained flight operations above 10,000 feet generally require supplemental oxygen (through a nasal cannula or oxygen mask) or pressurization.
[edit] Pressurized flight
Aircraft that routinely fly above 10,000 feet are generally equipped with an oxygen system fed through masks or canulas (typically for smaller aircraft), or are pressurized by an Environmental Control System (ECS) using air provided by compressors or bleed air. Bleed air extracted from the engines is compressively heated and extracted at approximately 400 degrees F and then cooled by passing it through a heat exchanger and air cycle machine (commonly referred to by aircrews and mechanics as 'the packs system'). Most modern commercial aircraft today have a dual channel electronic controller for maintaining pressurization along with a manual back-up system. These systems maintain air pressure equivalent to 8000 ft or less, even during flight at altitudes above 43,000 ft. Aircraft have a positive pressure relief valve in the event of excessive pressure in the cabin. This is to protect the aircraft structure from excessive loading. Normally the maximum pressure differential between the cabin and the outside ambient air is between 7.5 and 8 psid. If the cabin were maintained at sea level pressurization and then flown to 35,000+ feet, the pressurization differential would be greater than 9 psid and the structural life of the airplane would be limited. The traditional method of bleed air extraction from the engine comes at the expense of powerplant efficiency. Newer aircraft such as the Boeing 787 are using electric compressors to provide pressurization. This allows greater propulsive efficiency.
As the airplane pressurizes and decompresses, some passengers will experience discomfort as trapped gasses within their bodies expand or contract in response to the changing cabin pressure. The most common problems occur with gas trapped in the gastrointestinal tract, the middle ear and the paranasal sinuses. Note that in a pressurized aircraft, these effects are not due directly to climb and descent, but to changes in the pressure maintained inside the aircraft.
It is always an emergency if a pressurized aircraft suffers a pressurization failure above 10,000 feet. If this occurs the pilot must immediately place the plane in an emergency descent and activate oxygen masks for everyone aboard.
In most passenger jet aircraft (such as the Boeing 737 [1]), passenger oxygen masks are automatically deployed if the cabin altitude exceeds 14,000 feet. [2]
[edit] History and usage of cabin pressurization
While the piston fighters of World War II often flew at very high altitudes, they were not pressurised; instead pilots used oxygen. However, in a larger bomber where crew moved about the cabin, this was considerably less practical. Therefore, the first aircraft with cabin pressurization (though restricted to crew areas), was the B-29 Superfortress. Post-war piston airliners such as the Lockheed Constellation brought the technology to civilian service, and as jet airliners were always designed for high-altitude operation every jetliner features the technology.
Most turboprop aircraft also feature cabin pressurization due to their medium to high altitude operation. A very few piston-engined small private planes also do so; most do not routinely fly high enough to justify such a system.
[edit] Loss of pressurization
One consequence of cabin pressurization is that the pressure inside the airplane may be 10 psi (700 millibars or hPa), while the pressure outside is barely above 2 psi (15 kilopascals, 140 millibars). A harmless pinhole under these pressure differences will generate a high-pitched squeal as the air leaks out at supersonic speeds[citation needed]. A hole five feet across will depressurize a jetliner in a fraction of a second.
Gradual or slow decompression is dangerous because it may not be detected.
Rapid decompression is a change in cabin pressure where the lungs can decompress faster than the cabin.
Explosive decompression is a change in cabin pressure faster than the lungs can decompress (less than 0.5 seconds). This type of decompression is potentially dangerous and often results in lung damage and unsecured items / debris flying around the cabin.
Rapid decompression of commercial aircraft is extremely rare, but dangerous. People directly next to a very large hole may be blown out or injured by flying debris. Floors and internal panels may deform. Hypoxia will result in loss of consciousness without emergency oxygen. Onset of hypoxia-induced unconsciousness varies depending on the altitude:
| Altitude (feet) | Moderate Activity | Sitting quietly |
|---|---|---|
| 22,000 | 5 minutes | 10 minutes |
| 25,000 | 2 minutes | 3 minutes |
| 30,000 | 45 seconds | 75 seconds |
| 40,000 | 18 seconds | 30 seconds |
Additionally, the air temperature will plummet due to expansion, potentially resulting in frostbite.
- Contrary to Hollywood myth, as seen in the James Bond film, Goldfinger, people just a few feet from the hole are more at risk from hypoxia than from being sucked out.
- Contrary to Hollywood myth, a bullet hole in a window, or even the loss of an entire passenger window, will not cause explosive decompression. This was demonstrated on MythBusters. However, such damage may cascade into catastrophic aircraft failure, when metal fatigue is factored in.
[edit] Effects of cabin pressurization on an aircraft fuselage
As the airplane is pressurized and depressurized, the metal skin of the airplane expands and contracts, resulting in metal fatigue. Modern aircraft are designed to resist this compression cycle, but some early jetliners (see De Havilland Comet) had fatal accidents due to underdesign for fatigue.
[edit] Effects of cabin pressurization on the human body
- Ear and paranasal sinuses - One needs to adjust to the pressurized cabin air from the beginning. 1 in 3 passengers suffer ear discomfort, pain and temporary hearing loss on takeoff or landing, called "aerotitus" by the House Ear Institute in Los Angeles. Rapid changes in air pressure cause the air pocket inside the ear to expand during takeoff and contract during descent, stretching the eardrum. To equalize pressure, air must enter or escape through the Eustachian tube. "If a passenger has serious congestion, they risk ear drum damage", says Sigfrid Soli, Ph.D., head of the HCSD Department at the HSI(?).
- Tooth - Anyone with intestinal gas or gas trapped in an infected tooth may also experience Barodontalgia, a toothache provoked by exposure to changing atmospheric pressure.
- Pneumothorax - Anyone who has suffered a pneumothorax is recommended not to fly (even in a pressurised cabin) for at least 1 month and should obtain an x-ray prior to travelling.
As well as the more acute health effects experienced by some people, the cabin pressure altitude of 8,000 feet typical in most airliners contributes to the fatigue experienced in long flights. The in-development Boeing 787 airliner will feature pressuration to the equivalent of 6,000 feet, which Boeing claims will substantially increase passenger comfort.
Some people may still experience symptoms of altitude sickness despite the cabin pressure.
[edit] Noted incidents
- Aloha Airlines Flight 243 suffered from explosive decompression, a form of cabin depressurization, resulting in the loss of one flight attendent's life.
- Golfer Payne Stewart died in an aircraft accident as a result of loss of cabin pressure.
- Helios Airways Flight 522, a Cypriot Boeing 737, crashed in Greece on August 14, 2005, killing all 121 people on board. It is currently believed that the plane depressurized and the pilots fell unconscious.
[edit] See also
- Atmosphere (unit)
- Bleed air
- Compressed air
- Explosive decompression
- Gas compressor
- Rarefaction
- Spacesuit
[edit] References
- ^ K. Baillie and A. Simpson. Altitude oxygen calculator. Retrieved on 2006-08-13. - Online interactive altitude oxygen calculator
- Portions from the United States Naval Flight Surgeon's Manual
- CNN: 121 Dead in Greek Air Crash
- "Explosive Decompression" segment of MythBusters episode 10, January 11, 2004
- also shown as a segment of Beyond Tomorrow episode 12
