Cabin pressurization
When a aircraft reaches high altitudes, it becomes necessary for the cabin to be pressurised. This is because, as the plane gains altitude, the atmospheric pressure decreases, and the amount of oxygen becomes insufficient for humans to breathe properly. The maximum operational height of a plane includes considerations for cabin pressure.
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Unpressurized Flight
A lack of sufficient oxygen will bring on hypoxia by reducing the alveolar oxygen tension. In some individuals, symptoms may begin as low as 1500 metre (5000 feet) above MSL, although most passengers can tolerate altitudes of 8,000 feet without ill effect.
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 are generally not possible without pressurisation.
Pressurized Flight
Aircraft which routinely fly above 10,000 feet are generally pressurized by an Environmental Control System (ECS) using air provided by compressors or bleed air. These systems maintain air pressure equivalent to 8000 ft or less, even during flight at altitudes above 43,000 ft.
As the airplane climbs or descends, some passengers will experience discomfort as trapped gasses within their bodies respond to the changing cabin pressure. The most common problems occur with gas trapped in the gastrointestinal tract, the middle ear and the paranasal sinuses.
It is always an emergency if a pressurised aircraft suffers a pressurisation failure above 10,000 feet. If this occurs the pilot must immediately place the plane in an emergency descent and activate emergency oxygen for all passengers and crew.
Rapid Loss of Pressurization (Rapid Decompression)
One consequence of cabin pressurization is that the pressure inside the airplane may be 10 psi, while the pressure outside is barely above 2 psi. A harmless pinhole under these pressure differences will generate a high-pitched squeal as the air leaks out at supersonic speeds. A hole only five feet across will depressurize a jetliner in a fraction of a second.
Rapid decompression of commercial aircraft is extremely rare, but dangerous. People directly next to a hole may be sucked out or injured by flying debris. Floors and internal panels may deform. Hypoxia will result in loss of conciousness in seconds without emergency oxygen, and the air temperature will plummet due to expansion, potentially resulting in frostbite.
- Contrary to Hollywood myth, 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 rapid decompression. This was demonstrated on Mythbusters.
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, however some early jetliners (see De Havilland Comet) had fatal accidents due to underdesign for fatigue.
Effects of Ccabin 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.
Noted incidents
- Golfer Payne Stewart died in an aircraft accident as a result of loss of cabin pressure.
See also
References
Portions from the United States Naval Flight Surgeon's Manual