Cabin pressurization

Cabin pressurization is a process in which conditioned air is pumped into the cabin of an aircraft or spacecraft, in order to create a safe and comfortable environment for passengers and crew flying at high altitudes. For aircraft, this air is usually bled off from the gas turbine engines at the compressor stage, and for spacecraft, it is carried in high-pressure, often cryogenic tanks. The air is cooled, humidified, and mixed with recirculated air if necessary, before it is distributed to the cabin by one or more environmental control systems. The cabin pressure is regulated by the outflow valve.

Pressurization becomes increasingly necessary at altitudes above 12,500 feet (3,800 m) to 14,000 feet (4,300 m) above sea level to protect crew and passengers from the risk of a number of physiological problems caused by the low outside air pressure above that altitude. It also serves to generally increase passenger comfort and is a regulatory requirement above 15,000 feet (4,600 m). The principal physiological problems are listed below. Pressurization of the cargo hold is also required to prevent damage to pressure-sensitive goods that might leak, expand, burst or be crushed on re-pressurization.

The pressure inside the cabin is technically referred to as the equivalent effective cabin altitude or more commonly as the cabin altitude. This is defined as the equivalent altitude above mean sea level having the same atmospheric pressure according to a standard atmospheric model such as the International Standard Atmosphere. Thus a cabin altitude of zero would have the pressure found at mean sea level, which is taken to be 101,325 pascals (14.696 psi).

In airliners, cabin altitude during flight is kept above sea level in order to reduce stress on the pressurized part of the fuselage; this stress is proportional to the difference in pressure inside and outside the cabin. In a typical commercial passenger flight, the cabin altitude is programmed to rise gradually from the altitude of the airport of origin to a regulatory maximum of 8,000 ft (2,400 m). This cabin altitude is maintained while the aircraft is cruising at its maximum altitude and then reduced gradually during descent until the cabin pressure matches the ambient air pressure at the destination.

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