Airline Cabin Pressure

Regulations specify that the air pressure in the cabin of a commercial airliner must not be lower than that found at an altitude of 8,000 ft (2,438 m). The pressure at this altitude is 1,572 pounds per square foot or 75 kilopascals. This pressure is only about 75% that found at sea level, which is 2,116 psf (101 kPa).

Changes in atmospheric properties with altitude
Changes in atmospheric properties with altitude

This pressure was chosen for two reasons. First, the skin of the aircraft is designed to maintain its shape given the difference in pressure internal and external to the cabin. Aircraft manufacturers want to keep that difference as small as possible because it reduces the amount of structure needed to maintain the integrity of the aircraft's shape. The less structure required, the lighter and less expensive the plane will be.

Ideally, the internal and external pressures would always be equal to minimize the structural weight. However, the pressure cannot be too low or passengers could suffer from altitude sickness or pass out from oxygen deprivation. Most cases of altitude sickness occur at altitudes greater than 10,000 ft (3,050 m) and oxygen deprivation is typically not a concern below 14,000 ft (4,265 m).

The altitude of 8,000 ft was chosen as a tradeoff to satisfy these two requirements. The pressure at this altitude is low enough that it significantly reduces the amount of structure needed to maintain the plane's shape yet high enough that it prevents altitude sickness among the passengers. The pressure on a specific aircraft may vary as different manufacturers offer different environmental systems aboard their planes. In general, most airlines maintain an internal pressure comparable to that found at 6,000 to 8,000 ft (1,830 to 2,440 m). The pressure will obviously increase at lower altitudes to equalize with the external pressure encountered at takeoff and landing.
- answer by Jeff Scott, 12 December 2004

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