Cabin Pressurization - Cabin Altitude

The pressure inside the cabin is technically referred to as the equivalent effective cabin altitude or more commonly as the cabin altitude. The cabin altitude is the equivalent altitude having the same atmospheric pressure, so that if the cabin altitude were set to zero then the pressure inside would be the pressure found at sea level. In practice, it is almost never kept at zero, in order to keep within the design limits of the fuselage and to manage landing at airfields higher than sea level. The cabin altitude of an aircraft planning to cruise at 40,000 ft (12,000 m) is programmed to rise gradually from the altitude of the airport of origin to around a maximum of 8,000 ft (2,400 m) and to then reduce gently during descent until it matches the ambient air pressure of the destination.

A typical cabin altitude, such as the Boeing 767's, is maintained at 6,900 feet (2,100 m) when cruising at 39,000 feet (12,000 m). The trend in new aircraft is to lower the cabin altitude: the Airbus A380 features 5,000 ft (1,500 m) when cruising at 43,000 feet (13,000 m), while one of the lowest currently flying is the Bombardier Global Express business jet which features 4,500 ft (1,400 m) when cruising at 41,000 feet (12,000 m). The absolute lowest cabin altitude available on an aircraft is found on the Emivest SJ30 business jet which features a sea level cabin altitude when cruising at 41,000 feet (12,000 m).

Keeping the cabin altitude below 8,000 ft (2,400 m) generally avoids significant hypoxia, altitude sickness, decompression sickness, and barotrauma, and Federal Aviation Administration (FAA) regulations in the U.S. mandate that the cabin altitude may not exceed this at the maximum operating altitude of the aircraft under normal operating conditions. Unfortunately, this mandatory maximum cabin altitude does not eliminate all physiological problems: passengers with conditions such as pneumothorax are advised not to fly until fully healed; people suffering from a cold or other infection may still experience pain in the ears and sinuses; SCUBA divers flying within the 'no fly' period after a dive risk decompression sickness, because the accumulated nitrogen in their bodies can form bubbles when exposed to reduced cabin pressure.

Prior to 1996, approximately 6,000 large commercial transport airplanes were type-certificated to fly up to 45,000 ft without being required to meet high-altitude special conditions. 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 ft (7,600 m), it "must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of 15,000 ft (4,600 m) after any probable failure condition in the pressurization system." 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 ft (7,600 m) for more than 2 minutes, nor exceeding an altitude of 40,000 ft (12,000 m) at any time. In practice, that new Federal Aviation Regulations amendment imposes an operational ceiling of 40,000 ft (12,000 m) on the majority of newly designed commercial aircraft. Aircraft manufacturers can apply for a relaxation of this rule if the circumstances warrant it. In 2004, Airbus acquired an FAA exemption to allow the cabin altitude of the A380 to reach 43,000 ft (13,000 m) in the event of a decompression incident and to exceed 40,000 ft (12,000 m) feet for one minute. This allows the A380 to operate at a higher altitude than other newly designed civilian aircraft.