Cabin pressurization
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.
This increased airframe weight and saw the use of smaller cabin windows intended to slow the decompression rate if a depressurization event occurred.
[4] For increased passenger comfort, several modern airliners, such as the Boeing 787 Dreamliner and the Airbus A350 XWB, feature reduced operating cabin altitudes as well as greater humidity levels; the use of composite airframes has aided the adoption of such comfort-maximizing practices.
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.
[10] 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, and people suffering from a cold or other infection may still experience pain in the ears and sinuses.
[18] Before 1996, approximately 6,000 large commercial transport airplanes were assigned a type certificate to fly up to 45,000 ft (13,716 m) without having to meet high-altitude special conditions.
[19] In 1996, the FAA adopted Amendment 25-87, which imposed additional high-altitude cabin pressure specifications for new-type aircraft designs.
[20] 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.
[21] Russian engineers used an air-like nitrogen/oxygen mixture, kept at a cabin altitude near zero at all times, in their 1961 Vostok, 1964 Voskhod, and 1967 to present Soyuz spacecraft.
[24] By contrast, the United States used a pure oxygen atmosphere for its 1961 Mercury, 1965 Gemini, and 1967 Apollo spacecraft, mainly in order to avoid decompression sickness.
After this, NASA revised its procedure to use a nitrogen/oxygen mix at zero cabin altitude at launch, but kept the low-pressure pure oxygen atmosphere at 5 psi (0.34 bar) in space.
The control and selection of high or low bleed sources is fully automatic and is governed by the needs of various pneumatic systems at various stages of flight.
Failures range from sudden, catastrophic loss of airframe integrity (explosive decompression) to slow leaks or equipment malfunctions that allow cabin pressure to drop.
Modern airliners include a pressurized pure oxygen tank in the cockpit, giving the pilots more time to bring the aircraft to a safe altitude.
In jet fighter aircraft, the small size of the cockpit means that any decompression will be very rapid and would not allow the pilot time to put on an oxygen mask.
[39] On June 30, 1971, the crew of Soyuz 11, Soviet cosmonauts Georgy Dobrovolsky, Vladislav Volkov, and Viktor Patsayev were killed after the cabin vent valve accidentally opened before atmospheric re-entry.
Initially, the piston aircraft of World War II, though they often flew at very high altitudes, were not pressurized and relied on oxygen masks.
The first bomber built with a pressurised cabin for high altitude use was the Vickers Wellington Mark VI in 1941 but the RAF changed policy and instead of acting as Pathfinders the aircraft were used for other purposes.
The control system for this was designed by Garrett AiResearch Manufacturing Company, drawing in part on licensing of patents held by Boeing for the Stratoliner.
The world's first commercial jet airliner was the British de Havilland Comet (1949) designed with a service ceiling of 36,000 ft (11,000 m).
Initially, the design was very successful but two catastrophic airframe failures in 1954 resulting in the total loss of the aircraft, passengers and crew grounded what was then the entire world jet airliner fleet.
The critical engineering principles concerning metal fatigue learned from the Comet 1 program[48] were applied directly to the design of the Boeing 707 (1957) and all subsequent jet airliners.
[56] Aloha 243 was able to land despite the substantial damage inflicted by the decompression, which had resulted in the loss of one member of the cabin crew; the incident had far-reaching effects on aviation safety policies and led to changes in operating procedures.
[58] This combination, while providing for increasing comfort, necessitated making Concorde a significantly heavier aircraft, which in turn contributed to the relatively high cost of a flight.
The 787's internal cabin pressure is the equivalent of 6,000 ft (1,829 m) altitude resulting in a higher pressure than for the 8,000 ft (2,438 m) altitude of older conventional aircraft;[62] according to a joint study performed by Boeing and Oklahoma State University, such a level significantly improves comfort levels.
