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 humans 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 by one or more environmental control systems before it is distributed to the cabin.
An airliner fuselage, such as this Boeing 737, forms an almost cylindrical pressure vessel
The pressurization controls on a Boeing 737-800
An empty bottle, sealed at 11,000 m (37,000 ft), is crushed on descent to sea level, compared with one in its original state.
Outflow and pressure relief valve on a Boeing 737-800
The Boeing Model 307 Stratoliner is an American stressed-skin four-engine low-wing tailwheel monoplane airliner derived from the B-17 Flying Fortress bomber, which entered commercial service in July 1940. It was the first airliner in revenue service with a pressurized cabin, which with supercharged engines, allowed it to cruise above the weather. As such it represented a major advance over contemporaries, with a cruising speed of 220 mph (350 km/h) at 20,000 ft (6,100 m) compared to the Douglas DC-3's 160 mph (260 km/h), at 8,000 ft (2,400 m) then in service. When it entered commercial service it had a crew of five to six, including two pilots, a flight engineer, two flight attendants and an optional navigator, and had a capacity for 33 passengers, which later modifications increased, first to 38, and eventually to 60.
Boeing 307 Stratoliner
Boeing S-307 Stratoliner production line - note the early B-17s to the rear
Prototype Model 307 NX19901 with the small tail as initially designed, and which caused its loss
Wreck of the prototype, NX19901