Cabin Pressurization

Cabin pressurization is the process of pumping conditioned air into the cabin of an aircraft or spacecraft to create a safe, comfortable environment for people flying at high altitude; on an aircraft this air is usually bled from the gas turbine engines at the compressor stage, while a spacecraft carries it in pressurized, often cryogenic, tanks, and the air is cooled, humidified and mixed with recirculated air before being distributed through the cabin. Experimental pressurization systems were built in the 1920s and 1930s, the Boeing 307 Stratoliner became the first pressurized commercial airliner when it entered service in the 1940s, and pressurized cabins became widespread following the 1949 introduction of the British de Havilland Comet, though the Comet suffered two catastrophic in-flight breakups in 1954 caused by metal fatigue linked to its pressurization cycle. Concorde's high cruising altitude of 60,000 feet against a cabin altitude held near 6,000 feet required an unusually large pressure differential, forcing smaller windows and a heavier airframe, and the risks of pressurization were shown again in 1988 when Aloha Airlines Flight 243, a Boeing 737-200 that had exceeded its designed flight-cycle limit more than twofold, suffered a catastrophic cabin failure in flight; modern airliners such as the Boeing 787 and Airbus A350 use composite airframes to hold lower cabin altitudes and higher humidity for greater passenger comfort.