Why are aircraft cabins pressurised?

Commercial jets fly between 9,000 and 12,500 m because the thinner air there offers less drag and the engines run more efficiently. But that same thin air is lethal to people: at 11,000 m atmospheric pressure is around 230 hPa, less than a quarter of the 1,013 hPa at sea level, and the partial pressure of oxygen is so low that an unprotected adult loses consciousness in 30-60 seconds. Above 3,000 m the first effects of hypoxia already appear: headache, fatigue, impaired judgement. Pressurising the cabin is the only practical way to carry hundreds of passengers to that altitude without masks.

Air is drawn from the engine compressors (or, on the 787, from electric compressors), cooled in the air conditioning packs, and fed into the cabin. An outflow valve at the rear of the fuselage regulates how much air escapes, and with it the internal pressure. During cruise the cabin is kept at an equivalent altitude of 1,800-2,400 m (6,000-8,000 ft), about 750-800 hPa, which means a pressure difference with the outside of 0.5-0.6 bar. That difference pushes outward on every square metre of fuselage with about 5 tonnes of force, which is why the structure is designed as a pressure vessel with small, rounded windows.

Keeping 1,013 hPa inside would require a heavier fuselage and would shorten its fatigue life with every pressurisation cycle. The 2,400 m cabin altitude is a compromise: most people tolerate it without problems, though it explains the blocked ears, dryness and tiredness of long flights. Newer aircraft such as the A350 and 787, with carbon-fibre fuselages, lower the cabin altitude to around 1,800 m for improved comfort.