Why airliners have pressurised cabins
At 6,000 metres the air holds roughly half the oxygen of the air at sea level, and passengers feel dizzy and cold. Yet the higher an aircraft flies, the more it can avoid weather, and the less drag it meets. In the 1930s designers began to look for a way of keeping the cabin air at a comfortable pressure.
The first pressurised airliner was the Boeing 307 Stratoliner, which first flew in 1938. Air was drawn from the engines' superchargers, delivered to the cabin and let out through a valve that kept the pressure in the cabin higher than outside. It could fly at about 6,000 metres while the passengers sat at the equivalent of about 2,400 metres. Only ten were built, and the war ended its civil career.
A tube under stress
Pressurisation turns the fuselage into a pressure vessel. Every flight inflates it like a balloon, and every landing lets it down again, and the metal must withstand millions of such cycles. This is why airliner windows have rounded corners and why the structure is inspected so carefully. We describe a famous failure in our section on the Comet.
Modern cabins are kept at a pressure equivalent to roughly 2,000 metres, and the air is renewed every few minutes.