Why do airliners cruise at 35,000 feet?

Airliners cruise at about 35,000 feet because the thin air up there is a bargain: less drag, less weather, and engines that work well. The typical cruising altitude for commercial airliners is 31,000 to 38,000 feet. Cruise altitude is usually where the higher ground speed is balanced against the decrease in engine thrust and efficiency at higher altitudes, so planes climb until the gains stop paying.

The air is thinner at altitude, so there is less drag due to the lower air density. High altitude lets airliners fly faster and further, and above most of the weather. Most weather happens in the troposphere, the lowest layer of the atmosphere. Most commercial aircraft cruise in the lower stratosphere, just above it, where clouds and significant weather are usually absent. Gas turbine engines also operate efficiently at much higher altitudes than piston engines did.

The price is that the air outside could not keep you alive. So since the 1940s airliners have had pressurized cabins to carry passengers safely where low oxygen and air pressure would otherwise cause sickness or death. In a typical flight the cabin altitude rises gradually to a regulatory maximum of 8,000 feet, and a Boeing 767 at 37,000 feet typically keeps its cabin at about 7,000 feet. In other words, the cabin is not kept at sea-level pressure.

Pressurizing a plane was not easy at first. In 1954, two catastrophic failures of the de Havilland Comet grounded it worldwide. Investigators found a combination of progressive metal fatigue and stresses caused by pressurization, and the lessons were applied to later jet airliners.