Gate 41

Things that look scary but have an explanation

Why do my ears pop, and what is that hiss?

How can we even breathe at 35,000 feet?

At cruise altitude the outside air is far too thin to breathe and around -50 degrees Celsius. So the cabin is pressurised: air is drawn from the engines (or, on some aircraft, electric compressors), cooled and cleaned, then released through an outflow valve that sets the cabin pressure.

Inside, the pressure feels like roughly 6,000 to 8,000 feet above sea level (some newer aircraft keep it lower). As you climb, cabin pressure falls gently, so your ears pop to equalise. In descent it rises again. Swallowing, yawning or chewing helps.

The hiss is moving air and the system doing its job. Fresh air is exchanged many times an hour.

A little deeper

At 35,000 feet the outside pressure is about a quarter of sea level. The fuselage is a pressure vessel holding the difference, which is why windows are rounded and why every pressurisation cycle is counted for fatigue.

Cabin altitude changes are kept slow (a few hundred feet per minute) for comfort. Most aircraft mix fresh air with filtered recirculated air; the exact ratio varies.

Honest limits: if pressurisation is lost the masks deploy and the crew descend. See the oxygen masks discovery.

See it in Gate 41, in the cabin

Related questions

People involved