Why Do Planes Fly at 35,000 Feet？

Why do passenger jets cruise around 35,000 feet — and why don't they just keep climbing? This documentary follows one airliner through a flight to show how lift, drag, jet engines, fuel weight, winds and cabin pressurization decide its cruising altitude.

You'll see why planes fly so high in the first place: thinner air means less resistance, so an airliner can fly faster for about the same drag and cover more distance per kilogram of fuel. But the wings still need enough air to lift the aircraft, and the engines need enough air to make thrust — which is why there's a limit to how high an airliner can usefully go. We also look at step climbs (why the best altitude rises as fuel burns), how jet streams, weather and air traffic control shape the altitude a flight actually gets, and how cabin pressure keeps passengers comfortable more than ten kilometres up.