Science Puzzle
Why Do Airliners Fly So High?
A passenger jet spends about 20 minutes climbing to around 11 kilometres, higher than the top of Mount Everest, before levelling off for the rest of its journey. Climbing that high uses a lot of fuel.
Why do airliners bother to fly so high?
The Answer
Because the air is thin. At about 11 kilometres the air is less than a third as dense as at the ground, so there's far less of it to push out of the way and the plane can fly fast on much less fuel.
Drag, the air pushing back on a moving plane, depends on how much air the plane has to shove aside each second. Thinner air means less drag. A jet at cruising height travels at around 900 km/h; at the same speed near the ground, the drag would be three times as great and the fuel bill enormous.
There's a limit, though. A wing makes lift by pushing air downwards, and the engines need oxygen to burn fuel. Go too high and the air is too thin for either, so airliners settle at about 10 to 12 kilometres, the best balance between the two. Flying up there has a bonus: it's above most of the weather, so the ride is smoother.
Gravity barely changes. At 11 kilometres it's only about a third of 1 per cent weaker than at the ground, far too little to matter.
The principle: Drag depends on air density. Thin air high up pushes back far less on a moving plane, so it can fly fast on much less fuel.