Why do planes fly so high?
Thin air means less drag for the same lift, so an airliner burns far less fuel up high — and the altitude also buys smoother air, weather avoidance and time to react if something fails.
Simple intuition
The plain reason, in everyday words
Air resistance is what an aircraft spends most of its fuel fighting, and there is much less air to push through at 11 kilometres up — roughly a quarter of the density at sea level. Less air means less drag, so the same aircraft needs far less thrust to hold the same speed. There is a catch, of course: thin air also produces less lift, so the aircraft has to fly faster through it to stay up. That turns out to be a good bargain rather than a wash, because the speed gained is worth more than the extra effort. An airliner in thin air travels much faster over the ground for less fuel per kilometre than the same aircraft down low. Everything else — flying above the weather, above most turbulence, above other traffic — follows from a decision that was made on fuel economy.
Planes fly high mainly to avoid bad weather.
Weather avoidance is a real benefit but a secondary one. The altitude is chosen for fuel economy, and the aircraft is designed around cruising there.
Higher is always better, so aircraft climb as high as they can.
There is an optimum for each weight, above which the lift penalty outweighs the drag benefit and the safe speed range narrows dangerously. The optimum rises during a flight as fuel is burned.
The air is thinner so the engines struggle.
Thrust does fall with density, but fuel flow falls too and the cold improves efficiency. Engines are designed for these conditions; the thrust available is sized for take-off, not cruise.
If the engines stopped at cruise the aircraft would fall out of the sky.
It would glide, typically 100 to 150 kilometres from cruising altitude. That range is part of the safety case for long overwater routes.
It is a good demonstration that an engineering decision is usually one variable being optimised with several benefits following behind — and that the reason people assume for a design choice is often the least important one. It also connects directly to why an airliner is built the way it is: cruising high forces cabin pressurisation, which drives the fatigue loading on the fuselage that shaped window design and inspection regimes.
Who worked it out
Early airliners flew low because unpressurised cabins limited them to altitudes people could breathe at, which meant flying through weather rather than over it.
What problem forced it
The Boeing 307 Stratoliner introduced a pressurised cabin in 1938, making higher cruise possible, and the arrival of jet engines in the 1950s made it desirable, since a jet is far more efficient at altitude than a piston engine.
How it changed since
The de Havilland Comet's fatigue failures in 1954 showed the cost of the pressurisation that high cruise requires, and the resulting damage-tolerance rules reshaped airframe design. Optimum cruise altitude has since been a routine flight-planning calculation rather than a fixed number.
Why aircraft cabins are pressurised
The direct cost of cruising high, and the loading that governs fuselage design.
How far an airliner can glide
The safety margin that altitude buys, and how it shapes route planning over oceans.
Written for Curio rather than collected from a forum — it is part of the curated corpus that ships with the platform. The references it draws on are listed under Sources.