Flight levels: why planes fly so high
Why airliners cruise at 35,000 feet, what "flight level 350" means and how aircraft flying in opposite directions are kept at different heights.
The short answer
Jets cruise high because the air is thinner, so there's less drag and they burn less fuel, and they're above most of the weather. Up there, height is measured as a "flight level": FL350 is roughly 35,000 feet, measured against a standard pressure that every aircraft uses, so they all agree on who is higher.
The captain says you’re cruising at 37,000 feet. On a flight tracker it says FL370. Same thing, almost.
Why so high?
- Thinner air. Less air means less drag, so a jet engine uses less fuel to go the same speed.
- Above the weather. Most clouds and rain are lower down. Big thunderstorms are the exception.
- Space. Higher up there’s room to separate more aircraft from each other.
Short flights don’t climb as high, because they’d spend most of the trip climbing and descending.
What a flight level is
Air pressure changes with the weather, and an aircraft’s altimeter works by measuring pressure. If every aircraft used the local pressure, two of them could think they’re at different heights when they’re not.
So above a certain height, called the transition altitude, every aircraft sets the same standard pressure: 1013.25 hectopascals. The height it then shows, in hundreds of feet, is the flight level. FL350 means 35,000 feet on that standard setting.
Below the transition altitude, pilots use the local pressure, so their height above the airport is accurate for landing. In the UK the transition altitude is usually between 3,000 and 6,000 feet, depending on where you are.
East is odd, west is even
To keep aircraft flying in opposite directions apart, the direction of flight decides the level. Broadly, aircraft heading east fly at odd levels (FL350, FL370) and those heading west at even ones (FL360, FL380). Controllers can change that when the traffic needs it.
Between FL290 and FL410, aircraft with the right equipment are kept just 1,000 feet apart vertically. That’s how so many fit in the busiest part of the sky. Read more in how we keep planes apart.