What are flight levels?
Above a certain height, aircraft stop quoting feet above the ground and use a shared pressure scale called flight levels.
Why a shared scale
Air pressure changes with the weather. If every pilot used a different setting, heights would not agree. Above a set height, everyone uses the same standard setting, so that aircraft are separated properly.
Reading a flight level
A flight level is the height in hundreds of feet. FL350 means about 35,000 feet. Below the changeover, pilots use the local pressure setting and speak in feet.
Where the changeover is
It varies by country. In the USA it is 18,000 feet. In the UK and many other countries it is set by the airspace and is much lower.
Watch for it
Heights on AirLive’s map are in feet, taken from the aircraft’s own report. Cruising aircraft are typically between FL300 and FL400.