What is a flight level?
A flight level is an altitude expressed in hundreds of feet based on a standard pressure setting (29.92 inHg / 1013.25 hPa). FL350 means 35,000 feet. FL240 means 24,000 feet.
Why not just use feet?
Below 18,000 feet, pilots set their altimeters to the local barometric pressure. This gives accurate height above sea level. But pressure varies between locations - two aircraft at the "same" altitude could actually be at different heights if they're using different pressure settings.
Above the transition altitude, everyone uses the same standard pressure. The actual height doesn't matter as much - what matters is that all aircraft at FL350 are at the same pressure altitude and therefore separated from aircraft at FL340 or FL360.
Transition altitude around the world
The transition altitude - the point where pilots switch from local pressure to standard pressure (flight levels) - varies significantly by country:
| Region | Transition altitude | Notes |
|---|---|---|
| United States | 18,000 ft (FL180) | Highest in the world. All airspace above FL180 uses standard pressure. |
| Canada | 18,000 ft (FL180) | Same as the US |
| United Kingdom | 3,000 - 6,000 ft | Varies by location. London area is 6,000 ft. |
| Europe (most) | 3,000 - 10,000 ft | Varies by country and airport. Germany uses 5,000 ft at most airports. France ranges from 3,000 to 5,000 ft. |
| China | 600 m (~2,000 ft) | Uses meters for altitude below transition |
| Japan | 14,000 ft | Consistent nationwide |
| Australia | 10,000 ft | Consistent nationwide |
| India | 4,000 ft | At most major airports |
The transition level (lowest usable flight level) may be higher than you'd expect because it depends on the current pressure. When pressure is low, the gap between the transition altitude and transition level widens, creating a "transition layer" where neither local altitudes nor flight levels are assigned.
In countries with low transition altitudes, pilots switch to standard pressure much sooner. A pilot departing Heathrow might be on flight levels by 6,000 feet, while a pilot departing JFK stays on local pressure until 18,000 feet.
Semicircular rule
Aircraft heading east (0-179 degrees) fly at odd flight levels: FL310, FL330, FL350, FL370. Aircraft heading west (180-359 degrees) fly at even flight levels: FL320, FL340, FL360, FL380. This provides built-in separation for opposite-direction traffic.
In radarcontrol.io
Use cfl350 or c350 to assign a flight level. The sim automatically applies the semicircular rule when spawning aircraft. Below FL180, altitudes are in feet (c12000ft).