Flight levels explained: why 'FL350' is not 35,000 feet on the ground
What a flight level like FL350 means, how altimeter settings change it, the transition altitude and layer, and why a missed pressure setting causes level busts.
If you have heard a pilot say "climbing to flight level three-five-zero" you may have wondered why the number is not an altitude in feet. The short answer is that above a certain height, aircraft stop measuring altitude against local air pressure and use a standard setting instead, and flight levels are the result.
Altitude and flight level are different things
An altitude is a height above sea level, set using the local pressure reading, called QNH. A flight level is a surface of equal pressure, measured using a fixed standard setting of 1013.2 hPa (29.92 inHg). It is written in hundreds of feet, so FL350 means roughly 35,000 feet on the standard setting, not 35,000 feet above the ground.
Because everyone uses the same standard setting above the transition altitude, two aircraft flying at FL350 in the same airspace are on the same pressure surface. That makes spacing simpler for controllers than comparing local readings.
The transition altitude and the transition layer
The transition altitude is the height at which pilots switch from local QNH to the standard setting while climbing. It is fixed for a given area. In the United States and southern domestic Canadian airspace it is 18,000 ft. In most other countries it varies and can be as low as 3,000 ft.
The transition level is the flight level at or above which the standard setting applies, and it moves with the local pressure. Between the transition altitude and the transition level is the transition layer. Climbing aircraft pass through it using flight levels; descending aircraft use altitudes. Cruising inside the transition layer is not allowed unless air traffic control clears it.
Why the standard setting causes errors
Forgetting to switch to the standard setting, or switching at the wrong point, is a known cause of level busts, where an aircraft ends up at a different level from the one it was cleared to. The UK Civil Aviation Authority has published a safety notice on this. Its example: with a local QNH of 983 hPa, failing to set the standard 1013 hPa would leave an aircraft climbing 900 feet above its cleared level. The same notice also warns that a value such as 992 hPa can be misread as 29.92 inHg, so pilots check units carefully.
For professionals
- Procedures follow ICAO Doc 8168 (PANS-OPS). Each state publishes its own transition altitudes in its AIP, so check the figure for the airspace you are flying in.
- Standard-setting errors are one reason cockpit altimeter cross-checks and controller readbacks matter.
- We have not reviewed the current AIP of every country. For operational use, rely on the AIP and your operator's procedures.
Sources
- Flight level, overview
- UK CAA publication on altimeter setting and level busts
- Thailand AIP, ENR 1.7 altimeter setting procedures
Featured image: Photo: PattayaPatrol, CC BY-SA 4.0, via Wikimedia Commons. Source
