What is transition altitude and when do flight levels begin?
Learn what transition altitude means, when pilots select standard pressure and flight levels, and why the transition level may be different.
Transition altitude is the published altitude at or below which pilots use a local pressure reference, normally QNH, and report vertical position as an altitude. Climbing through it, they select standard pressure—1013.25 hPa, usually displayed as 1013, or 29.92 inHg—and use flight levels. Descending, they change back at the transition level.
What is the difference between transition altitude and transition level?
Transition altitude is the climb boundary; transition level is the descent boundary. They are deliberately separate because local atmospheric pressure changes, while flight levels are based on one common standard-pressure datum.
| Term | Meaning | Practical use |
|---|---|---|
| Transition altitude | Published altitude below which vertical position is expressed as an altitude | Change from QNH to STD while climbing |
| Transition level | Lowest usable flight level above the transition altitude | Change from STD to QNH while descending |
| Transition layer | Airspace between the transition altitude and transition level | Provides separation between altitude-based and flight-level traffic |
The transition altitude is normally fixed for an airport or area. The transition level may move as QNH changes so that the lowest usable flight level remains safely above the highest altitude. Level flight is not normally assigned within the transition layer.
A flight level describes a standard-pressure surface, not an exact height above mean sea level. Our explanation of the practical difference between altitude and flight level covers that distinction in more detail.
When exactly do pilots change the altimeter setting?
On climb, pilots select STD when crossing the transition altitude; on descent, they select the destination QNH when crossing the transition level.
- Before departure: Set the reported QNH and verify that the altimeter shows approximately the known aerodrome elevation.
- During climb: Keep QNH set until reaching the published transition altitude, then select STD and cross-check the standard-pressure indication.
- Above the transition altitude: State vertical position as a flight level. A standard-pressure indication of 7,000 feet is spoken as flight level 70, or FL70.
- Before descent: Obtain the destination QNH and transition level from the relevant weather broadcast, ATIS or controller.
- During descent: Keep STD set until passing the transition level, then set QNH and continue using altitude references.
For example, with a transition altitude of 6,000 feet and a transition level of FL70, the climbing change occurs at 6,000 feet on QNH. Descending traffic changes back to QNH when passing FL70. The space between those references is the transition layer.
For most civil flying, the local reference is QNH; QFE is an exception used under particular local procedures. Our guide to QNH and standard pressure explains what each setting makes the altimeter display.
Is transition altitude always 18,000 feet?
No. Eighteen thousand feet is the normal transition altitude in the United States, but it is not a worldwide standard.
Other countries may publish a national transition altitude or different values for individual airports and airspace areas. Values can be much lower than 18,000 feet, so pilots must use the applicable charts and local procedure rather than memory.
Even where transition altitude is fixed, transition level can vary with pressure. During low pressure in the United States, for example, the lowest usable flight level may be higher than FL180. Assuming that transition altitude and transition level always have matching numbers is a common and potentially serious error.
How should transition altitude be handled in a flight simulator?
Use the same sequence as real-world aviation: find the charted transition altitude, obtain the transition level and QNH, then change between QNH and STD at the correct boundary.
- Check the data source: An aircraft's FMS transition-altitude entry may disagree with an older chart or navigation database. Use one consistent data set rather than blindly following the first cue displayed.
- Expect the indication to move: Changing between QNH and STD can make the displayed value jump. That is a pressure-reference change, not necessarily an altimeter fault.
- Verify both sides: In a multi-crew aircraft, check that the captain's and first officer's altimeters show the same pressure mode and setting.
- Do not confuse unrelated changeovers: The switch from IAS to Mach is based on aircraft performance and altitude, not the transition altitude. See our IAS and Mach changeover explanation for that separate procedure.
Some simulated aircraft provide a flashing barometer indication or an FMS cue; others require the pilot to remember the change. Automation does not alter the rule. Microsoft Flight Simulator users who need the cockpit procedure can follow our MSFS 2024 pressure-setting instructions, bearing in mind that the exact control varies by aircraft.