Low down, you fly altitudes: feet above the sea, set on the local pressure. High up, you fly flight levels: a shared yardstick that ignores the local pressure entirely. The boundary where one gives way to the other is the transition altitude and transition level, and the small altimeter change you make there is one of the tidiest, most testable bits of procedure in flying.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
Two ways to measure height
Near the ground, what matters is your height above the terrain and obstacles, so you set the altimeter to the local QNH and read altitudes in feet above mean sea level. Aloft, what matters is staying separated from other aircraft, and for that everyone needs to be measuring from the same datum, regardless of whose local pressure is whose. So above a certain point all aircraft set the standard pressure of 1013 hectopascals, equivalently 29.92 inches of mercury, and read flight levels. With everyone on the same setting, a flight level is directly comparable between aircraft, which is exactly what vertical separation needs.
The altitude, the level and the layer
Three terms, defined in ICAO Doc 8168 and the FAA AIM:
- the transition altitude (TA) is the altitude at or below which you control your vertical position by reference to altitudes on QNH;
- the transition level (TL) is the lowest flight level available for use above the transition altitude, flown on the standard setting;
- the transition layer is the airspace between the two, which you pass through rather than cruise in.
Where the TA sits varies. In the United States it is fixed at 18,000 feet, so the lowest usable level above it is FL180. Across Europe and elsewhere the TA is lower and varies by aerodrome or region, published on the chart and often passed by ATC, which is why you confirm it rather than assume it. The European rules sit in the Standardised European Rules of the Air.
The rule for changing
The change-over is one line each way, and SKYbrary states it plainly:
- Climbing: set the standard pressure (1013 / 29.92) as you pass the transition altitude, and from then on report flight levels.
- Descending: set the local QNH as you pass the transition level, and from then on report altitudes.
A memory aid that holds in both hemispheres: going up, you reach the transition altitude first, so the altitude is where you swap to standard; coming down, you reach the transition level first, so the level is where you swap to QNH.
A worked example
Departing an aerodrome with a published transition altitude of 6000 feet on a QNH of 1004. You climb on QNH reading altitudes, and as you pass 6000 feet you wind the subscale to 1013 and call the next stop as a flight level, say "climbing FL080." You cruise at FL120 on the standard setting, directly comparable with the traffic around you.
Returning later, ATC passes a transition level of FL065 and a QNH of 1004. You descend on standard reading flight levels, and as you pass FL065 you set 1004 and revert to altitudes, "descending to 3000 feet on 1004." You did not level off inside the transition layer; you passed straight through it.
Common pitfalls
- Confirm the transition altitude; do not assume it. Outside the US it varies by place and is published or passed by ATC, so check it for the aerodrome you are at.
- Change at the right boundary. Standard at the transition altitude going up, QNH at the transition level coming down; mixing the two is a classic slip.
- Do not cruise in the layer. The transition layer is for passing through; level off above the TL or below the TA, not between them.
In Pilot EFB
Pilot EFB is a study and planning companion for altimetry and the altitude-to-level change-over, alongside the decoded pressure in your METAR and the rest of your briefing in one offline-first place. It does not set your altimeter or replace ATC and the published procedures, so fly the transition altitude and level from your official source of record. Pilot EFB is not a certified Electronic Flight Bag, so treat it as a study and planning aid and brief from your official source of record.