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. 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 American half of that is where the neat story usually breaks. It is very widely taught that because the US transition altitude is 18,000 feet, the lowest usable level above it is FL180, and that is not what the rule says. 14 CFR 91.121(b) makes FL180 the lowest usable flight level only when the local altimeter setting is 29.92 inHg or higher. As the pressure falls, the table in the rule steps the lowest usable flight level up with it in 500 ft increments, through FL185, FL190, FL195, FL200 and FL205, reaching FL210 in the lowest band the table covers (an altimeter setting of 27.41 to 26.92 inHg), precisely so that a real gap is preserved above the fixed transition altitude rather than allowing the flight-level band to sink into it. The transition altitude in the US is fixed; the transition level above it is not.
How deep the layer has to be
The transition layer is not just "whatever is left over". In the European region ICAO Doc 7030 requires it to be at least 1000 feet deep, and that requirement, rather than any tidy arithmetic, is what actually fixes the transition level for a given day. It has to, because the two boundaries are measured on different subscales: the transition altitude is an altitude on QNH, while the transition level is a flight level on 1013. When the QNH is below standard, an altimeter set to 1013 over-reads, at roughly 27 feet per hectopascal, so a given flight level sits lower above the sea than its number suggests, and the layer is thinner than it looks. The lower the pressure, the higher the transition level has to be pushed to keep 1000 feet in hand. This is why you take the transition level from ATC rather than working it out from the transition altitude alone.
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 a European 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 FL075 and a QNH of 1004. You descend on standard reading flight levels, and as you pass FL075 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.
Work FL075 out for yourself and the rule stops being arbitrary. The layer has to be at least 1000 feet, so its base must sit at 7000 feet above mean sea level or higher: the 6000-foot transition altitude plus a thousand. But the transition level is read on 1013, not on 1004. The QNH is 9 hPa below standard, and at about 27 feet to the hectopascal an altimeter on the standard setting over-reads by roughly 9 x 27 = 243 feet. So 7000 feet AMSL indicates as about 7243 feet on 1013, which is FL72 and a fraction: not a usable level.
Check the levels either side. FL070 sits at about 7000 - 243 = 6757 feet AMSL, which leaves a layer of only 757 feet above the transition altitude: too thin. FL075 sits at about 7500 - 243 = 7257 feet AMSL, which leaves 1257 feet: comfortably legal. FL075 is the lowest usable level, which is exactly the one ATC gave you. Raise the QNH towards 1013 and the same aerodrome's transition level would come back down; drop it further and it would climb again.
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.
- "US transition altitude 18,000 feet, so FL180" is not the rule. Under 14 CFR 91.121(b), FL180 is the lowest usable flight level only when the altimeter setting is 29.92 inHg or higher; below that it steps up in 500 ft increments, as far as FL210 in the lowest band the table covers.
- Do not compute the transition level from the transition altitude alone. The two sit on different subscales, and in Europe the layer must be at least 1000 ft, so the answer moves with the QNH. Take the level ATC gives you.
- 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.