Transition altitude explained — why it’s not always 18,000 ft

Published 3 June 2026Reference article

Transition altitude is the height at which an aircraft switches from a local barometric pressure setting to the international standard (29.92 inHg / 1013.25 hPa). In US airspace it is fixed at 18,000 feet. Everywhere else it is variable — sometimes by state, sometimes by aerodrome — and in the UK it is in the middle of a multi-year transition to a single higher value. The number a simmer needs is the one on the current chart, not the one in any reference book.

The mechanism

Below the transition altitude, every aircraft sets the local QNH (or, in the US, the local altimeter setting in inHg). At or above the transition altitude, every aircraft sets the standard 29.92 inHg / 1013.25 hPa and reports altitude as a Flight Level rather than as feet MSL.

The point of the convention is that aircraft cruising at high altitude all reference the same pressure datum and are therefore vertically separated by reading the same scale. Aircraft below the transition altitude, near terrain and airports, reference local pressure so their altimeters read a more accurate height above sea level for terrain clearance.

FAA — 18,000 feet, fixed

In US airspace the transition altitude is 18,000 feet MSL, anywhere, any aerodrome. At and above 18,000 feet all aircraft set 29.92 inHg and the altitude is FL180. This fixed boundary is one of the conventions that makes US ATC simpler than international ATC; a pilot never has to brief the transition altitude for a US destination because it never changes.

Source: FAA AIM Para 7-2.

UK CAA — variable, mid-harmonisation

UK transition altitudes are typically 3,000 to 6,000 feet and depend on aerodrome and surrounding airspace. Heathrow, Gatwick, Manchester, Glasgow, and Edinburgh each have their own value. The pilot is expected to read it off the current chart or the destination ATIS, not to memorise it.

The UK is harmonising the transition altitude across UK airspace at 18,000 ft to align with North America — set out in CAA DAP Policy 106 and delivered jointly with the Irish Aviation Authority across the UK/Ireland Functional Airspace Block, as part of the wider UK Future Airspace Strategy. The transition is happening aerodrome by aerodrome on independent timelines. The current transition altitude for any UK aerodrome is the one on the current chart, not the one in any reference book. This is one of the two areas where the book most quickly drifts out of date — it is listed on the errata page for that reason.

Source: CAP 413 Edition 24.1, Chapter 7; UK CAA DAP Policy 106 (Harmonisation of Transition Altitude in Controlled Airspace).

EASA — variable by state

SERA harmonises the procedure across EU member states but leaves the numerical value to each state. Most EU transition altitudes are in the 3,000 to 6,000 ft band; Spain and a few others run higher. Within the EU, the value is published in the AIP and on charts.

Source: SERA Annex Section 14 (© European Union).

Transport Canada — 18,000 feet, US-aligned

Canadian transition altitude is 18,000 feet, matching the FAA convention for North-American interoperability. A westbound flight from EGLL via Iceland and Greenland into CYYZ encounters two transition-altitude regimes: UK variable on departure, Canadian fixed on arrival.

Source: TC AIM 2026-1, RAC Chapter 7.

The crossing problem

An eastbound transatlantic flight from KJFK to LFPG transitions from FAA airspace (FL180 transition) to Gander Oceanic, then Shanwick Oceanic (managed jointly by NATS for the UK CAA), into French DSNA airspace under SERA. On the climb out of New York the aircraft sets standard at 18,000 ft. Descending into Paris the aircraft sets QNH in hectopascals back at the French transition altitude — typically around 5,000 ft.

That descent crosses two unit boundaries in a few minutes: the standard pressure scale changes from inHg to hPa, and the transition altitude changes from a familiar 18,000 ft to an unfamiliar local value that has to be briefed in advance. Most readback errors on transatlantic descents start here.

Online network implications

VATSIM and IVAO controllers expect pilots to brief the destination’s transition altitude before descent and to switch units appropriately. If a UK controller hears “set 29.92” from a pilot descending through FL180 into an airspace whose transition altitude is 5,000 ft, they will correct gently — but they will also note the pilot has not briefed the destination. Brief the destination chart or ATIS once on initial contact and the rest follows.