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The Pilots Desk
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ICAO Altimetry and Cruising Levels

QNH, QFE and QNE, the transition altitude and level, and how cruising levels are assigned — the altimetry a US pilot must relearn when flying abroad.

Altimetry is where a US-trained pilot most often trips up abroad. The physics are identical, but the settings, units and the point at which you switch differ from the FAA system.

The three settings

  • QNH — the setting that makes the altimeter read altitude above mean sea level. It is the everyday low-level setting, given by ATC or ATIS. Below the transition altitude you fly on QNH.
  • QNE — the standard setting, 1013.2 hPa (equivalently 29.92 inHg). Above the transition altitude everyone flies on standard, and the altimeter now reads a flight level.
  • QFE — a setting that makes the altimeter read height above a datum (often the aerodrome or runway). It is little used in the US but you will meet it at some ICAO airfields and in some military/gliding operations.

Units

Most ICAO states set pressure in hectopascals (hPa, formerly millibars); the US uses inches of mercury. The standard setting is the same pressure either way: 1013.2 hPa = 29.92 inHg. Be ready to set and cross-check in whichever unit the local altimeter and clearances use. Units of measurement are standardised by ICAO Annex 5.

Transition altitude vs transition level

  • The transition altitude is where, climbing, you switch from QNH to standard and start talking in flight levels.
  • The transition level is where, descending, you switch from standard back to QNH.
  • Between them is the transition layer.

In the US the transition altitude is a single value — 18,000 ft — everywhere. Under ICAO it is published per airport or region and is often much lower (commonly a few thousand feet), so you must check it for every arrival and departure.

Cruising levels

Cruising levels are assigned by direction of flight, but the convention and units vary: some states (China, Russia, Mongolia) use metric levels, and the semicircular/hemispheric rule can differ from the FAA convention. Fly the state's published rule, not your habit. For the wider picture see ICAO vs FAA key differences.

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*Reference and training only — not for navigation or as legal advice. Always consult the current 14 CFR, the AIM, your aircraft’s POH/AFM and a certificated flight instructor.*

Official sources
For reference and training only — verify current requirements with the official authority. Last reviewed July 24, 2026.