QNH

QFE

Description

Aircraft pressure altimeters indicate the elevation of the aircraft above a defined datum. The datum selected depends on the barometric pressure set on the altimeter sub-scale. Sound altimeter setting procedures are an essential tool in ensuring safe separation from the ground and from other aircraft.

The SI or metric unit of measurement for barometric pressure is the hectopascal (hPa) and this is adopted in respect of altimeter pressure settings in ICAO Annex 5. Variations from the standard shown in the Supplement to ICAO Annex 5 include:

• Millibar (mb), (1mb = 1 hPa);
• Inches of mercury (inHg) (in particular North America);
• Millimetres of mercury (CIS and some other south-east European States).

Three references for barometric pressure are in common usage: QNH, QFE and Standard Pressure.