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Altimeter Pressure Settings
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| Category: | General | 
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| Content source: | SKYbrary | 
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| Content control: | EUROCONTROL | 
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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 standard unit of measurement for barometric pressure is the hectopascal (hPa) (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) (especially North America);
- Millimetres of mercury (uncommon).
Three references for barometric pressure are in common usage: QNH, QFE and Standard Pressure (also referred to as QNE).
With QNH set, an aircraft altimeter indicates height above mean sea level. The QNH may be the pressure observed at the airfield, or the lowest pressure observed throughout a specified geographical area (see figure 1).
With QFE set, an aircraft altimeter indicates height above the QFE datum, which may be a point in the aerodrome environment or the touch-down zone of the runway in use (See Figure 2).
With Standard Pressure (1013.2 millibar) set, an aircraft altimeter indicates Pressure Altitude (Flight Level), and is used by all aircraft operating above the transition altitude to provide a common datum for vertical measurement. The Standard Pressure is equivalent to the air pressure at mean sea level (MSL) in the International Standard Atmosphere (ISA).
Definitions
- QNH. The altimeter sub-scale setting to obtain elevation when on the ground.
- QFE. The atmospheric pressure at aerodrome elevation (or at runway threshold).
(ICAO, e.g. Doc 4444: PANS-ATM and Doc 8168: PANS-OPS)
Related Articles
Further Reading
ICAO:
- Annex 5: Units of Measurement;
- Annex 3: Meteorological services;
- Doc 4444: PANS-ATM;
- Doc 8168: PANS-OPS.
Flight Safety Foundation ALAR Toolkit