QNH - Altitude Above Mean Sea Level

Altitude Above Mean Sea Level

QNH differs from QFE, which refers to the altimeter setting that will cause the altimeter to read the height above a specific aerodrome or ground level, and therefore read zero on landing. While using QFE is convenient while flying in the traffic circuit of an airfield, the most common procedure when flying 'cross country' is to set the altimeter to either the local QNH or the standard pressure setting – 1,013.25 hPa (29.92 inHg). When 1013 hPa (hectopascal or millibar) is set on an altimeter subscale the aircraft's vertical position (in feet, divided by one hundred) is referred to as a Flight level instead of an altitude.

Air Traffic Control will pass the QNH to pilots on clearing them to descend below the transition level, as part of air traffic control clearance, on request of the pilot or when the QNH changes. A typical radio conversation might be:-

• Pilot: , request Cotswold QNH
• ATC: , Cotswold QNH 1016
• Pilot: QNH 1016,

Here, the pilot requests the regional air pressure, which is given as 1016 hectopascals for the Cotswold Altimeter Setting Region ("ASR") (one of twenty ASRs into which UK Lower Airspace is divided). The pilot is required to read back the safety critical part of the transmission (in this case, the QNH value).

In most parts of the world, QNH is given in hectopascals (in the past in millibars) . In North America, QNH is given in hundredths of inches of mercury (in the example, ATC would say ", altimeter three zero zero one" meaning 30.01 inches of mercury).