Altitude: Difference between revisions

*The code [http://en.wikipedia.org/wiki/QNH '''QNH'''] is how a pilot would request the "atmospheric pressure at sea-level". In reply [[ATC]] will repeat QNH followed with the pressure. These days QNH is also transmitted automatically on [[ATIS]].

*If there is no [[ATIS]] available you can find the QNH value in Environment=>Global Weather.

To use a barometer as a tool to measure the altitude, the [[altimeter]], means it must be corrected for weather influences. For that the airfield has an accurate barometer and they know the height of the airfield above [[Mean Sea Level]]. From those two they will calculate the barometric pressure at [[Mean Sea Level]], the QNH. It will transmit the QNH and with that the altimeter in the aircraft can be corrected for weather influences and will display the [[#True height]]. This way the [[#Indicated altitude]] will be accurate for a certain area around the airfield as long as the weather remains stable.

QNH is a vital tool to use a height map of an area, hence it is vital during approach of an airfield. There have been dramatic accidents where pilots were flying using [[#Pressure altitude]] (during descend) while they should have used the [[#True altitude]] making the height map of the area useless. In bad visibility mountains have not been noticed and the ground was higher as expected.