THE SIMPLE ALTIMETER
The Altimeter gives the pilot information about the aircraft's height or altitude. This instrument relies on the fact that air pressure decreases with altitude. It works on the same principle as the barometer seen in many homes as a source of weather information. Air pressure decreases by around 1millibar per 30 feet and the altimeter shows this decrease in air pressure as an increase in height. However the air pressure at sea level is not constant: it varies from place to place and time to time. Therefore it is necessary to provide the instrument with a means of adjustment to allow for the changes in air pressure at sea level. Current air pressure information is given to the pilot, by air traffic control, before take off and at various stages of the flight.
For example before take-off the pilot might be informed that the QNH (one of several “Q” codes used in radio) is 1001 millibars. He will then set this figure on the altimeter by means of a small knob and small window which displays the figure set. The altimeter will then indicate the aircraft's height above sea level - useful for the en route phase. Prior to landing the air traffic control will give the pilot the “QFE” setting: this is the air pressure at airport level. Once the pilot sets this figure the altimeter will indicate the height above the airfield. The pilot can then use the height information given by the altimeter to ensure he is the correct height above the runway at the various stages of the landing.