Aircraft Altimeter

a device for measuring the altitude above the earth’s surface at which an aircraft is flying. Aircraft altimeters include barometric and radio altimeters.

The operating principle of the barometric altimeter is based on the unambiguous relationship between atmospheric pressure and flight altitude. The pressure is sensed by the aneroid capsule of the altimeter. The deformation of the aneroid capsule, which is proportional to changes in pressure and, consequently, to the flight altitude, works through a system of levers to cause a corresponding rotation of the indicator hands of the instrument. The instrument scale is calibrated in kilometers, hundreds of meters, and tens of meters of altitude. The altimeter has a knob for forced rotation of the hands. This can be used to set the instrument for readings of true altitude (relative to the level at which the pressure is 101,325 newtons per sq m, or 760 mm of mercury, and the temperature is 15° C), relative altitude (relative to the takeoff point), or absolute altitude (altitude above the area being overflown).

The operating principle of the radio altimeter is based on the measurement of the time between the sending and reception of electromagnetic waves propagating through space at a constant velocity. The radio altimeter’s readings correspond to the absolute flight altitude.