PRESSURE ALTITUDE

Pressure altitude is the height above a standard datum plane. The airplane altimeter is essentially a sensitive barometer calibrated to indicate altitude in the standard atmosphere. If the altimeter is set for 29.92 in. Hg Standard Datum Plane (SDP), the altitude indicated is the pressure altitude—the altitude in the standard atmosphere corresponding to the sensed pressure.

The SDP is a theoretical level where the weight of the atmosphere is 29.92 in. Hg as measured by a barometer.

As atmospheric pressure changes, the SDP may be below, at, or above sea level. Pressure altitude is important as a basis for determining airplane performance as well as for assigning flight levels to airplanes operating at above 18,000 feet.

The pressure altitude can be determined by either of two methods:

1. by setting the barometric scale of the altimeter to 29.92 and reading the indicated altitude, or
2. by applying a correction factor to the indicated altitude according to the reported "altimeter setting."