respect to the standard atmosphere, all aircraft instruments are calibrated for the standard atmosphere. In order to properly account for the nonstandard atmosphere, certain related terms must be defined. Pressure Altitude Pressure altitude is the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.92 "Hg (1,013.2 mb) as measured by a barometer. An altimeter is essentially a sensitive barometer calibrated to indicate altitude in the standard atmosphere. If the altimeter is set for 29.92 "Hg SDP, the altitude indicated is the pressure altitude. 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 or above 18,000 feet. The pressure altitude can be determined by one of the following methods: 1.
Setting the barometric scale of the altimeter to 29.92 and reading the indicated altitude
according to the reported altimeter setting
SDP is a theoretical pressure altitude, but aircraft operate in a nonstandard atmosphere and the term density altitude is used for correlating aerodynamic performance in the nonstandard atmosphere. Density altitude is the vertical distance above sea level in the standard atmosphere at which a given density is to be found. The density of air has significant effects on the aircraft’s performance because as air becomes less dense, it reduces:
Power because the engine takes in less air
Thrust because a propeller is less efficient in thin air
Lift because the thin air exerts less force on the airfoils