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Regarding aircraft performance, two types of altitude are of most interest to a pilot: pressure altitude and density altitude.

Pressure altitude is that corrected altitude in the standard atmosphere corresponding to a certain static pressure. Pressure altitude is the vertical distance above a standard datum plane where atmospheric pressure is 29.92″. In the United States, at FL180 and above, the altimeter is always set to 29.92″ unless abnormally low pressure exists in the area. Pressure altitude is used in performance calculations to compute true airspeed, density altitude, and takeoff and landing data.

Figure 2.3 provides a convenient method to determine pressure altitude. For example, if a given airport elevation is 3500 ft and the automated weather observation states a pressure value of 30.10″, use the altitude correction column to determine that 165 needs to be subtracted from 3500 ft. The pressure altitude in the given condition is 3335 ft, which makes sense as it is lower. The pressure given is higher than standard pressure, thus the air density is higher, resulting in a lower pressure altitude.

Figure 2.3 Field elevation versus pressure altitude.

Source: U.S. Department of Transportation Federal Aviation Administration (2008a).