Given an atmospheric pressure measurement, the pressure altitude is the imputed altitude that the International Standard Atmosphere (ISA) model predicts to have the same pressure as the observed value.
The National Oceanic and Atmospheric Administration (NOAA) published the following formula[1] for directly converting atmospheric pressure in millibars (mb) to pressure altitude in feet (ft):
In aviation, pressure altitude is the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.921 inches of mercury (1,013.2 mbar; 14.696 psi) as measured by a barometer.
[2] It indicates altitude obtained when an altimeter is set to an agreed baseline pressure under certain circumstances in which the aircraft’s altimeter would be unable to give a useful altitude readout.
Examples would be landing at a high altitude or near sea level under conditions of exceptionally high air pressure.
Standard pressure, the baseline used universally, is 1013.25 hectopascals (hPa), which is equivalent to 1013.25 mb or 29.92 inches of mercury (inHg).
This setting is equivalent to the atmospheric pressure at mean sea level (MSL) in the ISA.
This may be interpreted as the lowest terms of the Taylor expansion of
The term refers to the indicated altitude at the landing runway threshold when
It is the pressure altitude at the landing runway threshold.
Most aviation texts for PPL and CPL exams describe a process for finding the pressure altitude (in feet) using one of the following rule of thumb formulae.
Internationally, pressure altitude is approximated as: For example, if the airfield elevation is
, then If the altimeter uses inches of mercury, as common in the United States, Canada, and Japan, the following formula is used: For example, if the airfield elevation is
, then Aircraft Mode “C” transponders report the pressure altitude to air traffic control; corrections for atmospheric pressure variations are applied by the recipient of the data.