Pressure altitude
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 inHg (1,013.2 mbar, 14.696 psi) as measured by a barometer.
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.
Old altimeters were typically limited to displaying the altitude when set between 950 mb} and 1030 mb.
This setting is equivalent to the atmospheric pressure at mean sea level (MSL) in the ISA. Pressure altitude is primarily used in aircraft-performance calculations and in high-altitude flight (i.e., above the transition altitude).
QNE is an aeronautical code Q code. The term refers to the indicated altitude at the landing runway threshold when 1013.25 mb or 29.92 inHg is set in the altimeter’s Kollsman window. In other words, 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 ft) using the following rule of thumb formula:
Pressure Altitude (PA) = Elevation + 1000 * ( 29.92 – Altimeter Setting )
Pressure altitude is primarily used in aircraft-performance calculations and in high-altitude flight (i.e., above the transition altitude).
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