The National Oceanic and Atmospheric Administration (NOAA) has published the following formula for directly converting atmospheric pressure in millibars () to pressure altitude in feet ():
In aviation, the pressure altitude is the indicated 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 readout of the altitude. Examples would be landing at a very 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 and . Standard pressure, the baseline used universally, is hectopascals (), which is equivalent to or inches of mercury (). 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).
- The term QNE refers to the indicated altitude at the landing runway threshold when or is set in the altimeter’s Kollsman window. In other words, it is the pressure altitude at the landing runway threshold.
For example, if the airfield elevation is and the altimeter setting is , then
For example, if the airfield elevation is and the QNH 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.
The relationship between static pressure and pressure altitude is defined in terms of properties of the ISA.
- Flight level
- Density altitude
- Standard conditions for temperature and pressure
- Barometric formula
- "Pressure Altitude" (PDF).