The altitude shown by an altimeter with its subscale set to 1,013.25 mbar. It differs from the true altitude if the MSL (mean sea level) pressure changes or the temperatures aloft differ from those in the ICAO (International Civil Aviation Authority) standard atmosphere.
The altitude in standard atmosphere at which a given pressure will be observed. It is the indicated altitude of a pressure altimeter at an altitude setting of 29.92 inches of mercury, and is therefore the indicated altitude above the 29.92 constant pressure surface.
In aviation, pressure altitude is the indicated altitude when an altimeter is set to 101.3 kPa (29.92 inHg US and Canada). It is primarily used in aircraft performance calculations, and in high-altitude flight (Class A airspace in the United States, which is controlled airspace at or above 18,000 feet). In radio communication, it is referred to by the Q code QNE.
