In aviation, pressure altitude is the indicated altitude when an altimeter is set to an agreed pressure setting. This setting, 101.325 kPa - equivalent to 1013.25 millibar, or 1013.25 hectopascals, or 29.92 inches Hg, is a baseline pressure setting equivalent to the International Standard Atmosphere (ISA) at sea level. 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 MSL. In radio communication, it is referred to by the Q code QNE.
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