Transition Altitude
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Transition altitude is a term used in aviation. It is the highest altitude at which an aircraft in normal operation should use an altimeter pressure setting indicating height above mean sea level (QNH). Above the transition altitude (TA) the aircraft altimeter pressure setting should be adjusted to the standard pressure setting of 1013.2 hectopascals or 29.92 inches of mercury and aircraft altitude will be specified as a flight level.
This is distinct from the Transition Level, the flight level at which, on descending, the pilot starts to refer to altitude of the aircraft by setting the altimeter to the QNH for the region or airfield. Note that the Transition Level is the lowest flight level for the atmospheric conditions, but aircraft do not use the first 500 feet above the current Transition Level. Therefore the lowest usable flight level is the Transition Level plus 500 feet.
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