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transition altitude[tran′zish·ən ′al·tə‚tüd]
The altitude in the vicinity of an aerodrome at or below which the vertical position of an aircraft is controlled by reference to true altitude.
The altitude at or below which the vertical position of an aircraft is controlled by reference to altitudes (ICAO). The height above the aerodrome of the transition altitude is as low as possible but normally not less than 3000 ft (900 m). The calculated height of the transition altitude is rounded up to the next full 1000 ft (300 m). The transition altitude is always indicated on instrument approach charts. It is the altitude when altimeter settings are changed from QNH to QNE (i.e., 1013.2 hectopascals/29.92 in of mercury) when climbing and from QNE to QNH or QFE when descending. See also QFE, QNE and QNH.