Harmonisation of Transition Altitude in Controlled Airspace

The Transition Altitude (TA) is the altitude at or below which the vertical position of an aircraft is controlled by reference to altitudes. The vertical position of aircraft at, or below, any Transition Altitude will normally be expressed in terms of Altitude. The vertical position at, or above, any Transition Level will normally be expressed in terms of Flight Level. When descending through the Transition Layer the vertical position will be expressed in terms of Altitude, and when climbing in terms of Flight Level.