This article’s goal is to help the air traffic controller to use the correct altimetry in his airspace. You will learn the different terms and how to use them.
In their airspace, air traffic controllers must define the transition altitude and transition flight level. These data are available through airfield ATIS information or on charts.
The transition altitude is the upper limit from the surface to use the local QNH altimeter setting, applicable to all aircraft defined inside the associated TMA (terminal area) where it is published.
The transition altitude is also :
Transition Altitude abbreviation defined by ICAO is TA.
Example of typical transition altitudes:
- In Europe, dependent on the airfield (usually below 10000ft and above 4000ft).
- In USA, there is one unique transition altitude at 18000ft.
The transition altitude of an aerodrome should not be below 3000ft.
The transition level is the lower limit to use the standard 1013hPa altimeter setting, applicable to all aircraft defined inside the associated TMA (terminal area) where transition altitude is published.
The transition level is also:
Transition Level abbreviation defined by ICAO is TRL.
Note that in some charts or documentation, you may find the use of TL abbreviation instead of TRL.
The transition layer is the airspace located between the transition altitude and the transition level. The transition layer is defined inside the associated TMA (terminal area) where the transition altitude is published.
The altitude of the transition level shall always be greater than or equal to the transition altitude.
No cruise flight in the transition layer is permitted. An aircraft can only cross the transition layer.
The transition layer thickness is laid down in country regulations and can be:
Consult your national regulation documentation or ask your training or ATC operation staff in order to have this value. Per default use the value "Between 1000ft and 1999ft".
For flights in the vicinity of aerodromes and within terminal control areas the vertical position of aircraft shall be expressed in terms of:
While passing through the transition layer, the vertical position shall be expressed in terms of flight levels when climbing and in terms of altitudes when descending.
For flights en route, the vertical position of aircraft shall be expressed in terms of:
The default transition altitude, outside TMA or airspaces with no altitude transition published, should be 3000ft above the surface (height). In this case there is no transition level.