The transition altitude tells the pilot whether they should be using a QNH setting or the standard pressure setting of 1013hPa.
Below the transition altitude, QNH should be used – airport QNH where available and regional QNH if there is no nearby airport QNH available. Above the transition altitude, we use the standard pressure setting (SPS) of 1013hPa.
The transition altitude is set by each airport across Europe and is usually broadcast on the ATIS, but is also found in the AIP.
An area exists between the Transition Altitude and Transition Level, known as the Transition Layer. In controlled airspace, a pilot will never be assigned an altitude or flight level that would place them inside the transition layer.
Above the transition altitude, pilots set the standard pressure setting of 1013hPa in the altimeter. Altitudes are then referred to as Flight Levels (FL). If the altimeter reads 6,000ft with the SPS set, this is called “Flight Level Six-Zero” – written as FL60.
Flying below the transition altitude, the pilot has the QNH set and 3,000ft would be called “Three Thousand Feet”.