Transition altitude and transition level
...transition altitude is constant, while the transition level is changing?
While the transition altitude (TA) is fixed and firmly defined for the one FIR, the transition level (TL) is changing with actual atmospheric pressure.
For example, in the Czech Republic the TA is set firmly on the altutide 5000 ft AMSL. The TL is usualy changing between FL50 and FL60, it rarely happens that the fluctuation of pressure is so great that TL is set at a lower or higher level.
If the aircraft is flying at an altitude below the TL it must always set the altimeter to local pressure value (in Europe mostly marked as QNH and measured in hPa). If the aircraft flies above the TA it must always set the standard pressure is 1013 hPa. Otherwise, it could reduce the separation between aircraft flying overhead!
Altimeter pressure setting is switched during the climb to a flight level (switching from local to standard QNH 1013 hPa), while during descend through the TL the setting is switched to the actual local presure.
For more information on altitudes and levels refer to our Wiki.