TA and TRL
The TA (transition altitude) is determined by the CAA in every country, normally the same for the whole country, to make it easier for everybody. Here is where you fly on QNH.
Then we have the TRL (transition level). The TRL is the lowest flightlevel that can be used above the TA. Here is where you fly on standard setting, 1013,25. There are two schools. One is the US method where you have a fixed TRL and 2000 feet difference between the TA and the TRL. Here you never have to calculate the TRL but you lose an altitude (level).
In Europe we have a maximum of 499 feet between TA and TRL. This is calculated depending on air pressure and temperature. There is no separation between TA and TRL. This difference is called transition layer. For separation purposes you consider TA and TRL the same level. E.g. TA is 5000 feet and TRL FL 55 then the lowest flightlevel you can descend to is FL 70 to have separation.
As a general rule you can say that the higher up you fly the lower the pressure is.
If the QNH is 1013 or lower then the TRL is 55 or higher ( if TA is 5000 feet) and when the QNH is 1014 or higher the TRL is 50 or lower.
So, if you have e.g. QNH 1040 then TRL is FL 45 or lower but you are still flying above the TA which is 5000 feet on the QNH.
I hope this makes it clear for everybody!
Last edited by 2control; 18th Jun 2008 at 16:34.