I think the questioner has got it slightly confused; unless they have changed things in the last 16 years since my last visit. There is a nationwide TA of 18,000'. In EU there are numerous, and some at airfields quite close to each other, but just across an FIR/AIS boundary. Very confusing and perhaps not good in this TEM world. It has been discussed before why UK and/or continental EU does not have a single TA. I do not know the state of that debate.
In the USA, outside Alaska the highest spot is 15,325', according to wikipedia. I assume this was used to decide upon 18,000'. In UK I remember the highest MSA to be 5800'; I stand to be corrected. In Continental Europe, outside eastern Europe, it is 18,600'??
So UK could use a universal 8000' to allow for various weather corrections; or even 10,000'. Continental Europe, using the USA model would go for 20,000'. Both these should allow for pressure & cold weather corrections and be easy to remember.
I wonder if this is what the questioner was alluding to?
An aside: the USA 18,000' has always made me believe why the the Boeing QRH has the Emergency Descent to 10,000' or MSA if higher. It is the correct setting in USA i.e. QNH. In EU controlled airspace 99% of the time you will be above T.LVL and thus every other a/c will be on STD. Will ATC really let you fly around in their airspace on QNH when above TA? I very much doubt it, and stand to be corrected by an ATC-er. Please, please tell us what you would do with an a/c at 10,000' QNH in the Ambers?
Thus, if UK had TA = 10,000 and continental Europe 20,000' then you could descend to 10,000 QNH in all areas, MSA allowing.
By the way; I was severely bollocked, in the sim, for descending FL 80 over UK so not to cause an RA. WRONG according to QRH; but that's for another thread.
Last edited by RAT 5; 4th Nov 2015 at 20:05.