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In Jeppesen approach plates we see that the Transition Altitude of the aerodrome is fixed but the Transition Level is not fixed and is given by ATC. What is the reason for this?

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The point of flying on QNH is to keep you from flying into terrain, which is at a constant altitude above sea level, so TA is fixed.

Once you are high enough that terrain can’t be a problem and you only have to worry about hitting other planes, everyone can transition to QNE. However, because TL must be physically above TA, the difference between QNH and QNE will dictate what the lowest usable flight level is.

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    $\begingroup$ you might like to add this factoid: In some countries like the USA, they've addressed the issue of a different TLvl by having the TAlt and the TLvl = 18,000ft and FL180 respectively by choosing this value of TAlt so that it clears all terrain in the area of applicability. $\endgroup$
    – skipper44
    Dec 5, 2020 at 9:12
  • $\begingroup$ @skipper44 The FAA claims their TL is always FL180, but then they sometimes declare the lowest FLs to be “unusable” depending on QNH, which in practice is no different from having a variable TL. $\endgroup$
    – StephenS
    Dec 5, 2020 at 14:51
  • $\begingroup$ thanks for the elaboration. Here's more on lowest usable FL $\endgroup$
    – skipper44
    Dec 6, 2020 at 8:55

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