Definition
A notation found on instrument approach charts indicating that the transition level — the lowest flight level available for use above the transition altitude — will be assigned by Air Traffic Control rather than being fixed and published. The transition level marks the boundary above which aircraft fly using standard pressure setting (flight levels) and below which they use the local altimeter setting (altitudes in feet).
Plain English
On the chart, instead of telling you the exact level where you switch from flight levels back to altitudes, it says ATC will tell you. The controller gives you the number when you need it.
Context Anchor
Seen on instrument procedures, especially outside the United States or in procedures that use transition altitude and transition level information.
Derivation
‘Transition’ comes from Latin transire, ‘to go across.’ The transition level is the level you cross when changing between two altimeter-setting systems. ‘BY ATC’ simply means the value is supplied by Air Traffic Control rather than printed on the chart.
Why Pilots Care
Pilots must obtain the transition level from ATC before changing from altitude to flight-level references to maintain proper vertical separation.
Grounding Statement
This note tells you that the changeover point for the altimeter setting comes from ATC, not from a fixed number printed on the chart.
Intuition Check
Do not read level as meaning wings-level flight or a flat attitude. Here, level means an altitude or flight level used for vertical separation; BY ATC means ATC assigns it, not the pilot.
Example Sentence 1
The approach plate showed ‘TRANS LEVEL: BY ATC,’ so we waited for the controller to assign it during our descent into the terminal area.
Example Sentence 2
Because the airport uses variable transition levels, the procedure shows TRANS LEVEL: BY ATC instead of a specific number.