There are several SE Q&As that deal with the terms Transition Altitude, Transition Layer, and Transition Level. The very-brief, paraphrased definition of the Transition Level (TL) is:
The lowest usable flight level that can be assigned, and the point at which aircraft descending out of the flight levels will change their altimeters back to QNH.
Due to atmospheric pressure variations, the TL can vary from day to day and (maybe?) region to region.
My five questions about the Transition Level are thus:
- Is the entire US using the same TL at any given point in time?
- Or, are individual ARTCCs setting their own TL based on the lowest QNH reading in their geographical area?
- Or, are individual sectors within an ARTCC setting their own TL based on the lowest QNH reading in their even-smaller geographical area?
If the individual sector controllers are NOT making the determination "on the fly", I then assume that the TL is determined by a "higher authority" and published to all controllers/sectors/centers under that authority. Which leads to the fourth and fifth questions:
- Who (what level, position, etc.) determines the TL?
- When, or how often, is this determination made?