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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:

  1. Is the entire US using the same TL at any given point in time?
  2. Or, are individual ARTCCs setting their own TL based on the lowest QNH reading in their geographical area?
  3. 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:

  1. Who (what level, position, etc.) determines the TL?
  2. When, or how often, is this determination made?
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The "Lowest Usable Flight Level" ("Transition Level" [TL] - term used outside North America) is FL180 when the (local) altimeter setting within a controller's area of jurisdiction is 29.92 inHG or greater. This is normally, not necessarily always, the province of ARTCC controllers in the U.S.

Your Questions:

Is the entire US using the same TL at any given point in time?

  1. This is not the case because the local pressure in San Francisco can be (for example) 28.92 and the pressure in Miami could be 30.20. Therefore, the lowest assignable/usable FL over San Francisco would be FL 190 and the lowest assignable/usable FL over Miami would be FL 180.

From FAA JO 7110.65W, para. 4-5-4: JO 7110.65W

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Or, are individual ARTCCs setting their own TL based on the lowest QNH reading in their geographical area?

  1. Individual areas of jurisdiction within ATC Facilities would be responsible for determining the Lowest Usable Flight Levels based on local QNH/altimeter settings. Keep in mind that some sectors (areas of jurisdiction a single controller may be responsible for) can be enormous in size (part of Nevada west to near San Francisco, for example). There can be a significant difference in local QNH settings within that sector that may require adjustments to the Lowest Usable Flight Level.

Or, are individual sectors within an ARTCC setting their own TL based on the lowest QNH reading in their even-smaller geographical area?

  1. See the answer above.

  2. and 5. In my experience, an individual controller maintains all separation authority and responsibility within his/her assigned sector. There may be supervisory input or oversight, but the responsibility to separate aircraft in accordance with the JO 7110.65 rests with the assigned controller. There also may be internal SOP's that dictate how a facility will handle the flow of the information regarding lower than 29.92 local altimeter settings.

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  • $\begingroup$ Thanks, @757toga. Actually, #1 could theoretically be possible, and is why I asked the question. In your example (and in my theory), if the pressure over SFO was the lowest in the nation, then the ATCSCC would set the entire NAS to a Lowest Usable Flight Level of FL190. (your entire answer clears up that this is not the case -thank you!) Re: TBL 4-5-2, I found this table in my initial research. I just didn't go all the way to the source and read the words, "in your area of jurisdiction". Thanks for pointing that out. $\endgroup$ – Jimmy Nov 6 '18 at 23:37
  • $\begingroup$ Without much inside knowledge of ATC processes, I had envisioned someone at the ATCSCC using Table 4-5-2, perhaps every 6 hours, to determine a nation-wide LUFL and publishing that somewhere/somehow to all ARTCCs. Or, perhaps someone at a CWSU performing the task and publishing the LUFL to all of that center's sector controllers. $\endgroup$ – Jimmy Nov 7 '18 at 0:19
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    $\begingroup$ @Jimmy - consideration for TLs /LUFLs is much more active outside of North America because of the relatively large number of aircraft that are affected with the lower TLs/TAs. The number of aircraft in the U.S. that are at a cruise altitude (either temporarily or for long periods) near FL180/FL190 for which LUFL is at issue is comparatively small and fairly easily accommodated as necessary. $\endgroup$ – 757toga Nov 7 '18 at 0:51

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