In the US, is there any FAA Regulation that keeps all aircraft away from 18,000 feet MSL… Since theoretically, FL190 could be lower than 18,000 feet MSL, and create all kinds of confusion with keeping safe aircraft separation?

  • $\begingroup$ Can you perhaps make your question more specific? As it is right now, the title itself isn't a question and in the body you seem to ask two different things: a) are there regulations that prohibit flying at 18,000 MSL, and b) how is separation provided close to the transition level/altitude? It's not clear to me at least what your question really is. $\endgroup$
    – Pondlife
    Jul 23, 2018 at 13:19

2 Answers 2


It's not the altitude that is adjusted, but the flight level. Aircraft are allowed to fly at 17,500 feet MSL. Per the rules provided in other answers, this would be VFR flight to the north or east. an IFR aircraft could be flying at FL180, and this would provide 500 feet of separation.

However, 14 CFR 91.121 determines the lowest usable flight level. If the altimeter setting is 29.92 or higher, FL180 provides at least 500 feet separation from the next VFR altitude, and at least 1000 feet from the next IFR altitude. However, if the altimeter setting is lower, then the lowest usable flight level is higher than FL180.

Current altimeter setting   Lowest usable flight level
29.92 (or higher)           180
29.91 through 29.42         185
29.41 through 28.92         190
28.91 through 28.42         195
28.41 through 27.92         200
27.91 through 27.42         205
27.41 through 26.92         210
  • $\begingroup$ Thank you, Fooot. THIS is what I was looking for. At certain times, what I was theorizing is, in fact, a valid concern, and 14 CFR 91.121 speaks to it. $\endgroup$ Jul 24, 2018 at 15:08

That's where the transition level comes into play. When climbing through 18,000 feet MSL (transition altitude), the altimeter will be set to the standard pressure of 29.92 in.Hg or 1013 hPa. However, when descending, the local QNH will not be set at the TA, but the transition level (TL). The transition level is dynamic and depends on the local air pressure.

Following a table on how to determine transition level from Wikipedia:

How to determine TL

Source: Wikipedia

The switch to local QNH happens, when passing the determined TL.

The lowest flight level one can fly is TL + 500 ft but many countries also add a buffer to TA.

So that's how aircraft can fly at both TA and TL while still having sufficient vertical seperation. (Transition layer + buffer)

  • $\begingroup$ Thank you, Noah. I thought I made it clear within the context of my questions that I’m well aware 18,000 feet MSL is the transition to pressure altitude. While your answer is quite accurate, I don’t feel it addresses my question. The fact is that some VFR Aircraft may, for example, wish to fly eastbound at 17,500 feet MSL. If at the same time, a slightly higher IFR Aircraft were at flight level 190, absolute separation could be anywhere from 2,500 Feet to Zero, or even inverted ... with the aircraft at FL190 flying below the aircraft at 17,500 MSL. $\endgroup$ Jul 23, 2018 at 11:29
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    $\begingroup$ Once in the Class A airspace all a/c are IFR. Based on the QNH, ATC won't assign a flight level that puts an aircraft at an MSL altitude that could conflict with traffic at or below 18000 feet. With a QNH of 950mbar, the lowest FL ATC will assign is FL210. Flight levels below that are not usable for the reason you state. $\endgroup$
    – Gerry
    Jul 23, 2018 at 12:17
  • $\begingroup$ Many countries have a variable transition level ensuring the transition layer will always be at least 1000 ft (typically between 1000 and 1499 ft), meaning that the TRL and TA are vertically separated. For some reason, as with many other things, the USA feels the need to be different, in this case with a fixed TA of 18000ft and a fixed TRL of FL180. $\endgroup$ Jul 23, 2018 at 15:18
  • $\begingroup$ To clarify; air traffic procedures exist to avoid the problem. They can be found here. $\endgroup$
    – Gerry
    Jul 23, 2018 at 15:58

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