The US airway system uses a single transition level (or altitude), but some other countries like China separate a transtion altitude and a transition level with the transition layer in between.

Could anybody kindly explain why they use this type of separate transition system? I can guess it's there to prevent possible mid-air collisions between two aircraft using two different altimeter settings (QNH and QNE) but I don't understand how exactly that works.

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    $\begingroup$ USA has a transition layer as well. It is 0 feet if the QNH is 1013,25 hPa, but in all other cases it is larger than that $\endgroup$ – expeditedescent Sep 24 '17 at 9:37
  • $\begingroup$ @J. Hougaard You can call it a transition layer as well but that's a little different from the transition layer I'm talking about here in that it's between two different transition altitude and level, for example like the transition altitude being 14,000 and the transition level being FL160. $\endgroup$ – lemonincider Sep 24 '17 at 9:49
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    $\begingroup$ Related "3.9 Minimum usable flight levels" on VATEUD with this picture. $\endgroup$ – mins Sep 24 '17 at 11:17

By definition, the transition level is the lowest usable flight level above the transition altitude. So if there is a transition altitude, there is a transition level.

In the USA, the transition altitude is 18 000ft. If the QNH (altimeter setting) is higher than 1013.25 mbar then the transition level is FL180. This provides at least 1000 ft separation from aircraft flying at 17 000 ft (or 500 ft from the VFR at 17 500 ft).

If the QNH is lower than 1013.25 mbar then the transition level is FL190 or even FL200 to ensure there is at least 1000 ft separation from 17 000 ft.

The transition level in other countries is based on similar logic.

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    $\begingroup$ The 'flexible' transition level makes maximum use of the airspace by using all available altitudes and flight levels. It appears from the question that some countries use a fixed transition level that accounts for the worst case QNH. It sacrifices airspace utilization for operational expediency. They don't have to check the barometric pressure to see if the boundary area levels are available for use as they're always excluded. $\endgroup$ – Gerry Sep 25 '17 at 12:23
  • $\begingroup$ @Gerry - In the US, wouldn't a VFR flight choosing 17,500' be a bit dangerous. I'm missing something I'm sure. $\endgroup$ – ymb1 Sep 28 '17 at 17:12
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    $\begingroup$ @ymb1 No more so than any other VFR altitude. 500 feet vertical separation is the standard between IFR and VFR below Class A airspace with IFR traffic assigned on the thousands and VFR 500 feet above or below. $\endgroup$ – Gerry Sep 29 '17 at 0:15
  • $\begingroup$ When you say in the US "if the QNH is lower than 1013.25 mbar then the transition level is FL 910 or even FL 200," you mean the ATC instructs pilots to change their altimeter settings to the local QNH setting at FL 190 or FL 200 rather than at FL 180 for separation purposes? $\endgroup$ – lemonincider Sep 29 '17 at 13:19
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    $\begingroup$ It basically means that FL180 / FL190 do not exists under those conditions. Aircraft will not be cleared to those levels. ATC does not tell pilots at which altitudes they have to change their altimeter setting. Instead they clear / instruct the aircraft to a flight level or an altitude (+QNH). The altimeter setting is implied. $\endgroup$ – DeltaLima Sep 29 '17 at 13:24

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