Flight levels

Good morning I have just started studying for my CPL and I understand flight levels but the transition layer is really playing with my head. what does it mean when it says that a transition layer is not availably when the QNH is what ever it is(it doesn’t really matter what the QNH say it is such that it makes one of the FL unavailable). does it mean that the transition layer start at 10,000ft and goes to the level below the unavailable level or does it go to the level above I am very confused.
Thanks, Ethan.

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I thought it was FL180 when you go from the local airport altimeter setting to standard altimeter setting. But using foreflight it says transitioning through when I’m climbing 10,000’. @DeerCrusher might be able to help as he has is ATP

Transition layer or transition altitude is the altitude that you will change from the local altimeter (inHg, hPa, etc) to your standard setting of 29.92inHg or 1013hPa. This altitude varies at some countries. In the US, this occurs at FL180. In others such as yours, 10,000ft.

Hope that answers your question. If not, let me know and I can further explain.


Yeah I understand as to where it starts what I am having trouble is what it means by that FL 115 is not available when area QNH is below 997 HPA or FL125 is not available when QNH is below HPA. I tried to upload an image from the AIP which might make it easier to understand my question. And yeah I am in Australia.
Thanks, Ethan.

Is there a reasoning behind why that altitude would vary?

I’m not very familiar with Australian airspace rules. But one of the reasons could see those altitudes not being useable is as pressure increases/decreases so does your relative altitude to the ground. Think of the airspace around your aircraft as one vertical cylinder. Above you is a lot of lower pressure than your present altitude.

If the pressure is below the depicted hPa, then you’re going to have aircraft who are above you in this cylinder compressing closer and closer to you. Those altitudes that they have listed on that chart indicate problematic altitudes where aircraft separation may not be guaranteed. In otherwords, because the pressure is already so low, the change in pressure would be minimal enough to not provide adequate separation for you and others.

Again, this may or may not be totally correct as we don’t have situations like that in the US. And if we do, I can’t think of any areas where this would occur. This above is just my theory as to why this may be in place. Its interesting for sure.

Take the image below. These altimeters are in (inHg) but the same concept applies to (hPa). The aircraft at 4000ft goes from a high pressure to lower pressure resulting in a lower indicated true altitude. Only difference is instead of distance from the ground, you’re looking at distance from other aircraft.


Yeah that is what I am trying to find out.

Thanks mate that makes sense.

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Keep us up to date on training if you wish. Always enjoy hearing/seeing how people do. 🙂


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