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Could you clarify why you cite 20,000ft as the transition altitude? I've always been under the impression it is FL180.

From your wiki link:

> In the United States and Canada, the transition altitude is 18,000 ft.

Depending on conditions, FL180 could be below the transition altitude at 18000'. I doubt, therefore, that you would ever be given anything below FL200. That is the subtlety to which I'm referring (well that and the fact that different transition altitudes are used in other countries). I'll admit that this is stretching my knowledge. Prior to this thread, I merely "had it on good authority" that FL200 was the minimum.

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