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If you are at 17999 MSL VFR, you twist in 29.92 into the altimeter, and it raises your altitude to above 18000, did you bust the class A?

Then theoretically, could you be in class A airspace with a local altimeter setting if there is a very low pressure?

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  • $\begingroup$ I'm only a helicopter pilot so anything above a couple of thousand feet to me involves sitting in comfort with someone else doing the flying ;), hence the comment rather than a definitive answer. I believe that you cannot accidentally do this since Class A begins at the lowest flight level above the transition altitude, i.e. there is no FL18. You switch to 29.92 when passing 18000 but, the 500 feet between this and FL185, known as the translation layer, exists to provide that buffer between QNH (or whatever setting you might be on) and class A. $\endgroup$ – Simon Aug 4 '16 at 8:15
  • $\begingroup$ 91.121b has minimums for the lowest flight levels due to the altimeter setting. $\endgroup$ – Tango India Mike Aug 16 '16 at 2:29
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No, fiddling with your altimeter setting doesn't change the airspace you are flying in. So you can't bust the class A above 18 000 ft by changing the setting. The top and bottom boundaries of the airspace are defined by applying the altimeter setting local to that airspace.

Conversely, and perhaps more important, you can't weasel out of a busted airspace by quickly changing your altimeter setting.

Your transponder sends your altitude as if your altimeter was set to 1013.25 hPA / 29.92 inHg, regardless of what your actual setting is. The ATC system will do the conversion according to their setting, so you can't fool them.

Note that the altitude resolution of your transponder is either 100 feet (Mode C) or 25 feet (Mode S with appropriate altimeter encoder), so 1 feet difference doesn't really matter.

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    $\begingroup$ It may we worth adding that if you are flying at the vertical border between two airspaces, the airspace class of the least restrictive airspace applies. Say, if the airspace is class C above 5000ft and class E belof 5000ft, if you are flying at 5000ft, you are legally in class E airspace $\endgroup$ – J. Hougaard Aug 4 '16 at 11:28
  • $\begingroup$ @J.Hougaard That is not necessarily true. There are instances where the underlying airspace stops just short of the top altitude, in your case 5000. on sectional charts it is denoted as a "-" before the altitude. Check out class D airspace surrounding KSDL. That airspace extends up to but does not include 4,000 MSL. $\endgroup$ – wbeard52 Aug 4 '16 at 17:43
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Then theoretically, could you be in class A airspace with a local altimeter setting if there is a very low pressure?

Yes.

The answer lies in the FARs. Class A airspace starts at 18,000' MSL. When you set 29.92" Hg you are no longer flying a MSL altitude but a flight level. The minimum flight level available is dependent on the current altimeter setting as specified in §91.121

Altimeter Setting Procedures

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