The definition of pressure altitude is what's indicated when the altimeter setting window is adjusted to 29.92. If working up a flight plan I have always computed this. If you're setting this in the plane isn't a bit late to start working on performance data. I haven't ever had to find pressure altitude as I'm firing the plane up or had a reason to run out to the plane and dial it in. The only thing I can think is that you could find the pressure altitude you computed and the actual to see if there would be any differences. Just seems to me a bit late to be checking pressure altitude. Just doing some review and never really thought about it
Setting 29.92 into your altimeter to find pressure altitude is more of a conceptual aid than a practical technique that's used during flying. If you know, in addition, that
1) .1" Hg approximately equals 100 feet, and
2) changing the altimeter setting in the Kollsman window causes the indicated altitude to change in the same direction,
then you can solve simple questions of correcting for nonstandard pressure easily in your head.
For example, say you are entering the traffic pattern and you forget to listen to ATIS and set the altimeter. The altimeter setting is 29.97, but your altimeter is set at 30.12. Assuming everyone flies at the correct TPA, where will other planes be relative to you?
1) 30.12 - 29.97 = .15 = 150 feet
2) you would "dial down" your altimeter to 150 feet below TPA when correcting your altimeter setting
3) therefore, other traffic will be 150 feet above you
Or, if you are flying at 3500' indicated with an altimeter setting of 29.82, what is your pressure altitude?
1) 29.92 - 29.82 = .1 = 100 feet
2) You would "dial up" to get your altimeter setting to 29.92
3) therefore, your pressure altitude is 3600'
