Thx for the inputs!
@peekay4
As far as the density gradient not being linear is concerned, there's probably no way around that approximation. After all, the ICAO doc 8168 seems to create its height correction tables on an assumed linearity as well, so I guess this is will be close enough for a corrected FPA too.
I still would think that even when some of these calculations are based on approximations they still should be more accurate than a simple rule of thumb.
Your "What we want to find is the "inverse" of the above" got me thinking though (tell me if that's closer to what you suggested): if I want to correct the (wrong) height to reach the charted height, I've got to tackle the problem "from below" (as in, how much do I need to increase the altitude on my altimeter to actually fly the charted height), so instead of putting the dh correction on top of H, I've changed my
drawings to add dh to the pressure altitude so as to reach H
after correction. And after adding the threshold-to-TD correction I get similar results to those from the more simple formula suggested earlier.
@FlightDetent
Isn't going all the way to TD the more clean way to calculate this? Going all the way down to height 0 eliminates the need for another correction at threshold height (again, based on approximations, however small).
Funny thing about these FPA corrections is that our company has removed the table from the QRH, apparently after someone had messed up after using it. We're now to make the corrections to the FPA "as appropriate"
