Originally Posted by
FlightDetent
Guilty as charged HB, pardon me doubting then. Out of interest, given your "theatre" could your outfit actually do without it?
Murky side story: there is a HW/SW configuration of quite a few airframes, where for a LNAV+VNAV managed app mode executed with QNH you need to set (M)DHinto the FMS for all things to work. While with the same ship, on a non-VNAV apch, (M)DA goes into the box.
We fly from between the Canadian border to Peru, with Bogota being the highest elevation (of the top of my head),
AFAIK QFE is mostly former USSR, China? but no place we fly to.
We set DA for all vertical guided approaches (ILS, LNAV/VNAV), and a DDA (derived decision alt = MDA +50') for all non vetical guided approaces, and fly them all similar to an ILS as far as not leveling at the MDA anymore, but flying a 3deg path to the DA/DDA and GA if the required visual contact isn't there. Which things wouldn't work if you didn't set MDH?