I have no source for this, but it would be logical if the runway elevation comes from the navigation database, and the terrain from the EGPWS. Now, as Honeywell say in their brochure, the EGPWS uses 'geometric altitude' and is GPS derived. However: the display wouldn't show any actual measured data except for the aircraft symbol at indicated altitude plus current FPA - what you have set as QNH should not matter. The only reason for the runway to either float over or sink into the terrain is a mismatch of terrain elevation and runway elevation in their respective databases... Or am I missing something? Now, if you're shooting an ILS and your FPA vector is shown above or below the glide path, then you could assume that your QNH setting is incorrect, but that's about the only way I could imagine one could use the VSD for that purpose.
Last edited by STBYRUD; 28th June 2023 at 10:44.