B737NG VSD
I wonder whether someone can provide some insight in the finer details of the 737NG Vertical Situation Display (VSD). (link)
For reference, the VSD has a Boeing PFD-style altitude tape on the left hand side and an airplane symbol at the current altitude next to it. It appears this is supplied from the Air Data computor as Barometric Altitude.
Since a terrain profile view is also shown on the VSD, this information is from the EGPWS and the EGPWS has BAROMETRIC inputs.
So if all the alititude information supplied to the VSD is BAROMETRIC how would it be able to show an incorrect QNH setting for approaches other than an ILS?
The way I see it, the terrain, approach waypoints and runway data come from a data base. The aircrafts lateral postion comes from ADIRU postion inputs. Also altitude information is supplied by the ADIRU 1 which is also supplied to the PFD altitude tape, being BAROMETRIC altitude and subject to QNH corrections.
Therefore if BAROMETRIC ADIRU information is used the VSD will present the aircraft as all being on profile and above terrain even with a incorrect QNH setting.
So, what is actually shown? Anyone?