B787 VSD Runway Information

[frustratedpilot]

The B787, and maybe the B738, VSD has a runway symbol when the lateral path leads all the way to a landing runway.
This is a good tool to identify a gross error in the QNH setting. A higher QNH value than the correct one will bury the runway symbol underground, and a lower QNH setting will make the runway symbol float above ground.
Does anyone know what runway elevation information is used in the VSD to achieve this logic?
Obviously the terrain info is derived from the GPWS, and the runway info from the FMC.

[STBYRUD]

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.

[frustratedpilot]

Thanks for the info. I tried it in the SIM and it actually works as explained in my post. I just want to understand the logic behind it.

[FlyingStone]

Perhaps it’s different on the 787, but on the 737 the VSD uses “inertial altitude”, which enables the detection of incorrect altimeter setting via the position of the runway above or below the terrain.

You’d probably have to contact Boeing for a more accurate definition, but I would guess it’s a mix of IRS and GPS altitude data.