B737NG VSD

[mangatete]

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?

[de facto]

Good question,i never use it,i do non precision the old fashion way,CDA with altitude call outs and a VDP to MDA.

The aircrafts lateral postion comes from ADIRU postion inputs.
On the NG,it comes from the GPS,if the GPS fails then the IRU will provide lateral position to the FMS.
The vertical path profile is coming from the FMS,so proper QNH setting and temperature correction should be used in order to follow a proper path.

Its a good aid in challenging terrain but standard brief,call outs and navigating should be maintained,it is not a tool designed as a replacement for basic piloting skills.

[donkey123]

You will be able to tell as the runway shown on the VSD will sit either above, or below the terrain depending on the miss set QNH. So yes the terrain will not be accurate with miss set QNH but the runway will.

So if the runway is the same level as the terrain, you're in good shape!

Cheers,

[FlightPathOBN]

PowerPoint from Boeing on vsd...this may help answer your questions...

Boeing vsd powerpoint (http://operationsbasednavigation.com/wordpress/wp-content/uploads/2013/03/vsd.pps)

[mangatete]

Agree, the runway will display on the VSD in correct place even with an incorrect QNH set on the altimeter, however if the terrain that is displayed on the VSD is obtained from a data base (the EGPWS data base) and this is relative to the barometer derived aircrafts vertical position, which the EGPWS uses (as stated in Boeing Vol 2), then the terrain should also be depicted on the VSD in the same vertical position in relation to the runway. i.e not displaying the true aircrafts vertical position in relation to the terrain below.

Or is the aircraft symbol on the VSD's altitude in relation to the terrain obtained by some other altitude means that is not subject to miss set QNH??

Thanks for reply's

[framer]

If you set an incorrect QNH the runway will either appear to be suspended in the air or below the ground.

[mangatete]

can you explain how the runway can be above or below the ground if both terrain and the approach slope/runway are all tied to a common barometric altitude?

as far as I can tell, no radio altimeter inputs are used for the VSD.

The power point training referred to earlier in this thread does not discuss what the VSD presentation will do with an incorrect QNH setting when flying a non ILS type approach.

Any technical explanation on this would be appreciated.

[Hunter S Thompson]

The VSD is brilliant. As Donkey and Framer have stated, if the QNH is incorrectly set, the runway will definately appear above or below the ground level, if QNH is correct, runway will be at ground level.

The VSD uses TAWS info which does include radio altimeter inputs.

[framer]

can you explain how the runway can be above or below the ground if both terrain and the approach slope/runway are all tied to a common barometric altitude?
Sorry I can't but I have had a play with it on approach on a visual day and it does do it.

[mangatete]

Thanks again, I have also tried changing the altimeter in the simulator and it does initially displace the aircraft symbol from the magenta approach path, however after a short time the approach path, leading to the runway, adjusts back to give the correct looking VSD picture with the incorrect subscale setting??

In the simulator, the VSD still showed aircraft on the magenta approach slope and all lined up for correct landing point on the runway, with the altimeter's set with a 10hpa difference to the simulator's S/L pressure. The terrain appeared to also stay in correct relationship to the runway elevation.
If the subscale was changed during the approach the display would look incorrect initially but would slowly realign to the changed altimeter setting.
However if the QNH was incorrectly set at transition and left that way, the VSD display appeared to show the runway and terrain in the correct position.

Some uncontrolled training papers I have, indicate that the VSD should show an incorrect altimeter setting but I have not been able to see it in the sim yet or quite fathom out technically how this would be achieved... might have to leave it to Bill's Astronauts and Engineers. cheers and thanks for reply's

[mangatete]

Set up an RNP approach in SIM with incorrect QNH set passing through transition, (aircraft subscale set 30mb higher than airport QNH).

VSD showed everything being normal and OK. Magenta vertical path projected to correct runway touch down point, the white aircraft vertical track sat perfectly over the magenta path line, and terrain display showed terrain in the correct position in relation to runway.

Aircraft position also depicted the correct height above terrain as if correct subscale was set....

The only indication of the error was the radio altimeter on the PFD.

Appears the VSD will not assist in identifying incorrect set altimeters as this incorrect information is also feed into the VSD display and the EGPWS forward looking information.

[framer]

It definitely shows up in the aircraft that I fly. Simulator issue?

[FlightPathOBN]

Desktop SIM or full motion?

[mangatete]

It is probably quite difficult to test out in the aircraft, making a (short time) barometer adjustment will not allow the subscale change to be immediately presented to the VSD and EGPWS. This is seen when passing through transition. It takes some time for the new QNH to be recalculated by these devices. So unless an incorrect QNH was set from some distance out, the VSD display will initially use the previous inputs.

The simulator used was a late model full motion, airline simulator, but I guess there could be software issues.

Cheers

[framer]

That's not the experience I have had in the aircraft on a nice VMC day.

[mangatete]

Thanks everyone for your valuable advice,

So with an incorrect QNH set the VSD will show the non ILS approach slope back from the touchdown point on the runway (magenta line).

The VSD aircraft symbol, if in VNAV path, with be positioned on the this magenta line and showing this vertical track leading to the runway touch down point on the VSD runway symbol.

The "Aircraft symbol" and "Runway Symbol" will both APPEAR as being at the correct elevation in relation to the VSD altimeter tape at left side of VSD.

However the terrain picture at the bottom of VSD will not be at the correct elevation in relation to the runway symbol, due to this terrain information coming from the EGPWS (TAAS) which receives radio altimeter inputs when the aircraft is below 2500ft.

So with an incorrectly set QNH, it will only become apparent, on the VSD, when the runway symbol appears in view on the VSD, and this terrain will show above or below the runway symbol.

A altimeter error, creating aircraft actually 250ft low, would show the green terrain line about 3mm above the runway symbol...

Thank you.

[EW73]

Really....

What a great quote "... it is not a tool designed as a replacement for basic piloting skills"

I love that, it applies to almost everything in the "modern" flight deck!

:ok:

[Oakape]

What a great quote "... it is not a tool designed as a replacement for basic piloting skills". I love that, it applies to almost everything in the "modern" flight deck!

And it is the complete opposite of the belief that most airline management teams have these days. :ugh: