VDEV

[United744]

Hi,

I'll probably state the obvious, talk complete rubbish, or both, but I think I need to ask some questions to see if I actually understand this thing (it is now driving me crazy):

1) Does the vertical path, as computed by the FMS, represent a fixed vertical path for the segment computed (fixed in the same way an ILS glide slope is fixed)?

2) Can the vertical path adjust itself in response to change in conditions (e.g. wind)? I think no, because the aircraft would just adjust its forward speed and/or vertical speed to alter its descent path angle and maintain its position on the vertical path.

3) Can the vertical path adjust itself in response to change in aircraft parameters? Again I think no, because it is a fixed path through space and time...

So... where am I going wrong in my understanding?

I have tried across multiple builds to fly the VDEV manually, and it just doesn't behave as expected. The VDEV path indicator has a mind of its own, and sometimes even moves in the opposite direction to that expected.

How on earth does it work?!

[skelsey]

Are you talking generally in the descent or about RNAV approaches specifically?

1) Does the vertical path, as computed by the FMS, represent a fixed vertical path for the segment computed (fixed in the same way an ILS glide slope is fixed)?

Assuming you are referring to RNAV approaches with a vertical path angle coded in the database -- no, absolutely not! The vertical path is based on the barometric altimeter and therefore is affected by temperature error and, indeed, whether you have the correct QNH set. At lower than standard temperatures, the aircraft will fly a 'flatter' Baro-VNAV glidepath, wheras at higher than usual temperatures the actual glidepath flown will be steeper. This is why RNAV approaches are typically associated with a minimum temperature and it is absolutely essential that the correct QNH is set.

2) Can the vertical path adjust itself in response to change in conditions (e.g. wind)? I think no, because the aircraft would just adjust its forward speed and/or vertical speed to alter its descent path angle and maintain its position on the vertical path.

Not entirely sure what you're asking here... in the case of an approach VNAV path, the flight path angle (as far as the aeroplane is concerned) is fixed and the aeroplane will attempt to follow it (using pitch).

The VNAV descent profile that is generated from top of descent is calculated based on the entered VNAV descent speed, the entered (and, I think, element of the sensed) wind, temperature etc. If the outside conditions deviate from what is in the box (or you select a higher or lower speed on the MCP than is entered in the VNAV DES page), your actual profile will no longer match the computed profile (though that is not to say you will not get the right outcome -- you may just be providing a different solution to the same problem, if you see what I mean).

3) Can the vertical path adjust itself in response to change in aircraft parameters? Again I think no, because it is a fixed path through space and time...

As above really - the VNAV descent profile is calculated based on the information you give the box in relation to speed, wind, lateral path etc. It does not really change, unless you change the information you have plugged in (the big one being speed).

Does that make some sense? I don't doubt someone with a better understanding of the system than I might be along soon also!

Simon

[emerydc8]

Good information above. Just to add to it, even though we often think of the gradient path on a descent to be constant, this is not exactly true. For instance, if you have a descent that contains an airspeed restriction of, say, 260/190 and your econ descent speed is 300, as you approach that speed constraint, the aircraft descent rate will shallow to 500 -1000 FPM to meet the speed restriction, yet the VDI will continue to display that you are on-path. If it was a constant gradient path, this would be counterintuitive because you would be going above path when you reduced your descent rate. The FMC takes this into consideration, so that it will show that you are still on path as you reduce your descent rate to slow.