Originally Posted by
safelife
The VNAV path remains calculated for the managed speed.
The predictions are for the speed you selected, though.
Yup, this is right. It will give you the progress VDEV for the originally planned managed descent speed.
You can get it to recalculate a managed descent path at your new ATC speed although it is a little fiddly. Reset a lower cruise altitude in the progress page if you're able to level off. This puts it back into ALTCRZ and then you're able to reenter a new speed in the Descent PERF page according to the one you've been given by ATC. It will then assumedly tell you to DECELERATE and get descent clearance etc, then the PROG VDEV figures will be accurate for your new shallower descent profile.
Level Off and reset ALT CRZ to the current level.
Modify managed descent target on the Descent PERF page to the ATC speed.
Now you have the new profile in VDEV according to your assigned speed.
It's probably in FCOM-PRO-NOR-SUP somewhere if that still exists.
I personally find it very useful and saves the calculating for somebody that has always been crap at maths. It's only as useful as the info you put into it. Try using the secondary flight plan to plan your minimum track milage and work out the height you'd expect to be at your most likely short-cut divergence point and then stick that in as a hard height against that waypoint in the primary and then the VDEV info is useful.