Originally Posted by
SmokeAndNoise
Gents, sorry if my explanation was not clear, english is not my first language. I'll try with a more detailed example.
A:
Fl150 descending. You are in open des with 270 kts. Vs will be approximately 2000 fpm +/-. If you pull vs the speed bug will come back because the fmgc calculates you need a very long time to get the speed back to 250kts with -2000 fpm (actually you would never get to 250 kts at this rate of descent).
B:
Same scenario as above. You turn the vs knob to -500 before you pull. The speed bug will not come back because the fmgc calculates that you do not yet need to begin deceleration to reach 250 kts by fl100. Deceleration would begin at approximately fl110 in this case as you would expect with the other descent modes.
Make sense now?
It is a satisfactory explanation I was looking for!
It happens only below FL150 or so,
when the time would not be enough for the older DECEL point to work with the current V/S. Therefore, FMGC calculates new DECEL point and airplane is found to be already beyond that new DECEL point.