@ kijangnim
Are you rated on the A320 family? That the FMS in deed SHOULD be able to corretly calculate a descent profile is indeed true, however reality looks way different, at least on our aircraft. If yours are different, I would like to know why...
We fly CFM-powered A319s (2x23 klbs) and A320s (2x25 klbs) as well as IAE-powered A321s (-100: 2x 30 klbs/-200 2x33 klbs). We have ships with the Legacy FMS as well as many Pegasus and some Thales equipped.
Except for the Thales FMS (only a couple of sectors until now) my personal experience is, that even with all variables entered as good as it gets, the FMS calculates way too conservative.
You can fly well above the FMS profile an get back on it before Speed reduction a 10.000 ft during descent, even when the box calls for 'More Drag' or 'Too Steep Path Ahead' during Descent. The A319 will make up for some 2.000 ft during an Descent from the lower 30s, the A320 for 1.500 and the A321 for more than 1.000.
Only in case you have to fly the entire descent with ENG A/I on, you come close to the predictions and even then you are descending steeper than the box estimated in the mach-segment of the descent (seen by the VDEV-value on the PROG page decreasing during initial descent).
For myself I don't even ignore the descent predictions anymore and calculate for myself usually pretty well. I'm just wondering how other FMSs do it better? What is the experience of other bus-operators?
Regards, MAX