Yes I am rated on type. I fly an IAE A320.
I got no idea what type of FM we are using.
Yes, our A320's will often make up for 1500' during descent (meaning it commenced descent about 5 miles early).
This is from the FCOM :
IDLE FACTOR
IDLE and PERF factors follow the same principle.
The PERF factor is mainly used for prediction during the cruise phase.
The IDLE factor is dedicated to the FM descent segment.
The aim of the IDLE factor is to adjust FM descent predictions. In particular, the Top of Descent (TOD)
position, with the actual engine idle thrust used during descent.
A positive IDLE FACTOR gives an earlier Top of Descent (shallower path).
A negative IDLE FACTOR delays the Top of Descent (steeper path).
Example : An IDLE FACTOR of - 3 decreases the computed descent length by 5 NM.
IDLE Factor at delivery :
The IDLE Factor set at delivery is 0 %.
If a negative factor delays top of descent (as per FCOM), then wouldn't an idle factor of -3 now solve this problem and give a more accurate descent point (we commence descent and not gain the usual 1500 / not go 1500 below VDEV) ?