PPT Slide
The upper part of the descent path is called the idle path. The airplane uses pitch to stay on the path. Idle thrust is the amount of thrust required to produce the planned speed profile. That is, if all goes as planned, making the descent on this path at idle thrust will result in the airplane flying at the target speeds all the way down to the first altitude constraint. In this way, the FMGC computes for you the most economical descent possible.