Expert: Ned Dolan - 10/19/2004

Question
In a current  flight commercial jet,
How is calculated  the beginning  point of descent ?
Thanks.

Answer
The most efficient descent is to remain at altitude to a point where a power off descent to about 2000 feet is possible. In most cases the Air Traffic System will interfer with this efficient descent. However, in those cases where it is possible the pilot figures his expected rate of descent in a power off condition and uses his expected ground speed to determine the "top of descent". That is the geographic point of starting the descent.

Another consideration is the airspeed to be used in descent. The goal of most efficient fuel savings would require a rather slow airspeed for the descent whereas a goal of trying to save time would require a higher speed descent. So the speed selection would have to be made before the above calculations could be started.

Many new aircraft have flight managment computers that make all the above calculations for the pilot and grahpically show the top of descent on a screen with a map.

Hope this helps.  Ned