“N5678, cross LTOWN at and maintain one-zero thousand feet.” Center just issued you a crossing restriction. You are expected to plan and initiate your descent so as to cross some point at a given altitude. You are cruising at 300 knots 100 miles from LTOWN at flight level 240. When will you initiate your descent? Just how quickly do you need to come down?
Part 1: Compute your Top of Descent:
First, find out how many feet you need to lose:
24,000 – 10,000 = 14,000.
Now, discard those zeroes and multiply by 3:
14 x 3 = 42.
Your top of descent is 42 miles from LTOWN.
Part 2: Compute your Rate of Descent:
Begin by determining your ground speed.
Multiply your ground speed by 6:
300 x 6 = 1,800.
Your rate of descent is 1,800 fpm.
At 42 miles from LTOWN, you will have to maintain 1,800 fpm to meet the crossing restriction.
Why you should not (necessarily) descend right away.
Many a pilot would be tempted to initiate a descent immediately upon receiving a crossing restriction, however one should be patient as there are other factors at play.
Recall that fuel efficiency increases with altitude, particularly for turbine engines. By descending too early, the pilot will be shorting himself on fuel. Furthermore, it is usually easier to avoid unwanted weather conditions at higher altitudes. By descending into the muck too early a pilot may be inviting turbulence, icing, and other nastiness into the flight. Also consider the effect of altitude on true airspeed: as altitude increases, true airspeed increases. A premature descent can lead to a lower true airspeed and longer flight times.