Descent Calculation

For all those flight simmers and real life pilots who are unsure when to start descending and what descent rate to use, especially if you are at a good altitude, these calculations can help.. They are simple enough you might even be able to remember them when in the plane..

The first calculation is used to determine when to begin descending, the second gives you the rate of descent to use.

For these examples, I have used 10,000 ft altitude, and 120kt groundspeed.

1) To decide when to begin descending, take the last 3 zeros off your altitude then multiply by 3, which gives you the number of nm away to begin your descent. So, take 10,000 ft, knock the zeros off becomes 10, multiply by 3 becomes 30 nm away from your destination to begin descending.

2) To decide the rate of descent to use, divide your groundspeed by 2 then multiply by 10. So 120kt / 2 = 60, times 10 = 600 ft / min to descend at. Just remember your groundspeed is used, not your airspeed.

Of course this is all going to be a little irrelevant for me shortly because I have ordered a aviation GPS with VNAV built in, which should do all this for me, and allow me to plan descents to arrive at my target alt before I bust airspace, which is a little more difficult to do with these calculations, but I still will be using them for my X-plane flights