One of those cases is when you need to figure out your descent rate to arrive over a point. The easiest way to do this is to multiply the altitude you need to lose by three to find the number of miles prior to the point when you need to begin the descent. Then, take half your groundspeed and add a zero to find the descent rate in feet per minute. For example, let’s say you are flying cross-country in a Cessna 172 at 110 knots at 5,500 feet and you need to reach a traffic pattern at 1,500 feet. You know you need to lose 4,000 feet. Now multiply that by three to find that you should start your descent 12 miles from the airport (or 13 to give yourself a cushion). Half of your groundspeed is 55 knots. Add a zero and you’ll find you need to descend at 550 feet per minute to reach the point. Those quick rules of thumb are good for any airplane in any scenario.
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