Descent estimation

From FlightSim

1. Three Degree Descent Slope:
   • If you figure a 3º descent angle (same as a typical ILS glide slope), it works out to about 318 feet per nautical mile. Rounding this off to 300 (easier to work with in your head), you can divide the amount of altitude to be lost by 300 to get the distance in nautical miles that you'll cover in the descent.
   • To get the rate of descent needed for that 3º descent angle, multiply your groundspeed by 5.
   • So, as an example, if you are cruising at 9,000 feet and are descending to an airport with a pattern altitude of 1500 feet, you need to lose 7,500 feet. If you choose a 3º descent angle, you can divide 7,500 by 300 which gives you 25 nautical miles out that you need to start your descent. If your expected groundspeed during the descent will be 140 knots, multiply that by 5 to get 700 feet per minute (fpm) for your rate of descent.
2. In an unpressurized aircraft, a descent rate of 500 feet per minute (fpm) is a good rate to minimize ear problems for passengers. The rule of thumb is to double your groundspeed in miles-per-minute, then multiply by the altitude (in thousands of feet) that you need to lose.
   • First, of course, you must determine how much altitude to lose. So for this example, you are cruising at 12,000 feet at 140 knots groundspeed, intending to land at an airport with a pattern altitude of 1500 feet. This results in 10,500 feet to lose. Note that you must maintain the 140 knots groundspeed during descent for the figures below to work. If you'll increase your groundspeed during the descent, that increased speed should be used instead of 140.
   • 140 knots is 161 mph, divided by 60 gives about 2.7 miles per minute for your groundspeed. Times 2 is 5.4.
   • 5.4 times 10.5 (altitude to lose/1000) is 56.7 miles needed to lose 10,500 feet at 500 fpm. Of course you could have rounded off some of the numbers to make it easier to work in your head, so 5 (instead of 5.4) times 10 (instead of 10.5) is 50, so you might not quite be at your desired altitude in time.
3. An easier way to figure your 500 fpm descent. This is less accurate than the one above, and assumes a ground speed around 90 mph (or knots, if you're figuring nautical miles):
   
   • Divide the altitude to lose (10,500 as above) by 1,000 = 10.5.
   • Multiply by three (3) = 31.5 miles out to start descent.

   Note that you could make a chart using 2) above for several different groundspeeds, determining the multiplier for each to use in 3) above. So 140 knots would be 5.4.