Originally Posted by
propjunkie
altitude to lose (in thousands) x 3 = dme to start down
G/S divided by 2, add a 0 = FPM required for 3:1 calculation. (also works for FPM required for glideslopes)
example: you are 24000 need to cross nesto at 10000. Groundspeed is 420
14 x 3 = 42nm
420 / 2 = 210 add zero 2,100 FPM
start down at 42nm and use 2100 fpm
make sure to increase descent rate when GS increases
3:1 calculation gives you a 3-degree glideslope.
1-degree nose down = ~100 ft/nm down
Therefore, another, perhaps easier way (depends on how your brain works I guess) to come up with the same 2,100 fpm above would be:
420 GS = 7 nm/min
7 * 3 (your desired glideslope) = 21 (add the zeros to get 2,100 fpm)
For crossing restrictions, figure out the required glideslope, then use that x your NM/min to determine fpm.