This formula is especially useful for turbo pilots wanting to stay
out of icing and turbulance. It gives you a way to meet a crossing restriction
and stay high as long as possible.
Take the difference in altitude and multiply by three. This will give you
the distance from your crossing restriction to begin descent or Top of
Now multiply your ground speed time six. This will give you your rate of descent
in feet per minute.
example: You are level at FL250. ATC tells you to cross ABC at 7000 ft.
You are 70 NM from XYZ with a ground speed of 350 knots.
25000-7000 = 18000 (drop the zeros and use 18) x 3 = 54 nm
300 x 6 = 1800 or 1800 feet per minute descent
54 miles from ABC you must start down at 1800 fpm or greater to meet the restriction.