JustaNumber
Well-known member
- Joined
- Nov 10, 2005
- Posts
- 922
Thousands of feet to descend: 20
Times 3
=60 miles out from fix
Ground speed times 5
GS:500kts
=60 miles out at 2,500fpm
Your math is easy and works well. However, if you want to be even more precise (and be able to explain why the math works), divide the altitude you want to lose by 300. (In the example, you get 20,000/300=67 miles out). This is because if you are descending on a 3 degree glidepath, you are descending 300 feet down for every 1 NM you go forward. Hence, the altitude to lose divided by 300 feet gives you the miles it would take to lose that altitude.