Fri May 17, 2002 10:37 pm
Another formula for determining when to start a decent, for say a crossing restriction:
when to start decent = 3 x altitude to lose
rate of descent = 1/2 Groundspeed
Lets say you are told, Cross XYZ VOR at 10,000'. You are at FL350 and your groundspeed is 400 knots.
25,000' to lose, multiply 25 x 3 and you get 75. Start your descent 75 miles away from XYZ VOR.
400 Knots groundspeed divide by 2 and get 200. Put a zero at the end and get 2000, and decend at a rate of 2000 FPM.
This formula will work perfectly everytime, but I usually add 5 miles to the distance so you can do thing smoothly for the passengers.
Keep checking your progress along the way. as you get to 30,000' you should be no less than 60 miles away from XYZ VOR. If you are within 60 miles, increase your decent a little and check again at 25,000' to see if that helped.
Adjust your Rate of Decent also for Groundspeed along the way. As the winds change as you go lower, you may need to adjust for that as well.
Enjoy,
Jetpilot500