Rules of Thumb
Contents
En Route
Glide ratio calculation
Divide groundspeed by vertical speed (in hundreds)
Example: 100 kts and descending at 500 fpm. 100 : 5 or 20 : 1
Descent
Top of Descent calculation
This calculation can be used either to know when to start your descent to an IAF or ATC have given an instruction to cross at or below a certain altitude by a certain point.
Let's assume H = height between where you are now and where you have to be (in 1000's of ft),
and D = distance from where you are now to where you have to be (in NM)
then multiply H by 3 to give you the distance at which to start your descent
and multiply groundspeed by 5 to work out the required rate of descent.
Example. We are at 5000 and need to be at 2000ft crossing a VOR. We are doing 100 kts and the DME says we are currently 12 NM from the VOR,
5000 - 2000 = 3 x 3 = 9 NM
100 * 5 = 500 fpm
So 3 miles from now, we need to start descending at 500 fpm to cross the VOR at 2000ft.
Approach
Rate of descent down a 3° glideslope
Multiply groundspeed by 5 (or add a zero and half it) to give rate of descent
Height above ground vs distance to runway
As a glide slope check (or if G/S is not available), multiply nautical miles to go by 300 to give current height above the ground
