Hi there Mamakim,
Draw your track, add 10 mile markers and preselect 1 or 2 x-track features which cut your track like a river/road or powerlines.
Using your nav computer crossing one of the x-track features simply time the distance to the nearest 30 seconds and calculate your exact groundspeed. Easy to do with one hand.
The 10 mile markers can be used for a rough groundspeed and estimate at first though. If forecast h/wind is 20 kts and you're in a 120kt aircraft then groundspeed becomes 100kts ( 1.5 nm/min ). If you have a 20 kt t/w then your groundspeed becomes 140kts ( 2.5 nm/min ) etc.
Using the handy 10 nm markers you can simply estimate that:
10/1.5 = 10/3x2 = 6 mins
10/2.5 = 10/5x2 = 4 mins
Therefore you can estimate for planned track or a diversion and back it up with a x-track groundspeed check. Also if you're 5 miles left of track in 20 miles and it took 6 mins to fly 10 nm ( 60/5 = 12deg ) x 2 = 24 kts of wind from the right fwd quadrant. That way you know the direction and velocity to apply to subsequnt legs and 10 nm becomes the easiest way to calculate everything. I used to teach this method to Qantas cadets.
Hope it helps. Damo
