Number rule;
120kt (ground speed) = 4nm @ 500ft/min
150kt = 5nm
180kt = 6nm
and so on...........
The way it works (unpressurised A/C) take you alt ie 9000ft then multiply it by that GS figure above and you will get your TOD.
example: your at 9000ft doing 180kt GS what is your TOD?
9000 = 9
180 = 6
9x6=54 (TOD is at 54nm)
Nice and easy, good luck for your renewal
