Originally Posted by
cruisercruiser
Hi,
I was told in training as a general rule of thumb, 1% of G/S for 25-30 degree AOB for a 90 degree turn.
For example joining a DME arc at 90 deg with a G/S of 200kt, you need to initiate the turn 2nm before the arc so as not to overshoot the arc.
Of course we don't always join the DME arc at 90deg. Say if you join at 45deg, you need only 1nm in the above example.
Same applies for QDR intercepts if you are told to fly over a VOR to intercept a radial outbound.
That's a good way of estimating the lead for intercepting an arc, but it's not a very good way of estimating turning radius in an established turn. The rule of thumb allows for rolling in and out of the bank. The radius of an established turn with the same bank will be somewhat smaller than that. IOW, if you calculated precisely what your radius of turn would be ay X airspeed and y bank, and began your 90 degree turn to an arc at precisely that distance, you'd fly thru the arc by a fair amount.