Originally Posted by
Fl1ingfrog
No pilot can keep a constant track to one or two degrees. An airway, by example, has a minimum width of 10 NM and this allows for a track error of up to +/- 10 degrees, the mean track being the centre line. The semi-circular rule is a flight planning matter only and with reference to the intended magnetic track. Once in flight then do your best. A track error of up to 10 degrees is reasonable. Off track errors should be corrected using the method you have adopted or been taught. It is expected that the track that you achieve is a mean track and so very unlikely to be constant.
Agreed, and for airways in particular, the even-odd level direction is normally described in the AIP.