In a recent paper I endeavoured to give a statistical solution of the problem of the separation of aircraft tracks with safety. In it I assumed that the distribution of errors of navigation made by the aircraft was a normal gaussian one; but I added a proviso that the effects of blunders might modify the result. From enquiries of various navigators as to the frequency with which they considered they were likely to make blunders I gathered that with a good navigator an undetected blunder might occur on 1 per cent of occasions and further it seemed reasonable to assume that a blunder might be defined as an error greater than three times the standard error of normal navigation. These assumptions are of course quite arbitrary but they seemed to be suitable to give examples of the effect on track separation; this was done in an unpublished paper. In these examples it was supposed that aircraft flying in the same direction were using two parallel tracks distant L apart and on each track aircraft were flying at fixed intervals but quite independently of the aircraft on the other track.