This paper describes how it was found possible to estimate the accuracy, under operational conditions, of navigators' wind-determinations by the 3-course drift method. This was done by studying the self-consistency of the set of three drifts logged by the navigator; the technique which was developed, using the size of the circle inscribed in the ‘triangle of error,’ can be used expeditiously by persons with no special training. This technique is applicable to other position line problems. Of the total inaccuracy of the winds found by the navigators, three quarters was contributed by the errors of observation of the drifts, and one quarter by the process of deriving the wind from the set of drifts, using the Dalton Computor.

The validity of the method was checked by comparing wind measurements by the 3-course drift method with the value obtained from Air Position Indicator readings taken in the course of practice bombing exercises; the accuracy of the latter value was also estimated from internal consistency. The check was satisfactory; and in particular, contrary to widely held belief, showed that the navigators did not systematically underestimate drift angles.