Probably many members of the Mathematical Association are being called upon to help with the instruction of an Air Training Corps, or of attested men who have been accepted as pilots or observers and then sent home on “deferred service”. It may be of interest to give the experiences of a teacher who came into contact with this work at an early stage and so encountered some of the inherent difficulties in their most awkward form. As Klein remarked, mathematical teaching is a function of two variables, the subject and the pupil. The difficulties with the subject are not serious, for there are few ideas in air navigation which are not already known to those who teach elementary mathematics and geography. However, these ideas are clothed in unfamiliar dress, and it is necessary to learn a few technical terms, which will be given at the end of this article.