In this paper the basic principles of avoidance action are developed. It is shown that a unique set of rules exists which will guarantee the safe passage of converging aircraft; when two are involved, if either, or both simultaneously, act according to these rules, it is impossible to have an accident whatever the situation. (This assumes inerrant assessments on the part of Controllers, and it also assumes that the uncontrolled aircraft continues on course.) The rules are consistent with the Rules of the Air. Past attempts to formulate rules are compared.

The action taken by an aircraft in flight to avoid coming dangerously close to some other nearby aircraft we call ‘avoidance action’. This action may be made by the pilot should the other aircraft come within his field of view, or if ground controlled, it will be initiated by the Radar Controller. The pilot works to the Rules of the Air and the Radar Controller works to them whenever it is possible for him to do so. The Rules of the Air do not always help the Radar Controller because, in every case, except for two aircraft approaching each other head-on, one or other has the right-of-way and ‘an aircraft which has the right-of-way shall maintain its course and speed.’ This means that an aircraft which has the right-of-way must not take avoiding action. The object of this paper, therefore, is to attempt to propose a set of avoidance action rules which will assist Radar Controllers in their task.