It is likely that in the future aircraft designs will be increasingly tailored to optimize the traffic system rather than the economics of the aircraft considered in isolation. In this paper Mr. Attwooll, of the Mathematics Department at the Royal Aircraft Establishment, discusses the effect on the properties of air traffic systems of the individual characteristics of aircraft. This paper (published by permission of the Ministry of Defence), which reflects Mr. Attwooll's personal views rather than any official policy, was read at a meeting of the Institute in London on 28 November 1973 with the President in the Chair.