Introduction. The judgments discussed in this paper are mainly those which the pilot of an aircraft has to make in order to land. Some of these, as, for instance, the judgment of alignment, are similar to those made by the driver of a moving surface vehicle, but some, as, for instance, the judgment of approach slope, are peculiar to the pilot, and are very much more difficult. The main difficulties of the pilot arise, however, from the fact that an aircraft has six degrees of freedom and is controllable about all three axes, whereas the surface vehicle has only three degrees of freedom and is controllable only about the vertical axis. In bad visibility these extra freedoms confuse the visual indications in ways of which the person who has only driven a surface vehicle has no conception. The result of this has been that until recently most of the engineers concerned in the layout of airports have not realized that pilots have special difficulties; while the pilots, not having studied visual psychology and perspective, have been unable to analyse these difficulties, or even explain them in terms intelligible to the engineers. In this country, we have been able to find methods of bridging the gap, and enabling both parties to talk the same language. The practical result of this can be seen at London Airport.

It has been known for many years that in bad visibility the transition from instrument to visual flight is difficult, and may be highly dangerous if the pilot is suffering from fatigue, but it is only in the last five years or so that the problem of finding patterns for the visual aids which would make the transition easy and safe has been systematically studied. This paper describes some of the illusions which occur at transition and shows that they can be largely prevented by using bars of lights running transversely across the approach zone. A brief account is given of the results which have been obtained on the Crossbar System of Approach Lighting under operational conditions. A new theory of visual judgment, known as the “parafoveal streamer” theory is described, and from this the characteristics which are desirable in the visual aids are deduced. The crossbar and slopeline systems of approach lighting are compared in the light of this theory, and it is concluded that the ordinary run of pilots will find the crossbar system easier to use because the mental processes necessary for its interpretation are those which the pilot has built up in good weather flying. It is emphasised that in the absence of any coherent theory of how visual judgments are made, it is almost impossible to evaluate the various systems in a reasonable time, and that if international standardisation is to be achieved, some theory must eventually become generally accepted.

In the last number of the Journal (14, 243) Hollingdale showed mathematically that anticollision manoeuvres for the case of two craft moving in a plane can be formulated on a rigorous logical basis; and that a simple set of manoeuvres could be devised to meet all the geometrical requirements for anti-collision. The convention adopted was that each craft should manoeuvre so that if the other craft stood-on, the sight-line (the line joining the two craft) would always rotate in an anti-clockwise direction. The analysis showed that, mathematically, this was the complete and only answer to the basic problem.The author of the present paper compares systems, such as that represented by the Collision Regulations and the Rules of the Air, based on onus to avoid, with a system based on a convention as to the direction of rotation of the sight-line. The first type of system, he holds, is defective for visual sightings and almost useless for radar sightings. The system put forward in the paper can be applied to both visual and radar sightings whether unilateral or bilateral by orientating a simple diagram with own course. A means of inter-communication is desirable but even without it such a system can, the author believes, safely be employed in the greater proportion of encounters.The collision problem as a whole is beset by operational factors which are entirely distinct from the geometry of the situation, and for that reason no purely mathematical approach can, in isolation, be entirely valid. Nevertheless, there can be no doubt that studies such as the present one, based on a proper understanding of the geometry, can contribute greatly to the solution of this most pressing problem of navigation.

This paper is intended to bring the theory of the visual aids up to date, and to discuss various practical methods of increasing the safety of the approach and landing operation. Arguments are put forward to show that the guidance in the vertical plane needs to be improved, particularly in visibilities somewhat above the limiting visibility, and a new form of angle of approach indicator is described, which promises to achieve this result. The beam spreads of the approach lights should be increased, and fittings for providing guidance for circling approaches should be developed. The British design of flush-type runway light has proved! satisfactory in actual operations, and as a result, landing mats have now become practicable, even on existing runways.

When the problem of collision in the air is discussed, it is usual to start by pointing out the enormous closing speed of two modern aircraft meeting head-on, and to conclude from this that avoidance on the ‘see and be seen’ principle has ceased to be possible. The fact is, however, that the great majority of mid-air collisions (about 85 per cent) occur within five miles of an airport and the typical case is not the head-on one, but the case in which the two aircraft crab into one another from a direction which may be anywhere around the whole enclosing sphere. Since the field of view of the aircrew covers only about 20 per cent of the enclosing sphere, the aircrew of colliding aircraft seldom see each other. It would seem, therefore, that the ‘see and be seen’ principle never did afford much protection, even when speeds were low. In other words, the fact that the number of mid-air collisions in Europe has hitherto been small is not primarily due to seeing and evading, although this sometimes happens, but to the fact that the airspace is very large compared to the volume of all the aircraft in it at any given time. However, as traffic densities go up, the risk rapidly increases, and in congested airspace, such as that around New York, the problem of avoiding collision has already become acute. In the period 1948–57, there were 159 mid-air collisions in the United States, and many of these made headlines in the world press. One can imagine the public outcry if two large transports were to collide over a housing estate; but unless something effective is done, something like this will presumably happen eventually. At very high altitudes the ‘see and be seen’ principle certainly fails, by day, because the speed will be high, and in addition, the range at which a pilot can see an object the size of an aircraft may be less than 1½ miles due to what is sometimes called ‘high-altitude myopia’.