Abstract
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.
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