Even in good weather, visual collision avoidance by pilots is unsatisfactory for reasons which include the following:

1. (i) When on a collision course the other aircraft has no relative motion across the windscreen and is indistinguishable from other marks on the windscreen until danger is very close.

2. (ii) There is a tendency to focus the eyes on nearby objects on the windscreen instead of at infinity, especially in the absence of conspicuous cloud and at high altitude. This can cause ‘empty field myopia’ whereby a normal pilot becomes nearsighted, his eyes focusing on a location less than six feet away.

3. (iii) In modern jet fighter aircraft the pilot must give much of his visual attention to instruments in the cockpit, such as the altimeter, owing to the difficulty of maintaining longitudinal stability, especially when heavily loaded. In clear air above 35,000 ft, it is estimated the pilot flies half contact and half on instruments.