For a visual scan for traffic aircraft that are on a collision course remain at a constant relative bearing only increasing in size
not moving across the visual field.
The eye is constructed so that the field of view is around 120° horizontally and 150° vertically, however only the centre of the vision area (the fovea) can detect the sort of detail we need to see aircraft - the rest is designed to detect movement (like big nasty predators!).
In addition to this without anything to focus on (i.e. looking at an empty sky), an eye will relax to its natural focus point of around one to two metres. This makes the pilot short sighted (known as empty field myopia) so that aircraft out the window are out of focus, and even harder to detect.
Finally, the eye doesn't move in a smooth scan, but "jerks" from poistion to position (known as "saccades"), which each "jerk" taking about a third of a second.
Given these points, for a good visual scan you need to:
- Move your scan from point to point, covering about 10° of the field of view with each movement.
- regularly focus on distant objects (like hills or clouds) to avoid empty field myopia. and
- hope you have a good radar service / TCAS
