During night flying accidents occur because the pilot's visibility is determined by the greatest distance that lit obstacles can be seen. Darkness degrades or eliminates most of the visual cues so depth perception is degraded or totally removed. Lit objects are seen at a greater distance at night than by day. When a pilot approaches a runway over terrain that does not have any lights such as desert or water judgement becomes difficult - known as the Black Hole Effect.
Black Hole Effect
Where an approach is made over unlit terrain such as water or desert the pilot sees the runway lights at a greater distance than the runway would be seen during the day. The perception is that the aircraft is high on the approach to the runway. Under these conditions it is possible to misjudge the approach and land short of the runway. At night bright lights and good visibility lead to an under-estimation of the distance. Conversely low light and poor visibility lead to an over-estimation of the distance.
A pilot on a "Black Hole" approach varies the descent profile by reference to the visual perspective this can also be aggravated by other factors:
- If a long, straight approach is made to an airport located near to a small town.
- The pilot is not familiar with the runway length/width combination.
- The airport is at a slightly lower elevation than the surrounding terrain.
- The airport does not have a good lighting system.
- Small settlements are spread over an area around the airport.
Visual Factors at Night
Other factors that mislead pilots flying at night:
- A brightly lit runway will make the runway appear closer than it really is. This may cause the pilot to descend early.
- Flying in clear air at night, brightly lit objects appear closer than they really are.
- If the horizon is obscured scattered lights can be mistaken for stars. This can give the pilot the sense that the aircraft is nose high and a correction nose down is made.
- If the horizon is obscured then the distant lights of a city may make the horizon seem to be lower than it actually is.
- Rain on the windshield can convince a pilot that he is too high due to the refraction of light. It is possible that an error of 200 ft per nautical mile can occur.
- When an airport is viewed through a rain shower the runway lights bloom and appear bigger than they really are causing the pilot to believe that the aircraft is high.
- Flying over a dark sea at night when no stars are visible it is possible that the pilot may misinterpret fishing boat lights below the aircraft as stars. The misconception is that the aircraft is upside down and the pilot rolls the aircraft to put these "stars" above him.
Where an aircraft is approaching head on the retinal size of the approaching aircraft is small until a short time before impact. Where a target is moving across the visual field the "pick up" time is much shorter.
Peripheral vision is good at picking up the movement across the eye by using the rods.
In the diagram below two aircraft on a collision course are on a constant bearing at a constant speed.
If the constant bearing is maintained and there is no relative motion then aircraft B will be stationary in aircraft A's visual field. The movement needed to stimulate the rods is absent and the pilot in aircraft A will not see aircraft B until shortly before the collision. The aircraft subtends such a small angle on the retina till it is within 0.4 seconds of impact. Probably too late for any corrective action.
Visual acuity has been described as the capacity of the eye to resolve detail. The acuity across the eye reduces rapidly as soon as we are more than 2° away from the fovea.
The eye has a Blind Spot. In normal vision a person does not notice any deterioration in vision because of the position of the blind spot. Compensation is made by the saccadic motion (a jerk/rest cycle of ⅓ second) of the eye. The saccadic movement can be demonstrated by the following experiment. On a dark clear night, stand still and concentrate on a single bright star, after 5 to 10 seconds the star will start to move. A process known as Autokinesis.
To illustrate the blind spot look at the diagram below from about 30 cm.
- Close the left eye.
- Focus the right eye on the cross.
- Move the picture slowly towards the face.
As the picture gets closer the aircraft will disappear and then re-appear.
Rain on the windshield causes a distortion-refraction-that makes terrain appear lower than it actually is-a hilltop ½ nautical mile away may appear 200ft lower than it actually is.
Also, landing on a upslope runway presents a similar illusion-that you are higher than what you actually are.
Here is a summary of the landing illusions that pilots can experience: