Eyes provide the brain with a visual image of the environment. Each eye acts as a natural and very sophisticated digital camera. Its basic function is to collect light rays reflected from an object, using the lens to focus these rays into an image on a screen (the retina), and then converting this image into electrical signals that are sent via the optic nerve to the brain. This is how you see. The brain matches the image to previously stored data so you recognize (perceive) the object. The connection of the optic nerve to the brain is so close and integral, and the importance of the messages sent to the brain is so dominant, that the eyes can almost be considered an extension of the brain. Today we’ll talk more about vision with an excerpt from our textbook The Pilot’s Manual: Ground School (PM-2).
Scanning by Day
The central (foveal) region of the retina provides the best vision, and in full color but only during reasonable daylight. Objects are best seen by day if you can focus their image on the foveal region, and you do this by looking directly at them. The most effective method of scanning for other aircraft for collision avoidance during daylight hours is to use a series of short, regularly spaced eye movements to search each 10° sector of the sky. Systematically focusing on different segments of the sky for short intervals is a better technique than continuously sweeping the sky. This is sometimes called the saccade/fixation cycle, where the saccade or movement takes about one-third of a second.
Relative Movement
If there is no apparent relative motion between you and another aircraft, you may be on a collision course, especially if the other aircraft appears to be getting bigger and bigger in the windshield. Due to the lack of movement across your windshield, an aircraft on a collision course with you will be more difficult to spot than one that is not on a collision course.
Any relative movement of an object against its background usually makes it easier to notice in your peripheral vision. The image of the other aircraft may not increase in size much at first, but, shortly before impact, it would rapidly increase in size. The time available for you to avoid a collision may be quite brief, depending upon when you see the other aircraft and the rate of closure.
If you are flying at 100 knots and it is flying at 500 knots in the opposite direction, the rate of closure is 600 knots, i.e. ten nautical miles per minute. If you spot the other aircraft at a distance of one nautical mile, you only have 1/10 of a minute (six seconds) to potential impact. If you are a vigilant pilot and spot it at 3 nautical miles you have eighteen seconds in which to act.
In hazy or low-visibility conditions, your ability to see other aircraft and objects with edges that might be blurred will be diminished and, if you can see them, they may appear to be further away than their actual distance. You might be closer than you think.
Empty-Field Myopia
When trying to search for other aircraft in an empty sky, the natural tendency of a resting eye is to focus at about six feet. Consequently, distant aircraft may not be noticed. To avoid this empty-field myopia, you should focus on any available distant object, such as a cloud or a landmark, to lengthen your focus. If the sky is empty of clouds or other objects, then focus briefly on a relatively distant part of the airplane like a wing tip as a means of lengthening your focus. Having spotted an airplane in an otherwise empty sky, be aware that it could be closer to you than it appears to be, because you have no other object with which to compare its size.
Specks
A small, dark image formed on the retina could be a distant aircraft, or it could be a speck of dirt or dust, or an insect spot, on the windshield. Specks, dust particles, a scratch, or an insect on the windshield might be mistaken for a distant airplane. Simply moving your head will allow you to discriminate between marks on the windshield and distant objects.
Scanning by Night
The central (foveal) region of the retina containing mainly cones is not as effective at night, causing an area of reduced visual sensitivity in your central vision. Peripheral vision, provided by the rods in the outer band of the retina, is more effective albeit color blind. An object at night is more readily visible when you are looking to the side of it by ten or twenty degrees, rather than directly at it. Color is not perceived by the rods, and so your night vision will be in shades of gray. Objects will not be as sharply defined (focused) as in daytime foveal vision.
The most effective way to use your eyes during night flight is to scan small sectors of sky more slowly than in daylight to permit off-center viewing of objects in your peripheral vision, and to deliberately focus your perception (mind) a few degrees from your visual center of attention (that is, look at a point but look for objects around it). Since you may not be able to see the aircraft shape at night, you will have to determine its direction of travel making use of its visible lighting:
- the flashing red beacon;
- the red navigation light on the left wing tip;
- the green navigation light on the right wing tip; and
- a steady white light on the tail.
Visual Judgment on Approach
The eyes and brain use many clues and stored images of known objects to help in judging distance, size and height. The relative size and relative clarity of objects give clues to their relative distances: a bigger object is assumed to be nearer than a smaller one and a more clearly defined object nearer than a blurry one. When the object is near, binocular vision (the slightly different images of a nearby object relative to its background seen by each eye) assists in depth perception.
Texture also assists in depth perception: the more visible the texture, the closer the object appears to be. On final approach as you near the aim point, the surface texture will appear to flow outward in all directions from the point on which you are focused. This is one means by which you can visually maintain the flight path to the aim point: adjust the attitude and heading so that the point from which the texture appears to be moving outward remains the desired aim point.
Texture is also used for the estimation of height; for instance, as you approach flare height for a landing, the actual texture of the runway or the grass passing by the cockpit becomes increasingly noticeable. Relative motion also aids in depth perception. Near objects generally appear to pass by faster than more distant objects. This helps a visual pilot estimate height above the runway before and during the flare: the closer the airplane is to the runway, the faster the runway surface and the surrounding environment appears to pass by.
Depth perception can be difficult in hazy or misty conditions, where edges are blurred, colors are muted, and light rays may be refracted unusually. This gives the impression of greater distance, an impression reinforced by the fact that we often have to look at distant objects through a smoggy or hazy atmosphere. This illusion is referred to as environmental perspective. In hazy conditions, the object might be closer than it seems; in very clear conditions, the object might be further away than it seems. On hazy days, you might touch down earlier than expected; on very clear nights, you might flare a little too soon.
