Medical Handbook For Pilots Chapter 12 – Night Flight

A veteran pilot once remarked that night flying is no different from day flying-it’s just that at night you can’t see anything. Although his statement carries a good deal of truth, you can usually see something. To compensate for what you can’t see, you need proper instrumentation. To make the most of your vision at night, you need to understand how the eye operates in darkness.

“Autokinesis” (short for autokinetic visible light phenomenon) is one of the special visual hazards of night flying. It resembles vertigo in some ways. Autokinesis occurs when you stare at a pinpoint of light in a dark sky. After a while, you get the feeling that either you or the light is in motion. To prevent this from happening, keep your eyes moving. Don’t stare at a single light too long. Autokinesis used to be responsible for numerous aircraft disasters, until we discovered the cause for this optical illusion.

Night flying requires a different visual technique than day flying. You can see an object best during daylight by looking directly at it. At night, however, a scanning procedure is more effective-to permit “offcenter” viewing of the target. In other words, you will find after some practice that you can see things more clearly and definitely at night by looking slightly to one side of them, rather than straight at them.

The explanation for this lies in the dual structure of your eye. There are two kinds of light-sensitive nerve endings at the back of your eye: (1) the cones, which distinguish color and require considerable light to function, and (2) the rods, which detect objects only in shades of gray but can operate in very dim light.

The cones, because they need greater intensity of light to function, are used in day vision. In fact, the cones stop working altogether in semidarkness. Millions of these tiny structures are clustered at the back of the eyeball, directly behind the pupil. Not only do they distinguish colors, they pick up distant objects.

The rods are concentrated in a ring around the cones. Being colorblind, they see only in grays and are used in peripheral vision during the day-that is, to perceive objects in motion out of the corner of the eye. Because the rods can still function in light of 1/5,000 the intensity at which the cones cease to function, they are used for night vision. These structures are 100,000 times as sensitive in the dark as they are in sunlight. However, they do need more time to adjust to darkness than the cones do to bright light. Your eyes become adapted to sunlight in 10 seconds, whereas they need 30 minutes to fully adjust to a dark night.

The fact that the rods are distributed in a band around the cones, and, therefore, do not lie directly behind the pupils, makes “offcenter” viewing important to the pilot during night flight. If, in your attempts to practice the scanning procedure mentioned previously, you find that your eyes have a tendency to swing directly toward the target, force them to swing just past it so that the rods on the opposite side of the eyeball pick up the object.

Rods lose their sensitivity after short exposure to alight source, but regain it quickly after a moment of “rest.” Consequently, a prolonged blink may be enough to renew the effectiveness of your vision if you are simply using the “offcenter” technique, without scanning. Remember, too, that rods do not perceive objects while your eyes are in motion, only during the pauses.

Good sight depends upon your physical condition. Fatigue, colds, vitamin deficiency, alcohol, stimulants, smoking, or medication can seriously impair your vision. Keeping these simple principles in mind, you should be able to safeguard your night vision.