Generally, most pilots are poorly informed about night vision. Human eyes never function as effectively at night as the eyes of nocturnal animals, but if humans learn how to use their eyes correctly and know their limitations, night vision can be improved significantly. The human eye is constructed so that day vision is different from night vision. Therefore, it is important to understand the eye’s construction and how the eye is affected by darkness.
Innumerable light-sensitive nerves called cones and rods are located at the back of the eye or retina, a layer upon which all images are focused. These nerves connect to the cells of the optic nerve, which transmits messages directly to the brain. The cones are located in the center of the retina, and the rods are concentrated in a ring around the cones. [Figure 12-6]
The function of the cones is to detect color, details, and faraway objects. The rods function when something is seen out of the corner of the eye or peripheral vision. They detect objects, particularly those that are moving, but do not give detail or color—only shades of gray. Both the cones and the rods are used for vision during daylight.
Although there is not a clear-cut division of function, the rods make night vision possible. The rods and cones function in daylight and in moonlight; in the absence of normal light, the process of night vision is almost entirely a function of the rods.
The fact that the rods are distributed in a band around the cones and do not lie directly behind the pupils makes offcenter viewing (looking to one side of an object) important during night flight. During daylight, an object can be seen best by looking directly at it, but at night a scanning procedure to permit off-center viewing of the object is more effective. Therefore, the pilot should consciously practice this scanning procedure to improve night vision.
The eye’s adaptation to darkness is another important aspect of night vision. When a dark room is entered, it is difficult to see anything until the eyes become adjusted to the darkness. In the adaptation process, the pupils of the eyes first enlarge to receive as much of the available light as possible. After approximately 5 to 10 minutes, the cones become adjusted to the dim light and the eyes become 100 times more sensitive to light than they were before the dark room was entered. About 30 minutes is needed for the rods to become adjusted to darkness; when they do adjust, they are about 100,000 times more sensitive to light than in the lighted area. After the adaptation process is complete, much more can be seen, especially if the eyes are used correctly.
After the eyes have adapted to the dark, the entire process is reversed when entering a lighted room. The eyes are first dazzled by the brightness, but become completely adjusted in a few seconds, thereby losing their adaptation to the dark. Now, if the dark room is reentered, the eyes again go through the long process of adapting to the darkness.
Before and during night flight, the pilot must consider the adaptation process of the eyes. First, the eyes should be allowed to adapt to the low level of light. Then, the pilot should avoid exposing them to any bright white light that would cause temporary blindness and possibly result in serious consequences.
Temporary blindness, caused by an unusually bright light, may result in illusions or afterimages until the eyes recover from the brightness. The brain creates these illusions reported by the eyes. This results in misjudging or incorrectly identifying objects, such as mistaking slanted clouds for the horizon or a populated area for a landing field. Vertigo is experienced as a feeling of dizziness and imbalance that can create or increase illusions. The illusions seem very real and pilots at every level of experience and skill can be affected. Recognizing that the brain and eyes can play tricks in this manner is the best protection for flying at night.
Good eyesight depends upon physical condition. Fatigue, colds, vitamin deficiency, alcohol, stimulants, smoking, or medication can seriously impair vision. Keeping these facts in mind and taking appropriate precautions should help safeguard night vision.
In addition to the principles previously discussed, the following actions aid in increasing night vision effectiveness:
- Adapt the eyes to darkness prior to flight, and keep them adapted. About 30 minutes is needed to adjust the eyes to maximum efficiency after exposure to a bright light.
- Use oxygen during night flying, if available. Keep in mind that a significant deterioration in night vision can occur at altitudes as low as 5,000 feet.
- Close one eye when exposed to bright light to help avoid the blinding effect.
- Avoid wearing sunglasses after sunset.
- Move the eyes more slowly than in daylight.
- Blink the eyes if vision becomes blurred.
- Concentrate on seeing objects.
- Force the eyes to view off center.
- Maintain good physical condition.
- Avoid smoking, drinking, and using drugs that may be harmful.
Unique WSC Flight Characteristics
If the WSC aircraft is trimmed properly and the pilot is proficient in the basic flight maneuvers of climbs, cruise, and descent procedures, the WSC aircraft speed is easily determined with control bar pressure and position for normal flight conditions. A pilot can also determine basic climbs and descents through the feel of the aircraft with the airspeed and throttle positions. Therefore, basic pitch control can be done by a proficient pilot with his or her eyes closed.
As discussed in Chapter 2, Aerodynamics, WSC aircraft are generally not designed to be roll stable, and any engine turning effect or movement of the air can put the WSC aircraft into a roll, which it maintains unless corrected by the pilot. In other words, releasing the control bar in a WSC aircraft will not level a bank back to straight flight. The pilot must continually provide input to fly a constant heading even if this control is small corrections. In other words, the pilot cannot level the wings or fly a straight heading for very long with his or her eyes closed.
To maintain a constant heading or ground track, one of three instruments can be used: magnetic compass, global positioning system (GPS), and aircraft heading indicator. Without a visual reference, these can be used to fly straight. An attitude indicator can be used on WSC aircraft providing additional instrument reference. These instruments and others are discussed later in this chapter.
In addition to night vision limitations, pilots should be aware that night illusions could cause confusion and concerns during night flying. The following discussion covers some of the common situations that cause illusions associated with night flying.
A false horizon can occur when the natural horizon is obscured or not readily apparent. It can be generated by confusing bright stars and city lights. It can also occur while flying toward the shore of an ocean or a large lake. Because of the relative darkness of the water, the lights along the shoreline can be mistaken for stars in the sky. [Figure 12-7]
On a clear night, distant stationary lights can be mistaken for stars or other aircraft. Even the northern lights can confuse a pilot and indicate a false horizon. Certain geometrical patterns of ground lights, such as a freeway, runway, approach, or even lights on a moving train can cause confusion. Dark nights tend to eliminate reference to a visual horizon. As a result, pilots need to rely less on outside references at night and more on flight and navigation instruments.
Visual autokinesis can occur when a pilot stares at a single light source for several seconds on a dark night. The result is that the light appears to be moving. The autokinesis effect does not occur if the pilot expands the visual field. It is a good procedure to vary visual focus and not become fixed on one source of light.
Distractions and problems can result from a flickering light in the flight deck, such as anticollision lights, strobe lights, or other aircraft lights which can cause flicker vertigo. If continuous, the possible physical reactions can be nausea, dizziness, grogginess, confusion, headaches, or unconsciousness. The pilot should try to eliminate any light source causing blinking or flickering problems in the flight deck.
A black-hole approach occurs when the landing is made from over water or unlighted terrain on which runway lights are the only sources of light. Without peripheral visual cues to help, pilots have trouble orienting themselves relative to Earth. The runway can seem out of position (downsloping or upsloping) and, in the worst case, result in landing short of the runway. If an electronic glideslope or visual approach slope indicator (VASI) is available, it should be used. If navigation aids (NAVAIDs) are unavailable, careful attention should be given to using the flight instruments to assist in maintaining orientation and a normal approach. If at any time the pilot is unsure of his or her position or attitude, a go-around should be executed.
Bright runway and approach lighting systems, especially where few lights illuminate the surrounding terrain, may create the illusion of less distance to the runway. In this situation, the tendency is to fly a higher approach. Also, when flying over terrain with only a few lights, it makes the runway recede or appear farther away. With this situation, the tendency is common to fly a lower-than-normal approach. If the runway has a city in the distance on higher terrain, the tendency is to fly a lower-than-normal approach. A good review of the airfield layout and boundaries before initiating any approach helps the pilot maintain a safe approach angle.
Illusions created by runway lights result in a variety of problems. Bright lights or bold colors advance the runway, making it appear closer. Night landings are further complicated by the difficulty of judging distance and the possibility of confusing approach and runway lights. For example, when a double row of approach lights joins the boundary lights of the runway, there can be confusion where the approach lights terminate and runway lights begin. Under certain conditions, approach lights can make the aircraft seem higher in a turn to final than when its wings are level.