producing what appears to be a series of light fl ashes at regular intervals. These flashes may be one or two different colors that are used to identify various types of landing areas. For example: 

• Lighted civilian land airports—alternating white and green 

• Lighted civilian water airports—alternating white and yellow 

• Lighted military airports—alternating white and green, but are differentiated from civil airports by dual peaked (two quick) white flashes, then green 

Beacons producing red flashes indicate obstructions or areas considered hazardous to aerial navigation. Steady burning red lights are used to mark obstructions on or near airports and sometimes to supplement flashing lights on en route obstructions. High intensity flashing white lights are used to mark some supporting structures of overhead transmission lines that stretch across rivers, chasms, and gorges. These high intensity lights are also used to identify tall structures, such as chimneys and towers. 

As a result of technological advancements in aviation, runway lighting systems have become quite sophisticated to accommodate takeoffs and landings in various weather conditions. However, the pilot whose flying is limited to VFR needs to be concerned only with the following basic lighting of runways and taxiways. 

The basic runway lighting system consists of two straight parallel lines of runway-edge lights defining the lateral limits of the runway. These lights are aviation white, although aviation yellow may be substituted for a distance of 2,000 feet from the far end of the runway to indicate a caution zone. At some airports, the intensity of the runway-edge lights can be adjusted to satisfy the individual needs of the pilot. The length limits of the runway are defined by straight lines of lights across the runway ends. At some airports, the runway threshold lights are aviation green, and the runway end lights are aviation red. 

At many airports, the taxiways are also lighted. A taxiway- edge lighting system consists of blue lights that outline the usable limits of taxi paths. See the Pilot’s Handbook of Aeronautical Knowledge for additional information on airport lighting. 

##  Night Vision 

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 off- center 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 diffi cult to see anything until the eyes become adjusted to the darkness. In the adaptation process, the pupils of the eyes fi rst 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. 

