VISUAL APPROACH SLOPE INDICATORS (VASI) - The VASI system provides the pilot with visual approach slope angle information while on final approach. The VASI system is helpful during daylight or night operations. There are three standard VASI system configurations: VASI-4. VASI-12, and VASI- 16. We will discuss the VASI-12 system as it will appear on most Navy airfields.

The VASI system consists of twelve light boxes with three lights in each box. There is one complete system for each end of the runway. There are two pairs of bars-one pair of bars on each side of the runway. Each wing bar is composed of three light boxes (fig. 6-36). The set of bars nearest the threshold is called the downwind bars. and the other pair. the upwind bars. Each light box projects a beam of light that is white (clear) in its upper part and red in its lower part. The lights are arranged so that the pilot of an airplane, during the approach. sees all of the wing bar lights as red when below the glide slope. When on the glide slope. the pilot sees the downwind bar lights as white and the upwind bar as red. When above the glide slope, the pilot sees all the wing bar lights as white.

FRESNEL^® LENS OPTICAL LANDING SYSTEM (FLOLS). - Another system designed for continuous automatic operation is the FLOLS. (See fig. 6-37.) It also provides optical landing assistance by indicating the correct glide slope angle to the pilot of an approaching aircraft. This system contains two groups of horizontal datum lights set perpendicular to the approach path; two vertical bars of wave-off lights; two double types of cut lights; and a source light indicator assembly, consisting of five vertical cell assemblies. Each cell assembly contains source lights, a Fresnel^® lens. and a lenticular lens. The arrangement of these lenses gives the pilot the glide slope. The unit should be set up on the left side of the runway, from the pilot's perspective, about 10 feet from the edge of the pavement and 750 feet from the runway threshold.

Power for the system is provided by an installed field lighting supply or by an auxiliary, power unit capable of 20 kilowatt (kW). 60 hertz (Hz), three- phase, 120 volts phase to neutral.

RUNWAY DISTANCE MARKER. - With the use of high-speed aircraft, the runway distance marker system is needed to tell the pilots how much runway is left to take off or to land. The distance information, in thousands of feet, is given by numbers on the side of the marker. The numbers are on two sides of the signs, so that the distance left can be shown for both directions. There is one row of signs on each side of the runway. Each row is the same distance from the runway center

Figure 6-36. - Visual approach slope indicators (VASI).