There are many airports equipped with a Visual Approach Slope Indictor to assist pilots in the landing approach. The VASI provides a color coded visual glidepath using a system of lights positioned alongside the runway, near the designated touchdown point. It ensures safety by providing a visual glidepath which clears all obstructions in the final approach area. Once the principles and color code of the lighting system are understood, flying the VASI is a simple matter of noting the light's colors and adjusting the airplane's rate of descent to stay on the visual glide slope. The VASI is especially effective during approaches over water or featureless terrain where other sources of visual reference are lacking or misleading, and at night. It provides optimum descent guidance for landing and minimizes the possibility of undershooting or overshooting the designated touchdown area. There are several types of VASIs in use including the 2 bar, 3 bar, and tricolor systems. The basic principle of the 2 bar and 3 bar VASI is that of color differentiation between red and white. Each light unit projects a beam of light having a white segment in the upper part of the beam and red segment in the lower part of the beam (Fig. 7-9).

   When on the proper glidepath, the pilot will in effect, overshoot the downwind bars and undershoot the upwind bars (Fig. 7-8). Thus, the downwind bars will be seen as white and the upwind bars as red. From a position below the glidepath the pilot will see all the light bars as red, and from above the glidepath all the light bars will appear white. Passing through the glidepath from a low position is indicated to the pilot by a transition in color from red through pink to white. From a high position, passing through the path is indicated to the pilot by a transition in color from white through pink to red.

   When the pilot is below the glidepath, the red bars tend to merge into one distinct red signal. A safe obstruction clearance may not exist when this distinct red signal is visible. The visual glidepath will separate into individual lights as the pilot approaches the runway threshold. At this point, the approach should be continued by reference to the runway touchdown zone.

   Three bar VASI installations provide two visual glidepaths. The lower glidepath is provided by the near and middle bars and is normally set at a 3 degree incline while the upper glidepath, provided by the middle and far bars, is normally 1/4 degree higher. This higher glidepath is intended for use only by high cockpit aircraft to provide a sufficient threshold crossing height.

   When using a 3 bar VASI it is not necessary to use all three bars. The near and middle bars constitute a two bar VASI for using the lower glidepath. Also, the middle and far bars constitute a 2 bar VASI for using the upper glidepath. The Tricolor Approach Slope Indicator normally consists of a single light unit, projecting a three color visual approach path into the final approach area of the runway upon which the system is installed. In these systems, a below glidepath indication is red, the above glidepath indication is amber and the on path indication is green.