The VASI is a system of lights so arranged to provide visual descent guidance information during the approach to a runway. The light units are arranged so that the pilot using the VASIs during an approach will see the combination of lights shown below. 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. The basic principle of the VASI is that of color differentiation between red and white. Pilots of high performance aircraft are cautioned that use of VASI angles in excess of 3.5 degrees may cause an increase in runway length required for landing and rollout. Although normal glide path angles are three degrees, angles at some locations may be as high as 4.5 degrees to give proper obstacle clearance. This higher glide path is intended for use only by high cockpit aircraft to provide a sufficient threshold crossing height. The lower glide path is provided by the near and middle bars and is normally set at 3 degrees while the upper glide path, provided by the middle and far bars, is normally 1/ 4 degree higher. Three-bar VASI installations provide two visual glide paths. Two-bar VASI installations provide one visual glide path which is normally set at 3 degrees. Where the installation consists of 12 or 16 light units, the units are located on both sides of the runway. VASI installations consisting of 2, 4, or 6 light units are located on one side of the runway, usually the left. This installation may consist of either 6 or 16 light units. Some VASIs consist of three bars, near, middle, and far, which provide an additional visual glide path to accommodate high cockpit aircraft. Most VASI installations consist of 2 bars, near and far, and may consist of 2, 4, or 12 light units.
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