A system of lights arranged to provide visual descent guidance information during the approach to a runway. These lights are visible from 3 to 5 miles during the day and up to 20 miles or more at night. The visual glide path of the VASI provides a safe obstruction clearance within +/-10° of the extended centerline and up to 4 NM from the runway. A VASI installation may consist of 2, 4, 6, 12, or 16 light units arranged in bars referred to as near, middle
, and far bars
. A three-bar VASI is essentially a two-bar VASI with an additional pair of bars. The installation may consist of 6 or 16 light units. If it is 6 light units, then it is located on one side of the runway—usually the left. On the other hand, if it is a 16-light system, the units are located on both sides of the runway. Such a system is useful for large eye-wheel distance aircraft. An indication of redred means below the glide path, white-white means above the glide path, and red-white means on the glide path.