Helicopter approach path indicator (HAPI).
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 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. Most VASI installations consist of two bars—near and far—and may consist of 2, 4, or 12 light units. Some VASIs consist of three bars—near, middle, and far—which provide an additional visual glide path to accommodate high-cockpit aircraft. This installation may consist of either 6 or 16 light units. VASI installations consisting of 2, 4, or 6 light units are located on one side of the runway, usually the left. Where the installation consists of 12 or 16 light units, the units are located on both sides of the runway. Two-bar VASI installations provide one visual glide path that is normally set at 3°. Three-bar VASI installations provide two visual glide paths. The lower glide path is provided by the near and middle bars and is normally set at 3°, whereas the upper glide path, provided by the middle and far bars, is normally ½ degree higher. This higher glide path is intended for use only by high-cockpit aircraft to provide a sufficient threshold crossing height. Although normal glide-path angles are 3°, angles at some locations may be as high as 4.5° to give a proper obstacle clearance.
In most cases, an indication of red-red means below the glide path, white-white means above the glide path, and red-white means on the glide path. The various versions of VASI are the two-bar VASI, three-bar VASI, PAPI (precision approach path indicator), T-VASI, and AT-VASI.