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Visual Approach Slope Indicator
The visual approach slope indicator (VASI) is a system of lights which acts as an aid in defining the desired glide path in relatively good weather conditions. VASI lighting intensities are designed to be visible from 3 to 5 mi during the day and up to 20 mi at night.
There are a number of different VASI configurations depending on the desired visual range, the type of aircraft, and whether large wide bodied aircraft will be using the runway. Each group of lights transverse to the direction of the runway is referred to as a bar. The downwind bar is typically located between 125 and 800 ft from the runway threshold, each subsequent bar is located between 500 and 1000 ft from the previous bar. A bar is made up of one, two, or three light units, referred to as boxes. The basic VASI-2 system, illustrated in Fig. 8-6, is a two-bar system consisting of four boxes. The bar that is nearest to the runway threshold is referred to as the downwind bar, and the bar that is farthest from the runway threshold is referred to as the upwind bar. As illustrated in Fig. 8-6, if pilots are on the proper glide path, the downwind bar appears white and the upwind bar appears red; if pilots are too low, both bars appear red; and if they are too high both bars appear white.
In order to accommodate large wide bodied aircraft where the height of the eye of the pilot is much greater than in smaller jets, a third upwind bar is added. For wide bodied aircraft the middle bar becomes the downwind bar and the third bar is the upwind bar. In other words, pilots of large wide bodied aircraft ignore the bar closest to the runway threshold and use the other two bars for visual reference..
The more common systems in use in the United States are the VASI-2, VASI-4, VASI-12, and VASI-16. VASI systems are particularly useful on runways that do not have an instrument landing system or for aircraft not equipped to use an instrument landing system.
Precision Approach Path Indicator
The FAA presently prefers the use of another type of visual approach indicator called the precision approach path indicator (PAPI). This system gives more precise indications to the pilot of the approach path of the aircraft and utilizes only one bar as opposed to the minimum of two required by the VASI system.
The system consists of a unit with four lights on either side of the approach runway.
During the final approach for landing, pilots must make a decision to
complete the landing or ?execute a missed ap of the threshold is a major factor in pilot decisions to land or not to land. For this reason, the region near the threshold is given special lighting consideration. The threshold is identified at large airports by a complete line of green lights extending across the entire width of the runway, and at small airports by four green lights on each side of the threshold. The lights on either side of the runway threshold may be elevated. Threshold lights in the direction of landing are green but in the opposite direction these lights are red to indicate the end of the runway.
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