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Visual Approach Slope Indicator Systems (VASIS)
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A visual approach slope indicator system is a system consisting of four light units situated on the left side of the runway in the form of two wing bars referred to as the upwind and downwind wing bars. The aircraft is on slope if the upwind bar shows red and the downwind bar shows white, too high if both bars show white, and too low if both bars show red. Some aerodromes serving large aircraft have three-bar VASIS, which provide two visual glide paths (GP) to the same runway.
The main systems in use are the:
- AT-VASIS: abbreviated T visual approach slope indicator system
- T-VASIS : Visual Approach Slope Indicator System which can be installed flush with the runway pavement surface to provide approach guidance to that runway. A T-VASIS installation will span a large area on each side of the runway centreline extending along it from just beyond the runway threshold for approximately 500 metres
- PAPI: precision approach path indicator
- APAPI: abbreviated precision approach path indicator
Precision Approach Path Indicator
A precision approach path indicator (PAPI) uses lights similar to the VASI system except they are installed as four lights in a single row, normally on the left side of the runway. However, depending upon the runway / taxiway configuration, the PAPI can be located on the right as shown in the picture above. An aircraft is on the appropriate glide path when two of the lights are red and two are white. Three red lights indicate that the aircraft is below and four red lights indicate that the aircraft is well below the nominal flight path. Conversely, three white lights indicate that the aircraft is above and four white lights indicate that the aircraft is well above the flight path.
There are other, less common approach slope indicator systems.
A tri-color system consists of a single light unit projecting a three-color visual approach path. Below the glidepath is indicated by red, on the glidepath is indicated by green, and above the glidepath is indicated by amber. When descending below the glidepath, there is a small area of dark amber. Pilots should not mistake this area for an “above the glidepath” indication.
Pulsating visual approach slope indicators normally consist of a single light unit projecting a two-color visual approach path into the final approach area of the runway upon which the indicator is installed. The on glidepath indication is a steady white light. The slightly below glidepath indication is a steady red light. If the aircraft descends further below the glidepath, the red light starts to pulsate. The above glidepath indication is a pulsating white light. The pulsating rate increases as the aircraft gets further above or below the desired glideslope.
The useful range of these systems is about four miles during the day and up to ten miles at night.
- Final Approach Runway Occupancy Signal (FAROS) - Final Approach Runway Occupancy Signal (FAROS) works by providing a visual signal to aircraft on final approach to land that the runway ahead is occupied by another aircraft or a vehicle. This is done by adapting the PAPI or VASI system to alter from steady lights to flashing mode whilst the identified hazard remains.
- Pulse Light Approach Slope Indicator (PLASI)
- Visual Approach