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
Note: According to the recommendation in point 184.108.40.206 of ICAO Annex 14, Volume I, as of 1 January 2020, the use of T-VASIS and AT-VASIS as standard visual approach slope indicator systems should be discontinued.
Precision Approach Path Indicator
The PAPI can be seen to the right of the runway.
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.
PAPI minimum altitude (height above threshold) depends on the particular installation. There is no universal value and differences from runway to runway are common. A typical value is perhaps 50-60 ft. The lowest point where PAPI signal is useful is called MEHT (minimum eye height over threshold) and is included in the AIP for the particular runway. The declared MEHT makes assumptions regarding the position of the pilot's eyes and the ILS aerial of the aircraft, which of course varies from one aicraft type to another. In most cases PAPI is useable almost until touchdown but some states may have rules about switching them off if the ceiling or RVR is below a certain level, in which case the PAPI may be useable but not available.