A VASIS consists of a series of lights visible from approximately 4 NM and designed to provide visual indications of the desired approach slope to a runway (usually 3°). At a certified airport, aircraft following the on-slope signal are provided with safe obstruction clearance to a minimum of 6° on either side of the extended runway centreline out to 7.5 km (4.1 NM) from the runway threshold. Newly certified airports are commonly protected out to 8° on each side of the extended runway centreline. Exceptions will be noted in the CFS. Descent using VASIS should not be initiated until the aircraft is visually aligned with the runway centreline.
The VASI and PAPI have lights normally situated on the left side of the runway only. When available strip widths preclude the use of a full system, an abbreviated approach slope indicator, AV or AP, consisting of only two light units, may be installed.
Where a PAPI or VASI is provided on a precision approach runway and has not been harmonized with a vertical guidance signal, it will be turned off in weather conditions involving a ceiling of less than 500 ft (150 m) and/or visibility less than 1 mi., unless specifically requested by the pilot. This is to avoid possible contradiction between the electronic precision approach vertical guidance and visual (VASI/PAPI) glide slope signal.