Visual approach slope indicator

"[2] A tri-color system consists of a single-light unit projecting a three-color visual approach path.

[3] The pulsating visual approach slope indicator (PVASI) is a system similar in purpose to a VASI, but only consists of one piece of equipment.

It indicates the pilot's height on approach with a mix of red and white, steady and pulsating light.

[4] This type of system is less prevalent than other visual glide slope indicators like regular VASIs or PAPIs: as of May 2022 there were only 84 PVASIs installed at 51 airports and heliports in the United States and its territories according to Federal Aviation Administration data, compared to 6730 PAPIs and 623 VASIs.

[5] Some airports serving long-bodied aircraft have three-bar VASIs that provide two visual glide paths to the same runway.

Standard visual approach slope indicator (circled in red)
Working VASI
Comparison of PAPI , VASI, and OLS meatball and datum lights (not to scale)
Tri-color VASI
Pulsating VASI