Definition
A visual glide path system, typically installed beside a runway, that uses a single light unit to show whether an aircraft on final approach is on, above, or below the correct descent path. The light changes colour and pulsation pattern to indicate position relative to glide path: a steady white or red light usually means on path, a pulsating white light means above path, and a pulsating red light means below path.
Plain English
A single light next to the runway that tells a pilot on approach whether they are too high, too low, or correctly lined up for landing. The light flashes or changes colour to give the message.
Context Anchor
Seen during approach and landing, especially when using runway lights to judge the proper path down to the runway. In this chapter context, it may also appear in discussions of equipment or components that are not working.
Derivation
Pulsating refers to the flashing behaviour of the light, which is the key feature that distinguishes this system from a steady-light VASI or PAPI. The word comes from the Latin pulsare, meaning to beat or throb, the same root as pulse. The pulsing is what carries the off-path message.
Why Pilots Care
Gives immediate visual confirmation of glide path position during a visual landing when precision instruments or other lighting systems are unavailable.
Intuition Check
Do not assume a pulsing PVASI light means the light is broken. In this system, the pulsing light is one of the normal signals telling you whether you are above or below the proper path.
Example Sentence 1
On short final the pilot saw a pulsating red light on the PVASI and added power to regain the glide path.
Example Sentence 2
With the ILS out of service, the crew relied on the PVASI to maintain the proper descent angle to the runway.