Definition
A pulsating Visual Approach Slope Indicator is a single-light-source glidepath aid that shows the pilot whether the aircraft is on, above, or below the proper approach path by varying the color and pulse rate of the light. A steady white light indicates on glidepath, a pulsating white light indicates above glidepath, a steady red light indicates slightly below glidepath, and a pulsating red light indicates well below glidepath.
Plain English
A small light beside the runway that tells you if you are coming in too high, too low, or just right. The color and whether it flashes tells you which one.
Context Anchor
Seen on final approach to some runways as a visual guide for maintaining the proper descent path to landing.
Derivation
Pulsating' comes from the Latin pulsare, meaning to beat or throb. Here it describes the flashing behavior of the light, which is what distinguishes this system from a steady-light VASI.
Why Pilots Care
Gives immediate visual confirmation of the correct descent angle during visual approaches, helping prevent unstable landings or runway overruns.
Intuition Check
Do not assume a pulsing VASI light means the equipment is failing. In this system, pulsing is an intentional part of the guidance signal.
Example Sentence 1
On final approach, the pilot saw a steady white light from the pulsating VASI and knew the aircraft was on the correct glidepath.
Example Sentence 2
When the light began to pulse red we knew we had dropped slightly below the glide path and added a touch of power.