Definition
A ground-based system of lights positioned beside the runway that gives pilots a visual reference for maintaining the correct descent path on final approach. The standard VASI uses two bars of lights — a near bar and a far bar — that appear red or white depending on the aircraft's position relative to the proper glide path. Seeing white over red indicates the aircraft is on the correct slope; red over red means too low; white over white means too high.
Plain English
A set of lights next to the runway that tells the pilot whether they are coming in at the right angle for landing. The colours change based on whether the aircraft is too high, too low, or on the correct path.
Context Anchor
Seen near the landing end of some runways and used during final approach, especially at night or when judging height by sight is difficult.
Derivation
The name describes its job directly: a visual (you see it with your eyes) indicator of the approach slope (the angle of descent toward the runway). Worth noting because pilots sometimes confuse VASI with PAPI, which does the same job using a single row of four lights instead of two bars.
Why Pilots Care
Maintains a safe descent path that helps avoid landing short of the runway or floating too far down it.
Intuition Check
Do not think of a VASI as a landing target or an automatic landing aid. It is a visual light cue that helps you judge your descent path; the pilot still flies the airplane.
Example Sentence 1
On final approach, the pilot adjusted the descent rate after seeing the VASI showing red over red.
Example Sentence 2
Red over white on the VASI confirmed the correct glide path to the runway threshold.