Definition
A ground-based system of lights placed beside the runway that gives pilots a visual indication of whether they are on, above, or below the correct approach path to the landing threshold. A standard VASI uses two bars of lights; the colour combination seen by the pilot (red and white in various arrangements) shows the aircraft's position relative to the proper glide path.
Plain English
A set of lights next to the runway that tells you, just by their colour, whether you're coming in too high, too low, or just right.
Context Anchor
Seen beside some runways during landing, especially when the pilot is using outside visual references to judge the descent.
Derivation
The name is descriptive: 'Visual' (you see it with your eyes, not on an instrument), 'Approach Slope' (the angle you're descending toward the runway), 'Indicator' (it shows you something). Knowing this reminds you it's a visual aid for judging your descent angle, not a precision instrument.
Why Pilots Care
Helps maintain a safe descent angle to the runway, reducing the risk of landing short or long.
Grounding Statement
As you line up to land, the VASI gives a simple color cue that helps you adjust your descent before the runway gets close.
Intuition Check
Do not treat a VASI as an autopilot or a landing system that flies the airplane for you. It is only a visual aid; the pilot still controls the descent.
Example Sentence 1
On final approach, the pilot saw red over white on the VASI and held the descent steady, knowing she was on the correct glide path.
Example Sentence 2
At night the VASI gave clear visual guidance for the final approach to the runway.