Definition
A ground-based light system installed beside a runway that gives the pilot a visual indication of whether the aircraft is on, above, or below the correct descent path to the runway during approach. VGSI is a general category that includes systems such as VASI (Visual Approach Slope Indicator) and PAPI (Precision Approach Path Indicator).
Plain English
A set of lights next to the runway that tells the pilot, by color, whether they are too high, too low, or on the right slope coming in to land.
Context Anchor
Seen on instrument approach charts and during final approach to runways that have visual descent guidance lights.
Derivation
Visual (you see it with your eyes), Glide Slope (the angled path down to the runway), Indicator (something that shows you where you are). The name describes exactly what the lights do — show you the glide path visually.
Why Pilots Care
Allows immediate correction of glide path to avoid obstacles and achieve a safe touchdown point.
Intuition Check
Do not assume the VGSI always matches the instrument approach descent path. If the chart says the two do not line up exactly, treat them as separate guidance references.
Example Sentence 1
The chart noted that the runway was equipped with a VGSI, so the pilot used the lights to fine-tune the descent during the final approach.
Example Sentence 2
At night the VGSI confirmed the correct descent angle all the way to the runway threshold.