Definition
A ground-based system of lights installed beside the runway that shows a pilot, by means of color signals, whether the aircraft is on, above, or below the correct descent path to the runway threshold. Common types include VASI (Visual Approach Slope Indicator) and PAPI (Precision Approach Path Indicator).
Plain English
A set of lights next to the runway that tells you, just by their color, whether you are too high, too low, or right on the proper path down to the runway.
Context Anchor
Seen on instrument approach procedure charts and near runways during the final approach to landing.
Derivation
Visual means seen by eye. Glide slope is the angled path an aircraft follows down to the runway. Indicator means something that shows information. Together: a visual aid that shows the proper descent path.
Why Pilots Care
Helps ensure obstacle clearance and a safe touchdown point on the runway.
Intuition Check
Do not confuse a VGSI with an instrument-only signal. A VGSI gives visual light cues outside the aircraft; the pilot must look outside to use it.
Example Sentence 1
The chart noted that a VGSI was installed for runway 27, so the crew used it as a visual reference during the final portion of the approach.
Example Sentence 2
At night the VGSI provided clear glide path cues until touchdown.