Definition
A ground-based system of lights installed near a runway that shows an approaching pilot, by means of colored light 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 beside the runway that tells the pilot, just by their color, whether the airplane is too high, too low, or on the right path coming in to land.
Context Anchor
Seen during the final part of a visual approach to a runway, usually as colored lights near the side of the runway.
Derivation
Visual because the pilot reads it with their eyes, glide slope because it shows the angled descent path, and indicator because it signals the aircraft's position relative to that path.
Why Pilots Care
Helps maintain a safe descent angle so the aircraft touches down at the proper point on the runway, reducing the chance of landing short or long.
Intuition Check
Do not assume it controls the airplane or guarantees a safe approach. It only gives visual height guidance for the intended descent path to that runway.
Example Sentence 1
On final approach, the pilot scanned the visual glide slope indicator and saw two red and two white lights, confirming the aircraft was on the correct descent path.
Example Sentence 2
At the uncontrolled airport the Visual Glide Slope Indicator gave the arriving pilot reliable glide path guidance without needing radio calls.