Definition
A ground-based visual landing aid that projects a single light beam toward the approach path, where the color seen by the pilot indicates the aircraft's position relative to the correct glide slope to the runway. The pilot sees green when on the proper glide path, amber (or yellow) when above it, and red when below it. The transition between colors occurs through narrow bands, including a brief pink or amber zone just above and below on-glide-path, depending on the unit's design.
Plain English
A light near the runway that changes color based on whether you are too high, too low, or just right on your descent. Green means you're on the correct path down, red means too low, and amber means too high.
Context Anchor
You see a T-VASI while flying the final part of an approach to a runway, especially at night or when visual guidance is needed near the airport.
Derivation
"Tri-color" simply means three colors, from Latin tri- (three) and color. Visual Approach Slope Indicator describes its function: a visible aid showing the slope (angle) of the approach. The name tells you exactly what it does -- three colors signaling your slope on approach.
Why Pilots Care
Provides immediate visual feedback on glide path to support a stable descent and safe landing when electronic guidance is unavailable.
Grounding Statement
As you line up with the runway, the T-VASI color tells you whether to stay on your current descent, correct downward, or correct upward.
Intuition Check
Do not read the colors like a normal traffic light. In a T-VASI, green means on the desired descent path, amber means above it, and red means below it.
Example Sentence 1
Approaching the small uncontrolled field at dusk, the pilot watched the T-VASI shift from amber to green and held that descent rate to the threshold.
Example Sentence 2
With the ILS unavailable, the crew used the T-VASI to maintain the proper glide path to the runway.