Definition
A visual approach slope aid that uses a single light source projecting three colors along the approach path: red below the glidepath, green on the glidepath, and amber above the glidepath. The pilot judges vertical position relative to the correct descent angle by the color seen.
Plain English
A single light near the runway that changes color depending on whether you are too low, on the right path, or too high. Red means low, green means on path, amber means high.
Context Anchor
Seen on final approach at some airports when the pilot is using outside visual cues to judge the descent toward the runway.
Derivation
From Latin tri- (three) and color. The name simply describes how the system works: three colors signal three possible positions on the approach.
Why Pilots Care
Helps maintain a safe descent angle without relying on instruments during visual landings.
Grounding Statement
As the airplane moves above or below the intended path, the same light appears as a different color to the pilot.
Intuition Check
Do not read “tri-color system” as three separate runway lights. In this context, it is one visual glidepath aid that gives three possible color indications.
Example Sentence 1
On final approach, the tri-color system showed green, confirming the aircraft was on the correct glidepath.
Example Sentence 2
When the tri-color system turned green, the aircraft was confirmed on the proper glide path.