Definition
A ground-based visual landing aid arranged in the shape of a T, positioned beside the runway, that shows the pilot whether the aircraft is on, above, or below the correct approach slope to the runway threshold. The system uses a row of lights forming the crossbar of the T plus a stem of lights extending away from the runway. When on slope, only the crossbar is visible. Lights appearing above the crossbar indicate the aircraft is too low; lights appearing below the crossbar indicate the aircraft is too high. The number of visible stem lights shows how far off the correct slope the aircraft is.
Plain English
A pattern of lights next to the runway shaped like the letter T. It tells the pilot, just by looking, whether the aircraft is approaching at the right height, too high, or too low.
Context Anchor
Seen on instrument approach charts and at airports as a runway lighting aid near the landing end of a runway.
Derivation
Named for the shape the lights form on the ground. The crossbar of the T sits perpendicular to the runway, and the stem extends outward. The shape itself is the indicator: a clean T means on slope, extra lights above or below the crossbar mean off slope.
Why Pilots Care
Maintains a safe descent path that avoids obstacles and ensures touchdown at the intended point on the runway.
Intuition Check
Do not think of T-VASI as an instrument navigation signal. It is a visual runway light system that the pilot looks at outside the aircraft.
Example Sentence 1
On final approach, the pilot saw only the crossbar of the T-VASI and knew the aircraft was on the correct slope.
Example Sentence 2
When all lights showed white on the T-VASI, the crew knew they were slightly high and reduced power slightly.