Definition
A ground-based system of lights installed beside the runway that gives the pilot a visual indication of whether the aircraft is on, above, or below the correct approach slope to the runway. A standard VASI uses two bars of lights (near and far). When the pilot sees red over white, the aircraft is on the proper glide path. White over white indicates the aircraft is too high; red over red indicates too low. VASI guidance is normally usable from about 4 nautical miles out and provides obstruction clearance within plus or minus 10 degrees of the runway extended centerline.
Plain English
A set of lights next to the runway that tells you, just by their colour, whether your descent path is too high, too low, or just right. Red over white means you're on the correct slope. The phrase pilots use to remember it is 'red over white, you're alright.'
Context Anchor
Seen near the landing end of some runways and used while lining up and descending toward the runway.
Derivation
Visual' because the pilot reads it with their eyes, not an instrument. 'Approach Slope' refers to the descent path angle to the runway. 'Indicator' because it shows you where you are relative to that slope. The name is literal once each word is understood.
Why Pilots Care
Ensures pilots maintain the proper descent angle to touch down at the correct point on the runway, reducing the risk of landing short or floating too far.
Intuition Check
Don't confuse VASI with PAPI. PAPI is a single row of four lights; VASI uses bars of lights stacked near and far. Both do the same job, but they look different from the cockpit.
Example Sentence 1
On final approach, the pilot saw red over white on the VASI and held the descent steady to the runway.
Example Sentence 2
With all white lights visible on the VASI, the aircraft was slightly high on the glide path.