Definition
A Visual Approach Slope Indicator system that uses three rows (bars) of light units instead of the standard two, providing two separate visual glide paths to the same runway. The lower glide path (typically about 3 degrees) is shown by the near and middle bars and is used by smaller aircraft. The upper glide path (typically about 3.25 degrees) is shown by the middle and far bars and is used by pilots of high-cockpit aircraft, such as widebody jets, so the wheels still cross the threshold at a safe height.
Plain English
A runway light system with three rows of lights that gives two correct approach angles to the same runway — a normal one for most aircraft and a slightly steeper one for aircraft whose pilots sit very high above the wheels.
Context Anchor
Seen near the runway during final approach, and discussed in instrument approach and airport lighting information.
Derivation
VASI stands for Visual Approach Slope Indicator. The 'three-bar' simply describes the physical layout: three rows of light units beside the runway instead of the usual two. The extra bar adds a second, slightly higher glide path option.
Why Pilots Care
Helps large aircraft with high cockpits maintain a safe descent angle and avoid landing short or floating down the runway.
Grounding Statement
On final approach, the three-bar VASI gives the pilot a simple light picture to judge whether the aircraft is above, below, or on the intended descent path.
Intuition Check
Three-bar does not mean three individual lights. It means three rows of VASI lights arranged to provide two approach paths.
Example Sentence 1
Approaching a runway equipped with a three-bar VASI, the pilot of the light twin used the near and middle bars and disregarded the upper bar entirely.
Example Sentence 2
At this airport the three-bar VASI gave the crew an upper glide path option suited to their wide-body jet.