Definition
A Visual Approach Slope Indicator system that uses two sets (bars) of light units, one near and one far, beside the runway to show the pilot whether the aircraft is on, above, or below the proper visual glide path to the runway. The pilot reads the two bars as a pair: red over white means on the glide path, white over white means too high, and red over red means too low.
Plain English
A pair of light bars next to the runway that tell you, by their colors, if your descent angle to the runway is correct, too steep, or too shallow.
Context Anchor
Seen during the visual part of an instrument approach or during any runway approach where VASI lights are installed.
Derivation
VASI stands for Visual Approach Slope Indicator. "Two-bar" refers to the two rows of light units (a near bar and a far bar) used in this version of the system. A three-bar VASI exists for aircraft with a higher cockpit eye height and adds a third bar.
Why Pilots Care
Gives instant visual confirmation of glide path to help avoid landing short, long, or striking obstacles.
Grounding Statement
As you approach the runway, the two light bars change from your viewpoint, giving a simple color cue for your descent angle.
Intuition Check
Two-bar does not mean two individual lights. It means two rows, or bars, of lights used together to show your approach path.
Example Sentence 1
After breaking out of the clouds, the pilot saw red over white on the two-bar VASI and knew the approach was on glide path.
Example Sentence 2
With two white lights visible on the two-bar VASI, the aircraft remained on the correct glide path to the runway.