Definition
A Visual Approach Slope Indicator system using two rows of light units, called the near bar and the far bar, positioned beside the runway to show whether an aircraft is on, above, or below the correct approach path. When both bars appear white the aircraft is above the glide path, both red means below the glide path, and red over white indicates on the glide path.
Plain English
A simple set of two light bars next to the runway that tells the pilot, by their colors, whether they are coming in too high, too low, or just right.
Context Anchor
Seen during the visual part of an approach to a runway, especially when checking whether the airplane is on a normal descent path for landing.
Derivation
VASI stands for Visual Approach Slope Indicator. '2-bar' refers to the two light bars used; a 3-bar version exists for runways serving aircraft that need a higher cockpit-to-wheel reference.
Why Pilots Care
Maintains the correct descent path to the runway threshold and reduces the chance of landing short or long.
Intuition Check
Do not read “2-bar” as two separate systems or just two individual lights. Here, a “bar” is a row of lights, and the two rows work together to show your approach path.
Example Sentence 1
On final approach the pilot saw red over white on the 2-bar VASI and held the descent steady.
Example Sentence 2
On short final the 2-bar VASI displayed red over white, confirming the aircraft was on the proper glide path.