Definition
VASI and PAPI are ground-based light systems installed beside a runway that show a pilot, on final approach, whether the aircraft is on, above, or below the correct glide path to the touchdown zone. A VASI uses two or three sets of light bars and signals path information through red-and-white combinations (for example, red over white means on path). A PAPI uses a single row of four lights, also signaling path information through red-and-white combinations (for example, two red and two white means on path). Both systems are visible at night and during the day, and most are usable from several miles out on final.
Plain English
Lights next to the runway that tell you, just by their color pattern, whether you're coming in too high, too low, or just right.
Context Anchor
Seen when planning or flying a landing, especially when checking runway environment, lighting, and approach conditions.
Derivation
VASI = Visual Approach Slope Indicator (a visual cue for your approach slope). PAPI = Precision Approach Path Indicator (a more precise version of the same idea). PAPI replaced VASI at most larger airports because its four-light row gives finer resolution about how far above or below path you are.
Why Pilots Care
Allows pilots to maintain a safe, stabilized descent angle and avoid landing short or floating long.
Intuition Check
If you see more white than expected, you're high. If you see more red than expected, you're low. A simple memory aid: 'red over white, you're alright; red over red, you're dead; white over white, you'll fly all night.'
Example Sentence 1
Three miles out, the PAPI showed three whites and one red, so the pilot reduced power slightly to settle back onto the glide path.
Example Sentence 2
PAPI indications confirmed the aircraft was on the proper glide path during the night landing.