Plain English Meaning

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.

Full 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 and during the day, and most are usable from several miles on .