Plain English Meaning

A row of lights next to the runway that tells the pilot if they are coming in at the right angle. The mix of red and white lights shows whether the approach is on the correct slope, too high, or too low.

Full Definition

A visual landing aid located beside the runway that uses a row of light units to show the pilot whether the aircraft is on, above, or below the correct approach path during the final descent to the runway. Each light unit projects a beam that appears either white or red depending on the aircraft's vertical position relative to the designed glide path. A standard PAPI consists of four lights in a single row; the combination of red and white lights tells the pilot the angle of approach. Two white and two red indicates on the correct path; more white means too high; more red means too low.