Precision approach path indicator

The Precision Approach Path Indicator (PAPI) is a light system that consists of four boxes of lights that provide a visual indication of an airplane's position on the glidepath for the associated runway.

Each box of lights is equipped with a lens that, depending on viewing angle, colors the light output red or white.

A normal glidepath is indicated by two red and two white lights. If an airplane is beneath the glidepath, more red lights visible to the pilots; if an airplane is above the glidepath, more white lights are visible.

During aircraft descent, this system may be activated by the pilot by keying the airplane microphone with the airplane's communication radio tuned to the CTAF.

Sources

• An FAA Aircraft Accident Report of Federal Express Flight 1478