The Precision Approach Path Indicator (PAPI) is a light system positioned beside the runway that consists of two, three, or four boxes of lights that provide a visual indication of an airplane's position on the glidepath for the associated runway. The FAA standard for the PAPI is the same as the ICAO's standard Visual Glide Slope Indicator.
The PAPI is usually located on the left side of the runway and can be seen up to five miles during the day and twenty miles at night.
Each box of lights is equipped with an optical apparatus that splits light output into two segments: red (the upper segment, or right segment if lights are in series) and white. Depending on the viewing, and thus the descent, angle, a pilot will see either the red or the white portion.
A normal glidepath (usually 3%) is indicated by an even split in red and white lights. If an airplane is beneath the glidepath, more red lights are visible to the pilots; if an airplane is above the glidepath, more white lights are visible.
During aircraft descent, this system, along with other airport lights, may be activated by the pilot by keying the airplane microphone with the airplane's communication radio tuned to the CTAF.
The PAPI operates on the principle of the Fresnel lens.
See also
- Pilot Controlled Lighting (PCL)
- Visual Approach Slope Indicator (VASI)
- Runway End Indentification Lights (REIL)
- Runway Edge Lights (HIRL, MIRL, LIRL)