Precision Approach Path Indicators (PAPI)

An elevated unidirectional halogen unit, left of the runway, showing an aircraft on final whether it is high or low on the glide path by its white-to-red ratio — more white when high, more red when low. The optics keep the red/white transition line straight, so the crew reads a clean boundary; coated filters hold a firm, heat-stable coating that does not fade under halogen heat. A four-digit display reads elevation; operation and calibration-flight modes mean aiming and flight checks need no external instrument.

An elevated unidirectional LED unit, grouped left of the runway, marking the glide path by its white-to-red ratio — more white high, more red low. The optics keep the red/white transition band narrow and the boundary straight, so height reads off a line, not a color spread. Elevation, mode and fault codes read through the front glass without opening it, so inspection disturbs neither seal nor aim. The units link by CAN, and the group shuts down on a fault rather than show wrong guidance.