What is the main function of a PAPI system?

The main function of a PAPI (Precision Approach Path Indicator) system is to indicate the glide-slope path for aircraft during their approach to a runway. This system consists of a series of light units that provide visual guidance to pilots on the correct angle they should maintain while descending toward the runway. The lights are positioned beside the runway, and their color variations (typically red and white) indicate whether the aircraft is on the correct glide path.

When the lights are primarily white, it indicates that the aircraft is above the ideal glide slope, while a predominance of red suggests that the aircraft may be too low. By providing real-time visual feedback, the PAPI system enhances safety during landings, helping pilots to align their descent accurately and conform to flight protocols. This is crucial for efficient landings, especially in low visibility conditions.

The other functions mentioned, such as determining runway length, measuring aircraft speed, or illuminating the runway at night, do not relate to the primary purpose of a PAPI system.