What aspect does PAPI specifically reference in aviation?

PAPI, which stands for Precision Approach Path Indicator, specifically refers to the vertical aspect of approaching a runway. It is an essential visual aid used by pilots to maintain the correct approach angle for landing. The system consists of a series of lights that indicate whether the aircraft is above, below, or on the correct glide slope. When an aircraft is on the proper approach path, the pilot will see a specific combination of lights that indicates a safe descent.

This vertical guidance is crucial for ensuring safe landings, especially in conditions where visibility may be limited. By providing pilots with clear visual feedback on their approach angle, PAPI helps reduce the risk of accidents during the critical phase of landing.