What is the meaning of three whites and one red light in a PAPI system?

In a Precision Approach Path Indicator (PAPI) system, the configuration of lights provides crucial information to pilots about their altitude in relation to the optimal glideslope for landing. Specifically, three white lights and one red light indicate that the aircraft is "slightly high" on the approach.

This means the aircraft is above the ideal descent path and may need to descend slightly to ensure a safe and accurate landing. The PAPI system is designed to give visual cues to help pilots manage their approach and landing safely. A configuration of three white lights reflects that the majority of the lights indicate a higher than necessary altitude, while the presence of one red light serves as a warning that the aircraft is not on the desired descent path.

Understanding the PAPI light signals is essential for pilots to maintain proper altitude during the final approach to the runway, ensuring safety and adherence to the glide slope.