What does it indicate if one red light shows on the PAPI lights?

One red light on the Precision Approach Path Indicator (PAPI) signals that the aircraft is slightly above the ideal glide slope for landing. PAPI lights are used to provide visual guidance to pilots as they approach a runway, showing whether they are above, below, or on the correct flight path.

In the PAPI system, the lights are arranged to change color based on the aircraft's position relative to the optimal approach angle. A perfect approach typically illuminates four white lights, while two white and two red lights indicate an acceptable approach. Consequently, one red light among three white lights clearly indicates that the aircraft is slightly high on the approach slope.

Understanding the PAPI system helps pilots make real-time adjustments to their descent, ensuring a safer landing.