PAPI (precision approach path indicator) lights are a set of lights positioned beside the runway which provide pilots with a visual indicator of their aircraft’s position relative to the correct glidepath for the runway. … Two of each shows the pilot is on the correct path for landing.

Are PAPI lights required?

A PAPI can, if required, be located on the right-hand side of the runway. The red lights are always on the side closest to the runway. If the PAPI is on the right-hand side of the runway (non-standard), the red lights will be on the left. Each light unit consists of one or more light sources, red filters and lenses.

What is the difference between PAPI and Vasi lights?

The only functional differences between the VASI and PAPI is that the VASI has the red over the white, the PAPI the white actually goes to the right of the red, and the PAPI offers higher precision (or more glideslopes depending how you look at it a larger airliner with a high cockpit may elect to fly a slightly higher

A two-light PAPI system simplifies this: Two white lights: the plane is too high. One white light and one red light: the plane is on the correct glideslope. Two red lights: the plane is too low.