How many lights typically make up a PAPI installation?

A Precision Approach Path Indicator (PAPI) installation typically consists of four lights. This system is designed to provide visual guidance to pilots during the approach phase of landing by indicating whether they are on the correct glide path. The four lights work in pairs, with one side displaying red lights and the other showing white lights, helping pilots gauge their altitude in relation to the desired approach slope.

Regarding the other options, two lights would not provide enough visual information for pilots, and three lights are generally not standard for PAPI installations. Using five lights is also not typical, as it could lead to confusion and complicate the simple visual guidance that PAPI is designed to provide. Therefore, the correct number of lights for a standard PAPI system is indeed four.