precision approach path indicator

precision approach path indicator (PAPI)

A system of lights arranged to provide visual descent guidance information during the approach to a runway. The system is similar to the VASI (visual approach slope indicator). However, the light units are installed in a single row of either two or four light units on the left side of the runway. These systems have an effective visual range of about 5 miles during the day and up to 20 miles at night. The glide-path indications are as depicted in the illustration.