AIM 2-1-3 Runway End Identifier Lights (REIL)
Chapter 2. Aeronautical Lighting and Other Airport Visual Aids · Section 1. Airport Lighting Aids
A reference copy of the official AIM text for study. It is not legal advice or an FAA interpretation; always confirm against the current official source.
REILs are installed at many airfields to provide rapid and positive identification of the approach end of a particular runway. The system consists of a pair of synchronized flashing lights located laterally on each side of the runway threshold. REILs may be either omnidirectional or unidirectional facing the approach area. They are effective for:
a. Identification of a runway surrounded by a preponderance of other lighting.
b. Identification of a runway which lacks contrast with surrounding terrain.
c. Identification of a runway during reduced visibility.