AIM 2-1-1 Approach Light Systems (ALS)
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.
a. ALS provide the basic means to transition from instrument flight to visual flight for landing. Operational requirements dictate the sophistication and configuration of the approach light system for a particular runway.
b. ALS are a configuration of signal lights starting at the landing threshold and extending into the approach area a distance of 2400-3000 feet for precision instrument runways and 1400-1500 feet for nonprecision instrument runways. Some systems include sequenced flashing lights which appear to the pilot as a ball of light traveling towards the runway at high speed (twice a second). (See FIG 2-1-1.)