Approach Light Systems (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.
PAPI (Precision Approach Path Indicators) primarily assists by providing visual glide slope guidance in non-precision approaches environment. These systems have an effective visual range of at least 3 miles during the day and up to 20 miles at night. The row of light units is normally installed on the left side of the runway and the glide path indications are as two red and two white (2 red ) when on proper glide path angle of approach. Light combinations indicate when slightly high (3 white ), significantly high (4 white ), slightly low (3 red ) and significantly low (4 red ).
Video shows the side by side comparison of a series of approaches into an airport. The benefit of the comparison is to see and understand the superior clarity of LED lighting systems over the more traditional incandescent lighting systems. The video contains no audio.
FAA Navigation Services Technical Lead
Current Status: There are 938 PAPIs in the NAS. Major manufacturers include New Bedford Panoramex (709), Sonicraft (116) and AVW (101). PAPIs are used to replace Visual Approach Slope Indicator Lights (VASI), and to support Commercial Aviation Safety Team (CAST) and Land and Hold Short Operations (LASHO) new requirements. The FAA is researching replacing incandescent lamps with more efficient Light Emitting Diode (LED) lamps.