Approach Lighting System

An Approach Lighting System (ALS) is a configuration of signal lights installed on the approach end of a runway to provide visual guidance to pilots transitioning from instrument flight to visual conditions during an approach to landing.

Common ALS configurations include:

• ALSF-1 and ALSF-2: High-intensity approach lighting with sequenced flashers, extending up to 2,400 feet from the runway threshold. ALSF-2 includes red side row bars and is required for CAT II/III ILS approaches
• MALSR: Medium-intensity approach lighting with runway alignment indicator lights
• ODALS: Omnidirectional approach lighting, using a sequence of flashing lights
• REIL: Runway End Identifier Lights — a pair of synchronized flashing lights at the threshold

The presence and type of approach lighting affects the published approach minimums. Better lighting systems generally permit lower decision heights and visibility requirements.