Definition
A standard approach lighting system installed at the approach end of a runway, consisting of a 1,400-foot lane of steady white lights extending outward from the runway threshold, combined with a sequence of flashing strobe lights (the runway alignment indicator lights, or RAIL) along the outer portion. MALSR is the standard ALS for runways served by an ILS or other precision-style approach where a full ALSF system is not required, and it provides medium-intensity lighting suitable for guiding the pilot from the end of the instrument approach to the visual landing phase.
Plain English
A line of approach lights leading up to the runway, with flashing lights on the outer end that 'point' toward the runway. It helps you find and line up with the runway when you break out of cloud or low visibility on an instrument approach.
Context Anchor
Seen on instrument approach charts and outside the cockpit near the end of an instrument approach to a runway served by this lighting system.
Derivation
The name describes the system literally: 'medium intensity' refers to the brightness level (between low and high intensity systems), 'approach light system' is the lane of lights leading to the runway, and 'runway alignment indicator lights' are the sequenced flashers that visually point the pilot toward the runway centerline.
Why Pilots Care
Provides positive visual alignment cues that reduce the risk of runway misalignment or controlled flight into terrain during low-visibility landings.
Grounding Statement
At night or in low visibility, the MALSR can appear before the runway itself as a lighted path leading toward the runway threshold.
Intuition Check
Do not treat a MALSR as the runway lights themselves. It is an approach lighting aid before the runway that helps guide your eyes to the runway and confirm alignment.
Example Sentence 1
The runway was equipped with MALSR, so once we broke out of the clouds the flashing lead-in lights made the centerline easy to find.
Example Sentence 2
Many airports with ILS approaches install MALSR systems to give pilots reliable visual guidance once they break out of the clouds.