Definition
A standard approach lighting system installed on the runway approach end, consisting of a 1,400-foot configuration of steady-burning white light bars centered on the extended runway centerline, combined with a sequence of high-intensity flashing white lights (the Runway Alignment Indicator Lights, or RAIL) on the outer portion. MALSR is the FAA's standard approach light system for Category I precision instrument runways and is medium-intensity (lower output than the high-intensity ALSF systems used for Category II/III runways).
Plain English
A line of lights stretching out from the runway end that helps you find and line up with the runway as you break out of the clouds. The outer lights flash in sequence toward the runway, almost pulling your eyes in, while the inner lights stay steady to show the centerline.
Context Anchor
Seen on instrument approach procedure charts and at airports with runways approved for instrument approaches.
Derivation
The name is descriptive: 'Medium Intensity' distinguishes it from higher-output systems like ALSF-1 and ALSF-2; 'Approach Lighting System' identifies its purpose; and 'Runway Alignment Indicator Lights' (RAIL) names the sequenced flashing component on the outer end. Together the parts tell you exactly what you're getting — a medium-brightness approach lighting setup with the flashing alignment lights added.
Why Pilots Care
Tells the pilot what visual cues will appear once the runway environment is in sight, supporting the decision to continue or execute a missed approach.
Grounding Statement
Picture a lighted path leading toward the runway, with flashing lights drawing your eyes toward the runway centerline.
Intuition Check
Do not read “medium intensity” as meaning the system is minor or optional. It describes the brightness category of the approach lights; the system can still be very important for finding and aligning with the runway.
Example Sentence 1
As we descended through the clouds on the ILS, the flashing RAIL lights of the MALSR appeared first, leading our eyes straight to the runway threshold.
Example Sentence 2
With the MALSR in view, the pilot transitioned from instruments to visual references and completed the landing.