Definition
A system of flush-mounted lights installed along the centerline of a runway, spaced at regular intervals, to provide visual guidance during landing rollout and takeoff in low-visibility conditions. The lights are white for most of the runway, transition to alternating red and white as the runway end approaches, and become solid red for the final portion before the end.
Plain English
A line of lights set into the runway surface that shows the pilot exactly where the middle of the runway is, especially useful when visibility is poor. The colors change near the end of the runway to warn the pilot that the runway is running out.
Context Anchor
Seen on instrument approach charts, airport lighting descriptions, and in inoperative components tables that tell pilots what changes when certain runway lights are not working.
Why Pilots Care
An inoperative RCLS may raise approach minimums or require use of an alternate procedure to maintain safety margins.
Intuition Check
Do not confuse RCLS with runway edge lights. RCLS marks the middle of the runway; runway edge lights mark the sides.
Example Sentence 1
With the RCLS inoperative, we checked the inoperative components table and adjusted our required visibility for the approach.
Example Sentence 2
With the RCLS reported out of service, the approach required a higher decision altitude.