Definition
The portion of a runway's centerline lighting system, on precision instrument runways, where the lights alternate between red and white for the final 3,000 feet (or to the midpoint of the runway, whichever is less), warning the pilot that the end of the runway is approaching.
Plain English
A stretch of runway centerline lights, near the far end, that flashes red and white instead of all white. It tells the pilot, 'You're running out of runway — pay attention.'
Context Anchor
Seen during final approach when using runway lights that show whether the airplane is high, low, or on the proper path to the runway.
Derivation
Caution comes from a Latin word meaning care or warning. Zone means an area or region. In this aviation use, the “area” is not painted on the runway; it is the warning part of the light pattern you see from the cockpit.
Why Pilots Care
Recognizing the caution zone prevents runway incursions, collisions with obstacles, or landing short of a displaced threshold.
Grounding Statement
If the light display moves into the caution zone, treat it as an early warning to correct the approach before the airplane gets too low.
Intuition Check
Do not think of the caution zone as a marked physical area on the airport. Here, it means a warning part of the approach light indication seen from the airplane.
Example Sentence 1
As the centerline lights began alternating red and white, the pilot knew they had entered the caution zone and applied steady braking.
Example Sentence 2
During taxi, the crew confirmed they had passed the caution zone before turning onto the active runway.