Definition
A row of unidirectional, steady-burning red in-pavement lights installed across a taxiway at a runway holding position, used by air traffic control to give pilots a positive stop or go indication during low-visibility operations. When the stop bar is illuminated, the aircraft must hold; when controllers extinguish it, the aircraft is cleared to proceed across the holding position onto the runway. Green centerline lead-on lights beyond the stop bar typically illuminate when the stop bar is turned off, confirming the clearance to continue.
Plain English
A line of red lights set into the taxiway pavement at the runway hold line. Air traffic control turns them on to mean stop and turns them off to mean go. They are mainly used in low visibility so pilots have a clear, positive signal even when they cannot see well.
Context Anchor
Seen while taxiing at airports that use a Surface Movement Guidance and Control System, especially near runway entrances or taxiway intersections in fog, rain, snow, or other low-visibility conditions.
Derivation
Controllable means the lights can be switched on and off by air traffic control, rather than burning all the time. Stop bar describes the appearance: a bar of red lights stretched across the taxiway forming a visual line to stop at. The name reflects exactly what the system does and how it looks.
Why Pilots Care
Prevents runway incursions by giving pilots a clear visual stop point that controllers can actively manage.
Grounding Statement
If you are taxiing in fog and see a red line of lights across your path, stop before the lights and wait until the tower clearance and the lights both allow you to continue.
Intuition Check
Controllable does not mean the pilot controls the lights. It means the tower or airport control system can turn them on and off; a lighted stop bar is a stop instruction, not just a warning light.
Example Sentence 1
In low-visibility operations, the crew held short until tower extinguished the controllable stop bar lights and the green lead-on lights came on.
Example Sentence 2
During low-visibility operations the pilot held short until the controllable stop bar lights were extinguished.