How is a stop bar controlled?
A stop bar is only as good as its switching. Air traffic control turns the red bar on to hold an aircraft and off to release it, and the control system swaps the centreline lights beyond it at the same moment.
A stop bar is a command, not a decoration. The red row of lights across the taxiway at a runway holding position means stop for as long as it is lit, and it is the air traffic controller, not the pilot or the maintenance team, who decides when it goes out. Everything in the control chain exists to make that one decision reliable, visible and hard to get wrong.
1The controller decides
The controller switches the stop bar on to hold an aircraft or vehicle and off to clear it across the runway holding position. The command is given from the tower control panel or its touch screen. Where an advanced surface movement guidance and control system is fitted, parts of the routine can be automated, but the clearance itself stays with the controller.
2The centreline lights are interlocked
While the bar is lit, the taxiway centreline lights beyond it are extinguished, so there is no green path leading past a red bar. Annex 14 sets a minimum length for the extinguished section, 90 m in the wording most editions use. When the bar goes off, those centreline lights come on. A pilot therefore sees a clear change, red bar gone and green path lit, rather than relying on a radio call alone.
3Each bar is switched on its own
Stop bars are selectively switchable. Releasing one aircraft opens only the bar in front of it, and only its matching centreline section, while every other runway holding position stays red. In the field, each bar sits on a circuit or switched section of its own, fed through a constant current regulator and a switching unit that the control system commands.
4The bar comes back on
Once the aircraft has crossed, the bar is switched on again, by the controller or by automatic detection where it is installed, and the centreline lights beyond it go dark. The usual intent is that a stop bar fails safe: a lost command or a faulted control path should leave it lit rather than dark. The status of each bar is reported back to the controller, which is the subject of the next entry.
| Stage | Stop bar | Centreline beyond | Who acts |
|---|---|---|---|
| HoldDefault, and in low visibility | Red, on | Off | Controller sets or leaves it |
| ClearanceAircraft released | Off | Lit green | Controller command, interlock follows |
| Crossing | Off | Lit green | Pilot follows the green path |
| ReinstateAircraft clear of the bar | Red, on again | Off | Controller or automatic detection |
| FaultControl or circuit failure | Intended to remain red | Per design | Reported to the controller and maintenance |
Frequently Asked Questions
Q1Who controls a stop bar?
Air traffic control. The controller switches the stop bar on to hold an aircraft or vehicle and off to clear it across the runway holding position. At aerodromes with an advanced surface movement guidance and control system, parts of this can be automated, but the controller remains responsible for the clearance.
Q2Why are the centreline lights beyond a lit stop bar switched off?
It is a built-in cross-check. With the stop bar lit red, the taxiway centreline lights beyond it are extinguished, so there is no green path leading past the bar. ICAO Annex 14 sets a minimum length for this, 90 m in the wording most editions use. When the bar is switched off, those centreline lights come on.
Q3Can a pilot cross a lit stop bar if the controller gives a verbal clearance?
No. A lit stop bar is not crossed. If a pilot is given a clearance to enter the runway while the bar is still lit, the pilot stops and queries the controller. Air traffic procedures treat an unlit bar, not the words alone, as the signal that the way is clear.
Q4What should a stop bar do if its control fails?
The design intent is that a stop bar fails safe, meaning a lost command or a faulted control path should leave the bar lit rather than dark. How that is achieved depends on the control system and circuit design, so check it against the project specification and the applicable standard.
Q5Why is each stop bar switched separately?
Clearing one aircraft must not open every other runway holding position. Each stop bar is selectively switchable, so the controller releases only the bar in front of the aircraft being cleared, and the matching centreline section, while all others stay lit.
Q6Is stop bar control the same as stop bar monitoring?
No. Control is the command to switch the bar on or off. Monitoring is the feedback: whether the bar is actually lit, how many lamps have failed and whether the interlock behaved. Both run through the same control system. Monitoring is covered in the next entry.
See It. Understand It. Then Learn How to Engineer It.
Recognising an AGL system visually is the first step. The next step is knowing how to design, install, test, commission and maintain them correctly.
Build Practical AGL Engineering Skills
Go deeper with structured training from Precision AGL Academy, covering:
Learn how each light, sign and circuit works as part of a complete airport lighting system, not just as an isolated diagram.