How is a stop bar monitored?
A stop bar protects the runway only if it works when it is needed. Monitoring checks the lamps, confirms the bar did what was commanded and tells the controller and maintenance the moment something is wrong.
Control sends the command; monitoring proves the result. A red bar that has quietly lost lamps, or one that did not switch as asked, gives the pilot a false picture of the holding position. The control system therefore measures the bar, compares it with what was commanded, and reports the answer.
1Is each lamp working?
A failed lamp does not stop a series circuit, because the isolating transformers keep the circuit running, so circuit current alone can look normal. The failure has to be detected on purpose, from the small change in circuit load or by lamp-level monitoring at each fixture, depending on the system design.
2Did it do what was asked?
The system compares the measured state of the bar with the last command. A bar commanded on that is not lit, or one commanded off that is still lit, is an alarm, not just a lamp count.
3Do the pair agree?
The interlock from the previous entry is checked too. A lit bar should have the centreline lights beyond it off, and an unlit bar should have them lit. Any other combination is flagged, because the pilot should always see one clear signal.
4Who is told, and what then
The controller sees the status of every bar on the tower display with an alarm for faults, and maintenance receives lamp faults and an event log. When a bar is no longer serviceable, the aerodrome applies its own procedures to protect the runway until it is repaired.
| Condition | What is detected | Typical result |
|---|---|---|
| One lamp out | Lamp-level or load change | Maintenance alarm; bar still within limits |
| Lamps beyond the limitLimit set by the standard | Lamp count over threshold | Bar unserviceable; controller told |
| Bar not as commanded | Measured state differs from command | Alarm to the controller |
| Interlock mismatch | Bar and centreline not a valid pair | Alarm to the controller and maintenance |
| Communication lost | Status no longer received | Shown as unknown; bar intended to stay red |
Frequently Asked Questions
Q1What is stop bar monitoring?
It is the feedback side of stop bar control. The airfield lighting control and monitoring system checks that each stop bar is lit or off as commanded, counts failed lamps, confirms the centreline lights beyond it are consistent, and reports any fault to the controller and to maintenance.
Q2Why can a failed lamp go unnoticed on a series circuit?
Isolating transformers keep the series circuit running when a lamp fails, so the circuit current looks normal. A failed lamp therefore has to be detected deliberately, either from the small change in circuit load or by lamp-level monitoring at the fixture. Which method is used depends on the system design.
Q3What does the controller see?
A status for each stop bar, such as on, off or faulted, shown on the tower display, with an alarm when something is wrong. The display shows what the system has measured, not just what was commanded, so the controller can tell a bar that is really lit from one that only should be.
Q4What happens when too many lamps fail?
The stop bar is treated as unserviceable. The standards set limits on how many lamps may be out, and whether they may be adjacent, before that point. The controller is told, and the aerodrome applies its own procedures to protect the runway until the bar is repaired. Take the limits from the applicable standard, for example the maintenance chapter of ICAO Annex 14 or FAA AC 150/5340-26.
Q5What is an interlock mismatch?
It is when the stop bar and the centreline lights beyond it are not in the expected pair, for example the bar lit and the centreline beyond it also lit, or both dark. The system flags it as an alarm, because the interlock is meant to give the pilot one clear signal.
Q6Is monitoring the same as control?
No. Control switches the bar on or off. Monitoring does not switch anything: it reports, so that a fault is seen before an aircraft relies on the bar. The two run through the same control system, which is why the previous entry and this one go together.
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