How are obstruction lights monitored?
A failed light protects nobody. Monitoring finds the fault, raises an alarm and starts the report and the repair.
An obstruction light protects aircraft only while it works. A failed lamp, a light out of step, a photocell that leaves a light in day mode at night, or a power failure all leave a hazard unmarked, and on a remote mast or wind farm nobody may notice.
Monitoring closes that gap. It senses whether each light is working, raises an alarm when it is not, and starts the report and repair. Checks are either visual, with someone looking at the lights, or automatic, with sensors that alert the owner.
A typical system watches the light output, the flash and sync, the day, twilight and night mode, the power supply including any standby source, and its own communication link. A fault raises an alarm at a control centre. The owner then reports the outage to the authority so aviators can be warned, and repairs the light.
| Item | What is checked | Why it matters |
|---|---|---|
| Light output | Failure | Lamp or LED current and output show a failed or weak light |
| Flash and sync | Timing | Shows a light that stopped flashing or drifted out of step with its group |
| Day and night mode | Mode | Catches a light left at the wrong intensity for the light level |
| Power | Supply | Mains, standby battery or generator, so a supply fault is seen early |
| Communication link | Heartbeat | Shows when the monitor itself has gone silent, which is not the same as healthy |
| Alarm log | Record | Time-stamped faults, reports and repairs for proof and review |
| Method | How it works | Notes |
|---|---|---|
| Visual observation | Someone looks at the lights at set intervals | Simple, but a failure between checks can be missed, and remote sites are hard to cover |
| Local alarm | The light controller gives a status or alarm output when a light fails | Needs someone to see it, so it is usually paired with a remote link |
| Remote monitoring | The controller sends status to a control centre by wire, radio or cellular | Alarms reach people anywhere, which suits remote masts and wind farms |
| Central system | Many sites report to one screen with logs and reports | Useful for an owner with many structures |
Detect
Sensors or an observer find a failed, drifting or wrongly moded light.
Alarm
The monitor raises an alarm and logs the time and the light affected.
Report
The owner reports the outage to the authority, as the rules require, so aviators can be warned.
Restore
The light is repaired, its status is checked clear, and the authority is told it is working again.
Remote sites
Many masts and wind farms are unmanned, so automatic alarms matter more than at a staffed site.
Time matters
A failed light is an open hazard, so many rules set time limits for the alarm and the report.
Standby power
Monitoring should watch the standby supply too, so a mains failure does not leave the lights dark.
Monitor the monitor
A silent system is not a healthy one. A lost link should itself raise an alarm.
Three functions are easily confused:
- Monitoring→ checking that the lights work and raising an alarm when they do not
- Control→ switching the lights and changing their mode, which is a separate function from watching them
- Reporting→ telling the authority about an outage so a notice can be issued to aviators
Also note that an alarm is not a repair. Monitoring tells you about a failure, but the light stays a hazard until it is fixed and the outage is reported clear.
ICAO Obstruction-Light Monitoring
ICAO Annex 14 Volume I, Chapter 6, covers obstacle lights, and lights that are out of service are expected to be found and dealt with. When a light on a lit obstacle fails, the outage is normally reported through the aeronautical information service so a notice can be issued to aviators. Automatic monitoring of the lights and their controllers is a way of finding failures between visits. The duty to monitor, the checks and the time limits for reporting are set by the national regulator, so take them from the regulator's determination and the current editions of the standards.
FAA and FCC Obstruction-Light Monitoring
In the US, an extinguished or improperly working obstruction light is to be reported promptly so a Notice to Air Missions can be issued. For antenna structures the FCC rules call for the lights to be observed at set intervals, or for an automatic monitoring system that alerts the owner of a failure. The FAA advisory circular AC 70/7460-1, Obstruction Marking and Lighting, covers the lighting systems themselves. Confirm the current rules and the intervals and time limits that apply to your structure.
Frequently Asked Questions
Q1Why do obstruction lights need monitoring?
Because a light only protects aircraft while it works. A failed lamp, a light out of step, a light left in the wrong day or night mode, or a power failure all leave a hazard unmarked, and on a remote mast or wind farm nobody may notice. Monitoring finds the fault, raises an alarm and starts the report and repair.
Q2What does an obstruction-light monitor check?
Typically the light output (lamp or LED current), the flash timing and sync, the day, twilight and night mode, the power supply including any standby source, and its own communication link. The exact list depends on the equipment and on what the regulator requires.
Q3Is a daily visual check enough, or is automatic monitoring needed?
It depends on the rules that apply. Some rules allow either a visual observation at set intervals or an automatic alarm system. A visual check can miss a failure between visits and is hard at remote sites, so automatic monitoring is widely used on masts and wind farms. Confirm the regulation that applies to your structure.
Q4What happens when an obstruction light fails?
The monitor raises an alarm and logs it. The owner reports the outage to the authority within the time the rules set, so a notice can be issued to aviators, and then repairs the light. Once it works again the status is checked and the authority is told. The light remains a hazard until that is done.
Q5Who monitors the monitoring system itself?
The system should monitor itself. A lost link or a missing heartbeat from a site should raise an alarm just like a lamp fault. Owners also test the alarm path at set intervals and keep logs of faults, reports and repairs.
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.