Why do Obstruction Lights Flash Together?
Lights on one structure, or across a whole wind farm, flash in step so a pilot reads them as one object. A shared time reference keeps them there.
Flashing obstruction lights work best when every light on an object shows the same pattern at the same moment. Lights that flash together read as one object, so a pilot can judge its extent. The same lights flashing out of step look like several separate objects, and the shape of the hazard is lost.
On a single structure the lights at all levels normally flash simultaneously. This applies to medium-intensity Type A and Type B lights and to high-intensity Type A lights. The exception is high-intensity Type B, used on towers that carry overhead wires, where the lights flash in a sequence of middle, top, then bottom.
Across a wind farm or a group of nearby structures the same idea applies on a larger scale: the lights flash together, kept in step by a shared time reference, most often GPS time, or by a wired or radio synchronising signal. Each light typically keeps its own clock in case the signal drops, and monitoring flags a light that drifts out of step.
| Case | Pattern | Notes |
|---|---|---|
| One structure, medium-intensity | Simultaneous | Top, intermediate and lower lights flash at the same moment |
| One structure, high-intensity Type A | Simultaneous | All levels flash together |
| Tower carrying overhead wires, high-intensity Type B | Sequenced | Middle light, then top, then bottom, repeating. The ICAO intervals are 1:13 (middle to top), 2:13 (top to bottom) and 10:13 (bottom to middle) of the cycle; the diagram stretches them for clarity |
| Wind turbine tower tier, low-intensity Type E | Same rate as nacelle | The flashing red lights at half nacelle height flash at the same rate as the nacelle light, and are kept in step with the rest of the farm |
| Wind farm | Group in step | Lights across the farm flash at the same instant, within a tolerance set by the regulator |
| Nearby structures | Group in step | Where the study asks for it, neighbouring lit objects are kept in step as one group |
| Day, twilight and night change | Together | The group changes intensity or mode as one, so no light is left in the wrong mode |
| Method | How it works | Notes |
|---|---|---|
| GPS time | Each light or controller reads GPS time and starts every flash on the same time mark | No cable between lights. Needs a clear view of the sky, and lights free-run if the signal is lost |
| Wired sync | A master controller sends a timing signal over cable to every light | Dependable on one structure. Cable length and surge protection matter |
| Radio link | Timing is passed between controller and lights by radio | Suits spread-out sites. Check range and interference |
| Free running | Each light uses only its own clock | Lights drift apart over time, so it is not enough where synchronisation is required |
Pick the group
Decide which lights must flash together: every level of one structure, or every light in a wind farm or cluster.
Choose a time reference
Use GPS, or a wired or radio signal from a master controller. One source serves the whole group.
Start on the same beat
Every light begins each flash on the same time mark, at the flash rate for its type.
Watch and alarm
Monitoring flags a light that drifts, loses sync or fails, so it can be corrected.
Tight timing
Lights must flash within a small tolerance set by the regulator, so timing is controlled and not left to chance.
Signal loss
If the time source drops, lights free-run and slowly drift. A good system raises an alarm and restores sync when the signal returns.
Spread-out sites
A wind farm covers kilometres, so a shared reference such as GPS is easier than running timing cable between turbines.
Mixed equipment
Lights of different makes or types on one site can break sync, so one timing scheme is used across the group.
Three terms are easily confused:
- Synchronised→ every light in the group flashes at the same moment
- Sequenced→ the lights flash one after another in a set order, as with high-intensity Type B
- Flash Rate→ how many flashes a minute each light makes, which is a separate thing from timing
Also note that a shared flash rate does not make lights synchronised. Two lights at the same rate can flash half a cycle apart unless a common time reference starts them together.
ICAO Flash Synchronization
ICAO Annex 14 Volume I, Chapter 6, sets the flash rate for each flashing light type and says how lights at different levels of one object relate. Medium-intensity Type A and Type B lights, and high-intensity Type A lights, on one structure flash simultaneously, while high-intensity Type B lights, used on towers that carry overhead wires, flash in sequence: the middle light, then the top light, then the bottom light. For a group such as a wind farm, the standard recommends that the lights flash simultaneously. Take the flash rate, the timing and the group arrangement from the regulator's determination and the current edition of the standard.
FAA Flash Synchronization
The FAA covers this in AC 70/7460-1, Obstruction Marking and Lighting. Its flashing lights, such as the L-864 (flashing red), L-865 (flashing white, medium intensity) and L-856 (flashing white, high intensity), flash together within a lighting style, and across a wind farm the lights are kept in step, typically with a GPS or similar timing source. The circular sets the flash rate and the timing tolerance. Confirm the current edition of the advisory circular and what the FAA determination requires for your structure or farm.
Frequently Asked Questions
Q1Why do obstruction lights flash at the same time?
So a pilot reads them as one object. When every light on a structure or in a group flashes together, the pattern shows the extent of the hazard. When the same lights flash out of step they look like several separate objects, and the shape of the hazard is lost.
Q2Do all the lights on a tower flash together?
Usually. Medium-intensity Type A and Type B lights, and high-intensity Type A lights, flash simultaneously at every level of one structure. The exception is high-intensity Type B, used on towers that carry overhead wires, where the lights flash in a sequence of middle, top, then bottom. Take the pattern from the standard for the light type in use.
Q3How are the lights on a wind farm synchronised?
Each turbine's lights take their timing from a shared reference, most often GPS time, or from a wired or radio synchronising signal sent by a controller. Every light then starts each flash on the same time mark. The permitted timing tolerance comes from the regulator and the standard.
Q4What happens if the synchronising signal is lost?
Lights normally keep flashing on their own internal clocks, but they slowly drift out of step. A monitoring system should raise an alarm so the fault can be cleared, and sync should return when the signal does. Behaviour depends on the equipment, so check the manufacturer's data and the regulator's rules.
Q5Does the same flash rate mean the lights are synchronised?
No. Flash rate is how many flashes a minute each light makes. Synchronised means the flashes start at the same instant. Two lights can share a rate and still flash half a cycle apart unless a common time reference starts them together.
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