What is an alternate-circuit centreline arrangement?
If every centreline light sat on one circuit, one fault could put out the whole line. Feeding alternate lights from two circuits means a single failure only widens the gaps, and the taxiway still guides the aircraft.
In an alternate-circuit arrangement, neighbouring centreline lights are deliberately fed from different series circuits. It is a way of designing for failure: the loss of a circuit is expected to degrade the pattern, not remove it.
1The risk with one circuit
A series circuit is one loop. An open circuit or a regulator failure puts out every light on it. If a whole taxiway centreline were on that loop, the taxiway would go dark at once, which matters most in low visibility.
2Every other light on the other circuit
Odd-numbered lights go on circuit A and even-numbered lights on circuit B. Lose either circuit and the other still lights every second fixture along the whole line, with no dark stretch, only a gap that is twice as wide. Check that the wider gap is still acceptable: ICAO Annex 14 normally limits centreline spacing on straight sections to 30 m, and to 15 m on taxiways intended for operations below RVR 350 m, with closer spacing on curves. After one circuit is lost the spacing doubles. A 15 m line becomes 30 m, which is still within the general straight-section limit, so guidance is degraded but continuous. A 30 m line becomes 60 m, which is acceptable only where the standard allows it. Curves need their own check. The 15 m limit describes the healthy condition, so a lost circuit is a degraded state to be repaired promptly, not a normal way to run the taxiway.
3What it costs
Two circuits usually mean two regulators, each carrying about half the load, plus more cable runs and transformer connections to install and maintain. Where practical, the two circuits’ cables run in separate ducts or trenches, so one cable strike or duct failure does not take out both. Both circuits should be switched together and set to the same brightness step, or the line will look bright and dim in turn.
4How it fits with the rest
The arrangement still works with selective switching: each switched section simply has its two circuits switched together. Because a half-lit line can look acceptable at a glance, monitoring must report the loss of one circuit as an alarm so it is repaired rather than overlooked.
| Situation | Lights lit | Effect for the pilot |
|---|---|---|
| Both circuits healthy | All | Normal spacing |
| One circuit lost | Every second light | Spacing doubled; line still continuous |
| Both circuits lost | None | Taxiway dark; no protection from the arrangement |
| Circuits on different brightness steps | All, unevenly | Alternating bright and dim lights |
Frequently Asked Questions
Q1What is an alternate-circuit centreline arrangement?
It is a way of wiring taxiway centreline lights so that alternate fixtures, every other light along the line, are fed from two different series circuits. If one circuit fails, the other still lights every second fixture along the taxiway.
Q2Why alternate the lights instead of splitting the taxiway into two halves?
Splitting the line into halves would leave one half completely dark if its circuit failed. Alternating the fixtures spreads the loss along the whole line, so there is no dark stretch, only a wider gap between lit lights.
Q3What does the pilot see when one circuit fails?
The same green line with the lights spaced twice as far apart. It is harder to follow than the normal pattern but it is still continuous guidance, which is the point of the arrangement. Check the resulting spacing against the standard for your operation.
Q4Does it need two constant current regulators?
Usually yes, one for each series circuit. Each regulator carries roughly half of the load, so the regulators can be smaller, but there are two circuits, two sets of cable runs and more equipment to install and maintain.
Q5Do both circuits have to be switched and dimmed together?
Yes. Both circuits should be switched together and run on the same brightness step. If one circuit were on a different step, the lights would alternate bright and dim along the line.
Q6Why does monitoring matter more with this arrangement?
Because the failure is easy to miss. With half the lights still lit, the taxiway can look acceptable from the tower or the cab, so the control system must report the loss of one circuit as an alarm and maintenance must repair it.
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.