Taxiway Centreline Alternate-Circuit Fixture
A specialised centreline lighting fixture designed with dual-circuit capability, enabling alternating operation between two independent series circuits. This configuration provides redundancy, allows for staggered lighting patterns, and supports advanced guidance features such as directional sequencing or variable spacing for enhanced pilot awareness during ground movement.
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What Is It?
A Taxiway Centreline Alternate-Circuit Fixture is a specialised centreline lighting fixture designed with dual-circuit capability, enabling alternating operation between two independent series circuits. This configuration provides redundancy, allows for staggered lighting patterns, and supports advanced guidance features such as directional sequencing or variable spacing for enhanced pilot awareness during ground movement. The fixture typically contains two separate light sources (or two sets of LEDs) connected to two different isolating transformers and series circuits, allowing it to be illuminated by either circuit independently or in combination.
Purpose & Function
The taxiway centreline alternate-circuit fixture's purpose is to keep centreline guidance available even if one of its two feeding circuits fails, which matters specifically on the busy or precision taxiways where a gap in the centreline line could genuinely disrupt ground movement during low-visibility operations. Beyond straightforward redundancy, its dual-circuit design also enables staggered or sequenced lighting patterns that a single-circuit fixture couldn't produce — letting an airport reduce the visual 'dash' effect of ordinary centreline spacing or add directional sequencing through complex intersections, capabilities that exist specifically to sharpen guidance precision where taxiway routing gets complicated.
Where Is It Used?
Taxiway centreline alternate-circuit fixtures are installed on precision taxiways and busy airports where enhanced guidance, redundancy, or advanced lighting patterns are required. They are used on:
- Precision taxiways at major international airports requiring redundancy
- Complex taxiway intersections where directional guidance is beneficial
- Low visibility operations (CAT II/III) requiring enhanced centreline guidance
- Rapid exit taxiways where sequential lighting can guide pilots
- Apron taxi lanes with complex routing requirements
- Military airfields requiring flexible and redundant lighting configurations
Installation & Location
The fixture is installed flush into the pavement along the taxiway centreline at the same spacing as standard centreline fixtures, with its housing sized to accommodate the two separate light sources and their associated wiring within a single base can. Each of the fixture's two circuits is wired back to its own isolating transformer and series loop, run as physically separate cable paths from the fixture through to the field distribution point, so a fault on one circuit cannot propagate into the other and defeat the redundancy the fixture is installed to provide. Positioning along the centreline is tied to the same surveyed reference used for standard centreline fixtures, ensuring the alternate-circuit units integrate seamlessly into the existing lighting pattern rather than creating a visible discontinuity. Where the fixture is used to support sequenced or directional lighting patterns, its position within the overall array is set out specifically to achieve the intended visual effect, rather than at a generic centreline interval alone.
How Does It Work?
Taxiway centreline alternate-circuit fixtures operate using a dual-circuit design that provides flexibility and redundancy:
- Dual-Circuit Design: The fixture contains two independent light sources (LED arrays or lamps) connected to separate isolating transformers and series circuits (e.g., Circuit A and Circuit B).
- Alternating Operation: The fixture can be illuminated by Circuit A only, Circuit B only, or both circuits simultaneously, depending on the desired lighting pattern.
- Staggered Patterns: By alternating which fixtures are connected to which circuit, a staggered centreline pattern can be created (e.g., every other light on Circuit A, the rest on Circuit B), reducing the visual "dash" effect and providing a more continuous guidance line.
- Directional Sequencing: In advanced configurations, the circuits can be switched in sequence to provide directional guidance, with lights progressively illuminating along the taxiway to indicate the correct path.
- Redundancy: If one circuit fails, the fixture can still be illuminated by the other circuit, ensuring continuous centreline guidance.
System Architecture & Control Logic
Where a standard taxiway centreline fixture answers to one CCR, this fixture answers to two independent ones — Circuit A and Circuit B each run their own loop, their own constant current, and their own loop-level fault detection, joined only at the point where both feed the same physical light. This lets the ALCMS (or, at complex airports, an A-SMGCS route-sequencing system) command a taxiway centreline segment into one of several states — Circuit A only, Circuit B only, or both together — without needing per-fixture addressing, simply by deciding which of the two loops is energised. The practical uses are redundancy (if Circuit A develops a loop fault, ALCMS can switch the segment onto Circuit B alone rather than lose the guidance line entirely) and directional or route-specific patterning (staggering which fixtures sit on which circuit to signal a particular taxi route). Because the two circuits are electrically independent, a fault on one is reported and diagnosed exactly as it would be on any single-circuit fixture, with no cross-loop ambiguity for the CCR to resolve.
Dual-Circuit Overview
Fixture Illuminated by A, B, or Both — Independently Controlled
Enables staggered patterns, directional sequencing, and circuit redundancy if either loop fails
Physical Construction
The fixture houses two independent green LED arrays or filtered 45W–65W halogen lamps — one per circuit — within a single die-cast aluminium body, typically 200–250 mm in diameter, anodised or powder-coated against fuel spillage, de-icing fluid, and UV exposure. Each light source has its own gasket-sealed optical prism and its own set of internal leads, kept physically and electrically separate all the way back to two independent L-823 splice connectors, so a fault on Circuit A cannot bridge into Circuit B inside the fixture. The flush-mounted body seats into an L-868 base can rated for a 100,000 kg (220,000 lb) aircraft wheel load, with the top surface load-bearing regardless of which circuit is illuminated. Each circuit's leads route to its own dedicated 45W–65W isolating transformer housed separately nearby, and O-ring seals throughout give the assembly its IP67/IP68 rating.
Technical Parameters
| Parameter | Typical Value |
|---|---|
| Light Source (per circuit) | Green LED array or 45W – 65W halogen with green filter |
| Colour | Green (steady) |
| Luminous Intensity (per circuit) | 200 – 2,000 candela (depending on setting) |
| Intensity Steps | 5-step (2.8A – 6.6A) CCR controlled per circuit |
| Number of Circuits | 2 (Circuit A and Circuit B) |
| Beam Pattern | Uni-directional or bi-directional |
| Circuit Configuration | Alternating fixtures on Circuit A and Circuit B |
| Spacing (per circuit) | 15 – 30m (50 – 100ft) per circuit (alternating pattern) |
| Mounting Type | Flush-mounted (inset) in pavement |
| Load Capacity | 100,000 kg (220,000 lbs) wheel load |
| Isolating Transformer (per circuit) | 45W – 65W (6.6A series circuit) |
| Protection Class | IP67 / IP68 (submersible) |
| Operating Temperature | -40°C to +55°C |
| Fixture Diameter | 200 – 250 mm (typical) |
Electrical Requirements
| Requirement | Typical Value |
|---|---|
| Circuit Type | Series circuit, constant current (CCR fed) |
| Rated Series Current | 6.6A (5-step: 2.8A / 3.4A / 4.1A / 5.2A / 6.6A) |
| Isolation | Via dedicated isolating transformer (45W–65W) per unit, one per circuit |
| Dual-Circuit Wiring | Each fixture fed from only one of the two independent series loops (Circuit A or Circuit B), alternating along the taxiway |
| Transformer Secondary | Matched to fixture wattage; sized to series circuit rating |
| Cable Type | L-824 single-conductor, direct-burial rated, 5kV or 600V per circuit design |
| Connector | L-823 primary connector, submersible-rated splice |
| Insulation Resistance | ≥ 50 MΩ (megger test, cable-to-ground) |
| Earthing/Grounding | Fixture body and base can bonded to airfield earth grid |
| Fault Protection | CCR-level ground fault and open-circuit detection |
| Power Source | CCR output, regulated per ICAO/FAA intensity step schedule |
Photometric Requirements
| Parameter | Typical Value |
|---|---|
| Main Beam Intensity | 100 – 1,000 cd (green) per ICAO Annex 14 / FAA AC 150/5345-46 |
| Intensity Steps | 100% / 30% / 10% of rated peak intensity, matched across Circuit A and Circuit B |
| Vertical Beam Spread | Main beam 2°–10° above horizontal (peak ~3°–4°) |
| Horizontal Beam Spread | Omnidirectional (360°) or ± 5° either side of taxiway centreline for uni-directional variants |
| Circuit-to-Circuit Matching | Alternating fixtures on Circuit A and Circuit B produce equivalent intensity so the centreline appears continuous if either circuit fails |
| Isocandela Distribution | Per ICAO Annex 14 Attachment A / FAA AC 150/5345-46 taxiway centreline figures |
| Chromaticity (Green) | Within ICAO Annex 14 green boundary (CIE coordinates) |
| Test Method | Mobile photometric van/goniophotometer or fixed test bench per fixture, verified on each circuit independently |
Applicable Standards
ICAO Annex 14
Taxiway centreline light colour, spacing, and intensity requirements
FAA AC 150/5345-46
Specification for Runway, Taxiway, Heliport, and Vertiport Light Fixtures — covers this fixture's general photometric and electrical requirements
IEC 61822
Constant current regulators and monitoring device for the series circuit feeding this fixture
IEC 61821
Maintenance requirements for the AGL constant-current series circuit supplying this fixture
Installation Requirements
Base Can Recess & Grade
CivilRecess the base can into the pavement flush with the taxiway centreline, sized to accommodate the two separate light sources and their associated wiring within a single housing.
Centreline Survey Set-out
CivilSet out fixture positions against the same surveyed reference used for standard centreline fixtures, and where the fixture supports a sequenced or directional pattern, position it to achieve the intended visual effect rather than a generic interval.
Physically Separate Circuit Paths
ElectricalRun each of the fixture's two circuits as physically separate cable paths from the fixture through to the field distribution point, so a fault on one circuit cannot propagate into the other.
Dual Connector Termination
ElectricalTerminate and seal both circuits' connectors per manufacturer procedure, keeping each splice submersible-rated before the base can is closed.
Dual Transformer Matching
ElectricalConfirm each circuit's isolating transformer is wired back to its own isolating transformer and series loop, matched to the fixture and that loop's rated current step.
Load-Rated Housing Seating
MechanicalSeat the fixture housing fully home in the base can and confirm it sits flush and level, since the pavement-mounted design must carry full aircraft wheel load.
Circuit Independence Check
ElectricalVerify continuity on each circuit separately before backfill, confirming no cross-connection exists between the two paths.
Pre-Sealing Sign-off
DocumentationRecord base can position, both circuits' cable routes, and connector seals as-built before the pocket is finally sealed.
Testing & Commissioning
Insulation Resistance Test (Both Circuits)
Pre-EnergisationMegger-test each circuit's isolating transformer and series loop independently before either circuit is energised, confirming insulation resistance clears the acceptable minimum on both.
Loop Continuity Test (Both Circuits)
Pre-EnergisationVerify continuity of each series loop independently and confirm there is no cross-connection between Circuit A and Circuit B inside the fixture.
Independent Circuit Energisation Test
EnergisationEnergise Circuit A alone, then Circuit B alone, confirming the fixture illuminates correctly and independently on each before either is combined.
Circuit Isolation & Balance Verification
EnergisationConfirm the two circuits remain electrically isolated from one another and that energising one circuit does not induce voltage on the other.
CCR Brightness Step Response (Both Circuits)
EnergisationCycle each circuit's CCR through every intensity step and confirm the fixture responds correctly on each, ruling out a marginal transformer on either side.
Photometric Commissioning Test
Final AcceptanceMeasure beam intensity and colour on each circuit separately with a calibrated photometer against ICAO/FAA commissioning tolerances.
Base Can Verification
Pre-EnergisationConfirm base can depth and load-bearing pavement seal meet as-built tolerances before the surrounding surface is finally reinstated.
Documentation & As-Built Sign-off
Final AcceptanceRecord insulation, continuity, voltage, and photometric readings for both circuits alongside as-built position in the commissioning dossier for handover.
Inspection & Maintenance
Photometric & Circuit Balance Check
AnnualVerify both circuits produce matching intensity and correct any imbalance caused by cable length, transformer, or LED driver differences.
Circuit Wiring & Documentation Audit
AnnualReview circuit labelling and wiring documentation to keep troubleshooting straightforward given the dual-circuit design.
Green LED / Filter Inspection
AnnualCheck for uneven degradation between the two circuits' green light sources and replace modules showing colour or intensity shift.
Lens Cleaning
QuarterlyClean both circuit's lenses of dust and rubber deposits to maintain consistent light output across the fixture.
Housing & Seal Inspection
AnnualCheck the housing and seals for moisture ingress that could affect either circuit independently.
Common Field Issues
Circuit Imbalance
Alternate-circuit fixtures require careful balancing of the two circuits to ensure consistent intensity. Differences in cable length, transformer characteristics, or LED driver performance can cause one circuit to appear dimmer than the other. Regular photometric testing and circuit balancing are essential.
Complex Wiring and Troubleshooting
Dual-circuit fixtures require more complex wiring and connector arrangements, making installation and troubleshooting more challenging. Proper labelling, documentation, and testing are essential to avoid confusion and ensure correct circuit identification.
Green Filter/LED Degradation
Green light sources are susceptible to UV degradation and thermal stress. In alternate-circuit fixtures, degradation may occur unevenly between the two circuits, causing visible differences in colour or intensity. Regular photometric testing and replacement of LED modules is essential.
Troubleshooting Guide
One Circuit Dark, Other Circuit Normal
Visible Intensity Mismatch Between Circuits
Colour or Intensity Drift Specific to the Green Circuit
Confusion During Fault-Finding on Dual Circuits
Failure Modes & Root Causes
Single-Circuit Open Fault
Circuit Intensity Imbalance
Green LED/Filter Degradation
Circuit Identification / Labelling Fault
Safety & Operational Notes
Isolate Both Series Circuits
Electrical SafetyThe fixture is fed by two independent transformers and series loops; de-energise and lock out both CCRs and verify zero energy on each circuit before opening the base can, since isolating only one leaves the fixture live.
Circuit Identification & Tagging
Electrical SafetyPositively identify and tag each of the two circuits at the fixture and back to its transformer before disconnecting anything, to avoid cross-connecting the alternate circuits during reassembly.
Airside Work Coordination
OperationalCoordinate with ATC and raise a NOTAM for any period where the fixture, or the sequencing pattern it supports, will be dark or non-standard, keeping personnel and vehicles clear of active taxiway traffic.
Insulation Test Each Circuit Separately
Electrical SafetyMegger test each of the two series loops independently after any work, since a fault on the circuit not currently selected can go undetected until it is switched into use.
FOD Control at the Base Can
OperationalKeep tools, fasteners, and gaskets accounted for at all times when the base can is open, and secure or remove loose material before reinstating the fixture.
PPE & Grounding
Personnel SafetyUse insulated tools and appropriate PPE when working on either series-circuit connection, and confirm the fixture and base can are bonded/grounded before reconnecting.
Verify Correct Sequencing on Restoration
ComplianceAfter reassembly, confirm both circuits energise correctly and that the intended alternating or sequenced pattern is restored before closing out the work and cancelling the NOTAM.
Engineering & Expert Notes
Label Both Circuits at Every Point, Not Just the Fixture
ElectricalWith two independent series loops feeding one fixture, a mislabeled junction anywhere along either run — not just at the base can — can lead to a cross-connection during a completely unrelated repair months later; extend circuit tagging discipline to every accessible point on both loops.
Verify the Sequencing Logic End-to-End, Not Fixture-by-Fixture
ComplianceConfirming that this one fixture correctly alternates its output isn't the same as confirming the whole route's alternating pattern remains coherent; walk the full sequenced section after any work here, since one fixture's timing can look correct in isolation while conflicting with its neighbours.
Test Both Circuits, Even the One Not Currently Selected
TroubleshootingA fault on the standby circuit can sit undetected indefinitely if only the active circuit gets tested; make megger testing both loops routine, not just the one currently in service.
Two Transformers Means Two Chances for a Mismatch
ElectricalEach of the two isolation transformers feeding this fixture should be sized to its own circuit's actual load; reusing one transformer's rating for the other's replacement assumes a symmetry that dual-circuit designs don't always have.
Balance Loops Sharing One CCR
DesignWhere multiple series loops run off a single CCR, unequal loop lengths or lamp counts between loops can produce visibly uneven brightness even though each individual fixture tests within photometric tolerance. Balance loop impedance at the design stage, not after commissioning complaints.
Base Can Seal Ages Faster With Two Cable Entries
DurabilityA dual-circuit base can has twice the cable glands and connector points of a standard fixture, and each is a potential water-ingress path; inspect and reseal every entry point at overhaul, not just the one most recently worked on.
Frequently Asked Questions
Short answers about taxiway centreline alternate-circuit fixtures: dual-circuit operation, redundancy, standards and maintenance.
Q1What is a taxiway centreline alternate-circuit fixture?
It is a taxiway centreline light with two independent light sources fed from two separate series circuits (Circuit A and Circuit B). The fixture can be lit by either circuit alone or by both together.
Q2Why use an alternate-circuit centreline fixture?
Redundancy. If one circuit fails, the other keeps the centreline lit, which matters on busy or precision taxiways where a gap in centreline guidance is a safety risk.
Q3How does alternating operation work?
Alternating which fixtures connect to which circuit creates a staggered centreline, for example every other light on Circuit A and the rest on Circuit B. In advanced setups the circuits can be switched in sequence to give directional guidance.
Q4What colour is the fixture and how bright is it?
Green, steady, at roughly 200 to 2,000 cd per circuit depending on the setting. Each circuit is CCR-controlled through 2.8 A to 6.6 A steps, and intensity is matched across A and B.
Q5Where are these fixtures used?
On precision taxiways and busy airports needing redundancy, complex intersections, CAT II/III low-visibility operations, rapid exit taxiways and apron taxi lanes with complex routing.
Q6Which standards apply to alternate-circuit fixtures?
The main references are ICAO Annex 14 and FAA AC 150/5345-46, with IEC 61822 for the constant current regulators and IEC 61821 for maintenance of the series circuits. Always verify the current edition.
Q7How often should these fixtures be checked?
Typical checks are an annual photometric and circuit balance check, annual wiring and documentation audit, annual green LED and filter inspection and quarterly lens cleaning. Your airport's approved maintenance programme takes precedence.
Q8What should I check if only one circuit is dark?
Check the CCR status for each loop to see which circuit is de-energised, then megger the affected loop from the CCR outward. The usual cause is an open circuit from a failed transformer, connector or cable break confined to that loop.
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