Taxiway Centreline VS Alternate-Circuit Centreline
Guidance vs Redundancy — taxiway centreline lighting provides the normal visual guidance an aircraft follows along the taxi route, while an alternate-circuit centreline arrangement is the same centreline lighting wired across separate electrical circuits to maintain that guidance if the primary circuit fails. Same green light, different electrical architecture.
At a Glance
| Feature | Taxiway Centreline | Alternate-Circuit Centreline |
|---|---|---|
| Primary purpose | Taxiway route guidance | Maintain guidance through circuit redundancy |
| Location | Along taxiway centreline | Along the same centreline |
| Function | Normal visual guidance | Backup/alternate electrical circuit |
| Light colour | Green | Green |
| Pilot message | "Follow the taxiway centreline" | Same guidance message |
| Electrical arrangement | Normal circuit | Separate/alternate circuit |
| Main advantage | Normal operation | Improved continuity after circuit failure |
| Main distinction | Lighting function | Power/circuit configuration |
What Is a Taxiway Centreline & Alternate-Circuit Centreline?
Taxiway Centreline
Normal taxiway centreline lighting provides visual guidance along the aircraft's intended taxi path. The pilot uses the centreline as the visual route to follow.
Provides
- Continuous directional guidance
- A single line of green lights along the taxi route
- The pilot's primary "where to steer" reference
Alternate-Circuit Centreline
An alternate-circuit arrangement does not create a different centreline function — the same centreline lights are instead arranged across separate electrical circuits.
Purpose
- Improve continuity of centreline lighting
- Preserve availability if one circuit becomes unavailable
- Add electrical redundancy, not a new guidance function
Function vs Electrical Redundancy
Taxiway centreline asks "what does it do?" — the answer is taxi guidance. Alternate-circuit centreline asks "how is it powered?" — the answer is a separate/alternate circuit.
Therefore: Taxiway Centreline = function. Alternate-Circuit Centreline = electrical redundancy. The two describe different layers of the same system — what the light communicates to the pilot, and how reliably it stays powered.
How the Alternate Circuit Works
Imagine the centreline lights split across two independent circuits, A and B, alternating along the row.
Full centreline coverage — every light along the route is available.
Circuit B can preserve some centreline lighting coverage, subject to the actual system design.
Reducing the Effect of a Single Circuit Failure
Normal
Circuit A ✓ and Circuit B ✓ — full centreline lighting is available across the route.
One Circuit Fails
Circuit A ✗ and Circuit B ✓ — some centreline lighting remains available, providing greater system resilience than relying entirely on one circuit.
What the Pilot Actually Sees
From the cockpit, the pilot is primarily concerned with following the green centreline. The pilot normally does not need to know which electrical circuit is operating — the circuit arrangement is an engineering/system-reliability feature, not a piece of information conveyed to the flight crew.
Same Green, Same Guidance
Both remain part of the taxiway centreline lighting function: green centreline light means guidance, regardless of which circuit is currently supplying it. An alternate circuit does not mean a different colour or a different pilot instruction.
Not the Same as an Alternate Taxi Route
This distinction is important. Alternate-circuit centreline means the same taxiway, the same centreline, but a different electrical circuit. It does not mean an alternate taxiway or an alternate aircraft route. "Alternate-circuit" refers to the lighting electrical system, not a different path for the aircraft to follow.
Normal vs Alternate Circuit
| Situation | Normal Centreline Circuit | Alternate-Circuit Arrangement |
|---|---|---|
| Normal operation | Provides centreline lights | Provides centreline lights |
| Circuit failure | Coverage may be affected | Alternate circuit can maintain availability |
| Pilot guidance | Green centreline | Green centreline |
| Physical location | Taxiway centreline | Same centreline |
| Main engineering benefit | Normal operation | Redundancy/reliability |
Common Centreline Circuiting Confusion
It is the same green taxiway centreline light and function. The difference is purely electrical — how the fixtures are wired and supplied, not what colour or guidance message they provide.
Alternate-circuit refers strictly to the lighting electrical system. It does not signal a different taxiway or a different path for the aircraft to follow — the taxiway and centreline stay the same.
The pilot's task is simply to follow the green centreline. Circuit arrangement and redundancy are engineering/system-reliability considerations handled behind the scenes.
Taxiway Centreline vs Alternate-Circuit Centreline
Taxiway centreline and alternate-circuit centreline describe two different layers of the same system, not two competing lighting types. One defines what the light tells the pilot; the other defines how reliably that light stays powered.
Together, normal centreline lighting gives the aircraft its route to follow, while an alternate-circuit arrangement improves the continuity of that guidance if a single circuit fails — the guidance message never changes.
Easy memory: Centreline → where to taxi. Alternate circuit → how the lighting stays available.
Engineering & Expert Notes
Verify the Alternate Circuit Assignment
VerificationCheck that lights are on the intended circuits.
Test Circuit Loss Scenarios
RedundancyProve continuity of guidance.
Keep Drawings Aligned With the Field
RecordsUpdate after changes.
Inspect at Night
InspectionCheck the pattern.
Frequently Asked Questions
Taxiway centreline describes the lighting function — taxi guidance. Alternate-circuit centreline describes the electrical arrangement — the same lights supplied across separate circuits for redundancy.
No. The lights remain green taxiway centreline lights with the same guidance message, regardless of which circuit is currently supplying them.
No. It refers only to the lighting electrical system — the same taxiway and the same centreline, just wired across separate circuits. It does not indicate a different aircraft route.
Depending on the system design, the alternate circuit can maintain some centreline lighting coverage, providing greater resilience than relying entirely on a single circuit.
No. The pilot's concern is following the green centreline. Circuit arrangement and redundancy are engineering/system-reliability features handled independently of flight-crew guidance.
To reduce the effect of a single electrical circuit failure and improve the continuity and availability of centreline lighting, particularly on busy or complex taxi routes.
Governing Documents
For an actual project design, always verify the current edition of the applicable documents.
FAA AC 150/5340-30
Design and Installation Details for Airport Visual Aids — taxiway centreline lighting configuration, circuiting and electrical system requirements.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — taxiway centreline lighting colour, siting, intensity and operational requirements.
Series Circuit & CCR Design References
Governing electrical design documentation for series circuits, constant current regulators and alternate/redundant circuit arrangements feeding centreline lighting.
Final Engineering Note
Do not design a centreline circuiting arrangement from a generic web diagram. The final installation drawing should always be developed from the current governing standard, approved airport design and project-specific requirements.
Taxiway centreline provides visual guidance along the taxi route. Alternate-circuit centreline is that same lighting arranged with alternate/separate circuit supply to improve continuity and redundancy.
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