Green Centreline Light

A Green Centreline Light is a flush-mounted (inset) AGL luminaire installed along the centreline of a taxiway. It emits a steady green light, providing pilots with a clear visual reference to maintain the aircraft precisely on the taxiway centreline during ground movement. Unlike taxiway edge lights (blue) which define the lateral boundaries, centreline lights guide the pilot along the optimal path, particularly in low visibility operations (RVR < 550m). These lights are an essential component of the taxiway guidance system, especially at busy airports with complex taxiway layouts.

The green centreline light exists because knowing you are somewhere between two taxiway edges is not the same as knowing you are safely centred between them — and for a large aircraft, the difference matters. Wingspan and engine clearance from taxiway-edge obstacles, parked aircraft, or other traffic depend on the aircraft tracking the centreline closely, something a pilot cannot judge reliably from the edge lights alone, especially at night or in low RVR conditions where peripheral references are hard to use. By giving a precise, continuous line to follow rather than just a boundary to stay inside, the fixture reduces the pilot's workload during taxi and removes the guesswork from an otherwise purely visual judgement, which is why it becomes indispensable at busy airports where taxiway layouts are complex and the tolerance for lateral drift is small.

Green centreline lights are installed on taxiways at airports that support low visibility operations. They are typically used on:

• Precision approach runways with CAT II/III capability requiring enhanced taxiway guidance
• Major international airports with complex taxiway networks and high traffic density
• Taxiway straight sections where centreline guidance is critical for accurate tracking
• Taxiway curves and intersections where precise path following is essential
• Apron taxi lanes where aircraft manoeuvring in confined areas requires accurate guidance
• Rapid exit taxiways where aircraft exit the runway at high speed and need clear centreline guidance

Fixtures are recessed into a saw-cut or cored pocket along the taxiway centreline marking, using the standard load-rated inset base can set flush with the pavement surface, at 15–30 m spacing on straight sections tightening to 7.5–15 m through curves and intersections. Cabling is run in duct beneath the pavement from each base can back to its isolating transformer and on to the taxiway centreline series circuit, with fixture position checked against pavement slab joints during setting-out so no base can straddles a joint or crack; layout follows the taxiway centreline and marking survey along the full route the green lights guide.

Green centreline lights are powered from the series circuit via an isolating transformer. Each light typically contains an LED array or a halogen lamp with a green filter, producing a steady green light. The optical system is designed to project a uni-directional beam along the taxiway centreline, visible to the pilot from the cockpit. The lights are spaced at intervals that provide continuous guidance:

• Spacing: Typically 15m to 30m apart on straight sections, closer on curves.
• Intensity Control: Brightness is controlled by the CCR with 5 intensity steps, allowing adjustment for visibility conditions.
• Optical Design: Precision optics (reflector and prism) shape the beam to be visible from the cockpit angle.
• Integration: The lights are integrated with the ALCMS and can be dimmed or switched off as required.

Green centreline lights are not individually addressable — control is exercised at the loop level through the constant current series circuit assigned to a given taxiway route or section. The ALCMS selects which taxiway centreline circuits are energised and at which CCR step, either as a straightforward on/off routing decision or, on airports fitted with a follow-the-greens system, as a dynamically illuminated path that switches on ahead of a taxiing aircraft and off behind it as the aircraft progresses — in that case the "loop" the ALCMS is switching may be a short, sectionalised run of fixtures rather than the whole taxiway. Whichever mode is used, every fixture within an active section, via its own isolating transformer, draws the same current and burns at the same intensity simultaneously; the ALCMS cannot dim or brighten one green light independently of the others sharing its circuit. Fault detection remains loop-based: the CCR tracks loop voltage against commanded current, and a voltage rise — typically an open-circuit lamp or a failed isolating transformer — is reported to the ALCMS as a loop fault, isolated to a specific fixture only through sectionalised field testing.

The fixture body is a die-cast aluminium housing set into an L-868 base can flush with the taxiway pavement, load-rated for aircraft wheel loading up to 100,000kg. A green-filtered lens — glass or high-strength polycarbonate — sits over the LED array or halogen lamp, gasket-sealed with an O-ring for an IP67/IP68 rating against water and de-icing fluid ingress. Internal wiring runs to an L-823 primary connector, sealed for direct burial, which mates to the secondary leads of the isolating transformer housed in its own nearby base can; stainless-steel fasteners and a drainage channel in the base can keep water from pooling around the transformer and cable entries.