Lead-off Light

A Lead-off Light is the length of taxiway centreline lighting, coloured alternately green and yellow, that runs along the curved exit path from the runway centreline to the point where an aircraft is fully established on the taxiway. It is the same L-852D-type in-pavement fixture used for ordinary green taxiway centreline lighting, laid as a single row rather than paired left/right — the yellow segments simply mark the portion of the path that is still, functionally, on the runway. Lead-off lights are essential for ensuring efficient runway occupancy and safe aircraft movement during the post-landing phase.

The lead-off light exists to manage one of the highest-workload transitions in a landing sequence: the moment an aircraft leaves the runway and must correctly track the taxiway centreline while still moving at a meaningful groundspeed, often at night or in reduced visibility. The colour itself carries the guidance: yellow tells the pilot they are still tracking a centreline that is technically part of the runway environment, and the switch to green is the unambiguous cue that the aircraft has cleared onto the taxiway proper. That single, unmistakable colour change is easier to read under time pressure than judging position against markings alone. Its purpose is ultimately about runway throughput as well as safety: guiding aircraft off the runway quickly and correctly minimizes the time the runway stays occupied by a landing aircraft before the next arrival or departure can use it.

Lead-off lights are installed at runway exits where taxiways connect to the runway. They are used on:

• Precision approach runways (CAT II and III) where efficient runway exit is critical
• Busy airports with high traffic volumes requiring rapid runway clearance
• Rapid exit taxiways where high-speed exits are required
• Complex taxiway layouts where accurate guidance is essential
• Low visibility operations where visual guidance is critical

Fixtures are installed as a single row on the taxiway centreline, beginning at the point where the exit curve leaves the runway centreline and continuing along the curved portion until the aircraft is established on the taxiway centreline, typically spaced closer together than straight-taxiway centreline lighting to better define the curve. Each light mounts on an L-852 base can recessed flush with the pavement, fed by cable run in pavement ducting back to the taxiway lighting circuit, typically on its own isolating transformer tap so intensity can be stepped independently of the general taxiway lighting. Fixtures within the runway strip/runway pavement portion of the curve show yellow; fixtures beyond the point where the aircraft is clear of the runway show green, with that colour-change point set by the runway/taxiway geometry rather than a fixed distance. Placement is coordinated with the runway exit design (angle, radius, and speed category) so the row of lights follows the intended exit path rather than a simplified constant-radius curve.

Lead-off lights operate as part of the taxiway and runway guidance system:

• Installation: Lead-off lights form a single row on the taxiway centreline at the runway exit point, typically at 15-30 metre intervals along the curved path from the runway to the taxiway.
• Lighting: Each fixture is yellow within the runway strip and green beyond it, providing colour-coded guidance to the pilot. The lights may be steady burning or flashing, depending on the application.
• Guidance: The alternating green/yellow colour, and the point where it switches to solid green, guides the pilot to safely exit the runway and track onto the taxiway.
• Integration: Lead-off lights are integrated with taxiway centreline lights, edge lights, and other taxiway guidance systems.

Lead-off lights are not individually addressable — like the rest of the taxiway lighting circuit, they are controlled at the loop level. An Airfield Lighting Control and Monitoring System (ALCMS), operated by the tower or a routine linked to runway occupancy, selects the intensity step for the taxiway guidance circuit that the lead-off row belongs to; on smaller aerodromes this may instead be driven by pilot-controlled lighting (PCL). The command is passed to a CCR, which regulates the loop to a fixed current, and every fixture on that loop — every yellow and green light along the row, via their own isolating transformers — burns at the same intensity simultaneously. The green/yellow colour split is fixed by which lamp or filter each fixture carries, not by any independent switching logic, so intensity stepping is uniform along the whole row even though the colour is not. Where lead-off lights flash rather than burn steady, a separate sequential flasher control unit superimposes the flash timing on top of the loop's steady current; this flasher logic sits alongside, not instead of, the CCR-driven intensity control. Fault detection remains loop-based: the CCR monitors loop voltage against the commanded current, and a voltage rise (typically an open-circuit lamp or failed transformer) is reported to the ALCMS as a loop fault rather than pinpointed to a specific fixture without further sectionalised testing.

Each lead-off light draws power from the CCR through its own isolating transformer on the series circuit, so one fixture can fail without breaking the loop. The fixtures form a single row along the taxiway centreline; it's the colour of each fixture — yellow inside the runway strip, green beyond it — not the geometry of a paired layout, that guides the pilot off the runway.

The fixture body is typically a die-cast aluminium alloy housing, machined and anodised or powder-coated for corrosion resistance against de-icing fluids, fuel spillage, and UV exposure. The green (and where fitted, yellow) lens is a borosilicate glass or high-strength polycarbonate prism, gasket-sealed to the housing with an O-ring or silicone seal to achieve an IP67/IP68 rating against water and dust ingress, and shaped to project a uni- or bi-directional beam back towards the exiting aircraft. Elevated units bolt to a frangible coupling engineered to shear cleanly under aircraft impact loads, seated on an L-867 base can set flush with the surrounding grade; flush-mounted (inset) units instead seat directly into an L-868 base can recessed into the pavement, with the top surface load-rated for aircraft wheel loading of up to 100,000 kg. Internal wiring terminates at L-823 primary connectors, sealed splice connectors rated for direct burial and continuous immersion, which mate to the secondary leads of the isolating transformer housed in its own base can nearby. Stainless-steel fasteners are used throughout to resist corrosion, and the base can assembly includes a drainage provision to prevent water pooling around the transformer and cable entry points.