Lead-on Light

A Lead-on Light is the length of taxiway centreline lighting, coloured alternately green and yellow, that runs along the taxiway centreline where it crosses onto the runway at an entry point or intersection departure position. It is the same L-852D-type in-pavement fixture used for ordinary green taxiway centreline lighting, laid as a single row — the yellow segments mark the portion of the path that is already, functionally, on the runway. Lead-on lights are essential for ensuring that aircraft are properly aligned with the runway centreline before beginning their take-off roll, particularly during low visibility operations.

The lead-on light exists to ensure an aircraft is precisely aligned with the runway centreline before takeoff power is applied, a requirement that becomes far harder to judge visually at night or in low-visibility conditions when the pilot is transitioning from a taxiway onto the runway. The colour itself carries the guidance: yellow tells the pilot the centreline they are tracking has crossed onto the runway, giving an intuitive cue to line up rather than requiring them to estimate alignment from taxiway markings alone. Getting this alignment right before the takeoff roll begins matters because correcting a misaligned aircraft after power is applied and the aircraft is accelerating is far more difficult and time-critical than correcting it while still taxiing at low speed.

Lead-on lights are installed at runway holding positions where taxiways intersect the runway. They are used on:

• Precision approach runways (CAT II and III) where accurate alignment is critical
• Busy airports with complex taxiway layouts and multiple runway entry points
• Runway intersections where taxiways enter the runway at an angle
• Low visibility operations where visual guidance is essential
• Military airfields requiring rapid and accurate aircraft positioning

Fixtures are installed as a single row on the taxiway centreline where it crosses the runway holding position and continues onto the runway, with the final light positioned at or just before the point the aircraft lines up for departure. 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 on a dedicated isolating transformer tap so the lead-on sequence can be controlled or intensity-stepped independently of the general taxiway lighting. Fixtures on the runway side of the holding position show yellow, reinforcing to the pilot that they are in the transition zone on the runway rather than on ordinary taxiway alignment. Placement is coordinated with the holding position location and the runway entry geometry so the row of lights delivers the aircraft onto the runway centreline at the correct heading for departure.

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

• Installation: Lead-on lights form a single row on the taxiway centreline, typically at 15-30 metre intervals leading to the runway.
• Lighting: The lights are typically green or yellow, providing directional guidance to the pilot. The lights may be steady burning or flashing, depending on the application.
• Guidance: The switch to yellow along the centreline guides the pilot to align the aircraft with the runway centreline.
• Integration: Lead-on lights are integrated with stop bar lights, runway guard lights, and other taxiway guidance systems.

Lead-on lights sit on the same kind of loop-level architecture as the rest of the taxiway lighting circuit: an Airfield Lighting Control and Monitoring System (ALCMS) selects the intensity step, a CCR regulates the whole series loop to the corresponding fixed current, and every fixture on that loop — the whole row of lead-on lights, via their own isolating transformers — burns at the same intensity together. There is no per-fixture dimming; the yellow/green colour split comes from the lamp or filter fitted to each fixture, not from any independent control of individual lights. The distinguishing piece of control logic for lead-on lights is the stop bar interlock: where a lead-on light run leads up to a runway-holding position protected by a stop bar, the ALCMS suppresses the lead-on command whenever that stop bar is illuminated red, so a pilot is never shown a "proceed" cue while still required to hold. Only when the stop bar is selectively switched off to clear the aircraft does the interlock release, and the lead-on lights energize to guide the aircraft on to the runway. This interlock is implemented in the control system logic rather than in the fixture itself, so it applies uniformly to the whole lead-on run regardless of which CCR loop feeds it.

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 towards the approaching 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.