Taxiway Lead-on Light

A Taxiway Lead-on Light is a taxiway centreline light, colour-coded and independently switched, that guides pilots and vehicle drivers from a runway holding position onto the runway. Lead-on lights are installed in a single row following the taxiway centreline itself — the same line the aircraft already tracks — rather than as a separate paired fixture either side of it. They carry the same alternating green/yellow colour pattern as lead-off lights, extending from the stop bar toward the runway, so that a pilot who has just been cleared to enter or cross the active runway sees a continuous, unambiguous path forward the moment the stop bar goes dark.

The lead-on light's purpose is to resolve exactly the moment where a taxiing aircraft stops being at a holding position and becomes cleared to enter or cross an active runway — a transition where the consequence of confusion is a runway incursion. A controlled stop bar confirms ATC's clearance by going dark, but a dark stop bar on its own tells the pilot nothing about where to go next. The lead-on lights answer that: as the stop bar extinguishes, the taxiway centreline lights immediately ahead illuminate in the same alternating green/yellow pattern used for lead-off, marking that this stretch of centreline lies within the runway or ILS critical area boundary. The colour coding, not a change in fixture layout, is what carries the safety information — it tells the pilot they are now inside the runway environment, rather than relying on the pilot to judge that boundary from signage or memory under workload.

Taxiway lead-on lights are installed on the taxiway centreline stretch between a runway-holding position and the runway itself:

• Runway holding positions served by a controlled stop bar, extending from the stop bar toward the runway
• ILS critical area boundaries where the alternating colour pattern marks the sensitive area
• Low visibility operations (typically below 1,200 ft RVR) where visual confirmation of the runway boundary is essential
• Precision approach runways (CAT II and III) with controlled stop bar installations
• Complex or high-movement airfields using an Advanced Surface Movement Guidance and Control System (A-SMGCS) for selective route switching

Fixtures are set out in a single row directly along the existing taxiway centreline, using the same inset base can spacing as ordinary taxiway centreline lights, tightened where the centreline curves toward the runway. The lead-on stretch begins at the stop bar and extends toward the runway centreline (commonly around 45–50 m for the first segment on a controlled stop bar installation), with individual fixtures alternating green and yellow lenses along that stretch. Because this segment must be switched independently of the general taxiway circuit — illuminating only once ATC clears the aircraft to enter or cross, in step with the stop bar going dark — it is wired on its own controlled segment rather than the parent taxiway centreline series loop. Cabling runs in duct beneath the pavement back to the isolating transformers and the stop bar/lead-on control cubicle, with setting-out tied to the runway holding position's surveyed location.

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

• Installation: Lead-on lights are installed in a single row along the taxiway centreline itself, spaced as for ordinary taxiway centreline lights (tighter through curves), extending from the stop bar toward the runway.
• Lighting: Each fixture is steady-burning and fixed to either a green or yellow lens, alternating along the row, beginning with green nearest the stop bar — the same colour-coding used for lead-off lights.
• Guidance: The alternating colour pattern tells the pilot the centreline ahead lies within the runway or ILS critical area boundary, confirming the route is clear to proceed once the stop bar extinguishes.
• Integration: Lead-on lights are switched together with the controlled stop bar they serve, and are coordinated with taxiway centreline lights and, where fitted, runway guard lights and A-SMGCS routing logic.

Lead-on lights share the same constant-current series circuit and CCR intensity step as the taxiway centreline lights they extend, so brightness always follows the loop-level step — there is no independent per-fixture dimming. What is distinct is the switching logic layered on top: the lead-on segment is controlled together with its stop bar, either by a dedicated selector relay or, at airports with selective switching, by the ALCMS/A-SMGCS route logic, so the segment stays dark while the stop bar is illuminated and lights only on the specific route ATC has cleared. Fault reporting stays loop-based — a voltage rise at the CCR flags an open circuit or failed transformer somewhere on the loop, but pinpointing it to a specific lead-on fixture still requires sectionalised field testing.

Taxiway lead-on light fixtures follow standard flush taxiway centreline light construction: a die-cast aluminium housing set into an L-852 base can, with a hardened glass or polycarbonate lens gasket-sealed to achieve an IP67/IP68 rating against fuel spillage and standing water. Each fixture is fitted with a fixed-colour lens — green or yellow depending on its position in the alternating sequence — rather than a colour-changing element, since the pattern is set once by fixture position and does not need to vary. Internal wiring terminates at sealed L-823 connectors feeding the fixture's isolating transformer within the same base can, and the fixture body is kept low-profile and load-rated to withstand aircraft wheel loading, since the row sits directly on the taxiway centreline through a high-traffic transition zone into the runway. Stainless-steel fasteners and corrosion-resistant coatings are used throughout given this siting.