Runway Entrance Light (REL)

A Runway Entrance Light (REL) is a Runway Status Light (RWSL) fixture installed at taxiway intersections with the runway. These in-pavement lights indicate the precise location where a taxiway enters the runway, illuminating steady red to warn pilots and vehicle operators when it is unsafe to enter or cross. They are particularly valuable at complex airport layouts where multiple taxiways intersect the runway.

The REL exists to give a pilot or vehicle operator a dynamic, real-time warning at exactly the point where a taxiing aircraft is closest to entering an active runway — an intersection where the consequence of proceeding at the wrong moment is a runway incursion. Rather than marking a fixed boundary the way holding-position markings and stop bars do, the REL row stays dark under normal conditions and illuminates steady red only when the runway ahead is genuinely unsafe to enter or cross, driven by surface surveillance data on runway traffic and approaching aircraft. Its purpose is best understood as an automated, occupancy-aware layer of protection at complex intersections, reinforcing — not replacing — ATC clearance and the pilot's own judgement.

REL is deployed as part of an RWSL installation at taxiway/runway intersections, which requires airport surface surveillance data to drive it. They are used on:

• Busy, surveillance-equipped airports with high traffic density and frequent runway crossings
• Complex taxiway layouts with multiple runway entry points where the safe-to-enter moment is hard to judge unaided
• Runway/taxiway intersections with a documented history or elevated risk of incursion
• Airports with surface movement radar or multilateration (e.g. ASDE-X/ASSC), since RWSL depends on that surveillance feed
• Night and low-visibility operations, where a red illuminated cue is more reliable than judging runway occupancy visually

Fixtures are installed in a row across the taxiway at the runway holding position, spaced evenly across the taxiway width so an aircraft crossing the position sees a continuous line of lights rather than isolated points, with each unit recessed flush into the pavement on an L-868 base can. Cabling runs in pavement ducting back to the REL control cabinet, which interfaces with the Airport Lighting Control and Monitoring System (ALCMS) and, on RWSL-equipped fields, with the surface surveillance sensor feed that determines runway occupancy in real time. The control cabinet is typically located near the taxiway edge, clear of aircraft movement areas, and includes the interlock logic that illuminates the lights steady red only when the adjoining runway is occupied or a conflict is detected. Setting-out is tied to the holding position marking survey so the light row aligns precisely with the painted holding position line, and installation is commissioned alongside the surveillance system to confirm detection accuracy before the lights are placed in service.

RELs operate as part of the taxiway guidance system:

• Lighting: RELs are steady-burning red, unidirectional, in-pavement lights arranged in a row across the taxiway, illuminated only when the runway ahead is unsafe to enter or cross.
• Indication: When lit, the lights give a clear "do not enter/cross" signal at that taxiway entrance; unlit conveys no meaning and is not an instruction to proceed.
• Integration: RELs are commanded together with the co-located Stop Bar Light so the two always present the same state, and operate alongside — but independently of — Runway Guard Lights at the same holding position.
• Control: The lights are controlled by the ALCMS and can be activated or deactivated based on runway status.

Runway entrance lights are not powered from the constant-current runway edge circuit — they run on a dedicated switched circuit, a row of steady-burning red LED fixtures fed through their own isolating transformers from a REL-specific feeder. Control originates in the ALCMS, which derives a runway-occupied or runway-clear state from tower input, the Runway Status Light (RWSL) surveillance feed, or local pilot-controlled procedure, and issues a single ON/OFF command for the entire REL row at that taxiway entrance. This command is passed to the REL switching controller, which energises or de-energises every fixture in the row simultaneously — there is no way to switch a single REL independently of the rest of the row. The same ALCMS occupancy decision is sent at the same time to the co-located Stop Bar Light, which is why the two are always seen in the same state by an approaching pilot (see the interlock logic below). Fault supervision works at the circuit level: the switching controller or ALCMS interface monitors current draw on the REL feeder, and any anomaly — an open circuit, a failed transformer, a fixture that has failed to illuminate with the rest of the row — is reported as a REL circuit fault for follow-up field testing rather than pinpointed automatically to one fixture.

The fixture is a low-profile, inset (flush) unit that seats into an L-868 base can recessed into the taxiway pavement, with the top surface load-rated for repeated aircraft wheel loading at the runway-taxiway intersection. The housing is a die-cast aluminium alloy body, anodised or powder-coated for corrosion resistance against de-icing fluid, fuel spillage, and tyre wear. The red LED array sits under a high-strength polycarbonate or borosilicate glass lens, gasket-sealed to the housing with an O-ring to achieve an IP67/IP68 rating against water and dust ingress, and shaped to project the uni-directional beam pattern focused toward the taxiway holding position. Internal wiring terminates at L-823 primary connectors, which mate to the secondary leads of the isolating transformer housed in an adjacent base can, with control cabling routed separately to the ALCMS interface. Stainless-steel fasteners are used throughout, and the base can assembly includes a drainage provision to keep water from pooling around the transformer and cable entry points.