Runway Guard Light

A Runway Guard Light (RGL), commonly called a "wig-wag," is a visual warning system installed at runway holding positions to alert pilots and vehicle operators that they are approaching an active runway. Unlike the automated, occupancy-driven Runway Status Light (RWSL) system, an RGL is a standalone fixture that flashes continuously — typically alternating amber/yellow, either as a pair of elevated units either side of the taxiway (Configuration A) or a row of in-pavement units across the taxiway (Configuration B) — for as long as the associated circuit is switched on, independent of whether the runway is actually occupied. It serves as a constant, unmissable reminder of the holding position, not a dynamic go/no-go signal.

The runway guard light exists as the last visual checkpoint before a pilot or vehicle operator physically crosses onto an active runway, placed precisely at the holding position where a moment's inattention has the most serious consequence. Its flashing amber pattern is deliberately distinct from steady-burning taxiway and runway lighting, drawing attention specifically because a flashing signal is harder to overlook than a static one, especially in a high-workload taxi phase where attention may be divided. Because the fixture flashes continuously rather than only in response to a detected conflict, its purpose is best understood as a constant, unambiguous marker of where the runway environment begins, reinforcing — rather than replacing — a pilot's own judgement and any ATC clearance to enter or cross. At airports where RWSL is also installed, RGL is a complementary but functionally separate layer, not a component driven by the same occupancy logic.

Installed at runway holding positions on taxiways, typically at the intersection where a taxiway meets the runway. Runway guard lights are used on:

• Precision approach runways (CAT II and III) where enhanced safety is required
• Busy airports with high traffic volumes where runway incursion risk is elevated
• Runway crossings where taxiways intersect active runways
• Intersecting runways where crossing traffic needs to be managed

A pair of Runway Guard Light fixtures (the elevated "wig-wag" configuration) is installed on either side of the taxiway at the runway holding position, mounted on frangible stems set back from the pavement edge so the alternating flashing beams are visible to a pilot or vehicle operator approaching the holding point; where an in-pavement configuration is used instead, a row of flush units is set across the taxiway following the same layout as Runway Entrance Lights. Each fixture is fed by cable run in a trench or duct back to a control/flasher unit that drives the alternating flash pattern, typically switched on with the taxiway edge lighting circuit (for night or low-visibility operation, or continuously per local operating policy) rather than by any occupancy sensor. The control unit and any associated transformer are housed in an enclosure clear of the taxiway shoulder, with setting-out tied to the holding position marking so the fixtures sit at the same reference line as the hold line.

Runway guard lights are typically powered from the series circuit via isolating transformers, with a dedicated flasher interface producing the alternating amber/yellow wig-wag pattern. Unlike RWSL fixtures, RGL activation is not driven by surveillance data or runway occupancy: the lights flash continuously at a fixed rate whenever the circuit is energised, on a schedule set by airport operating procedure (commonly linked to darkness/low-visibility operation, or run 24/7 at busier fields). This constant flashing is what makes the fixture effective as an always-present reminder of the holding position, rather than a conditional warning.

Runway guard lights are wired onto the airfield's constant-current series circuit like other elevated and in-pavement AGL fixtures, each fed through its own isolating transformer, but their flash behaviour is not set by the CCR's brightness steps alone. A dedicated flasher/switching interface taps the same loop current and converts it into a rapid alternating (A/B) flash pattern between the two lamps of a Configuration A pair, or in sequence across the fixtures of a Configuration B row. The command to energise that interface comes from the ALCMS on a schedule tied to airport operating procedure — commonly switched with the taxiway lighting circuit for darkness or low-visibility operation, or left continuously on — and is entirely independent of any runway-occupancy surveillance system such as RWSL; on fields where RWSL is also present, the two operate side by side with no shared activation logic. Once energised, the fixture flashes continuously until the ALCMS commands it off. Fault supervision therefore centres on the flasher interface and the series circuit itself: a flasher stuck in one state, a lamp that fails to alternate in step, or a loss of series current is reported as an RGL fault requiring field follow-up.

The fixture is typically an inset (flush) unit seating into an L-868 base can recessed into the taxiway pavement at the runway-holding position, with the top surface load-rated for aircraft wheel loading; elevated (wig-wag) configurations instead mount two paired units on frangible fittings either side of the taxiway. The housing is a die-cast aluminium alloy body, anodised or powder-coated for corrosion resistance against de-icing fluid, fuel spillage, and UV exposure. The amber 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. 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 a separate control link carrying the ALCMS on/off command to the flasher interface. Stainless-steel fasteners are used throughout, and the base can assembly includes a drainage provision to prevent water pooling around the transformer and cable entry points.