Threshold Wing Bar Light

A Threshold Wing Bar Light is a green light fixture installed on precision approach runways (CAT II and III) to supplement the threshold lights. These lights are arranged in two rows, one on each side of the runway, extending outward from the runway edge lights. The wing bar lights project a uni-directional beam towards the approaching aircraft, providing a clear visual indication of the threshold location, particularly in low visibility conditions. They are also known as threshold bar lights or threshold wing lights.

Wing bar lights exist to widen and reinforce the threshold signal on precision approach runways, where a narrow row of threshold lights alone may not stand out strongly enough against background lighting or a low-visibility approach environment. By extending outward from the runway edge lights on both sides, the wing bars make the threshold location visually larger and harder to miss from a distance, giving pilots an earlier and more certain confirmation of exactly where the landing surface begins. Their purpose is concentrated specifically on CAT II/III operations, where the margin for misjudging the threshold position during a low-visibility landing is smallest and the value of an unmistakable, wide visual reference is highest.

Installed on precision approach runways (CAT II and CAT III) where enhanced threshold visibility is required. Each wing bar extends outward from the line of the runway edge lights, with the innermost light of the bar in line with the edge lights and the bar reaching at least 10 metres (33 feet) further out. Per ICAO Annex 14, each wing bar is formed by at least five lights on each side, typically spaced at roughly 2–3 metre intervals to span that minimum 10 m extent. Wing bar lights are used in conjunction with threshold lights and approach lighting systems to provide comprehensive visual guidance during low visibility approaches.

Fixtures are installed in two groups, one group on each side of the runway, each formed by at least five lights per ICAO Annex 14, with the innermost light of each bar in line with the runway edge lights and the bar extending at least 10 m further outward, spaced at roughly 2–3 m intervals to form a compact bar rather than a continuous line. Each light mounts on an L-868 base can recessed flush with the pavement or shoulder, sealed against surface water, with cabling run in pavement ducting or shoulder trenching back to the threshold circuit's isolating transformer, typically sharing a feed with the adjacent threshold lights. The unidirectional beam is angled to project toward the approach direction only, and setting-out is tied to the threshold survey mark and the runway edge light line so the wing bars sit at the correct offset and remain square to the runway centreline. Where the wing bar is added to an existing threshold installation, the new base cans are positioned to keep clear of the runway edge light circuit trenching already in place.

Each wing bar light is powered from the series circuit via an isolating transformer. The light projects a uni-directional beam towards the approaching aircraft using a precision optical system with a reflector and prism. The green colour is achieved through a green filter or dichroic coating, matching the threshold lights. The intensity is controlled by the CCR's current steps, with higher intensities used during low visibility operations. The lights are typically elevated or flush-mounted, depending on the runway configuration.

Wing bar lights are not individually addressable — control is exercised at the loop level through the constant current series circuit that also feeds the associated threshold lights. An Airfield Lighting Control and Monitoring System (ALCMS), operated by the tower or an automated visibility-linked routine, selects the required brightness step based on prevailing RVR, ATC instruction, or (at smaller aerodromes) pilot-controlled lighting input. This command is passed to the CCR, which regulates the entire loop to a single fixed current. Because the wing bars typically share the threshold circuit rather than run on a dedicated loop of their own, every wing bar light steps up and down in intensity in lock-step with the threshold lights at that runway end — there is no way to command the wing bars to a different brightness independently. Fault detection remains loop-based: the CCR continuously monitors loop voltage against the commanded current, and a voltage rise at constant current signals increased circuit impedance, typically an open-circuit lamp or a failed isolating transformer, which is reported to the ALCMS as a loop fault rather than pinpointed to an exact fixture without further sectionalised testing.

The fixture body is a compact die-cast aluminium housing finished for corrosion resistance, glazed with a green lens or filter over an LED array or halogen lamp behind a unidirectional optical system angled to project only toward the approach direction. The lens is gasket-sealed to the housing with an O-ring or silicone seal for an IP67/IP68 rating; elevated units bolt to a frangible coupling seated on an L-867 base can, while flush-mounted units seat directly into an L-868 base can recessed into the shoulder or pavement. Internal wiring terminates at L-823 primary connectors sealed for direct burial, which mate to the secondary leads of the isolating transformer shared with the adjacent threshold circuit. Stainless-steel fasteners are used throughout, and because wing bar units are grouped tightly in bars of at least five lights per ICAO Annex 14, housings and base cans are dimensioned to permit the close 2–3 m spacing without cable congestion in the shared trench.