Runway Threshold Light

A Runway Threshold Light is a green light fixture installed at the landing threshold of a runway. These lights are arranged in a single row extending across the full runway width, with the number of fixtures set by that width (spacing must not exceed 3 m, per ICAO Annex 14 / FAA AC 150/5345-46), and are uni-directional, projecting light only in the approach direction. The green colour signifies the start of the runway available for landing, distinguishing it from the runway end lights which are red. Threshold lights are a critical visual aid for pilots, especially during night operations and low visibility conditions.

The threshold light's purpose is to mark, with certainty, the exact point at which the declared landing surface begins, so a pilot on final approach knows precisely where touchdown may safely occur. This distinction matters because runway pavement can extend beyond the usable landing threshold (for displaced thresholds, overrun areas, or blast pads), and without a clear visual marker a pilot could misjudge the approach and touch down early or late. The green colour is reserved exclusively for this function across the airfield lighting system, giving an unambiguous "start of runway" signal that complements the red runway end lights at the far end, and is especially critical during night approaches and low-visibility landings when the physical threshold markings on the pavement are hard to distinguish.

Installed at the landing threshold of all paved runways that support night operations. The lights extend as a single row across the full runway width, on a line at right angles to the centreline, with spacing between adjacent lights not exceeding 3 metres; the outermost lights sit on, or up to about 3 metres outside, the runway edge to match the runway edge light line. For precision approach runways, the threshold lights are supplemented by wing bar lights to enhance visibility. Threshold lights are used on both precision and non-precision runways, and are also installed on displaced thresholds where applicable.

Fixtures are set out in a row across the full runway width at the threshold, on a line perpendicular to the runway centreline, with individual units spaced no more than 3 m apart and the outermost lights positioned on, or up to about 3 m outside, the runway edge to align with the edge light line. Units are almost always inset (flush) fixtures seated into an L-868 base can recessed into the pavement, sealed against surface water ingress, since the threshold sits within the touchdown zone where an elevated fitting would be a hazard. Where wing bar lights are fitted, the additional units are set out symmetrically either side of the runway edge lights at the threshold, following the same row alignment. Each base can is fed by armoured cable run in a duct back to its isolating transformer, with the transformer housed in a separate secondary base can nearby to keep the series-circuit joint clear of standing water. Setting-out is tied to the surveyed threshold position, which is re-confirmed whenever the threshold is displaced, so the light row remains an accurate marker of the usable landing surface.

Each threshold light is powered from the series circuit via an isolating transformer. The light projects a narrow, 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. The intensity is controlled by the CCR's current steps, with higher intensities used during low visibility operations. The lights are typically flush-mounted (inset) in the pavement, though elevated versions may be used in some configurations.

Threshold lights are not individually addressable — control is exercised at the loop level, and they are typically wired into the same series circuit as the runway edge lights (and often the runway end lights), so the threshold row switches and dims in step with the rest of the runway edge lighting system rather than on its own schedule. Where wing bar lights are fitted, these are carried on the same loop and step together with the threshold row. 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 shared loop to the corresponding fixed current. Every fixture on that loop — threshold, wing bar, edge, or end light alike — draws from the same current via its own isolating transformer and therefore burns at the same intensity simultaneously; the threshold row cannot be commanded independently unless it has been isolated onto its own circuit at the design stage. Fault detection is likewise 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 die-cast aluminium alloy housing, anodised or powder-coated for corrosion resistance against fuel and de-icing fluid exposure, with an internal reflector and baffle arrangement that restricts the beam to a uni-directional pattern facing the approach. The green colour is produced by a filtered lens or coloured LED array rather than a dyed housing, so output stays consistent as the fixture ages; the lens is toughened glass or high-strength polycarbonate, gasket-sealed with an O-ring to achieve an IP67/IP68 rating. Because the threshold sits in the touchdown zone, the fixture is almost always an inset (flush) unit, seating into an L-868 base can recessed into the pavement with a wheel-load-rated top plate rather than an elevated frangible mount. Where wing bar lights are fitted, the wing units share the same housing and optical construction as the main row. Internal leads terminate at L-823 primary connectors, sealed for direct burial and continuous immersion, mating to the isolating transformer's secondary in an adjacent base can, with stainless-steel fasteners and a base-can drainage path completing the assembly.