Green Threshold Light

A Green Threshold Light is a specialized runway threshold lighting fixture that provides green visual guidance to pilots approaching a runway. These lights are installed at the runway threshold and are part of the approach lighting system. The green colour indicates to pilots that they are aligned with the runway centreline and are on the correct approach path. Green threshold lights are a critical component of the runway lighting system, marking the landing threshold and providing a clear visual reference during final approach and landing.

The green colour of this fixture exists to give pilots an unambiguous "start of usable landing surface" cue that cannot be mistaken for any other signal on the field, since green is reserved specifically for this role across the runway lighting system. This is critical because the runway threshold does not always coincide with the physical start of the pavement — a displaced threshold, overrun, or blast pad may extend beyond it — so a purely visual judgement of where the pavement begins is not reliable enough on its own. During final approach, particularly at night or in reduced visibility, this dedicated marker lets the pilot confirm correct alignment with the landing surface and time the flare and touchdown against a fixed, trustworthy reference rather than an estimate.

Green threshold lights are installed at the runway threshold on precision approach runways. They are used on:

• Precision approach runways (CAT II and III) where approach guidance is critical
• Runways with approach lighting systems requiring threshold identification
• Low visibility operations where visual cues are essential
• Runways with complex approach paths requiring enhanced guidance
• Military airfields requiring precise approach guidance

Fixtures are set out in a single row across the full width of the runway at the threshold, spaced evenly on each side of the centreline in line with the runway edge lights, with additional units forming the wing bars where fitted. 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 isolating transformer serving the threshold circuit. Where the runway is used in both directions, the fixtures are unidirectional units angled so the green beam faces only the approach direction, with the reverse face fitted with a red lens or blanked so it does not confuse traffic departing from the opposite end. Setting-out is tied to the threshold survey mark so the light line remains square to the runway centreline, and the row is coordinated with the approach lighting system so the last approach light bar and the threshold row read as optically continuous to the approaching pilot.

Green threshold lights operate as part of the approach and runway lighting system:

• Installation: Green threshold lights are installed at the runway threshold, typically in a row across the runway width.
• Lighting: The lights are green and project a uni-directional beam towards the approaching aircraft, providing a clear visual reference of the runway threshold.
• Guidance: The green lights indicate to the pilot that they are aligned with the runway centreline and are on the correct approach path.
• Integration: Green threshold lights are integrated with approach lighting systems, runway edge lights, and other runway lighting components.

Electrically, green threshold lights are controlled exactly like runway edge lights — they are not individually addressable, and control is exercised at the loop level. An ALCMS, operated by the tower or an automated visibility-linked routine, selects the required brightness step and passes that command to the CCR, which is commonly wired in series with the adjacent runway edge circuit so the threshold row and the edge lights change intensity together. Every threshold fixture on that loop draws from the same fixed current through its own isolating transformer and therefore burns at the same intensity simultaneously; there is no way to dim or brighten one fixture independently of the row. Fault detection is likewise loop-based — a voltage rise at constant current signals increased impedance, typically an open-circuit lamp or a failed isolating transformer, and is reported to the ALCMS as a loop fault rather than pinpointed to an exact fixture. The unidirectional beam described in the section below is a purely optical property of the fixture housing; it plays no part in how the light is switched or dimmed.

The fixture body is typically a die-cast aluminium alloy housing, machined and anodised or powder-coated for corrosion resistance against runway de-icing fluids, fuel spillage, and UV exposure. The green uni-directional lens is a borosilicate glass or high-strength polycarbonate prism, gasket-sealed to the housing with an O-ring or silicone seal to achieve an IP67/IP68 rating against water and dust ingress, and shaped internally to filter light to the approach side only. Elevated units (FAA Type L-862E on precision-approach runways, L-861E on non-precision) bolt to a frangible coupling engineered to shear cleanly under aircraft impact loads, seated on an L-867 base can set flush with the surrounding grade; flush-mounted (inset) units (FAA Type L-850D) instead seat directly into an L-868 base can recessed into the pavement, with the top surface load-rated for in-pavement aircraft wheel and jet-blast loading per the FAA AC 150/5345-42 light base specification, and its retaining bolts torqued to the correct clamping force per FAA Engineering Brief 83A (In-Pavement Light Fixture Bolts). Internal wiring terminates at L-823 primary connectors, sealed splice connectors rated for direct burial and continuous immersion, which mate to the secondary leads of the isolating transformer housed in its own base can nearby. Stainless-steel fasteners are used throughout to resist corrosion, and the base can assembly includes a drainage provision to prevent water pooling around the transformer and cable entry points.