Threshold/Runway End Combination Fixture

A Threshold/Runway End Combination Fixture (also known as a combination threshold/end light) is a single light unit that incorporates both a green threshold light (visible from the approach side) and a red runway end light (visible from the opposite side). This dual‑colour design is used at runways where the same physical location serves as the threshold for one direction and the end for the other – common on runways with displaced thresholds or where the runway is used in both directions. The fixture typically contains two optical systems or a bi‑colour dichroic filter that produces green light when viewed from the approach side and red light when viewed from the opposite side. This eliminates the need for separate green and red fixtures at the same location, reducing installation complexity, cable runs, and maintenance costs.

This combination fixture exists to solve a specific geometric problem: at a displaced threshold or a runway used in both directions, one exact physical location has to mean two completely different things depending on which way you're looking at it — the start of usable landing surface to one pilot, and the end of pavement to another approaching from the opposite direction. Using two separate fixtures at that single point would be redundant and would double the cabling, mounting, and maintenance burden for guidance that has to occupy the same physical spot anyway.

By producing green in one direction and red in the other from a single housing, the fixture gives each approaching pilot exactly the colour their situation calls for, without any risk of the two messages being visible from the wrong side and causing confusion. This precision matters because threshold and end lighting carry very different implications — land here is the opposite message from stop here — and the whole point of the dual-colour design is making sure each pilot only ever sees the one that applies to them.

Installed at the ends of runways where the threshold and the runway end are co‑located – typically at both ends of a runway that is used for take‑off and landing in both directions (i.e., non‑directional runways). They are also used at displaced thresholds, where the actual landing threshold is located further down the runway, and the area before it serves as a stopway or overrun area. Combination fixtures are used wherever the threshold coincides with the runway extremity, on precision (CAT I, II, III) and non-precision runways alike. They are not appropriate where the threshold is displaced — there the green threshold row and the red runway end row sit at different locations and must be separate fixtures. They are arranged in a row across the runway width, with the green side facing the approaching aircraft and the red side facing the opposite direction. The spacing and mounting (elevated or inset) follow the same standards as individual threshold and end lights.

Fixtures are set out in a single row across the full runway width at the runway end or displaced threshold, using the same base can and spacing convention as separate threshold/end light installations — an inset base can where the surface is paved and load-rated, or an elevated frangible mount where conditions allow. Because a single fixture serves both directions (green facing the approach, red facing the departure/overrun side), its orientation is critical during setting-out: the fixture is aligned squarely across the runway centreline rather than skewed, so neither colour is compromised for the direction it serves. Cabling and isolating transformer arrangements follow the same convention as standard threshold and runway end lights, with position tied to the threshold or displaced-threshold survey rather than a generic runway-end offset.

Each combination fixture is powered from the series circuit via an isolating transformer. The fixture contains a single lamp or LED array whose light is split or filtered to produce two distinct colours. A common design uses a dichroic reflector or prism that reflects green light towards the approach side and transmits red light towards the opposite side, or vice versa. Alternatively, the fixture may have two separate optical cavities with independent colour filters. The intensity is controlled by the CCR's current steps, with the green and red outputs adjusted simultaneously to match visibility conditions. The fixture is uni‑directional for each colour: green is visible only from the approach side, and red only from the opposite side. This ensures that pilots on approach see the green threshold, while pilots on the opposite end see the red end light.

The combination fixture is not individually addressable — control is exercised at the loop level through the constant current series circuit it is wired into, generally the same runway edge lighting loop that continues along the pavement on both sides of it. An ALCMS, operated by the tower or an automated visibility-linked routine, selects the required brightness step for that loop based on prevailing RVR or ATC instruction, and this command is passed to the CCR, which regulates the entire loop — and therefore both the green and red outputs of every combination fixture on it — to a single fixed current. Because the fixture's colour split is achieved optically (by the dichroic reflector or separate filtered cavities) rather than electrically, the CCR has no way to command green and red independently: raising or lowering the loop's current step changes the intensity of both colours together, in lock-step with every other edge or threshold fixture sharing the circuit. Fault detection is likewise loop-based: the CCR monitors loop voltage against the commanded current, and a voltage rise — typically an open-circuit source or a failed isolating transformer — is reported to the ALCMS as a loop fault, isolated to the specific fixture only through sectionalised field testing; because a single lamp or LED array feeds both colour outputs, a failure at the source extinguishes green and red simultaneously rather than one colour independently of the other.

The fixture body is a die-cast aluminium alloy housing, anodised or powder-coated against runway de-icing fluid and UV exposure, with a single lamp module viewed through two separately masked lens segments — one green, one red — rather than two independent optics, since the colour separation here is optical rather than electrical. The lens assembly is gasket-sealed to the housing with an O-ring or silicone seal for an IP67/IP68 rating, and the unit mounts either on a frangible coupling seated on an L-867 base can for elevated installations, or directly into an L-868 base can for flush/inset units rated for aircraft wheel loading. Internal wiring terminates at an L-823 primary connector feeding the isolating transformer's secondary leads in its own adjacent base can, with stainless-steel fasteners and a drainage provision at the base to keep water off the transformer and cable entries.