Combination Threshold/End Light

A Combination Threshold/End Light is a versatile bi-directional runway lighting fixture that serves as both a threshold light and an end light depending on the direction of approach. These lights are installed at both ends of the runway and are equipped with two optical systems: one projecting green light in the landing direction (threshold) and the other projecting red light in the opposite direction (runway end). This dual-purpose design is essential for runways that are used in both directions, providing appropriate visual cues for landings from either end without requiring separate fixtures.

The combination fixture exists to solve a specific operational problem on runways used in both directions: each end of the runway must simultaneously act as a "start of landing surface" reference (green, seen by traffic landing towards it) and an "end of runway" warning (red, seen by traffic landing away from it). Rather than installing two separate fixtures at each end to cover both roles, the dual-optic design lets a single physical unit serve both functions at once, reducing the number of fixtures, circuits, and maintenance points on the field while still preserving the strict colour-coded meaning pilots rely on from either direction of approach. This makes it purpose-built for bidirectional runway operations, where the same physical location has a different lighting meaning depending on which way traffic is moving.

Combination threshold/end lights are installed on runways that are used in both directions (bi-directional runways). They are used on:

• Bi-directional runways where operations occur from both ends
• Precision approach runways (CAT I, II, and III) with reciprocal approaches
• Non-precision runways with operations in both directions
• Runways with displaced thresholds requiring dual-colour indication
• Military airfields requiring flexible runway directionality

Fixtures are installed in a single row across the full width of the runway at each runway end, spaced evenly on each side of the centreline in line with the runway edge lights, with one combination unit serving as both the threshold light for landings in one direction and the end light for operations in the other. Each unit 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 that runway end's circuit. The bi-directional optical assembly is set out so the green element faces the approach direction for landing traffic and the red element faces back along the runway for aircraft on take-off roll or landing rollout in the opposite direction, with careful orientation during installation critical to avoid the colours being reversed. Setting-out is tied to the runway end survey mark at both ends of the runway so each combination row remains square to the centreline and consistent with its counterpart at the opposite threshold.

Combination threshold/end lights operate as a bi-directional lighting system:

• Design: Each fixture contains two optical systems: one projecting green light in one direction (threshold) and the other projecting red light in the opposite direction (runway end).
• Installation: The lights are installed at both ends of the runway, typically in a row across the runway width at each end.
• Operation: When approaching from one direction, the pilot sees green lights (threshold). When approaching from the opposite direction, the pilot sees red lights (runway end).
• Integration: The lights are integrated with the runway lighting control system, which ensures the correct colour is displayed based on the active runway direction.

Like other AGL fixtures, the combination threshold/end light is not individually addressable — control is exercised at the loop level through the constant current series circuit serving that runway end. 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 pilot-controlled lighting input. This command is passed to the CCR, which regulates the entire loop to a single fixed current shared by every fixture on it. Because the green and red optical systems inside the fixture are fed from the same isolating transformer secondary, both are energised and stepped together at all times — there is no switching logic that turns one colour on and the other off; the colour a pilot perceives depends purely on which side of the fixture they are viewing, not on any command from the control system. 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 as a loop fault rather than pinpointed to a specific fixture or optic without further sectionalised testing.

The housing is a die-cast aluminium body split internally into two optical chambers, one glazed with a green filter or lens facing the approach direction and the other with a red filter or lens facing back along the runway, each fed by its own LED array or halogen lamp so the two colours cannot bleed into one another. The dual-lens assembly is gasket-sealed to the body with an O-ring or silicone seal to achieve an IP67/IP68 rating; where the unit is elevated it bolts to a frangible coupling engineered to shear on impact, seated on an L-867 base can, while flush-mounted units seat directly into an L-868 base can rated for aircraft wheel loading. Internal wiring for both optical chambers terminates at L-823 primary connectors sealed for direct burial, which mate to the secondary leads of the isolating transformer serving that runway end's circuit. Stainless-steel fasteners are used throughout, and careful attention is paid during assembly to keep the green and red lens chambers correctly oriented relative to the base can's fixed index, since a housing rotated 180° would present the wrong colour to each direction of traffic.