Bidirectional Runway Centreline Light

A Bidirectional Runway Centreline Light is an inset light fixture installed along the exact centre of the runway, engineered to project light in two opposite directions along the runway centreline. This design ensures that the light is visible to pilots approaching from either end of the runway, as well as to crews during the landing roll and take‑off run. The colour coding is identical to standard centreline lights: white from the threshold up to 900 m from the runway end, alternating red and white between 900 m and 300 m, and red in the last 300 m. This provides pilots with a clear visual indication of remaining runway length, regardless of the direction of travel.

The bidirectional design exists because many runways genuinely operate in both directions depending on the day's wind conditions, and a centreline light visible from only one end would leave the opposite-direction traffic without the same precise remaining-distance guidance. By projecting light both ways from a single fixture, the runway gets full centreline tracking and distance-remaining information regardless of which direction is currently active, without needing to install and maintain two separate light strings.

The same white/red-white/red colour progression works from either end because the pattern is symmetric relative to each threshold, meaning a pilot lands or departs and always sees the same meaningful sequence of colour transitions leading toward the runway end they're approaching, no matter which physical direction that happens to be on a given day.

Bidirectional centreline lights are installed on runways that are used for take‑off and landing in both directions (i.e., non‑directional runways). They are common on precision approach runways (CAT I, II, and III) where centreline guidance is required for both approach directions. The lights are spaced at 15‑metre intervals for CAT I runways and 7.5‑metre intervals for CAT II/III runways. They are used in conjunction with bidirectional runway edge lights and, where applicable, with touchdown zone lights that are also bi‑directional. The bidirectional optical design eliminates the need for separate unidirectional fixtures for each runway end, simplifying installation and maintenance.

Bidirectional centreline lights are set flush into the runway pavement along the physical centreline, tied into the runway centreline survey the same way as unidirectional fixtures, at 15 m intervals for CAT I runways and 7.5 m intervals through the CAT II/III zone. Because each fixture projects light both ways along the runway, only a single row of lights is needed regardless of which direction is in use, simplifying the setting-out compared with runways that would otherwise need separate fixtures or optics for each landing direction. Each unit seats into an inset base can sealed against water and jet-blast erosion, fed by cable run in pavement ducting back to its isolating transformer, with installation following the same colour-zone layout (white body, red/white transition, solid red) used for the equivalent unidirectional centreline lighting.

Each bidirectional centreline light is powered from the constant current series circuit via an isolating transformer. The fixture contains two optical systems – one facing each direction – or a single bi‑directional optical assembly (often using a prism or dual‑reflector design) that splits the light output into two equal beams along the runway centreline. Each beam typically has a narrow vertical spread (4°) and a horizontal spread (6°) to ensure visibility over a long distance. The colour transition is achieved using dichroic filters or coloured lenses that are selected based on the fixture's position along the runway. The intensity is controlled by the CCR's current steps, with higher intensities (up to 6.6 A) used during low‑visibility operations. The bidirectional nature is particularly useful during runway changes, as the same fixtures provide guidance for both approach directions without requiring reconfiguration.

Bidirectional centreline lights are not individually addressable — control is exercised at the loop level through the constant current series circuit. 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 the corresponding fixed current (one of 5 steps between 2.8A and 6.6A). Every fixture on that loop, via its own isolating transformer, draws from the same current and burns at the same intensity simultaneously in both directions — there is no way to intensify or dim one beam of a fixture, or one fixture, independently of the rest of the circuit. Colour is fixed at manufacture by the dichroic filter or lens selected for each fixture's position, so it is set by physical location on the loop rather than by any command from the ALCMS. Fault detection remains loop-based: the CCR monitors loop voltage against the commanded current, and a voltage rise signals increased circuit impedance — typically an open-circuit lamp or a failed isolating transformer — reported to the ALCMS as a loop fault rather than pinpointed to an exact fixture or beam direction without further sectionalised testing.

The fixture uses a die-cast aluminium body, precision-machined and anodised or powder-coated for corrosion resistance, set flush into an L-867 base can (fixture qualified under the L-850 series, FAA runway centreline light designation) recessed into the runway pavement. Because the unit must project light in two opposite directions along the centreline, it incorporates two separate lamp/LED and prism assemblies (or a single bi-lobed reflector arrangement) arranged back-to-back within a common housing, each independently aimed along the runway axis. The optical elements sit beneath a toughened borosilicate glass or polycarbonate top lens, load-rated for aircraft wheel loading and gasket-sealed with an O-ring to achieve an IP67/IP68 rating. Colour changes along the fixture's length are achieved with coloured filter segments or dual-colour LED arrays (white/red) built into each optical path rather than by physically relocating fixtures. Internal wiring from both optical assemblies terminates at sealed L-823 primary connectors mating to the secondary leads of a nearby isolating transformer housed in its own base can, with stainless-steel fasteners and a drainage channel provided to keep the base can free of standing water.