Unidirectional Runway Centreline Light

A Unidirectional Runway Centreline Light is an inset fixture that emits light in only one direction along the runway centreline. It is typically oriented towards the approaching aircraft, providing a clear visual reference for the runway centreline during take‑off and landing. The colour coding follows the standard ICAO/FAA convention: white from the threshold 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 unidirectional design reduces cost and complexity compared to bidirectional versions, making it suitable for runways where operations are predominantly in one direction or for non‑precision approaches where bidirectional guidance is not mandated.

The unidirectional centreline light's purpose is to keep an aircraft tracking precisely down the centre of the runway during the roll or takeoff, in situations where operations predominantly happen in one direction and the added cost of full bidirectional visibility isn't justified. Aiming the beam specifically toward the approaching or departing aircraft concentrates intensity where it's actually needed, rather than splitting light output in a direction that won't be used.

The colour transitions along its length — white, then alternating red and white, then solid red — exist to give the pilot a continuous, built-in sense of remaining runway distance without needing to consult an instrument, which becomes essential in low visibility landing rolls where visually judging distance to the runway end by eye alone is unreliable.

Unidirectional centreline lights are installed on runways where the primary approach and take‑off direction is fixed, such as at airports with prevailing wind patterns or where terrain constraints dictate a single operational direction. They are common on non‑precision runways (e.g., VFR or CAT I) where the lower cost of unidirectional fixtures is acceptable. They are also used as supplementary guidance on the departure end of bidirectional runways where only approach guidance is required from one side. The lights are spaced at 15‑metre intervals for CAT I runways and can be used on CAT II/III runways if bidirectional is not required, though typically high‑intensity bidirectional fixtures are preferred for low‑visibility operations.

Fixtures are recessed into the runway pavement along the centreline at 15 m intervals (CAT I) using a load-rated inset base can, with the fixture's single-direction optical assembly oriented squarely to face the predominant approach/landing direction the runway serves. Because the fixture emits no useful beam in the reverse direction, setting-out and orientation checks are more important here than for bidirectional fixtures — a rotated or misaligned base can produces a centreline light invisible from the intended approach even though the fixture itself is functioning correctly. Cabling and isolating transformer arrangements follow the standard runway centreline convention, with position tied to the runway centreline survey for the served approach direction.

Each unidirectional centreline light is powered from the constant current series circuit via an isolating transformer. The fixture contains a single optical system (reflector, prism, and lens) that projects a narrow beam (typically 4° vertical × 6° horizontal) in one direction along the runway centreline. This beam is carefully aligned with the runway axis to ensure it is visible to pilots on approach over a long distance. The colour is determined by a dichroic filter or coloured lens that corresponds to 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. Because the light only emits in one direction, it is not visible to pilots approaching from the opposite end, which is acceptable for runways with a defined operational direction.

Unidirectional centreline lights are not individually addressable — control is exercised at the loop level through the constant current series circuit serving the runway centreline. An ALCMS, driven by tower instruction or an automated visibility-linked routine, selects the required brightness step, and the CCR regulates the entire loop to a single fixed current (up to 6.6A for the lowest RVR conditions the runway supports). Every fixture on the loop, via its own isolating transformer, draws the same current and burns at the same intensity simultaneously — the electrical control is identical to a bidirectional centreline circuit, since the current step has no bearing on which way the fixture's single optical assembly is aimed. What is unique to this fixture is that orientation, not electrical control, determines visibility: because the beam only reaches pilots approaching from the served direction, correct alignment at installation is what makes the loop-level current command meaningful for traffic in that direction, and there is no CCR command that can compensate for a fixture facing the wrong way. Fault detection remains loop-based: the CCR monitors loop voltage against commanded current, and a voltage rise — typically an open-circuit source or failed isolating transformer — is reported to the ALCMS as a loop fault, isolated to a specific fixture only through sectionalised field testing.

The fixture uses a die-cast aluminium body, machined and anodised or powder-coated for corrosion resistance, set flush into an L-852-series base can recessed into the runway pavement. A single lamp or LED array and prism assembly is aimed along the runway axis toward the approach direction, with coloured filter segments or dual-colour LED elements built into the optical path to produce the white, alternating red/white, or red output required at different points along the runway. The optical elements sit beneath a toughened glass or polycarbonate top lens, load-rated for aircraft wheel loading and gasket-sealed with an O-ring to achieve an IP67/IP68 rating. Internal wiring terminates at a sealed L-823 primary connector mating to the secondary lead of a nearby isolating transformer housed in its own base can, with stainless-steel fasteners and a drainage channel in the base can preventing standing water around the connector and transformer.