Runway Centreline Light

A Runway Centreline Light is a series of flush-mounted inset lights installed along the exact centre of the runway. These lights provide critical longitudinal guidance to pilots, particularly during low-visibility operations. The colour coding is standardized: white lights from the threshold to 900 metres from the runway end, alternating red and white from 900 to 300 metres from the runway end, and all red from 300 metres to the runway end. This colour transition gives pilots a visual cue of the remaining runway length.

Centreline lights exist to keep an aircraft tracking the exact longitudinal axis of the runway when the pilot's outside visual references are degraded, most critically during CAT II/III landings and low-visibility takeoffs where edge lights alone cannot provide fine enough lateral guidance. Because the fixture sits directly on the line the aircraft's nose and main gear must follow, it lets the pilot correct drift immediately rather than relying on peripheral edge references, which is essential for staying within the narrow safety margins of a precision approach. The colour transition from white, to alternating red/white, to solid red is itself a purpose-built distance cue: it tells the pilot how much runway remains without requiring them to consult instruments, supporting the decision to continue, brake, or abort during the high-workload final stage of landing or takeoff roll.

Installed on precision approach runways (CAT I, II, and III) and many non-precision runways where centreline guidance is required. Per ICAO Annex 14, the standard longitudinal spacing is 15 metres; a wider 30-metre spacing may only be used where the centreline lighting's serviceability meets the specified maintenance objectives and the runway is intended for use in RVR conditions of 350 m or greater (i.e., less demanding, non-CAT II/III operations). CAT II/III precision approach and low-visibility runways are therefore kept at the 15-metre spacing. They are used in conjunction with runway edge lights and touchdown zone lights to provide comprehensive visual guidance during all phases of the approach and landing.

Fixtures are set into the runway pavement along the physical centreline, tied to the runway centreline survey so the line of lights remains true along the full runway length. Inset (flush) L-850-series fittings are used almost exclusively on the centreline, since an elevated fixture on the landing surface itself would present a strike hazard, and each unit is seated into a base can recessed into the pavement and sealed against surface water and jet-blast erosion. Standard spacing is 15 m, with a wider 30 m spacing permitted only where centreline light serviceability meets the maintenance objectives and the runway supports RVR ≥ 350 m; CAT II/III and other low-visibility runways are kept at 15 m throughout. Colour transitions are built into the setting-out: white along the body of the runway, alternating red/white through the last 900 m, and solid red for the final 300 m. Each fixture is fed by cable run in pavement ducting back to its isolating transformer, with transformers housed clear of the light base to keep the series-circuit joint out of standing water, and installation must maintain the pavement grade and load-bearing strength around each cut-in.

Each centreline light is powered from the constant current series circuit via an isolating transformer. The lights use a bi-directional optical system to project a narrow beam (typically 4° vertical x 6° horizontal) both ahead and behind the fixture, ensuring visibility to pilots on approach and during roll-out. The colour transition is achieved using coloured filters or dichroic lenses. The intensity is controlled by the CCR's current steps, with higher intensities used during low visibility operations.

Centreline lights are wired on their own dedicated series circuit, kept separate from the runway edge, threshold, and end light loops, since their close 15 m standard spacing and role in low-visibility operations call for brightness stepping that is independent of the rest of the runway lighting system. Colour is not a real-time control function: the white, alternating red/white, and solid red zones are fixed by the coloured filter or dichroic lens built into each fixture according to its distance from the runway end, so the colour pattern along the runway never changes regardless of which CCR step is commanded — only the overall intensity of every fixture on the loop rises or falls together. An Airfield Lighting Control and Monitoring System (ALCMS), operated by the tower or an automated visibility-linked routine, selects the intensity step appropriate to prevailing RVR; the CCR regulates the entire centreline loop to that fixed current, and every fixture's isolating transformer draws from the same current and burns at the same intensity simultaneously — there is no way to command an individual fixture, or a single colour zone, independently of the rest of the line. Fault detection is 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 to the ALCMS as a loop fault to be sectionalised by further field testing rather than resolved to an exact fixture from the control room.

Centreline fixtures use a low-profile, load-rated aluminium alloy housing sunk into the pavement so the top surface sits flush with the runway and tolerates repeated aircraft wheel loading without cracking or displacement. The lens is a shallow, toughened-glass or polycarbonate dome shaped to shed water and resist jet-blast erosion while still projecting a usable beam angle. Where the fixture must show two colours (the alternating red/white and solid-red zones), the optical assembly is split internally by a fixed filter or divided LED array, so red and white segments share one sealed unit rather than requiring separate fixtures. The housing seats into an L-868 base can recessed into the pavement, gasket-sealed against surface water and fuel ingress for an IP67/IP68 rating. Internal leads terminate at L-823 primary connectors, sealed for direct burial and continuous immersion, mating to the isolating transformer's secondary in a base can set clear of the light itself, with stainless-steel fasteners used throughout to resist corrosion.