Approach Centreline Light

An Approach Centreline Light is a high‑intensity light fixture installed on the approach centreline, typically spaced at regular intervals (e.g., 30 m or 60 m) as part of an approach lighting system. These lights form a continuous row that extends from the runway threshold into the approach area, providing pilots with a clear visual reference for lateral alignment and distance‑to‑go. They are often steady‑burning white lights, but may also flash in some configurations to enhance conspicuity.

Approach centreline lights are a standard component of both precision (ALSF‑1/2) and medium‑intensity (MALSF/MALSR) approach lighting systems. They work in conjunction with barrettes, crossbars, and sequenced flashing lights to create a comprehensive visual guidance system, particularly critical during low‑visibility (CAT I/II/III) operations.

The approach centreline light exists to give a pilot a continuous, un-interrupted line to fly before the runway itself is close enough to provide any visual reference at all. In the seconds after breaking out of cloud or fog on an instrument approach, this row of lights is often the very first thing a pilot sees, and it needs to answer two questions immediately: am I aligned, and how far do I still have to go? Lateral alignment comes from the row appearing straight ahead rather than skewed to one side; distance comes from the perceptual effect of the evenly spaced lights appearing to spread apart as the aircraft closes in, which a trained eye reads almost automatically as closure rate. Because it forms the visual spine that barrettes, crossbars, and flashing lights are all built around, its role is less about any single light and more about maintaining that unbroken reference line all the way to the runway environment.

Approach centreline lights are deployed on precision and non‑precision approach runways and are used in:

• CAT I precision approach runways (ALSF‑1) – providing essential centreline guidance for approaches with 200 ft decision height.
• CAT II and III precision approach runways (ALSF‑2) – critical for low‑visibility operations with decision heights as low as 100 ft or less.
• Medium‑intensity approach lighting systems (MALSF, MALSR) – providing enhanced centreline cues for non‑precision runways with higher traffic volumes.
• Runways with low‑visibility operations (RVR below 800 m) – ensuring pilots have continuous centreline alignment cues.
• Major international airports – where high traffic volumes and adverse weather demand reliable approach guidance.
• Military airfields – requiring all‑weather precision approach capability.

Barrettes are set out on the extended runway centreline on the approach lighting pier or gantry structure, spaced at 30 m intervals from 300 m out to 900 m from the threshold (per ICAO Annex 14 CAT II/III precision approach lighting; FAA ALSF configurations typically extend 2,400–3,000 ft, roughly 730–915 m, depending on terrain), each mounted on its own frangible pole and pier foundation. Where the approach terrain is level, piers stand individually; where the approach crosses low-lying ground or water, piers are raised on a stilted platform or bridge structure to keep the light row at the correct beam elevation. Cabling runs along the pier line back to the approach lighting control cabinet, with each barrette's isolating transformer at its pole base, and pier setout tied to the ILS/runway centreline survey so the row stays true to the extended centreline across the full approach length.

Approach centreline lights function as a continuous visual reference along the approach path:

• Configuration: A row of lights is installed along the approach centreline, typically spaced at 30 m or 60 m intervals, extending from the runway threshold to the outer end of the approach lighting system (e.g., 720 m or 900 m).
• Lighting: The lights are high‑intensity white, steady‑burning or flashing, with intensity controlled by the CCR (5‑step). In some systems, the centreline lights may flash in sequence with other components to create a "running" effect.
• Lateral Guidance: The row of lights provides a clear centreline reference. If the aircraft is off‑centreline, the lights will appear to shift left or right, indicating the need for lateral correction.
• Distance Reference: The spacing of the lights provides distance‑to‑go information. As the aircraft approaches, the lights appear to spread out, helping the pilot judge descent rate and remaining distance.
• Integration: Approach centreline lights are integrated with barrettes, crossbars, side row bars, and sequenced flashing lights to form a complete visual approach system. They are typically the most numerous lights on the approach lighting structure.

Approach centreline lights are not individually addressable — steady-burning intensity is controlled 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 or ATC instruction. 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), and every fixture on that loop, via its own isolating transformer, burns at the same intensity simultaneously. Where a row of centreline lights is layered with a sequenced flashing "running" effect, that timing is a separate control path: the ALCMS or an operator command starts the Flasher Control Unit's sequence, which triggers each Sequenced Flashing Light in turn, independent of the steady-burning current loop beneath it. Fault detection on the steady-burning loop remains loop-based — a voltage rise at constant current signals an open-circuit lamp or failed isolating transformer, reported to the ALCMS as a loop fault rather than pinpointed to an exact fixture without further sectionalised testing.

The fixture is typically a die-cast aluminium housing, anodised or powder-coated for corrosion resistance, mounted on a frangible pole (elevated) or set into an L-850-series base can flush with the approach area grade (inset). The optical assembly consists of a high-intensity lamp or LED array behind a borosilicate glass or polycarbonate lens/prism, gasket-sealed to achieve an IP66/IP67 rating against rain, dust, and de-icing fluid. Elevated units use a frangible coupling engineered to shear cleanly under aircraft impact, mounted atop a base can set in the approach light bridge or stand structure; inset units seat directly in a load-rated base can recessed into the approach area surface. Internal wiring terminates at sealed L-823 primary connectors that mate to the isolating transformer's secondary leads, with the transformer typically housed in its own base can nearby. Stainless-steel fasteners, corrosion-resistant hardware, and a drainage provision in the base can are standard to cope with the exposed, often unpaved, approach area environment.