Centreline Barrette

A Centreline Barrette is a short row of lights installed on the approach centreline, typically consisting of 3 to 5 lights spaced 1–2 metres apart, forming a “barrette” (short bar). These barrettes are placed at fixed distances from the runway threshold (e.g., 500 ft, 1,000 ft, 1,500 ft, etc.) as part of the approach lighting system. They provide a series of bright, white light points that help pilots maintain precise lateral alignment and judge distance to the runway during final approach.

Centreline barrettes are a standard component of ALSF‑1 and ALSF‑2 configurations, as well as other precision approach lighting systems. They work in conjunction with crossbar lights, side row bars, and the decision bar to create a comprehensive visual reference for the pilot, particularly in low visibility (CAT I/II/III) operations.

The centreline barrette exists to give a pilot a series of discrete, evenly spaced fix points along the approach path, rather than a single continuous line, which matters because discrete points at known distances let the pilot judge both lateral drift and remaining distance to the threshold at a glance. If a barrette appears shifted relative to the pilot's expected sightline, that's an immediate, intuitive cue that the aircraft has drifted off centreline, correction that would be much harder to detect from a continuous light line alone. Its purpose sharpens under CAT I/II/III conditions specifically, where visibility may only allow the pilot to see one or two barrettes ahead at a time, making the fixed, known spacing between barrettes a critical distance reference in addition to a lateral one.

Centreline barrettes are deployed on 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.
• Runways with low visibility operations (RVR below 800 m) – ensuring pilots have continuous centreline 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 that carries the full ALSF configuration, spaced at 30 m (100 ft) intervals extending from near the threshold out to 731 m (2,400 ft) — or 914 m (3,000 ft) where the approach glide slope is below 2.75°. Where the approach terrain is level, each barrette mounts on its own frangible pole set into an individual pier foundation; where the approach crosses low-lying ground, water, or a road, the piers are raised on a stilted platform or bridge structure to keep the barrette at the correct beam elevation above grade. Cabling runs along the pier line in duct or cable tray back to the approach lighting control cabinet, with each barrette's isolating transformer housed at the base of its pole. Pole height and pier spacing are set out from the ILS/PAPI survey so the barrette row stays true to the extended runway centreline across the full approach length.

Centreline barrettes function as a series of visual reference points along the approach path:

• Configuration: Each barrette is a short row of 3‑5 lights, typically spaced 1–2 m apart, installed perpendicular to the centreline. Multiple barrettes are placed at specific distances from the threshold (e.g., 500 ft, 1,000 ft, 1,500 ft, 2,000 ft, etc.) to provide a regular visual pattern.
• Lighting: The lights are high‑intensity white, steady‑burning or flashing, with intensity controlled by the CCR (5‑step). In some systems, barrettes may flash in sequence to create a “running” effect that guides the pilot toward the runway.
• Guidance: As the aircraft descends, the pilot sees the barrettes as short horizontal lines. If the aircraft is off‑centreline, the barrettes will appear asymmetrical, indicating the need for lateral correction. The spacing between barrettes also provides distance‑to‑go information, helping the pilot judge the approach progress.
• Integration: Centreline barrettes are integrated with crossbar lights (perpendicular bars), side row bars, and the decision bar to form a complete visual approach system. They are typically the most numerous lights on the approach lighting structure.

Centreline barrettes are not individually addressable — control is exercised at the loop level through the approach lighting series circuit, following the same constant-current principle as runway and taxiway lighting. 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 approach lighting CCR, which regulates the entire barrette loop to the corresponding fixed current (5-step). Every barrette on that loop, via its own isolating transformer at the base of its pole, draws from the same current and therefore burns at the same intensity simultaneously — there is no way to dim or brighten one barrette independently of the row. Where a system uses sequenced flashing to create a "running" effect toward the threshold, that timing is layered on by a separate approach lighting sequencer/flasher controller and is distinct from the steady-burning current-step control described here. Fault detection remains 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. Because barrettes are the most numerous fixture on the approach lighting pier line, a single lamp failure has minimal visual effect on the overall pattern, but the fault is still flagged for the affected loop as a whole pending sectionalised testing to locate the specific pole.

Every barrette is electrically identical — 3–5 lights, 1–2m apart, on the same series circuit — and the pattern's only job is repeating at a fixed interval so distance-to-go reads consistently. The actual guidance cue comes from the pilot's eye, not the wiring: a barrette centred on the extended centreline reads as symmetric and aligned, while one that's shifted off it signals drift and calls for lateral correction until it re-centres.

Each barrette is built as a short crossarm carrying 3 to 5 sealed-beam or LED lamp heads, mounted at a fixed height atop a frangible pole set into its own pier foundation. Individual lamp heads use a die-cast aluminium housing with a toughened-glass or polycarbonate lens, gasket-sealed for an IP65/IP66 rating against rain and windblown debris at the exposed approach-lighting site. The pole itself incorporates a frangible coupling near its base, engineered to shear on vehicle or aircraft impact without bringing down the adjacent piers in the approach lighting structure. Where piers sit on raised platforms over water or low ground, the crossarm and lamp heads are the same construction, only the supporting structure changes to a stilted or bridge-mounted frame. Cabling from each lamp head runs down the pole to an isolating transformer housed at the pole base, then out along the pier line in duct or cable tray, with stainless-steel fasteners and weatherproof junction boxes used throughout to withstand prolonged outdoor exposure.