Crossbar Lights

Crossbar Lights are a series of white lights arranged in a row perpendicular to the runway centreline, positioned at specific fixed distances from the runway threshold along the approach lighting system. A crossbar is distinct from a centreline barrette: barrettes are the narrow, closely‑spaced light rows repeated every 30 m (100 ft) along the full centreline, while a crossbar is a wider bar occurring at only one or two defined stations in the layout (for example the 150 m and 300 m crossbars of the ICAO/UK Calvert system, or the 500 ft and 1,000 ft "decision"/"roll" bars of the US ALSF‑2 system) — the terms are sometimes used loosely in the field, but they refer to different fixtures within the same approach lighting layout.

These lights are a defining feature of precision approach lighting systems such as ALSF‑1, ALSF‑2, and the Calvert System, and are also used in medium‑intensity systems like MALS and MALSF. The crossbars provide critical distance and lateral guidance, helping pilots maintain the correct glide path and align with the runway centreline, particularly in low visibility conditions where external visual references are limited.

The crossbar exists to give a pilot a visual width reference at fixed points along the approach, since a single point of light conveys distance but not scale, while a short perpendicular row conveys both at once — the pilot can judge range partly from how the bar's apparent width changes as the aircraft closes on the threshold. Positioned at known, fixed distances from the threshold, the crossbars effectively function as visual milestones along the approach, letting the pilot cross-check descent progress and glide path against distance in a way a continuous centreline alone cannot provide as distinctly. This combination of distance and lateral cues is why crossbars are a defining, recurring feature across precision and medium-intensity systems alike, from full ALSF configurations down to simpler medium-intensity setups.

Crossbar lights are installed as part of approach lighting systems at airports that require enhanced visual guidance. They are typically used in:

• ALSF‑1 and ALSF‑2 systems (precision approach runways with CAT I, II, III capability)
• Calvert I and II systems (British approach lighting configuration)
• MALS and MALSF systems (medium‑intensity approach lighting with crossbars for distance reference)
• Precision approach runways requiring enhanced depth perception and glide‑slope monitoring
• Major international airports with CAT II/III operations
• Military airbases requiring all‑weather approach capability

Crossbar assemblies are erected on a dedicated row of piers set perpendicular to the extended runway centreline, at the 300 m bar (CAT I) and/or the 150 m bar position within the approach lighting layout. Each crossbar spans several frangible poles set at the correct lateral spacing either side of the centreline, all piers tied to a common foundation line so the bar reads as a single continuous span to the approaching pilot. On approaches over water, marsh, or uneven terrain the crossbar piers are mounted on a raised platform or bridge structure matching the adjacent centreline piers. Cabling for the full crossbar is looped from pole to pole and back to the approach lighting control cabinet, with isolating transformers at each pole base; pier setout is surveyed off the runway centreline extension to keep the bar square to the approach path.

Crossbar lights provide essential visual guidance through their distinctive arrangement and placement:

• Distance Reference: Crossbars are placed at specific fixed distances from the threshold — 150 m and 300 m from threshold in the ICAO/UK Calvert system, or the 500 ft and 1,000 ft ("decision"/"roll") bars in the US ALSF‑2 system. As the pilot passes each crossbar, they gain a clear sense of their distance from the runway.
• Lateral Guidance: The row of lights across the approach path helps the pilot maintain lateral alignment with the runway centreline — if the crossbar appears centred in the cockpit view, the aircraft is on the correct lateral path.
• Depth Perception: The progression of crossbars at different distances creates a "ladder" or "ruler" effect that enhances depth perception and helps pilots judge their height and closure rate.
• Glide‑Slope Monitoring: By observing the spacing and position of crossbars relative to the aircraft, pilots can assess whether they are on the correct glide slope — appearing too close together may indicate being too high or too low.
• Steady‑Burning: Crossbar lights are typically steady‑burning (white) to provide a stable reference against the dynamic flashing lights (SFL) in the system.
• Intensity Control: The lights are controlled by CCRs with multiple intensity steps (typically 5‑step), allowing ATC to adjust brightness for visibility conditions.

Crossbar lights are not individually addressable — control is exercised at the loop level through the approach lighting 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 approach lighting CCR, which regulates the entire crossbar loop to the corresponding fixed current (5-step). Every light across the crossbar span, via its own isolating transformer at the base of its pole, draws from the same current and therefore burns at the same intensity simultaneously — the whole bar brightens or dims as one unit, and there is no way to intensify one end of the crossbar relative to the other. Because a crossbar spans several poles wired into a single continuous loop, the fixture's role as a lateral reference depends on every lamp along that span staying lit together; the loop-level design that keeps intensity uniform is the same mechanism that determines how a fault presents. 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. A single lamp outage within a long crossbar span can quietly shorten the perceived bar without tripping an obvious visual gap, so field verification of full-span continuity is part of routine crossbar inspection.

A crossbar spans several individual lamp heads mounted across multiple frangible poles, each head built the same way as a barrette unit: a die-cast aluminium housing with a toughened-glass or polycarbonate lens, gasket-sealed for an IP65/IP66 rating against the weather exposure typical of an open approach lighting site. Every pole in the bar shares a common foundation line and elevation setting so the assembly reads as one continuous span rather than a series of separate lights, and each pole incorporates a frangible coupling near its base to shear cleanly if struck. On approaches over water, marsh, or uneven terrain, the same lamp-head and pole hardware is instead carried on a raised platform or bridge structure tied to the adjacent centreline piers. Cabling loops from pole to pole along the bar and back to the approach lighting control cabinet, with an isolating transformer housed at each pole base, and stainless-steel fasteners and sealed junction boxes used throughout for long-term outdoor durability.