Approach Crossbar

An Approach Crossbar is a row of high‑intensity lights installed perpendicular to the approach centreline, typically consisting of 3 to 5 lights spaced evenly across the approach path. These crossbars are placed at fixed distances from the runway threshold (e.g., 150 m, 300 m, 450 m, 600 m, etc.) as part of the approach lighting system (ALSF‑1, ALSF‑2, MALSF, Calvert, etc.). They provide pilots with a series of bright visual "bars" that help maintain precise lateral alignment, judge distance‑to‑go, and monitor glide‑slope position during the final approach segment.

Approach crossbars are a fundamental component of both precision and medium‑intensity approach lighting systems. They work in conjunction with centreline lights, barrettes, side row bars, and sequenced flashing lights (SFL) to create a comprehensive visual reference for the pilot, particularly critical during low‑visibility (CAT I/II/III) operations where accurate glide‑path and centreline tracking are essential.

The approach crossbar's purpose is to give a pilot a repeated, unmistakable reference for both lateral alignment and distance-to-go during the final approach, at exactly the phase of flight where precise positional awareness matters most and visual cues are otherwise sparse. Because each crossbar spans the approach path at a known distance from the threshold, a pilot can read their lateral position from whether the bar appears symmetrical, and their progress along the approach from which crossbars are visible and how far apart they appear — a form of guidance that becomes essential, not just helpful, in the low-visibility CAT I/II/III conditions the approach lighting system exists to support.

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

• CAT I precision approach runways (ALSF‑1) – providing essential distance and lateral 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 visual cues for non‑precision runways with higher traffic volumes.
• Calvert I and II systems – a distinctive approach lighting configuration using crossbars as a key visual element.
• Runways with low‑visibility operations (RVR below 800 m) – ensuring pilots have continuous distance and alignment cues.
• Major international airports – where high traffic volumes and adverse weather demand reliable approach guidance.
• Military airfields – requiring all‑weather precision approach capability.

Crossbars are mounted on the approach lighting structures at their designated stations along the extended runway centreline, set perpendicular to the approach path at the widths and distances specified for the particular ALS configuration (ALSF-1, ALSF-2, MALSF, MALSR, or Calvert). Each crossbar sits on its own frangible support at the height called for by the system design, with the array of individual light units on the bar levelled and aimed during commissioning so the crossbar reads correctly from the cockpit. Cabling is run out from the approach light junction boxes along the structure to each unit, and the crossbar's position is tied into the overall approach lighting survey so its distance from the threshold and offset from the runway centreline extension match the design drawing exactly.

Approach crossbars function as a series of visual reference points along the approach path:

• Configuration: Each crossbar is a row of 3‑5 lights, typically spaced 3–5 m apart, installed perpendicular to the approach centreline. Multiple crossbars are placed at specific distances from the threshold (e.g., 150 m, 300 m, 450 m, 600 m, etc.) to provide a regular visual pattern along the approach path.
• Lighting: The lights are high‑intensity white, steady‑burning or flashing, with intensity controlled by the CCR (5‑step). In some systems, crossbars may flash in sequence to create a "running" effect that guides the pilot toward the runway.
• Distance Reference: As the aircraft descends, the pilot sees the crossbars as horizontal lines. The spacing between crossbars provides distance‑to‑go information, helping the pilot judge approach progress and descent rate.
• Lateral Guidance: If the aircraft is off‑centreline, the crossbars will appear asymmetrical, indicating the need for lateral correction. The perpendicular orientation of the crossbars provides a clear reference for maintaining centreline alignment.
• 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.
• Integration: Approach crossbars are integrated with centreline lights, barrettes, side row bars, and sequenced flashing lights (SFL) to form a comprehensive visual approach system. They are typically positioned at regular intervals along the approach lighting structure.

Approach crossbars are not individually addressable — they sit on a dedicated approach lighting series circuit, kept separate from the runway edge and centreline loops so the ALS can be intensity-stepped on its own schedule. An Airfield Lighting Control and Monitoring System (ALCMS), driven by prevailing RVR, ATC instruction, or pilot-controlled lighting at smaller fields, selects the required brightness step and passes it to the approach lighting CCR, which sets the entire loop to that fixed current (typically 5-step, up to 6.6A). Every crossbar station on that loop — along with the barrettes, side rows, and any sequenced flashers sharing the circuit — burns at the identical step simultaneously; there is no mechanism to brighten or dim one crossbar independently of the rest of the approach lighting array. Fault detection is loop-based: the CCR watches loop voltage against the commanded current, and a voltage rise flags an open-circuit condition — a burnt-out lamp, a failed isolating transformer, or a sheared frangible support beneath one of the crossbar's individual units — as a single approach-lighting-loop fault. The ALCMS reports the fault to the loop, not to a specific crossbar station, so a field walk of the structure is what actually locates the failed unit.

Each light in a crossbar is a self-contained high-intensity fixture — an LED array or 45W/65W/100W halogen lamp behind a precision optical prism, housed in a die-cast aluminium body that is anodised or powder-coated against jet fuel, de-icing fluid, and UV exposure. Elevated crossbars mount each fixture on a frangible coupling and pole seated in an L-867 base can, so an individual light shears cleanly off on impact rather than transmitting the load along the bar; flush-mounted crossbars instead seat each fixture in an L-868 base can rated for aircraft wheel loading. What makes a crossbar a single component rather than a set of unrelated fixtures is the shared civil layout: all 3–5 lights are set to a common width and spacing, wired back to individual isolating transformers of matched wattage, and aligned so their beams present as one continuous bar rather than a scatter of independent points. Gasket-sealed top caps and O-ring seals on each fixture give the assembly its IP67/IP68 rating, and sealed L-823 splice connectors carry the series circuit between fixtures and back to the primary cable.