Approach Light Structure

An Approach Light Structure is the physical support system that elevates and positions approach lighting fixtures (such as centreline lights, barrettes, crossbars, and sequenced flashing lights) at the correct height and orientation above the ground or pavement. These structures include frangible poles, masts, towers, base plates, and mounting hardware, and are designed to meet stringent safety and performance requirements. The primary function of the structure is to hold the light fixture at the required height (typically 0.3–1.0 m above ground for elevated lights) and at the precise angle to project the light beam towards approaching aircraft.

A critical design feature of approach light structures is frangibility — the ability to break away cleanly on impact with an aircraft, minimising damage to the aircraft and preventing catastrophic incidents. Structures are typically made from lightweight but strong materials such as aluminium, composite materials, or high‑strength steel, with frangible couplings designed to shear under a specified load. The structure must also withstand environmental loads including wind, ice, snow, and thermal expansion, while maintaining precise alignment of the light fixture.

The approach light structure's purpose is to hold each fixture at the exact height, angle, and position the approach lighting design calls for — since the whole system's value to a pilot depends on the crossbars, barrettes, and centreline lights forming a precise, repeatable visual pattern that indicates distance, alignment, and glide path. If a structure allowed a fixture to sag, tilt, or shift over time, the visual pattern the pilot relies on would become distorted exactly where accuracy matters most, on final approach in low visibility. Its frangible design serves a second, equally critical purpose alongside this positioning role: ensuring that the same structure holding the fixture precisely in place under normal conditions will still break away safely if an aircraft strikes it, so structural rigidity during normal operation and safe failure on impact are both designed into the same component rather than being in tension with each other.

Approach light structures are deployed in approach lighting systems wherever elevated fixtures are required. They are used in:

• ALSF‑1 and ALSF‑2 systems – supporting centreline, crossbar, barrette, and SFL fixtures for CAT I, II, and III precision approaches.
• MALSF and MALSR systems – providing elevated support for medium‑intensity approach lighting.
• Calvert I and II systems – supporting the characteristic barrette and crossbar configurations.
• Runways with snow or uneven terrain – where elevated structures are preferred over inset fixtures.
• Major international airports – requiring robust, reliable support for critical approach lighting.
• Military airbases – where rugged and easily maintainable structures are essential.
• Temporary or expeditionary airfields – where lightweight, modular structures are used for rapid deployment.

Approach light structures are erected at each fixture station along the extended runway centreline, set out from the approach lighting survey so distance from the threshold, lateral offset, and height all match the design for the specific ALS configuration. Foundations or ground anchors are sized for the structure's height and the wind loading it will see, and the structure itself sits within the terrain the approach lighting system crosses — which may include roads, wetlands, or uneven ground beyond the paved airfield — so access for construction and later maintenance is planned alongside the lighting layout itself. Each structure carries its crossbar, barrette, or SFL unit on frangible supports at the top, with cabling routed up from an underground or nearby junction box, and structures are spaced and aligned so the completed array reads as a coherent line of guidance from the approach.

Approach light structures operate by providing a stable, accurately positioned mounting platform for light fixtures, while incorporating safety features:

• Support and Elevation: The structure elevates the light fixture to the required height above ground (typically 0.3–1.0 m) to ensure the light beam clears snow, grass, or pavement irregularities, and is correctly oriented towards the approach path.
• Frangible Coupling: A breakable coupling is incorporated at the base or at a point near the ground. This coupling is designed to shear when subjected to a specific impact force (e.g., from an aircraft wing or landing gear), allowing the structure to break away cleanly and minimise damage to the aircraft.
• Alignment and Orientation: The structure includes adjustment mechanisms (e.g., slotted holes, levelling screws) that allow precise alignment of the light fixture in both horizontal and vertical planes. This ensures the light beam is directed accurately towards the approach path.
• Load Resistance: The structure is designed to withstand environmental loads including wind (up to 150 mph or more), ice accumulation, snow, and thermal expansion. The materials and cross‑section are chosen to provide adequate strength while remaining light enough to break away on impact.
• Cable Management: The structure often includes integrated cable ducts or clips to route the electrical cables from the ground to the light fixture, protecting them from environmental damage and maintaining a tidy installation.
• Corrosion Protection: Structures are typically galvanised, anodised, or coated with corrosion‑resistant finishes to withstand the harsh outdoor environment, including de‑icing chemicals and salt spray.
• Grounding and Lightning Protection: The structure provides a path for lightning current to safely discharge to earth, with grounding terminals and connections to the airfield earth network.

The structure itself carries no control logic — it is the mechanical means by which a fixture's designed photometric output is actually delivered at the correct height, aim, and offset the CCR's fixed current step assumes. Because loop-level control sets every fixture on a circuit to the same commanded intensity, the control system has no way to compensate for a fixture that is misaligned or mounted at the wrong height; the structure's levelling and adjustment features are what make the assumption of uniform performance across a loop valid in practice. Its frangible coupling is the deliberate exception to "stable" — it is engineered to be the electrical loop's designated failure point as well as the structure's, so that an aircraft strike registers as a single, predictable open-circuit event on the CCR rather than as unpredictable damage propagating into the cable or transformer. In that sense the structure defines where, mechanically, a fault is allowed to happen, while the CCR and ALCMS define how that fault is reported once it does.

The structure is built from aluminium alloy, galvanised steel, or composite fibreglass sections rising 0.3–1.0 m above grade (up to 3 m for sequenced flashing light towers), anchored to a base plate on a concrete foundation or set into a base can embedded in the pavement. Hot-dip galvanising, anodising, or a polymer coating protects the base material against corrosion, and the structure is engineered to hold its alignment under wind loads up to 150 mph gust and up to 13 mm of radial ice without deflecting the fixture off its aim point. The frangible coupling — the section's designed weak point — shears at a specified 2,500–5,000 N load, letting the pole and fixture break away cleanly on impact rather than transmitting force into the foundation. Cable enters through an integrated gland or conduit sized for the 6.6A series circuit, a grounding terminal ties the structure into the site earthing system, and a standard flange or bracket at the top accepts the 100–200W fixture the structure is built to support.