Approach Light Frangible Support

An Approach Light Frangible Support is a critical safety component that mounts elevated approach light fixtures to their foundations while providing a controlled breakaway function on impact. The support typically consists of a frangible coupling, a mounting pole or mast, and a base plate, all designed to safely separate when struck by an aircraft. The frangible coupling is the key element — a precisely engineered weak point that shears or fractures under a specific load, allowing the light fixture and pole to break away cleanly without causing structural damage to the aircraft.

The frangible support is a mandatory requirement for all elevated approach lighting fixtures under ICAO Annex 14 and FAA standards. The design must balance two conflicting requirements: sufficient strength to withstand environmental loads (wind, ice, vibration) and predictable breakaway performance at a specified impact force (typically 2,500–5,000 N shear). Modern frangible supports use advanced materials such as aluminium alloys, composites, or specially machined steel couplings with precise fracture characteristics, ensuring consistent and reliable performance across a wide temperature range.

The approach light frangible support's purpose is to resolve the same safety conflict addressed by a frangible coupling on any elevated AGL fixture, but for approach lighting specifically — structures that sit directly in an aircraft's path during the most critical phase of landing, where an undercarriage strike or a low approach is a realistic possibility. Because the support must be strong enough to survive wind, ice, and routine handling yet fracture cleanly at a tightly controlled load if struck, its engineered breakaway point is what prevents an approach light structure from becoming a fixed obstacle capable of damaging a wing, engine, or landing gear during an excursion. This is why the design is a mandatory ICAO/FAA requirement rather than an optional feature, and why the fracture is engineered to produce no sharp edges or large fragments — protecting the aircraft is the entire point of the component, not a secondary benefit.

Approach light frangible supports are deployed wherever elevated approach lighting fixtures are installed. They are used in:

• ALSF‑1 and ALSF‑2 systems – supporting all elevated centreline, crossbar, barrette, and SFL fixtures for CAT I, II, and III precision approaches.
• MALSF and MALSR systems – providing frangible support for elevated medium‑intensity approach lighting.
• Calvert I and II systems – supporting barrette and crossbar configurations with frangible couplings.
• Runway edge lighting – where elevated runway edge lights are used in snow-prone areas.
• Taxiway edge lighting – where frangible supports are required for elevated taxiway edge lights.
• Obstruction lighting – on masts and structures where frangible breakaway is required.
• Military airbases – where rugged, reliable frangible supports are essential for operational safety.

Frangible supports are installed at each fixture station along the approach lighting structure, footed on a base that anchors the support to the ground or to the main structure frame while allowing the coupling itself to shear away cleanly on impact. Height and positioning follow the design drawing for the specific ALS configuration, since the fixture it carries must sit at the correct elevation relative to the approach light structure and the rest of the array. Cabling is run up through or alongside the support to the fixture above, with enough slack left near the coupling that a frangible break doesn't immediately pull the conductor apart, and the support's base is set out so its failure zone doesn't intrude on any area aircraft might traverse.

The approach light frangible support operates through a carefully engineered breakaway mechanism:

• Frangible Coupling: The core component is a precisely machined coupling that connects the pole or mast to the base. The coupling is designed with a specific shear or fracture strength, typically achieved through a reduced cross‑section, score lines, or a frangible insert material. When an impact force exceeds the design threshold, the coupling fractures cleanly.
• Impact Force Threshold: The breakaway load is carefully specified to be high enough to withstand operational loads (wind, ice, vibration) but low enough to break on aircraft impact without causing significant damage. Typical shear loads range from 2,500 N to 5,000 N, depending on the fixture weight and height.
• Controlled Separation: On impact, the coupling fractures at the predetermined point, allowing the pole and light fixture to separate from the base. The design ensures the separation is clean — no sharp edges or large fragments are produced — minimising the risk of foreign object debris (FOD) or aircraft damage.
• Residual Strength: After the frangible coupling breaks, the remaining base and foundation remain intact and can be reused. The pole and fixture are typically replaced, but the base can be quickly re‑fitted with a new frangible coupling and pole.
• Environmental Resistance: The frangible support is designed to maintain its breakaway characteristics over a wide temperature range (‑40°C to +70°C) and after years of exposure to weather, de‑icing chemicals, and UV radiation. Materials are chosen for corrosion resistance and fatigue endurance.
• Alignment and Adjustment: The support includes adjustment features (e.g., slotted holes, levelling nuts) that allow precise alignment of the light fixture during installation, ensuring the light beam is correctly directed towards the approach path.
• Grounding: The frangible support typically includes a grounding terminal or connection point to ensure the light fixture is properly earthed, providing lightning protection and electrical safety.

The frangible support has no electronics of its own, but its mechanical failure mode is deliberately designed to plug into the same loop-level control logic that governs every other fixture on the approach lighting circuit. When the coupling shears, the light fixture it carries separates from its cable connection (or the cable itself parts at a designed weak point), which the CCR sees as an open circuit on its loop — an identical voltage-rise signature to a burnt-out lamp or a failed isolating transformer. The ALCMS therefore reports a sheared support the same way it reports any other loop fault: as a fault on that circuit, not as a structural event at a named location. This is an intentional consequence of keeping the electrical and mechanical failure modes aligned — a broken support never leaves a "live" stub of cable presenting a shock or arcing hazard, and it never fails silently, since any separation immediately registers as a control-system-visible fault. Confirming that the cause was impact rather than an electrical failure still requires a field inspection of the structure itself.

The assembly comprises three parts working together: a base plate anchored to a concrete foundation or embedded base can, a short mounting pole or mast typically 0.3–1.0 m tall (up to 3 m for sequenced flashing light towers), and the frangible coupling itself — the engineered weak point machined from aluminium alloy (6061-T6 or 7075-T6), ductile iron, or composite material to shear at a precise 2,500–5,000 N shear load. The coupling's fracture geometry is what distinguishes it from an ordinary joint: it is machined to break cleanly with no sharp edges or fragments that could cause foreign object damage, while still carrying a 200–500 N·m bending moment under normal wind and ice loading without prematurely failing. Anodising, hot-dip galvanising, or a polymer coating protects the base material from corrosion over a service life rated beyond 100,000 fatigue cycles, and a grounding terminal on the base plate ties the structure into the airfield earthing system. The top of the pole carries a standard flange or bracket sized to accept the 45W–200W light fixture it supports.