Frangible Pole

A Frangible Pole (also known as a breakaway pole, shear pole, or frangible mast) is a specially engineered support structure designed to hold elevated AGL light fixtures, signs, or other airfield equipment at the required height above the pavement, while incorporating a deliberate weak point that causes the pole to break away cleanly upon impact by an aircraft. This frangible feature is a critical safety requirement specified by ICAO Annex 14 and FAA standards, aimed at minimising damage to the aircraft and protecting lives in the event of a runway or taxiway excursion.

Key characteristics of frangible poles include:

• Controlled Breakaway: The pole is designed with a frangible coupling or a weakened section that fractures in a predictable manner when subjected to a defined impact force, typically at or near the base.
• Structural Integrity: Under normal operating conditions (wind, vibration, fixture weight), the pole behaves as a rigid, stable structure, supporting the fixture securely.
• Corrosion Protection: Hot‑dip galvanised or stainless steel construction ensures long‑term durability in the harsh airfield environment.
• Fixture Compatibility: Designed to accommodate standard AGL fixtures and mounting hardware, with appropriate bolt patterns and cable routing.

Frangible poles are used in the following applications:

• Elevated Runway Edge Lights: Supporting runway edge light fixtures, especially on curves and intersections.
• Elevated Taxiway Edge Lights: Supporting taxiway edge lights where flush mounting is not feasible.
• Approach Lighting: Supporting elevated approach light fixtures and barrettes.
• Obstruction Lighting: Supporting obstruction lights on poles within the airfield.
• Airfield Signage: Supporting elevated mandatory instruction signs and direction signs.

The frangible pole is typically mounted on a base plate or base can, and is designed to break at a specific height above the pavement (usually 25‑50mm) when subjected to a lateral impact force of a defined magnitude, ensuring that the aircraft sustains minimal damage.

Certain fixtures — approach lights, elevated edge lights, obstruction lights — only work if they are raised above the pavement or surrounding terrain, but any object raised into an aircraft's operating envelope is, by definition, a potential obstacle. The frangible pole's purpose is to let a fixture keep the height it needs for correct photometric coverage while removing the danger that height would otherwise create. Under normal conditions the pole must behave as an ordinary rigid structure, holding the fixture steady against wind and vibration so its beam stays correctly aimed; the moment it is struck by an aircraft, that same structure is designed to fail at a known, predictable point and load, breaking away cleanly rather than absorbing the impact rigidly. This is what allows AGL to place fixtures in positions — close to runway edges, on approach paths, at low elevations near the aircraft's flight path — that would otherwise be unacceptable safety risks.

Frangible poles are installed in areas where elevated AGL fixtures are required and where aircraft may potentially impact them, including:

• Runway Edge Lighting: Supporting elevated runway edge light fixtures on the runway strip, especially on curves, runway intersections, and where flush mounting is not possible due to snow or terrain.
• Taxiway Edge Lighting: Supporting elevated taxiway edge lights, particularly on curves, at intersections, and on taxiway shoulders.
• Approach Lighting Systems: Supporting elevated approach light fixtures, barrettes, and flashing light units in the approach area.
• Obstruction Lighting: Supporting obstruction lights on poles located within the runway strip, taxiway strip, or approach areas.
• Airfield Signage: Supporting elevated mandatory instruction signs, direction signs, and information signs that are located within aircraft movement areas.
• Sensor Mounting: Supporting anemometers, wind cones, and other instrumentation in aircraft movement areas.

The poles are designed to meet the frangibility requirements specified in ICAO Annex 14, which mandates that any fixture or structure located within the runway strip or taxiway strip must break away or yield upon impact to prevent aircraft damage. The poles are available in various heights (typically 300mm to 1500mm) and diameters to accommodate different fixture types and operational requirements.

The pole's frangible coupling seats on a foundation set flush with or just below grade — typically the same base can (L-867 type) that carries the fixture's electrical connection — so the break point sits at the designed low-fracture-force height rather than at ground level where it would be less predictable. Foundation position is set out from the runway/taxiway centreline and edge survey, kept within the required setback from the pavement edge while staying clear of the graded strip's load-bearing width, and the pole's fracture height is checked against the obstacle-limitation surface for that runway category so the fitting on top stays within the allowable envelope. Cabling from the base can up to the fixture runs inside the pole itself, sized with enough slack at the coupling to allow reconnection after a frangible break without needing to re-run the underground cable.

The frangible pole functions as a support structure that provides the required height for the fixture while incorporating a controlled failure mechanism that activates upon impact:

• Normal Operation: Under normal conditions, the pole acts as a rigid, stable column, supporting the fixture's weight and resisting wind loads, vibration, and thermal expansion. The frangible coupling or weakened section is designed to withstand all expected operational loads without failure.
• Impact Activation: When an aircraft contacts the pole (typically during a runway excursion or taxiway incursion), the impact force is transmitted through the pole to the frangible section. The frangible section is designed to fracture at a specific force threshold, typically at the base of the pole (25‑50mm above the mounting plate).
• Clean Breakaway: The pole shears cleanly at the frangible point, allowing the fixture and the upper portion of the pole to separate from the base. The design ensures that the pole does not become a projectile or cause additional damage to the aircraft.
• Energy Absorption: The frangible coupling is often designed with a shear‑type mechanism that absorbs some of the impact energy, further reducing the force transmitted to the aircraft.

The frangible coupling is typically a precision‑machined component that incorporates shear pins, a reduced‑diameter section, or a notched design that ensures a predictable fracture. The coupling is designed to meet specific frangibility criteria:

• Shear Force Threshold: The force required to break the pole is typically set between 2 kN and 10 kN (depending on the pole size and fixture type), ensuring that the pole will break before causing significant aircraft damage.
• Breakaway Height: The pole is designed to break at a height of 25‑50mm above the pavement, leaving a low‑profile base that does not pose a hazard to aircraft.
• Directional Performance: The frangible coupling is designed to break regardless of the direction of impact, providing protection from any angle.

The pole is typically installed on a base plate or base can, with the frangible coupling positioned between the base and the pole shaft. The fixture is mounted on top of the pole, with the electrical cable routed through the hollow pole or along its exterior.

The frangible pole carries no electrical or control function — it is a support structure with a deliberately engineered breakaway point, positioned between the foundation and whatever it elevates (an approach light, a sign, or other equipment requiring height). Under normal wind and service loads it behaves as a rigid structural member; under an impact load above a defined threshold, it is designed to fail cleanly at the engineered point rather than transmitting the impact energy into the foundation or into whatever struck it. As with the breakable coupling used on lower fixtures, everything below the frangible point — the base and foundation — is sized so it is never the weak point during that failure event.

The pole is a hollow tubular section of structural steel or stainless steel, hot-dip galvanised for corrosion protection, sized in diameter and wall thickness to resist the specified wind loading while still yielding predictably under aircraft impact. Frangibility is built in near the base, either through a shear-pin coupling, a deliberately reduced-diameter neck, or a notched circumferential groove, engineered so the pole shears cleanly at the design threshold rather than bending or embedding fragments, minimising secondary hazard to an errant aircraft. A flat top plate with a standard AGL bolt pattern is welded or bolted to the pole's upper end to accept the light fixture, while the base is designed to mount into a below-grade footing or base can via a matching bolt circle. The galvanised or stainless finish is applied after all machining and welding to ensure full corrosion protection at the frangible section, since any localised weakening there would compromise the calibrated shear threshold.