What is frangible mounting in approach lighting?
How an elevated approach light is held up so that it breaks away at a planned point if an aircraft lands short and strikes it — leaving the foundation, duct and cable in the ground.
Frangible mounting holds an elevated approach light strong enough for wind and service, but built to break away at a planned weak point — the frangible coupling just above the foundation — if an aircraft strikes it. Approach lights stand in the path of an aircraft that lands short, so the light should give way and leave a short, low stump.
The rule is wider in the inner 300 m, where the whole structure is frangible. Beyond 300 m, where a support is taller than 12 m, only the top 12 m must be frangible. Below the break everything is built to stay: foundation, anchor bolts, base plate, duct and cable. The secondary lead is left slack with a separable connector, so the head can leave without tearing the lead out.
Frequently Asked Questions
Q1What is frangible mounting in approach lighting?
Frangible mounting is how an elevated approach light is held up so that it gives way if an aircraft strikes it. The light and its stem are strong enough for wind and service, but they separate at a planned weak point, the frangible coupling, which sits just above the foundation. Only a short, low stump is left behind.
Q2Why must approach lights be frangible?
Approach lights stand in the path of an aircraft that lands short of the runway. If a light or its support is rigid, it can damage the aircraft or its crew. A frangible mounting yields on impact, so the aircraft meets as little resistance as possible. ICAO Annex 14 Volume I, Chapter 5, asks for elevated approach lights and their supports to be frangible.
Q3Does the whole structure have to be frangible?
In the inner 300 m from the threshold, yes, the whole structure. Beyond 300 m the rule is relaxed for tall supports: where a structure is higher than 12 m, only the top 12 m must be frangible, and where it is surrounded by non-frangible objects, only the part that rises above them. The mass of the light and its support is kept as low as possible.
Q4What stays in the ground after the break?
The foundation, anchor bolts, base plate and duct stay put, and so do the cable and the transformer. The secondary lead is left slack, with a separable connector, so the head can leave without tearing the lead out or damaging the base. Below the break the unit is built to stay.
Q5What can stop a mounting from being frangible?
Anything that bridges or stiffens the coupling: a collar of grout or concrete, a foundation top set too high, built-up soil against the stem, rigid conduit or an earth rod beside the unit, extra bracing, a wrong or home-made replacement part, or a coupling over-tightened beyond the specified torque. Keep the unit as the manufacturer made it.
Q6What is done after a light has been struck?
Treat it as damaged. Under the permit-to-work, make the circuit safe, replace the broken parts with the correct manufacturer parts, refit to the specified torque, and check the lead, connector and base. Then re-aim the light, test it and record the work.
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