Different mounting arrangements
Can, shallow base, stem or legs: four ways to hold a light or sign at the right height and position, yet let a raised one give way if an aircraft strikes it.
A light or sign has to be held at the right height and position, strong enough for wind and jet blast, and carried on something that stays put below ground. Where it stands above the surface, one point is made to fail first: a frangible coupling, so the upper part breaks away if an aircraft strikes it and the base or foundation stays in the ground.
The arrangement follows the equipment. An elevated light sits on a coupling above a base can. An inset light bolts to a shallow base in the pavement and needs no break point. An approach light rides a stem on a foundation, aimed toward the approach. A sign stands on legs fixed to a concrete foundation. In every case the supply cable is arranged so it does not hold the equipment back, and the bottom of any can is kept free-draining.
Base can, stem and sign legs. Each has a frangible coupling that breaks first, above a fixed base or foundation.
Shallow inset base. Not a raised object, so no break point, but the base must carry wheel loads and be re-set after resurfacing.
| Aspect | Base can | Shallow base | Stem mount | Sign legs |
|---|---|---|---|---|
| Used for | Elevated edge, threshold and end lights | Inset centreline, touchdown zone and in-pavement lights | Elevated approach lights | Airfield signs |
| Below ground | Can with top flange and spacer rings; a deep can may hold the transformer | Shallow base cut into the pavement structure | Foundation with a base plate bolted down | Concrete foundation with anchor bolts |
| Above ground | Coupling, short riser and low lamp head | Thin top with the optical window only | Coupling, stem or mast, lamp housing with aiming adjustment | Base plate, legs, coupling and sign housing |
| Break point | Frangible coupling at grade | None: flush, not a raised object | Frangible coupling; whole structure frangible in the inner 300 m | Frangible coupling low on the leg |
| Height set by | Spacer rings on the can flange | Adjusted at the base; re-set after resurfacing | Structure height, to sit in the approach plane | Design height, with the legend facing the pilot squarely |
| Power | Secondary lead from the transformer below | Secondary lead; transformer usually in a nearby housing | Short lead from the transformer can below ground | Cable through the foundation; parts at a separable connector |
| Watch points | Keep the can drained; keep the correct coupling as built | Height after resurfacing; sealed joint at the flange | Aiming re-checked after maintenance; keep mass low so the stem stays frangible | Tilt, loose or corroded fasteners, cracked coupling or foundation; bonding |
What to check on a mounting
Break point as built
Use the manufacturer’s coupling and fasteners. Never add bracing, a stronger bolt or a second fixing across the break point.
Drainage
Keep the bottom of every can free-draining. Water around the transformer and connectors damages insulation.
Fasteners and torque
Check bolts and anchors for looseness and corrosion, and tighten to the manufacturer’s values.
Height and alignment
Check the fixture or sign height and aim. Re-check inset height after resurfacing, and aiming after maintenance.
Cable and connector
Make sure the lead has enough slack and parts as designed, so it does not hold a broken mount back.
After a strike
Replace the coupling, then check the foundation, anchors, cable and connector before re-aligning and re-energising.
Frequently Asked Questions
Q1What mounting arrangements are used for airfield lights and signs?
Four common ones: an elevated light on a frangible coupling above a base can, an inset light on a shallow base set in the pavement, an elevated approach light on a stem or mast bolted to a foundation, and a sign on legs fixed to a concrete foundation. Details vary by product and standard.
Q2Why do raised fixtures and signs use a frangible coupling?
An aircraft that leaves the pavement or lands short can strike a raised object. The mounting is strong enough for wind and jet blast, but one point is made to fail first, so the upper part breaks away and the aircraft meets as little resistance as possible. The base and foundation stay in the ground.
Q3What is the difference between a deep can and a shallow can?
A deep base can often holds the isolation transformer directly beneath the fixture. A shallow can carries only the fixture, with the transformer in a separate housing or pit nearby. In every case the bottom is kept free-draining, because water damages insulation and connectors.
Q4How is the height of an inset fixture set?
The fixture bolts to a flange on a shallow base, and the height is adjusted at the base, typically with spacer rings. The top face must be level with the surface or only marginally above it, within the standard's projection limit. It is re-checked after resurfacing.
Q5What must never be done to a frangible mounting?
Never add bracing, a stronger bolt or a second fixing across the break point, and never swap the coupling for an ordinary fastener. That can stop it breaking as designed. Use the manufacturer's coupling and fasteners, tightened as instructed.
Q6How are approach light structures treated?
Approach lights are placed as nearly as practicable in a horizontal plane through the threshold, and each elevated light and its support must be frangible. In the inner 300 m the whole structure is frangible; beyond that, only the top 12 m need be where a structure is taller than 12 m (ICAO Annex 14, 5.3.4).
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