What is a frangible mounting?
The planned weak point in a sign’s mounting. In wind it holds firm. If an aircraft hits the sign, it gives way first, so the sign breaks off and the aircraft takes less damage.
An aircraft that leaves the pavement can hit a sign. A rigid sign would damage the aircraft, so Annex 14 asks for signs to be frangible: built to break, distort or yield on impact, so they present the least hazard to an aircraft.
1A weak point, on purpose
The mounting is strong enough for wind and jet blast, but one point is made to fail first. That is usually a frangible coupling, or a weakened section of leg, set low, just above the base. Below it, the base plate and foundation stay in the ground. Above it, the legs and housing separate and fall clear.
2The cable must let go too
If the supply lead stayed rigid, it would hold the sign back or tear out of the ground. So the lead is arranged to part, commonly through a separable connector, and the lead is left with enough slack. The connector has its own entry later.
3Keep it frangible
The coupling only works as designed if it is left as the maker built it. Use the manufacturer’s coupling and fasteners, tightened as instructed, and never add bracing, a stronger bolt or a second fixing across the break point. Any earth bond must be a slack, flexible lead that does not bridge or restrain the coupling. After a strike, replace the coupling, check the foundation, anchors, legs, cable and connector, then realign the sign.
| Part | What happens in a strike |
|---|---|
| Frangible coupling | Parts at a set load, low on the leg |
| Upper legs and housing | Separate and fall clear |
| Lower stubs, base plate | Stay fixed to the foundation |
| Foundation | Stays in the ground, ready for a new sign |
| Separable connector | Lets the supply lead part without damage |
| Fasteners | Maker's type and torque only |
| Standard | Annex 14: signs must be frangible |
Frequently Asked Questions
Q1What does frangible mean for an airfield sign?
It means the sign is designed to break, distort or yield when an aircraft hits it, so it does as little harm to the aircraft as possible. Annex 14 asks for signs to be frangible.
Q2Where does a frangible mounting break?
At a coupling or weakened section placed low on the legs, just above the base. The housing and upper legs separate, and the base plate and foundation stay in the ground.
Q3Can I use an ordinary bolt or add bracing to a sign?
No. A stronger fastener or extra bracing can stop the sign breaking away. Use only the manufacturer's coupling and fasteners, tightened as they instruct.
Q4What should be done after a sign has been struck?
Replace the broken coupling with the manufacturer's part, check the foundation, anchors, legs and housing for damage, check the cable and connector, then realign the sign and tighten the fixings as instructed.
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