Streamlined wire breaks in flight. Text of Mandatory Occurence Report to UK CAA.
“On 22nd September 2002 at 1435hrs, Stampe SV4c no.665 G-FORC had just departed Old Buckenham for Little Gransden with 2 on board. Heading South at 1000ft to avoid the Honington/Mildenhall MATZ, a jolt was felt and the aircraft skewed slightly. The pilot noticed that the port front landing streamline wire was broken, and the broken ends were trailing in the slipstream. A field was selected for an emergency landing, and power was reduced. In the descent to around 200ft, controls were tested, and the airframe found to be rigid , with streamline flying wires intact. A course was set at low altitude direct to Little Gransden, some 30 miles WSW, where the aircraft landed safely.”
The wire, p.n.BR48314, was part of a batch of 6 sets produced by Brunton Aero Products in February 1999, released under JAA Form 1 ref: A1813 . There were less than 50 flying hours on the wires fitted to G-FORC. The initial report from an independant test house suggested that “chlorides”caused corrosion pits , which act as a local stress concentration. The fracture itself was due to fatigue.
The complete set of wires were subsequently removed from G-FORC to Bruntons for examination. A further six of the wires from this set were then scrapped due to corrosion pitting.
A stone chip at the leading edge is another possible cause of a fracture. Those with wires manufactured from S80 should clean them and look for nicks which form a stress point from which a crack might develope. If a nick is found, it may be dressed with a stone, under guidance from or by a licensed engineer.The wires should then be cleaned and lightly oiled after every flight.
Another very possible cause of cracking could originate from the poor tooling used in the rolling process, i.e. the worn rollers which cold work the material into the streamline shape. Look down the length of a wire at the streamline section to see the many abrupt changes of direction at which stress concentrations are inevitably set up.
Since the above incident, other cases of breaking of Brunton S80 wires have been reported. Brunton has stopped producing S80 wires and now use 316 spec. Below is a photo taken in 1999 of Brunton’s rolling machine. Bar is fed into the contra rotating rollers to be cold rolled. The operator’s head is simultaneously drawn into the gears and occasionally jams the machine.