What happened
On 13 September 2008, a Pegasus Quantum 15-912, registration G-BZMI, was conducting a training flight approximately 7 miles east of Sandy, Bedfordshire. During the eighth flight of the day, the aircraft was performing a 30-degree right-hand banked turn when the crew heard a loud bang. The instructor observed that the bracket securing the top of the front strut to the monopole had failed, causing the trike to adopt a nose-down attitude approximately 10 degrees steeper than normal. The instructor took control of the aircraft and performed a successful precautionary landing in a field. There were no injuries to the two crew members on board.
The investigation
Investigators examined the aircraft and found that the front strut upper supporting bracket had failed, which subsequently caused the monopole to bend rearwards at the overcentre catch location. The investigation revealed that the bracket had been installed upside down. During the rigging process, it is possible to fit the front strut without the wing attached while the bracket is in this incorrect orientation. Because the structure can flex, the pins can be fitted even when the bracket is upside down, but this configuration subjects the front strut to an unintended tensile preload when the monopole is locked into position.
Engineering tests conducted on brackets with the same incorrect orientation showed that the rear face began to bow at 310 kgf, with cracking appearing at 610 kgf. This was significantly lower than the loads sustained by a correctly oriented bracket, which showed no signs of failure at 1,100 kgf.
Findings
- The primary cause of the incident was that the front strut upper bracket was oriented upside down.
- The incorrect installation introduced an additional tensile preload to the front strut.
- This additional load, combined with standard flight loads, caused the bracket to fail during flight.
- The failure of the bracket resulted in the rearward bending of the monopole.