What happened
On 12 July 2008, a pilot was operating a private flight in a Replica Sopwith Triplane, registration G-BWRA, traveling from White Waltham Airfield in Berkshire toward Rendcomb in Gloucestershire. While flying at approximately 1,200 ft, the pilot observed the engine cowling beginning to detach from its mounts. Shortly after noticing this, loud noises and vibrations originated from the nose of the aircraft, prompting the pilot to execute an immediate forced landing to avoid potential engine damage or propeller contact.
Seeking to avoid a rain shower and finding the available landing areas covered in standing crops, the pilot selected a field with a gentle upward slope. The aircraft touched down at a low speed, and the crops helped decelerate the plane. However, the combination of the soft ground and the resistance from the landing gear caused the aircraft to pitch forward, resulting in it coming to rest inverted. There were no injuries to the pilot.
The investigation
Investigators examined the engine cowling attachment system, which uses four anti-vibration mountings to secure the cowling to the engine. They determined that the front four mountings had failed, allowing the cowling to move forward and release from its rear retaining wire. This movement placed the cowling directly in the path of the propeller.
Testing revealed that the rubber in the failed mountings had failed between the bolt heads, causing the bolts to separate. Further analysis of the rubber components showed that the two lower mountings were harder than the upper two. The investigation also noted that the rubber used in the mountings was of an unknown specification, as they had been sourced from an automotive supplier. The rubber from the failed components exhibited numerous cracks, suggesting the parts may have been old stock.