What happened
During a flight, the pilot experienced significant engine vibrations accompanied by the smell of burnt pine. While the engine appeared to return to a normal state following these symptoms, the situation escalated when the propeller separated from the aircraft. Shortly after the separation, the pilot encountered wildlife crossing the runway and initiated a go-around. However, due to the loss of the propeller, the pilot was forced to execute an emergency off-field landing. The pilot managed the landing calmly and safely, aided by recent emergency procedure training.
The investigation
Investigators examined the hardware and found that the propeller bolts had been installed in the correct orientation, leaving insufficient clearance for the bolts to back out of the flange. Because the bolts were no longer present for physical examination, the exact cause of their failure could not be confirmed. However, evidence of fretting and movement on the flange suggested a loss of bolt tension or clamping force. The vibrations and the smell of burnt wood observed by the pilot further supported the conclusion that relative movement had occurred between the wooden propeller and the mounting hardware. The investigation considered whether installation errors, defective hardware, or propeller issues caused the tension loss, but no conclusive evidence was found to support a specific cause.
Findings
- The propeller separation was caused by the fracture of the propeller bolts.
- The fracture was likely linked to a reduction in bolt tension.
- The pilot's recent training in emergency maneuvers contributed to the safe outcome of the off-field landing.