What happened
The aircraft was engaged in agricultural operations, specifically dispersing chemicals over a peanut crop. During a maneuver to reverse direction, the engine suddenly lost power. Recognizing the emergency, the pilot immediately dumped the chemical load to reduce weight and attempted to maintain control. The pilot executed a hard full stall forced landing within the peanut field.
The investigation
Post-acc examination of the aircraft revealed that the left magneto impulse coupling had failed. Fragments from the broken coupling became lodged in the engine accessory case. This obstruction separated the camshaft drive gear, which rendered the engine powerless. Records indicated that the subject magneto had accumulated 576 hours time in service at the time of the accident and had not been inspected. Regulatory requirements specified inspection intervals not to exceed 500 hours.
Findings
The mechanical failure was directly linked to the lack of required maintenance. The impulse coupling failure caused the camshaft drive gear separation, leading to total engine power loss. The magneto exceeded the mandated inspection interval by 76 hours.