Incident Description
A pilot reported that after completing maneuvers in a practice area, while returning to Sarasota/Bradenton Airport in Sarasota, Florida, at an altitude of 1,600 feet, the engine suddenly ceased operating. The pilot executed a forced landing. During the landing rollout, the airplane collided with a curb and a sign, resulting in damage to the wings.
Examination Findings
Postaccident examination of the airplane revealed that the Bendix magneto had become displaced from its mount. Additionally, there was a discontinuity within the engine accessory section. Oil was present in the area of the airframe/engine tubular mount structure and on the lower left portion of the fuselage. The crankshaft gear bolt was loose and exhibited a flat mark on one of its threads. The lock tab was bent and twisted, and the dowel pin had sheared. The gears within the accessory section, including the impulse coupling for the left magneto, were undamaged. The crankshaft gear's teeth showed no damage, but the center internal bore displayed signatures consistent with having experienced a side load.
The NTSB Metallurgical Laboratory in Washington, DC, conducted an examination of the left Bendix magneto, the bottom magneto clamp, the dowel pin, the crankshaft gear, the lock tab, the crankshaft, and the crankshaft bolt. The damage observed was consistent with a loss of torque on the crankshaft assembly. The dowel pin had fatigue fractures emanating from multiple origins at the outer diameter surface, consistent with damage produced by grinding and wear/fretting.
Background
Airplane records indicated that on September 1, 1993, the engine had undergone a field overhaul and had accumulated 1,450.35 hours since that overhaul. On October 30, 2002, the engine had undergone maintenance, during which both the left and right crankcase halves were replaced, approximately 200 hours before the accident.