Narrative
The pilot reported that no engine discrepancies were noted during the preflight inspection or the initial cruise portion of the flight, and all engine instrument readings were normal. While climbing between 500 and 1,100 feet mean sea level, the engine experienced a significant loss of power accompanied by sputtering, vibration, and fluctuating rpm. The pilot declared an emergency, completed the emergency checklist with no change in engine performance, and executed a forced landing on a road. During the landing rollout, the airplane struck a road sign that the pilot had not seen.
Postaccident Examination
A postaccident inspection of the engine revealed that the gear at the back of the crankshaft was detached and moving freely between the corresponding gears. The crankshaft gear attaching bolt and threads were undamaged, but the bolt had backed out of the crankshaft and was resting in the crankshaft gear bore. The bolt locking plate remained at the head of the bolt and exhibited fretting signatures. Fretting signatures were also observed around the mating surfaces of the gear and the crankshaft bore. The crankshaft gear dowel pin had failed, with fatigue striations visible to the naked eye.
Airworthiness Directive Compliance
An engine overhaul was completed one month after an airworthiness directive (AD) was published. The AD required inspection of the crankshaft gear and the bolt to prevent the bolt from backing out and liberating the crankshaft gear. No evidence of compliance with this AD was found in the engine maintenance records.