Incident Overview
Following engine start, the pilot heard an abnormal mechanical noise from the engine. The engine was immediately shut down, and a mechanic was called to investigate. During a subsequent engine start attempt, the mechanic confirmed the presence of the abnormal mechanical noise, leading to another shutdown.
Examination Findings
Hand propping of the engine revealed that it would sometimes mechanically jam. A detailed examination of the engine components disclosed the fracture of two crankshaft gear teeth. Both fractured teeth exhibited signatures consistent with overload failure. Additionally, corresponding teeth on the starter adapter gear, which would have meshed with the crankshaft gear teeth during startup, were found to be cracked on the loaded side.
Metallurgical Analysis
All metallurgical requirements evaluated, including core hardness, surface hardness, and case depth, were found to be within specified limits.
Potential Cause
Although a single kickback while cranking could cause failure of components of the cranking system, there was no evidence to determine why a kickback may have occurred.