Incident Overview
During cruise flight, the left engine experienced a loss of power and a drop in oil pressure. The crew responded accordingly, and the aircraft subsequently landed without further incident. A subsequent teardown analysis of the engine was conducted to determine the root cause of the failure.
Investigation Findings
The engine teardown revealed that the crankshaft counterweights had two incorrect rollers out of the four installed. All four rollers were part number 73648. According to the engine manufacturer, Textron Lycoming, the correct configuration should have consisted of two part number 73648 5.1 order rollers and two part number 76788 6.3 order rollers. The mismatched roller configuration resulted in unacceptable 6th order torsional vibrations within the engine, which likely led to the power loss and oil pressure drop.
Engine Teardown Analysis
The analysis focused on the crankshaft assembly and its counterweight system. The counterweights are designed to balance torsional forces, and the incorrect roller combination disrupted the intended damping characteristics. With two identical rollers on the same side, the engine experienced excessive vibration at the 6th order, a known critical frequency. This vibration likely caused mechanical stress and eventual loss of oil pressure and power. No other anomalies were found in the engine components, confirming the installation error as the sole cause of the failure.