Incident Overview
A 1974 Cessna 421B, modified by a Riley conversion kit and registered as N1969J, was descending through approximately 1,900 ft MSL when the aircraft unexpectedly yawed to the left. Without additional warning, the aircraft then yawed severely to the right and began producing a high-pitched whining noise. Moments later, the right engine exploded. Debris from the engine penetrated the cabin wall and windows, striking the pilot and resulting in minor injuries. The pilot secured the right engine, feathered the propeller, and landed at El Paso without further incident.
Engine Examination
Post-flight examination of the right engine revealed that the power turbine wheel had disintegrated in flight. Internal inspection identified a broken N2 accessory idler gear that exhibited a fatigue fracture. Further examination determined that a loose nut securing the N2 accessory idler gear and N2 speed disc had allowed relative movement and wear of these components. The fatigue fracture of the idler gear led to a loss of drive to the overspeed governor, which in turn allowed the engine to overspeed and ultimately fail. The manufacturer is reportedly redesigning the assembly to address the issue.
Conclusion
The investigation determined that the engine overspeed and subsequent disintegration were caused by the fatigue fracture of the N2 accessory idler gear, which had been weakened by a loose nut allowing movement and wear.