What happened
During the descent for landing, the aircraft's right engine, a TPE331-14B, experienced an uncontained gearcase failure.
The investigation
Upon disassembly of the engine, investigators found that the gearcase had ruptured in the plane of rotation of the 1st bull gear. Additionally, a 90-degree arc of the rim and gear teeth on the 1st bull gear had separated.
A metallurgical examination of the 1st bull gear identified a fatigue fracture that originated within the gear's bore splines. This fatigue was traced to an area where the thickness of the spline was significantly reduced due to indentations located on the pressure surface of the splines.
An analysis of the engine overhaul manual showed that a tie-rod is utilized to maintain clamping force between the spur gear and the 1st bull gear. The established assembly procedures require the tie-rod to be stretched with a load of 22,000 pounds and secured using a hand-tightened nut.
During the disassembly of the gearcase, investigators were unable to determine the specific amount of stretch applied to the tie-rod or the breakaway torque of the retaining nut. Maintenance records indicated that the gearcase had been previously disassembled and reassembled during repairs conducted by the engine manufacturer.