Engine Failure
While descending for landing, the right engine of an airplane experienced an uncontained gearcase failure. The engine was a Honeywell TPE331-14B turboprop. Disassembly of the engine showed that the gearcase had ruptured along the plane of rotation of the 1st bull gear. The 1st bull gear itself had a 90-degree arc of its rim and gear teeth separated.
Metallurgical Examination
Metallurgical examination of the failed 1st bull gear identified a fatigue fracture as the cause of the separation. The fracture originated in the gear's bore splines, specifically in an area where the spline thickness was significantly reduced by indentations on the pressure surface of the splines.
Maintenance Review
Review of the engine's overhaul manual indicated that a tie-rod provided clamping force between the spur gear and the 1st bull gear. The assembly procedure required stretching the tie-rod with a load of 22,000 pounds, then securing it with a hand-tightened nut. During disassembly of the gearcase, it was not possible to determine the amount of stretch on the tie-rod or the breakaway torque on the retaining nut. Maintenance records showed that the gearcase had been previously disassembled and reassembled during repairs performed by the engine manufacturer.