Event Description
During climb to cruise altitude, the number 4 engine of an airplane experienced an uncontained failure.
Examination Findings
Post-incident examination of the engine revealed that the number 4 turbine disc had lost all but one of its blades. All blades except one separated just above the blade platform; the exceptional blade separated just below the platform. Metallurgical analysis identified a high-cycle fatigue crack in the blade that separated below the platform. The fatigue crack originated from a fretting damage mark located at the fir-tree portion of the blade.
Maintenance History
According to maintenance records, the number 4 turbine disc assembly had been installed only 22.5 hours or 22 cycles prior to the event. The manufacturer of the turbine disc stated that the wheel was new; however, all 60 blades had seen previous use.
Component Issues
The manufacturer reported that nine blade retention pins, designed to prevent radial movement of the blades, did not meet the requirement of no more than 0.005 inch clearance between the pin head and the wheel. Additionally, the manufacturer noted that stress concentration was induced by sharp edges in the wheel broach slots.