What happened
The flight proceeded without incident during the initial climb phase, reaching an assigned cruise altitude of Flight Level 260. Approximately forty minutes into the journey, the number three engine suffered a catastrophic mechanical failure. Specifically, the turbine blades on the number four power turbine wheel detached from the disc in an uncontained failure. Despite debris from the engine penetrating the pressure vessel, the pilot successfully executed an unscheduled landing at a nearby airport with no further damage to the airframe or injuries to those onboard.
The investigation
Post-accident examination of the engine revealed critical maintenance deficiencies. Ferroous chips were found contaminating the oil, which had been bypassing the engine oil filter for at least forty days prior to the event. Notably, the oil and filter had been replaced three times in the three months preceding the accident, yet no corrective action was taken by company maintenance personnel to address the underlying issue.
Findings
The investigation determined that the contaminated oil contributed directly to the failure of the number four turbine bearing. This bearing failure led to overheating and subsequent damage to the overspeed sensors. The resulting turbine overspeed caused the blades to depart the disc, initiating the chain of events that led to the engine failure.