Incident Summary
The flight was uneventful during takeoff and climb to an assigned cruise altitude of FL260. Approximately 40 minutes after takeoff, the number 3 engine suffered an uncontained failure of the turbine blades on the number 4 power turbine wheel. Debris from the engine penetrated the pressure vessel, forcing the pilot to make an unscheduled landing at an airport. No further complications were reported during the landing.
Engine Examination
Post-incident examination of the engine revealed that ferrous chips were present in the oil, indicating contamination. Additionally, oil was bypassing the engine oil filter, a condition that had persisted for at least 40 days. Despite this, no corrective actions were taken by company maintenance personnel. The oil and filter had been changed three times in the three months preceding the incident.
Contributing Factors
The examination concluded that the contaminated oil contributed to the failure of the number four turbine bearing. This failure subsequently led to overheating and damage to the overspeed sensors. Without proper overspeed protection, the turbine oversped, causing the turbine blades to depart the turbine disc.