Incident Overview
A detailed technical examination of a gas turbine engine identified that the protective coating on the stage four nozzle guide vane (NGV) assembly, specifically the No. 5 segment, had undergone oxidation and deterioration. This allowed inter-granular oxidation to develop in the base material of the aerofoils. Consequently, cracking occurred at the highly stressed leading edge of the No. 6 aerofoil, followed by its fracture and liberation.
Sequence of Events
The liberated aerofoil initially caused no damage ahead of the stage four NGV assembly, indicating it was the initiating event. As the aerofoil moved aft in the gas stream, it damaged the stage four and five low-pressure turbines and the stage five NGV. The resulting loss of turbine blade material created a rotor imbalance, which was felt as airframe vibration. The vibration persisted even after the flight crew reduced the power lever to idle. Due to the residual vibration level, the crew decided to shut the engine down in flight and return to the departure airport.
Mechanical Findings
Examination revealed that the failure originated from the stage four NGV due to coating breakdown and subsequent material degradation. No other pre-existing damage was found ahead of that stage. The fracture of the No. 6 aerofoil was a direct result of oxidation-induced inter-granular cracking under high stress.