What happened
During flight, a gas turbine engine experienced a mechanical failure that resulted in significant internal damage. The loss of a turbine component caused a rotor imbalance, which manifested as intense airframe vibration. This vibration persisted even after the flight crew reduced the power lever to idle. To prevent further damage to the engine, the crew elected to shut down the engine while still in flight and initiated a return to the departure airport.
The investigation
Technical analysis of the engine focused on the condition of the nozzle guide vanes (NGV) and turbine assemblies. Investigators examined the stage four NGV, specifically the No. 5 segment, and found that the protective coating had undergone oxidation and deterioration. This degradation allowed inter-granular oxidation to develop within the base material of the aerofoils.
Further examination of the downstream components showed that the No. 6 aerofoil's highly stressed leading edge had developed cracks, eventually leading to its fracture and liberation. Because no damage was found ahead of the stage four NGV assembly, investigators determined that the liberation of this specific aerofoil was the primary event. As the liberated piece moved through the gas stream, it struck and damaged the stage four and stage five low-pressure turbines, as well as the stage five NGV.
Findings
- The primary cause of the failure was the oxidation and deterioration of the protective coating on the stage four No. 5 NGV segment.
- This coating failure led to inter-granular oxidation in the parent material of the aerofoils.
- The resulting fracture and liberation of the No. 6 aerofoil caused a cascade of damage to downstream turbine components.
- The loss of material from the turbine blades created a rotor imbalance, resulting in the observed airframe vibration.