During climb from FL320 to FL340, the crew reported that a bang was heard from the vicinity of number 2 engine and vibration was reported by the cabin crew at the forward galley. The vibration indication for number 2 engine increased to 4 units, on a scale of 1-5, before stabilising around 2.5 units. All other engine parameters remained normal. During subsequent cruise at FL380, the vibration increased incrementally to 4 units over a period of about 90 minutes. The thrust was reduced to idle on the engine and the aircraft descended to FL370. The vibration remained at 2.5 units with the engine at idle for the remainder of the flight.
The operator subsequently reported that a visual inspection of the engine revealed that a section of the fan case attrition liner was missing. A borescope inspection revealed that a stage one compressor blade had failed at the midspan location. This had probably resulted in the increased vibration level. Further investigation revealed that all remaining stage one blades exhibited trailing edge tip curling and several blades had sustained impact damage. Severe damage and heat discolouration of the stage one rotor path was also reported. The engine was changed and the aircraft returned to service.
The source of the damage could not be positively established but may have been the result of leading edge blending that was evident on some of the blades, including the failed blade. The blade set was returned to the manufacturer for further investigation.