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 boroscope 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.