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An operator's proactive replacement of worn bushings components on an Airbus A330 inadvertently contributed to the engine experiencing excessive vibration, an ATSB investigation found.
Shortly after departure from Brisbane Airport on a scheduled flight to Auckland on 15 April 2018, the flight crew of a Qantas Airbus A330-200 received an advisory notification indicating excessive vibration from the left engine. The crew reduced thrust on the left engine to idle, and the noise and vibrations ceased.
The crew elected to return to Brisbane, where the aircraft landed uneventfully. The thrust on the left engine remained at idle during the air turn back.
The ATSB investigation found that worn bushings had led to fretting damage on a lever arm in the fourth-stage variable stator vanes within the General Electric CF6‑80E1 engine's high-pressure compressor. The lever arm fractured, allowing the variable stator vanes to become off schedule (misaligned), affecting the airflow entering the stage four high-pressure compressor.
The airflow disturbance resulted in abnormal aerodynamic loading and ultimately, fatigue failure of a fourth stage compressor blade. The downstream turbomachinery was then damaged due to the progression of blade debris through the engine.
The ATSB found that three non-mandatory variable stator vane lever arm inspections were carried out prior to the occurrence but were not effective in detecting the bushing wear.
General Electric intended that replacement of the complete set of bushings was required when more than half of the accessible bushings were worn. However, the operator had proactively replaced worn bushings individually when found during maintenance. As a result, the threshold to replace the complete set would not be reached and inaccessible bushings would not be replaced.
As a result of this occurrence, Qantas inspected all CF6-80E1 engines in its A330 fleet for similar defects, with none identified. Additionally, Qantas issued a maintenance memo to service personnel, highlighting the maintenance actions for the variable stator vane system and precautions to be aware of when carrying out work in this area.
The investigation report's safety message warns that when maintenance organisations carry out additional activities to what is required, they should consider checking with the manufacturer to confirm that no unintended consequences could be introduced.
You can find here the investigation report AO-2018-033: Engine surge and high vibration involving Airbus A330, VH-EBR, 44 km north east of Gold Coast Airport, Queensland, on 15 April 2018
