The Australian Transport Safety Bureau (ATSB) says a fan blade failure was involved in a 25 June incident involving an AirAsia X Airbus A330-300 aircraft.
In an update to its investigation, the ATSB says that as 9M-XXE (MSN 1075) climbed to 40,000ft (flight level 400) from 38,000ft, "a fan blade on the number 1 engine failed resulting in engine damage and significant airframe vibration".
The aircraft was operating the Perth-Kuala Lumpur route when the incident occurred, reportedly carrying 359 passengers.
"The shutdown continued to rotate in the airflow, resulting in ongoing airframe vibration. The aircraft returned to Perth and landed safely."
Rolls Royce's subsequent inspection of the Trent 772B-60EP engine discovered that a fan blade separated roughly one quarter of the length from its base.
"The blade section and associated engine debris were retained within the engine cowls," says ATSB.
"Examination of the retained fan blade section identified that the separation was likely due to metal fatigue that originated within the blade’s internal structure. Investigation and analysis of the failure mode are on-going."
The ATSB adds that this finding will be included in its draft report. It has classified the occurrence as a serious incident.
The aircraft was cruising just off the coast of Carnarvon, Western Australia, when passengers and cabin crew reported a loud explosion from the left-hand engine, followed by strong vibrations and shudders.