AirAsia X A333 near Learmonth on Jun 25th 2017, engine shut down in flight after severe vibrations
Last Update: June 30, 2020 / 15:18:37 GMT/Zulu time
Passengers reported a large bang was heard followed by severe vibrations and sounds like a spinning washing machine. They were instructed to brace for landing.
Australia's emergency services reported they put their marine emergency services on stand by for a possible water landing of the aircraft north of Perth.
The airline reported the aircraft returned to Perth due to a technical problem without confirming an engine issue. Emergency services were called as precaution only as part of the standard operating procedures. The passengers were rebooked onto other flights.
On Jun 26th 2017 Australia's Transport Safety Bureau (ATSB) reported the aircraft suffered moderate airframe vibration as result of an engine fault. The ATSB opened an investigation into the engine malfunction rated a serious incident. The investigation is estimated to conclude by November 2017.
On Aug 23rd 2017 the ATSB reported the aircraft was step climbing from FL380 to FL400 when a fan blade of the left hand engine failed resulting in engine damage and significant airframe vibration. The engine was shut down but continued to rotate in the airflow (wind milled) resulting in continuing vibrations. The engine manufacturer found that the blade fractured about one quarter of its length from the blade base and separated. The liberated blade section and associated engine debris were contained within the engine cowls. Analysis of the retained blade section identified the blade likely separated due to metal fatigue that originated within the blade's internal structure. Investigation of the failure mode is still continuing.
On Jun 30th 2020 the ATSB released their final report concluding the probable causes of the incident were:
- A fan blade in the left engine failed due to fatigue cracking, which had initiated at the corner of a membrane to panel bond, leading to the engine failure, shutdown and diversion.
- The Trent 700 blade manufacturing process produced a variation in internal membrane-to-panel acute corner geometry that, in combination with the inherent high level of blade panel stress, could lead to increased localised stresses in those corner areas and the initiation and propagation of fatigue cracking. [Safety issue]
- The scheduled inspections recommended by Rolls-Royce to detect cracking in Trent 700 fan blades, were insufficient to detect early onset fatigue cracks in the membrane to panel bond before those cracks could progress to failure. [Safety issue]
- Following a fan blade failure on the left engine, the flight crew returned to Perth as it was the nearest suitable aerodrome according to the operator’s procedures.
Emphasis added by ATSB to highlight importance of safety issues.
The ATSB analysed:
While enroute from Perth, Western Australia, to Kuala Lumpur, Malaysia, a lowpressure compressor (fan) blade from the left engine of an AirAsia X A330, registration 9M-XXE, failed. The blade failure in turn disrupted the operation of that engine. The flight crew completed the ENG 1 STALL procedure, shutting the engine down, and then initiated a diversion (in this case return) to Perth. The diversion to Perth was appropriate considering the operator’s procedures, their location and the status of other aerodromes in the area. As a result, this analysis will only examine the reasons behind the engine fan blade failure.
The failed fan blade
The fan blade failed at 4,701 cycles when a fatigue crack, which had initiated at an internal membrane-to-panel acute corner, grew to a size where the remaining strength in the blade could not sustain operational loads. The engine manufacturer, Rolls-Royce, carried out significant analysis to determine why this, and other Trent 700 blades had failed prematurely.
Detailed examinations did not identify any manufacturing defects or operational damage that would explain why fatigue cracking initiated in the blade. Nor did they identify any operational conditions that may have contributed to the failure.
Structural analysis of normal variations in blade geometry (within manufacturing tolerances) found that the stresses in the convex panel of the blade was about four per cent higher than the initially modelled blade stress. This placed the stress values in the top three per cent of the blade population produced since 2003; however, the level of stress was not sufficient, in itself, to account for the initiation of the fatigue crack.
Further analysis found that the radius in the internal membrane-to-panel acute corner produced a stress concentration, and that, in combination with the increased general stress state, could explain why a fatigue crack initiated at that location.
The crack shape (aspect ratio) was also examined to determine the effect that it had on growth rate. The early stages of the crack growth were predominantly along the internal surface of the panel, initially growing along the chord of the panel, before penetrating into its thickness. It was identified that this growth was characteristic of growth through a macrozone in the material texture. When assessing the effect of this initial crack propagation on the expected life of a fan blade, Rolls-Royce found that the elongated crack shape resulting from growth along a macrozone could, based on typical material properties, reduce the propagation life of the blade by up to 30%.
Rolls-Royce identified that the complex manufacturing processes used in the production of Trent 700 fan blades can result in a variation in the membrane-to-panel geometry; in particular the acute corner radius. Thus, there was potentially a population of Trent 700 fan blades with acute corner radii that made them susceptible to early onset fatigue cracking. The acute corner radius was an artefact of the super plastic forming process and could not be measured using normal production techniques, so Rolls-Royce was not able to identify the size of this population. As such, susceptible blades across the fleet were not able to be identified from the manufacturing records and individually managed.
At the time that the blade failed, there was an existing inspection regime to check for cracking in the fan blades of Trent 700 engines. That inspection had been completed for the failed blade 1,099 cycles prior to the in-service failure, at which time the blade was considered acceptable and returned to service.
The next inspection for the blade was not due for another 1,301 cycles. As such, the existing inspection interval was too great to capture the initiation and growth of the crack that developed in this blade due to the specific crack shape. Thus, the 2,400 cycle inspection interval that was in force at the time of this blade release event may have resulted in an in-service population of fan blades, with similar high stresses and small membrane-to-panel acute corner radii, at risk of premature in-flight failure.
This article is published under license from Avherald.com. © of text by Avherald.com.
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