Following the occurrence, the engine manufacturer issued non-modification service bulletin, NMSB 72-G739, on 6 October 2011. The NMSB directed a borescope inspection of the IP turbine blades to confirm that the interlocks were in an acceptable condition. It was recommended that the inspections be carried out before 30 June 2012.
The service bulletin was re-issued on 11 July 2012 to include crack acceptance and re-inspection criteria. The engine manufacturer advised that they were running a series of stress and modal analyses to look at the effect of a loss of the interlocking shroud feature on the system, in particular the stress in the IP turbine blades. The analysis was run with fixed-fixed and fixed-free configurations, representative of blades with full interlock functionality and with zero interlock functionality. It was reported that the fixed-fixed configuration showed good correlation to the certification tests which validated the model, and while the fixed-free test showed changes to the stress levels and distribution, it did not indicate a significant stress increase on the convex (suction) side of the aerofoil as likely in the occurrence event.
The fixed-half free model analysis was completed in the second half of 2012; however the engine manufacturer reported that they remained unable to verify the mechanism that led to the IP turbine fracture. The manufacturer reported that they were committed to reaching a root-cause understanding of the failure, and ongoing work included a rig-based fatigue test that involved high-life service-run blades against new blades.
It was thought that this would provide important material property data for service aged material with thermal degradation. Analyses were also commissioned to assess the dual in flight shut down (DIFSD) risk for the fleet (which included both 747 and 767 aircraft). In respect of the IP turbine blade release, the DIFSD risk was a factor of 10³ below the regulatory threshold of one such event in 1 billion (109) aircraft flight hours.
The ATSB notes that Rolls-Royce is continuing work to better understand the contributory mechanisms associated with the interlocking shroud wear.
The ATSB acknowledges the technical complexity associated with the issue, and in light of the measures already in place to mitigate the risks associated with future shroud-wear related engine failure events, the ATSB is satisfied that the work being undertaken is an appropriate response to the safety issue.