Wear of the interlocking shrouds of the intermediate-pressure turbine blades had the potential to reduce the dampening effects of the feature, and may have led to the development of conditions suitable for fatigue cracking of the IP turbine blades.
Following the occurrence, the engine manufacturer issued non-modification service bulletin, NMSB 72-G739, on 6 October 2001. 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 engine manufacturer also advised that the repair engineering department was currently looking at the effects and suitability of numerous repairs. At the time of this report they were in the process of clarifying and amending some of the acceptance limits, which they expected would result in an increase in the blade rejection rate. This work was due to be completed in the second half of 2012. The operator advised that as of early May 2012, 63% of the fleet had been inspected with no findings to date. All inspections were planned to be completed by 30 June 2012 in accordance with NMSB 72-G739
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.
The operator indicated that they had completed inspections across the fleet in accordance with the requirements of NMSB 72-G739 with no instances of excessive wear identified.
The ATSB is satified that this action has appropriately addressed the risks associated with this safety issue.
