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Analysis

Summary

The documented history of both engines showed that all engine operating parameters matched and were within the expected normal ranges until the loss of thrust event for the right engine, which was observed by the crew and recorded on the Digital Flight Data Recorder.

The damage to the engine was consistent with the failure of a 6th stage HPC blade. This blade, when liberated from its platform, generated debris that caused the trailing edge ricochet damage evident in the 5th stage blades, then travelled downstream in the gas path creating a cascade effect of damage as it collided with other structures.

The absence of integrity in the VSV ring arm pins at the 10 o'clock and 12 o'clock attachment points, due to incorrect or inadequate pin flaring technique and subsequent tack weld breakage, probably allowed the vanes at those positions to move independently of the ring and remain at, or adopt, angles other than those commanded by the engine thrust management computer. This movement was most probably within a range equivalent to the slot machined in the unison ring for the vane arm.

Off-schedule VSV operation can cause higher than normal vibratory stresses in the HPC 6th stage rotor blades and subsequent fatigue fracture of the root attachment. A single vane arm disengaged from the unison ring at any circumferential location, from any of the four variable vane stages, can produce a severe, once per revolution, vibratory excitation within the HPC. These turbulent pulses in turn induced the cyclic stresses on the blades that ultimately led to the blade failure in this occurrence.

The processes of flaring the vane arm pins and then tack welding to provide retention of the pins appears, either to not have been accomplished appropriately by the contracted engine repair facility, or the processes themselves were unsuccessful in meeting their intended purpose. Both of these aspects should be reviewed and addressed by the manufacturer.

 
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