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The manufacturer advised that previous research into the mid-span blade failures indicated that the failures were always associated with the presence of vibratory loads. The loads were found to have resulted from the higher levels of non-synchronous engine vibration that were evidenced by the power turbine shaft contacting and rubbing against the compressor tie rod.

The failed engine power turbine shaft was reported to be exhibiting evidence of rubbing against the compressor tie rod. The manufacturer advised that, although the rubbing was not as severe as observed during the previous first-stage compressor blisk mid-span blade separations, the required frequency, severity, and duration of the rub resulting in the blade separation was not known.

Previous research into the problem attributed onset of non-synchronous vibration to friction in the rotor system at the power turbine shaft forward spline due to either lack of lubrication, spline wear, misalignment, or reduced damping at the number-2 bearing in the input drive assembly. The NTSB engine examination report mentioned no deficiency in any of those areas.

On completion of the examination by the ATSB, the blisk was returned to the manufacturer. The ATSB requested to be informed of any further development in the investigation and research into the onset of the engine non-synchronous vibration by the manufacturer's specialists. Any results from that research will be published on the ATSB website at: www.atsb.gov.au.

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