CH149906 Cormorant - Epilogue

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Report / August 7, 2013 / Project number: ch149906-D-Cat

Location: Kamloops Airport, BC
Date: 7 August 2013
Status: Investigation Complete

Helicopter CH149906 was conducting a normal take off to a 10 foot hover.  Three seconds into the take-off, at approximately three to four feet above ground, the crew heard a clunk followed by "Master Warning" and "Engine Fail" tones.  As the helicopter yawed slightly, the Flying Pilot briefly paused his application of power before slowly reducing it as the aircraft settled back onto the runway.  The torque on No. 2 engine indicated zero and the Turbine Inlet Temperature was high at 1115 °C.  A No. 2 engine emergency shutdown was then conducted before the helicopter was taxied back to the ramp and shut down.

A boroscope inspection confirmed damage to the No. 2 engine downstream of the second stage compressor.  The engine was then routed to the third line contractor for a comprehensive teardown inspection with oversight from the engine original equipment manufacturer (OEM).  The teardown found two adjacent airfoils on the second stage compressor bladed disk that separated at the root, causing secondary damage to engine components downstream in the gas path.  The airfoils were not recovered, but the remaining bladed disk fracture surfaces clearly indicated signs of fatigue.

The investigation identified non-typical airfoil rub against the engine casing to be the most likely cause of the high amplitude fatigue that led to the crack initiation.  Once initiated, the crack propagated under normal high cycle fatigue loading until failure.  The investigation could not determine the exact conditions causing the non-typical rub, but two conditions may have contributed to such rubbing events: first, the tighter clearance of stage-2 Blisk replaced at third line maintenance, and second, the location of the engine on the airframe.

The principle safety recommendation called for all CH149 engine compressor casings to be refurbished so that adequate clearances exist between bladed disk edges and engine casings.  Additionally, it was recommended that the planned ground and flight testing on CH149 airframe be used to document potential variances between the various engine positions which would explain damage found in this occurrence.

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