CH149910 Cormorant - Epilogue

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Report / November 16, 2012 / Project number: CH149910-C-Cat

Location: Greenwood, Nova Scotia
Date: 2012-11-16
Status: Investigation Complete

A technician was carrying out a torque check and nut replacement of the bolted connection between the CH149 Cormorant helicopter main gearbox (MGB) upper case and the main case when a lock-ring stud failed in overload.  Additionally, several other lock-ring studs at the bolted connection were overtorqued and, consequently, the MGB was declared unserviceable and returned to the Original Equipment Manufacturer (OEM) for strip-down inspection and repair.  The torque check was part of an on-going recurring inspection, detailed in an OEM-issued Mandatory Service Bulletin (CSH-A63-206), and was being conducted during a 300 hour periodic inspection.

The extent of the damage and the complexity of the ground accident circumstances required the Directorate of Flight Safety to investigate.  The investigation determined that the lock-ring stud failed in overload due to application of excessive torque.  A number of errors contributed to the overload failure, including misidentification of the MGB main case, inadvertent confusion between metric and imperial torque units, and inappropriate technique.  The investigation also determined that the torque check procedure had created a significant maintenance burden and was poorly understood by technicians, resulting in numerous routine short-cuts and unauthorized deviations to the procedure, and that similar errors had occurred on other MGBs.  A number of collateral observations were also made, including lack of feedback of data to the OEM, the determination of the approved parts list for the CH149 MGB, and unit quarantine procedures.

Preventive measures included improvements to the torque check procedure, upgrading of the MGB studs, converting to metric for maintenance activities, and improving OEM processes and interaction with the CH149 In-Service Support Contractor.

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