1st Stage Carrier Bolt Failures
Transport Canada Civil Aviation (TCCA) would like to raise awareness of the potential for engine failure due to torque relaxation and fracture of PT6 reduction gear 1st stage carrier bolts.
Pratt & Whitney Canada (P&WC) has experienced engine events that have been related to bolt torque relaxation. This issue has been on-going for some time. TCCA released a service difficulty advisory, AV-2008-05, wherein P&WC concluded that inadequate bolt lubrication during assembly was the possible cause of the loss of torque. Further investigation has revealed that 1st stage carrier bolt torque relaxation can lead to bolt failure and possible engine failure. This condition is most likely to occur on low time (since overhaul) engines that were overhauled prior to 4 September 2011.
An investigation of the assembly techniques has revealed that P&WC standard practices for keywasher retention during torquing are not always followed, possibly creating a loss of torque. P&WC Service Information Letter (SIL) GEN-113 has been issued to remind technicians of the applicable standard practices. The overhaul and maintenance manual instructions are being revised to include more detailed information on the importance of retaining the keywasher’s anti-rotation tangs in a counter clockwise direction during bolt torquing. Not adhering to these standard practices may affect engine reliability. In addition to this, P&WC has developed a new bolt specification for this application available since February 2012.
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