Recommendation

Transport Canada (TC) has assessed the need for regular
borescope inspections of PW 118A reduction gear-box with below
recommended thickness of carburised case depth (pre SB 21323),
regardless of the Service Bulletin state of the engine.



The shaft failure, as investigated by Pratt & Whitney Canada,
was deemed to have been caused by the gradual spalling of the shaft
gear teeth over an extended period of time. Since this spalling
has, in the past, been linked to input shaft failures, the periodic
requirement to check gearbox chip detectors and their associated
circuitry continues to be in place in the engine maintenance
schedule. Pratt & Whitney Canada confirmed that there is no
non-destructive test that may be applied to determine the depth of
the input shaft carburised case, thus precluding the introduction
of a reliable cost effective check to determine which shafts would
be candidates for additional inspections.



The Service Difficulty Report (SDR) database revealed no other
reported incidents associated with PW118A gearbox input shaft
failures. This leads Transport Canada to conclude that the shaft
failure incident detailed in the ATSB report to be an isolated
case. Consequently Transport Canada does not deem it necessary to
reinstate previously applied borescope inspections. The industry
will be best served by relying on the existing chip detector
inspections to verify the extent of gradual wear leading to
eventual failure. The existing manufacturer's recommendations also
provide the necessary precautions, which indicate a need for
in-depth investigation in the event that chip detectors show
evidence of gradual degradation.