ATSB Note: On 3 March 2001, the CASA, in its reply to the draft
investigation report, made the following statement in regard to the
`Unfortunately, it would not be possible to determine the case
depth without considerable work. The recommendation is therefore
difficult to implement.
`At this time, we intend to respond to the problem identified by
providing information on the problem to each operator of the
4edngine. This would be reevaluated if the manufacturer provides
you with information which would justify mandatory action.'
The following response was received on 17 April 2001:
`ATSB Recommendation 20000111 recommends the Civil Aviation Safety
Authority investigate the need for regular borescope inspections of
PW118A reduction gearbox (RGB) input shaft with below recommended
thickness carburised case depth (pre SB 21323) regardless of the
service bulletin status of the engine.
`PW118A engine, S/No PC-E 115093 experienced a failure of the 1st
stage reduction input gearshaft. Failure of the gearshaft was
attributed to spalling and subsequent fatigue fracture of the
gearshaft teeth. While damage to the engine was extensive, the
failure was contained. The aircraft is certificated to operate
safely with one engine failed.
`The engine was returned to Pratt and Whitney Canada (PWC) for
investigation. That investigation found the case hardening at the
root of the input gearshaft teeth was below minimum requirements
both in depth and hardness.
`The RGB chip detector was also found to be defective. Although it
passed the continuity test, the poles of the detector had
fractured. The investigation did not identify if the chip detector
damage was secondary. PWC strongly believes a serviceable chip
detector would have identified gear tooth spalling during routine
chip detector inspection and well before the failure event.
`Spalling of gear teeth in the reduction gear is a known problem in
certain models of the PW100 engines. Initially such spalling was
controlled by routine borescope inspection of the gear teeth.
Modification to the oil supply to the gear teeth (SB 20246 for the
PW118A engine model) has been effective in reducing the incidents
of gear tooth spalling. As such, the requirement for regular
borescope inspection was deleted by PWC for engines modified by SB
20246. The RGB magnetic chip detector system has been reliable in
identifying subsequent gearshaft spalling events.
`An improved input gearshaft featuring greater case depth and a
larger gear tooth root radius is also available to overcome the
problem (SB 21323 for the PW118A engine model). The engine involved
in the incident had a pre-SB 21323 input shaft installed and was
modified to incorporate SB 20246. As such borescope inspection was
not required for this reduction gearbox configuration.
`The primary cause of the failure was that the case hardening at
the root of the gear teeth of the input gearshaft did not meet
specification. The ATSB recommendation addresses this by
recommending borescope inspection of those shafts that are known to
have inadequate case hardening. While NDT techniques to measure
case hardening depth are available, the gearshaft would need to be
removed from the engine for such an inspection. It would be
impractical to identify affected gearshafts for in-service engines.
Thus, to address the ATSB recommendation, it would be necessary to
include all pre-SB21323 gearshafts in the population to be
`The ATSB recommendation should also be applied to other affected
models of the PW100. While the exact number of affected shafts in
the Australian fleet is not known, advice from the manufacturer
suggests that few engines remain to be modified. Australian PW100
operators are aware of the severity of this engine failure and the
circumstances surrounding the event.
`PWC advises that events of this nature are extremely rare, 1
incident in 20 million engine operating hours. It is therefore
understandable that the manufacturer deleted borescope inspection
requirements if the modified oil supply system is incorporated.
Indeed, the way the engine manual calls up the borescope
requirement, it appears that the shaft may have failed before the
first inspection was due.
`Given that such events are rare, that the ATSB recommendation
would impose a significant maintenance burden on operators without
any confidence of preventing future problems, and that the RGB chip
detector should provide adequate warning of such events, CASA does
not consider mandatory action to address the ATSB recommendation is
justified. However, CASA will advise each operator of these engines
about the incident and recommend expedited incorporation of the
modified RGB input gearshaft.'