Recommendation R20000111

Recommendation issued to: Civil Aviation Safety Authority

Recommendation details
Output No: R20000111
Date issued: 01 March 2001
Safety action status:
Background: Why this Recommendation was developed

Output text

The Australian Transport Safety Bureau recommends that Civil Aviation Safety Authority investigate the need for regular borescope inspections of PW118A reduction gear-box input shafts with below recommended thickness carburised case depth (pre SB 21323), regardless of the Service Bulletin state of the engine.

Initial response
Date issued: 20 April 2001
Response from: Civil Aviation Safety Authority
Action status: Monitor
Response text:

ATSB Note: On 3 March 2001, the CASA, in its reply to the draft investigation report, made the following statement in regard to the draft recommendation:

`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 borescoped.

`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.'

ATSB response:

The ATSB wrote to CASA on 28 September 2002 as follows:

Your faxed response to my recent enquiries about CASA's action on the recommendations was received on the 26 August 2002. This response detailed only CASA's original responses to the recommendations and did not provide any evidence of CASA's proposed action being completed.

Could you please advise the ATSB as soon as possible of the action CASA has taken on this matter to enable us to finalise the safety deficiency report.

Further correspondence
Date issued: 06 October 2002
Response from: Civil Aviation Safety Authority
Response status: Closed - Accepted
Response text:

Thank you for your letter of 28 August 2002 concerning Recommendation R20000111. This Recommendation arises from the incident involving a Brasilia aircraft VH-ASN which occurred 87km north west of Tindal Aerodrome, Northern Territory on 1 June 1999 - ATSB Occurrence Brief 199902600.

The Bureau seeks advice on the action CASA has taken to inform operators of the incident and to recommend to them the expedited incorporation of the modified reduction gearbox input shaft for the Pratt & Whitney engine.

On 10-11 December 2001, a CASA officer attended the Embraer Operators Conference which was held in Brisbane. All Australian operators of the Embraer 120 aircraft and a number of potential operators attended the conference. The issue was raised at that conference, including the advantages of incorporating Pratt & Whitney's SB 21323 to install an improved input shaft.

CASA believes that the undertaking to advise each operator of the engine type has been fulfilled by this action. CASA also notes that there has been a considerable elapse of time since the incident without any indication of further problems. Therefore CASA regards the issue is concluded.

However, should either Transport Canada or the manufacturer of the engine provide a response to ATSB Recommendations R20000109 and R20000110, respectively, which indicates a need for more formal action, that would be considered by CASA.

Last update 05 April 2012