The power turbine shaft in Pratt & Whitney Canada PW100 series engines operating in certain marine environments is susceptible to corrosion pitting, which can grow undetected between scheduled inspections. This increases the risk of shaft fracture resulting in engine failure.
The ATSB welcomes the action taken by P&WC and acknowledges the difficulties in the practical implementation of a borescope inspection and the results of the risk assessment conducted.
While the actual magnitude of the risk remains unknown, given the ongoing commitment to better understand in-service PT shaft corrosion and the related risk, the safety issue will be closed – partially addressed.
In September 2020, Pratt & Whitney Canada (P&WC) advised the ATSB that it was ‘currently looking at different options’ to address this safety issue and ‘finding a way to remove the contaminants from inside the shaft is one of them’. In addition, P&WC was ‘looking at adding an internal inspection of the shaft during a hot section inspection [HSI] using a borescope’ as well as ‘evaluating how to define criteria to be used if corrosion or damages are found during that inspection’. It considers these criteria will also assist maintenance organisations in evaluating power turbine (PT) shaft corrosion findings during overhaul inspections.
In February 2021, P&WC advised the ATSB that it had commenced a review of historical overhaul experience of the PT shaft in an effort to identify which engines and operators are potentially exposed to an increased risk of PT shaft corrosion. P&WC also advised that it was assessing the requirement for additional mitigating actions for engines within the PW100 engine fleet that have already completed an HSI, but are potentially exposed to the risk of PT shaft corrosion.
Pratt & Whitney Canada did not provide a timeline for the review and implementation of any related safety action.
The ATSB acknowledges the safety action taken and proposed by P&WC to address this safety issue and considers that, if implemented, such action will probably address the issue. However, as a timeline for implementation was not provided, the ATSB remains concerned with resolution of the safety issue. Accordingly, the ATSB issues the following safety recommendation to support P&WC’s proposed safety action.
The Australian Transport Safety Bureau recommends that Pratt & Whitney Canada takes safety action to address the risk of corrosion-related fracture of the power turbine shaft in its PW100 series engines.
Pratt & Whitney Canada (P&WC) agrees with the safety recommendation. In order to mitigate the risk, P&WC will incorporate an additional inspection of the PT shaft during the Hot Section Inspection (HSI) into the PW100 engine Cleaning, Inspection and Repair manual (CIR). The inspection criteria at overhaul will also be revised accordingly. P&WC plans to incorporate these changes in the manuals by October 2021.
P&WC have reviewed historical overhaul experience which has shown that engines being operated in a coastal or salt laden environment seem to be more prone to corrosion due to the salty air flowing through the shaft. P&WC are still evaluating if the borescope inspection requirement will be implemented on all engines or on engines operating in these type of environments. P&WC do not intend to add a borescope inspection for engines that have already completed a HSI before going back to a shop for another HSI or overhaul.
With regard to removing contaminants from inside PT shaft during service, a small quantity of water and detergent flows through the PT shaft during engine washes but the quantity is impossible to control. Therefore, P&WC cannot implement any in‐service cleaning of the shaft.
The ATSB notes P&WC's response and encourages P&WC to review the risk related to higher time engines that have already completed a HSI and may not be subject to another PT shaft inspection before overhaul. No safety action has been implemented yet so the ATSB will continue to monitor the safety issue.
A P&WC investigation has shown that the probability of another in‐flight shut down event due to failure of the PT shaft from corrosion is relatively low. The analysis indicates that the next event would be expected in 10 years assuming no corrective action is put in place.
Reviewing the results of this analysis, P&WC do not believe safety action is necessary at this time to address higher time engines, whether they have had a hot section inspection or not. Looking at the data over the last 10 years, the occurrence event is the only confirmed PT shaft fracture event due to corrosion on a PW100 series engine with flying hours totalling close to 62 million hours.
By adding an inspection at HSI, P&WC are confident the risk of having another event will be even lower. P&WC have decided to include the inspection on all PW100 engines going for a hot section inspection, no matter where the engine is being operated. P&WC believe this way we make sure all shafts are inspected, no matter if the engine or aircraft is sold to a customer operating aircraft in a salt laden or coastal environment.
The ATSB acknowledges P&WC's response and will assess any safety action once implemented. The ATSB will continue to monitor the safety issue
Pratt & Whitney Canada conducted a safety assessment of the PW100 fleet relating to the failure of the power turbine (PT) shaft from corrosion. The most severe outcome of such an event was an in-flight shutdown (IFSD). The entire PT shaft population was taken into consideration for the assessment as the historical overhaul experience showed that there was no specific distinction between the removal of the PT shafts with corrosion in benign environments compared with those in corrosive environments. Based on PW100 field experience, the rate of IFSD associated with the corrosion failure mode was within the Federal Aviation Administration (FAA) Advisory Circular (AC) 39-8 guidelines for acceptable risk and does not require corrective action. In addition, the failure rate was below the P&WC Reliability Group recommended rate. Nonetheless, P&WC evaluated the possibility of performing a borescope inspection during a Hot Section Inspection (HSI) to address potential power turbine shaft corrosion, but the inspection was not considered practical due to:
P&WC acknowledged that the lack of a statistical difference between environments affecting PT shaft corrosion was not perfect as the historical corrosion was not quantified. For that reason, P&WC will continue to collect data on the PW100 PT shafts operating in corrosive and benign environments to better characterise the PT shaft corrosion and assess if specific corrective actions as a function of environment need to be developed at some future time.
The ATSB welcomes the action taken by P&WC and acknowledges the difficulties in the practical implementation of a borescope inspection and the results of the risk assessment conducted. Although the statistical risk of unserviceable corrosion between benign and corrosive environments was found to be similar, as P&WC have acknowledged, the degree of corrosion found during overhauls was not previously quantified and so the actual magnitude of the risk remains unknown.
However, given the ongoing commitment and timeframe to better understand inservice PT shaft corrosion and the related risk the safety recommendation will be closed, and the safety issue will be closed – partially addressed.