Summary
We propose to supersede airworthiness directive (AD) 2015-20- 04, which applies to certain Pratt & Whitney Canada Corp. (P&WC) PT6B- 37A turboshaft engines. AD 2015-20-04 requires initial and repetitive inspections until replacement of the No. 10 bearing, and eventual replacement of the No. 9 bearing, both located in the engine reduction gearbox (RGB) assembly. Since we issued AD 2015-20-04, P&WC has determined that the repetitive inspection of the bearings has an associated risk of gearbox damage or contamination and that the bearing installation required by AD 2015-20-04 does not adequately address the issue of bearing axial movement. This proposed AD would require removal from service and replacement of the No. 9 and No. 10 position bearings. We are proposing this AD to address the unsafe condition on these products.