On July 25, 2019, about 1925 eastern daylight time, an Aero Commander S2R, N5680X, was substantially damaged when it was involved in an accident near Cuthbert, Georgia. The commercial pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight.
The pilot performed a preflight inspection of the airplane at 0700 with no anomalies noted. He departed on the 14th flight of the day and set the engine power at 50 lbs of torque producing 96% gas generator speed (NG). About 5 minutes into the flight, at 500 ft above ground level, he recalled a loss of airspeed with the sound of the engine winding down. He stated the oil pressure was still in the green arc and the NG had reduced to 35%. He performed a forced landing to a field, during which the airplane sustained substantial damage.
Review of the maintenance logbooks revealed that the engine was installed on the accident airplane in 2013 and operated 2,189 hours until an unscheduled removal for a compressor problem in December 2015. The repair involved a vibration workscope and included replacement of both the Nos. 1 and 2 (compressor support) bearings. The engine was reinstalled and was operated 2,287 hours before the accident.
Examination of the engine revealed that the gas generator system did not turn when rotated at the starter drive pad. The accessory gearbox magnetic chip detector was not shipped with the engine, but heavy ferrous and non-ferrous debris was captured when the accessory gearbox oil was drained through filter paper. Fragments identified as tangs from the No. 1 bearing internal key washer were identified in the debris. The gas generator rotor remained seized after the accessory gearbox was removed from the engine. The accessory gearbox drivetrain could be rotated when the coupling shaft was turned with force. Scoring damage was noted in the main pressure pump, pressure relief valve and housing, and other oil-wetted components, consistent with hard debris circulating in the oil.
An approximate 120° arc of material was missing from the forward edge of the No. 1 bearing retention nut cup washer. The missing section included one of the two retention (stake) features. Oil was observed on bearing surfaces and there was no evidence of distress from lack of lubrication. The No. 1 bearing cage was fractured into four sections. All 10 ball elements were present and showed local material transfer. The thrust inner ring ball path was heavily worn. The non-thrust inner ring ball path was worn to a lesser extent. The No. 1 bearing outer ring and the internal key washer were in place inside the compressor bearing adapter; but the 17 internal key washer tangs were separated. There were oval impressions in the ball path, including deep imprints in five consecutive locations. The stationary rear air seal was locally gouged. The No. 1 bearing oil jet was undamaged. A flow bench check found its spray pattern acceptable.
Metallurgical examination of the bearing by the engine manufacturer found evidence of fatigue on two bearing cage fracture surfaces at the pocket rounded edge; however, the extensive secondary damage prevented further evaluation. The cage microstructure was consistent with the drawing specification, and hardness measurements conformed to the drawing requirements. No material anomalies were noted. Energy dispersive x-ray spectroscopy spot analyses found no concentrations of nickel to suggest that key washer or cup washer material had migrated into the bearing and initiated the failure.
Manufacturer records showed that the No. 1 bearing was sold new as a spare in 1985. It was installed in the accident engine during the December 2015 compressor repair. According to the repair shop, it had undergone inspection and been determined serviceable after removal from an engine that was disassembled for spares.