CEN21LA287

The pilot was conducting an agricultural flight when the airplane had a sudden total loss of engine power. The pilot stated that the sound of the engine went from normal to silent, with no indication of an engine issue before the loss of engine power. The airplane impacted a levee and nosed over in a flooded rice field during the forced landing. The airplane’s empennage, aft fuselage, and engine mounts were substantially damaged during the forced landing. A postaccident engine examination and disassembly revealed the torque sensor assembly front spur gear was disengaged from the splined shouldered shaft on which it was installed. Additionally, the self-locking nut that secured the front spur gear to the shaft had backed off which allowed the front spur gear to disengage from the shaft. The examination and testing were unable to determine if the front spur gear self-locking nut was improperly torqued at installation or if it became loose during operation due a loss of retention. However, had the self-locking nut not been seated properly at installation, the disengagement of the front spur gear would likely have occurred well before the accident (2,541.5 hours since the overhauled torque sensor was installed on the engine). Additionally, no anomalies were found with the self-locking nut to suggest why it would have lost retention during operation. The engine-driven fuel pump and fuel control unit (FCU) are driven through a series of gears in the accessory gearbox, including the front spur gear, and any failure in the gear train that drives the fuel pump and FCU would prevent the fuel pump from delivering fuel to the FCU to sustain engine operation, resulting in an uncommanded engine shutdown.

On June 20, 2021, about 0700 central daylight time, a Grumman G-164B airplane, N62375, was substantially damaged when it was involved in an accident near Sedgwick, Arkansas. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial-application flight. The pilot stated that he had completed 5 to 6 aerial-application passes over the intended rice field when the airplane had a total loss of engine power while in level flight. The pilot stated that the sound of the engine went from normal to silent, with no indication of an issue before the loss of engine power. The airplane impacted a levee and nosed over in a flooded rice field during the forced landing. The airplane’s empennage, aft fuselage, and engine mounts were substantially damaged during the forced landing. A postaccident engine examination and disassembly revealed the torque sensor assembly front spur gear was loose and disengaged from the splined shouldered shaft on which it was installed. Furthermore, the self-locking nut that secured the front spur gear to the shaft had backed off which allowed the front spur gear to disengage from the shaft. A disassembly of the torque sensor assembly revealed that the internal spline teeth of the front spur gear and the corresponding external forward spline teeth of the splined shouldered shaft were heavily damaged, smeared over, and exhibited loss of teeth material. Metallurgical examination of the splined shouldered shaft found stepped wear and deformation consistent with meshing with the fretted internal spline teeth of the front spur gear. Wear and deformation were also observed on the last (rearward) threads of the splined shouldered shaft consistent with contact with the worn threads of the self-locking nut. The internal spline teeth of the front spur gear exhibited corresponding wear and deformation consistent with meshing with the fretted external forward spline teeth of the splined shouldered shaft. Dimensional inspection of the splined shouldered shaft away from the damaged region was consistent with the manufacturing drawing. The average hardness of the splined shouldered shaft and front spur gear were within their respective manufacturing drawing ranges. The self-locking nut that secured the front spur gear to the splined shouldered shaft has slightly ovalized threads so that the nut must elastically deform to mate with the threads of the splined shouldered shaft, causing increased surface friction that resists loosening when fully and properly torqued. Metallurgical examination of the self-locking nut exhibited wear marks on one side of each wrenching flat consistent with contact from the installation/torque tool, wear marks on the aft face of the nut consistent with contact with the mating washer, and wear of the internal threads consistent with engagement with the splined shouldered shaft. The average hardness of the self-locking nut met the minimum hardness design requirement.However, the examination and testing were unable to determine if the self-locking nut had been improperly torqued at installation or if it became loose during operation due a loss of retention. According to overhaul documentation, a new self-locking nut was used when the torque sensor assembly, part number 3101726-3, serial number P-4184C, series 2, was last overhauled on October 3, 2013. The engine logbook indicated that the overhauled torque sensor assembly was installed on the engine on February 11, 2015. At the time of the accident, the torque sensor assembly had accumulated 2,541.5 hours since its last overhaul. The engine-driven fuel pump and fuel control unit (FCU) are driven through a series of gears in the accessory gearbox, including the front spur gear, and any failure in the gear train that drives the fuel pump and FCU would prevent the fuel pump from delivering fuel to the FCU to sustain engine operation, resulting in an uncommanded engine shutdown.