History of Flight
On June 20, 2021, at about 0700 central daylight time, a Grumman G-164B airplane, registration N62375, was substantially damaged during a forced landing near Sedgwick, Arkansas. The pilot, who was not injured, was operating the aircraft as a Title 14 Code of Federal Regulations Part 137 aerial-application flight. According to the pilot, after completing 5 to 6 passes over a rice field, the engine experienced a total loss of power while in level flight. He reported that the engine sound changed from normal to silent without any prior indication. During the forced landing, the airplane struck a levee and nosed over in a flooded rice field, resulting in substantial damage to the empennage, aft fuselage, and engine mounts.
Wreckage and Impact Information
Postaccident examination of the engine and disassembly revealed that the torque sensor assembly front spur gear was loose and had disengaged from the splined shouldered shaft on which it was installed. The self-locking nut that secured the front spur gear to the shaft had backed off, allowing the gear to disengage. Internal spline teeth of the front spur gear and the corresponding external forward spline teeth of the splined shouldered shaft exhibited heavy damage, smearing, and loss of tooth material.
Metallurgical examination of the splined shouldered shaft showed stepped wear and deformation consistent with prior meshing with the damaged internal spline teeth. Wear and deformation were also present on the last (rearward) threads of the shaft, consistent with contact from the worn threads of the self-locking nut. The internal spline teeth of the front spur gear had corresponding wear. Dimensional inspection of the shaft away from the damaged region matched manufacturing drawings, and the average hardness of both the shaft and the spur gear was within specified ranges.
The self-locking nut, designed with slightly ovalized threads to resist loosening when properly torqued, exhibited wear marks on one side of its wrenching flats (consistent with tool contact), on its aft face (consistent with washer contact), and on its internal threads (consistent with shaft engagement). Its average hardness met minimum design requirements. However, examination could not determine whether the nut had been improperly torqued at installation or had loosened during operation due to 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 showed the overhauled torque sensor assembly was installed on the engine on February 11, 2015. At the time of the accident, the assembly had accumulated 2,541.5 hours since overhaul. The engine-driven fuel pump and fuel control unit (FCU) are driven through a gear train that includes the front spur gear; failure in this train would prevent fuel delivery to the FCU, resulting in an uncommanded engine shutdown.