On September 17, 2019, about 1230 eastern daylight time, a Robinson R44, N827JE, was substantially damaged when it was involved in an accident near Sardis, Georgia. The pilot was seriously injured. The helicopter was operated as Title 14 Code of Federal Regulations Part 137 aerial application flight. The pilot reported that he was flying about 100 ft above ground level, applying spray, when he heard a "pop," followed by alarms and lights activating in the cockpit. The helicopter then lost engine power and the pilot performed an autorotation. During the autorotation, the helicopter impacted trees and terrain. Examination of the helicopter by a Federal Aviation Administration inspector revealed that it came to rest on its left side. The inspector observed damage to the fuselage and the tail boom had separated. Subsequent teardown examination of the engine revealed that the left crankshaft idler gear was not engaged in the teeth of the crankshaft gear or the camshaft gear. The idler gear was removed and the idler gear shaft was observed not attached to its left crankcase half mounting location. According to an engine diagram, the idler gear shaft was supposed to secured to the left crankcase half by a bolt and a crankcase stud. Worn fragments of the bolt and crankcase stud, along with the stud nut and fragments of safety wire, were recovered from the oil sump. Metallurgical examination revealed evidence of wear on crankshaft idler gear, shaft flange, and gear shaft fasteners (bolt and stud). The worn gear shaft fasteners exhibited a rotating bending fatigue failure (for more information, see Materials Laboratory Factual Report in the public docket for this accident). According to maintenance records and information from the operator, the engine had been operated for about 1,900 hours over a 20-year-period since major overhaul. Review of Lycoming Service Instruction (SI) 1009BD, superseded by SI 1009BE, revealed a recommended time between overhauls as 12 calendar years or 1,500 hours of operation in the case of engines used in crop dusting or other chemical application, whichever occurs first.