ERA19FA118 · Hughes 369 D

The helicopter pilot was using an aerial saw to trim trees along power lines. A witness reported that the pilot performed one pass, then returned for the second pass when the helicopter began to spin counterclockwise around its main rotor mast. During the second rotation, the helicopter impacted trees and terrain before coming to rest on its right side. Postaccident examination revealed a fracture of the overrunning clutch outer race and cracks on three of the engine mounts. While two of the three engine mounts cracks were likely a result of impact forces, the cracks on a third engine mount were likely present prior to the accident. The failure of the overrunning clutch subassembly outer race resulted in a loss of power to the main rotor system. Given the helicopter's low altitude and airspeed at the time of the loss of power, the pilot was likely unable to successfully initiate and perform an autorotative landing.

Before its installation onto the helicopter, the overrunning clutch subassembly had been repaired, which included the installation of a new outer race. The accident helicopter operated with the overrunning clutch subassembly for about 33 flight hours until the accident occurred. Examination of the fracture surfaces revealed signatures consistent with fatigue and subsequent overload. The orientation of the crack in the circumferential plane was consistent with an anomalous bending load, such as an angular misalignment of the drivetrain, that drove fatigue crack initiation.

Directionality of the deformation and fracture of the center engine mount was primarily in the vertical direction and the fracture surfaces exhibited evidence of oxidation, indicating that the cracks were present before the accident. Therefore, it is likely that the damaged center engine mount was the source of the anomalous bending loads that led to fatigue crack initiation and subsequent failure of the overrunning clutch outer race.

The 100-hour/annual inspection of the engine mounts could be performed with the engine installed on the airframe, and the operator had done so; however, the presence of the engine could present difficulties for a mechanic in performing a visual inspection, particularly of the forward side of the center engine mount, where the cracks were located. Maintenance records indicated that, in the 9 months preceding the accident, the engine mounts were inspected six times with no anomalies observed. However, when the engine mount cracks may have developed and whether they would have been visible during any of the previous inspections could not be determined based on the available information..

Verbatim NTSB analysis and probable cause from the NTSB dataset