WPR24LA041

The pilot was conducting agricultural operations at night and had just departed from a company loading area. About 150 ft above ground level (agl), the pilot experienced a sudden power surge, abnormal engine noise, and vibration. The pilot believed that engine failure was imminent and turned back toward his point of departure. Engine power decayed, and the pilot bled off rotor rpm to maneuver over terrain toward flat ground. The helicopter landed hard, entered a pylon whirl condition, and the main rotor hub assembly separated from the main rotor mast. Postaccident examination of the airframe revealed no evidence of preaccident mechanical malfunction or failure. However, postaccident examination revealed a sheen of oil beneath the engine and that the engine’s compressor (N1) and power turbine (N2) systems were seized. Further examination of the engine revealed multiple failed bearings, failed drives, and fractures within the turbine scavenge oil pump assembly. The engine had been installed on the helicopter about 10 flight hours before the accident. Based on metallurgical analysis of the engine components, the No. 8 bearing likely failed and shed debris to the oil pump, which bound the first level oil pump gears and led a gear tooth to fracture and the pump driveshaft to fail. Subsequently, the engine was starved of oil and lost all power. The cause of the bearing failure could not be determined.

On November 20, 2023, about 2005 Pacific standard time, a Bell OH-58A helicopter, N158TR, was substantially damaged when it was involved in an accident near Brawley, California. The pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. According to the pilot, shortly after departure, about 150 ft agl, there was a sudden power surge followed by abnormal engine noise and vibration. He believed that engine failure was imminent and turned the helicopter toward flat terrain near his departure point. The engine power decayed and the pilot “bled off some of the available rotor rpm” to maneuver over an embankment. About 5 ft agl, he applied forward cyclic to level the skids. The pilot estimated that the helicopter “fell through” from about 4-5 ft agl before it landed hard and the main rotor hub assembly separated from the main rotor mast. The helicopter came to rest upright and the main rotor hub assembly came to rest about 46 ft from the helicopter. Postaccident examination of the airframe revealed no evidence of preaccident mechanical malfunction or failure. Examination of the airframe revealed damage to the main rotor mast, transmission mounts, and the K-flex driveshaft that was consistent with excessive main rotor flapping, pylon whirl, and spike knock. According to the helicopter operator’s manual, “Pylon whirl is a condition which occurs after blade flapping and mast bumping. The resultant motion of the pylon is elliptical, and spike knock is apt to occur…Spike knock occurs when the round pin in the drag-pin fitting contacts the side of the square hole of the pylon stop.” Postaccident examination of the engine revealed seized N1 and N2 systems, a sheen of oil beneath the engine, and ferrous debris on the upper and lower chip detectors. The security of all lubrication lines between the airframe and engine was verified. All engine external oil lines, fuel lines, air lines, and connections were secure. Engine control continuity was established between the cockpit controls and the engine fuel control system. The engine was removed and sent to the manufacturer for further examination and teardown. Teardown revealed multiple failed bearings, failed drives, and fractures within the turbine scavenge oil pump assembly. Metallurgical examination conducted by the manufacturer and reviewed by the NTSB Metallurgical Laboratory revealed no evidence of significant heat distress on the failed No. 8 bearing. Debris from the bearing was located within the turbine scavenge area of the oil pump. The condition of the debris was consistent with bearing deterioration while oil flow was present. Failure of the turbine scavenge oil pump was consistent with the presence of debris from the No. 8 bearing during pump operation, and additional bearing failures were consistent with deterioration due to oil starvation. Damage to the other engine components was consistent with rubbing damage and operationunder oil-starvation conditions. Maintenance records indicate that the most recent annual inspection was completed on April 18, 2023. On November 16, 2023, the previous engine assembly installed on the accident helicopter was removed, and the accident engine assembly was installed. After the accident engine installation, 10.3 hours were flown before the accident.