Accident Overview
On November 20, 2023, at approximately 2005 Pacific standard time, a Bell OH-58A helicopter (registration N158TR) operating under 14 CFR Part 137 as an aerial application flight was substantially damaged during an accident near Brawley, California. The pilot reported no injuries.
Pilot Account
According to the pilot, shortly after departure at about 150 feet above ground level (agl), the helicopter experienced a sudden power surge accompanied by abnormal engine noise and vibration. Believing an engine failure was imminent, the pilot maneuvered the helicopter toward flat terrain near the departure point. As engine power decayed, the pilot reduced rotor rpm to facilitate maneuvering over an embankment. At approximately 5 feet agl, the pilot applied forward cyclic to level the skids. The pilot estimated that the helicopter “fell through” from about 4 to 5 feet agl before landing hard, causing the main rotor hub assembly to separate from the main rotor mast. The helicopter came to rest upright, with the main rotor hub assembly found approximately 46 feet away.
Airframe Examination
Postaccident examination of the airframe revealed no evidence of preaccident mechanical malfunction or failure. Damage was observed to the main rotor mast, transmission mounts, and the K-flex driveshaft, consistent with excessive main rotor flapping, pylon whirl, and spike knock. The helicopter operator’s manual describes pylon whirl as a condition occurring after blade flapping and mast bumping, leading to elliptical pylon motion and spike knock—contact between the drag-pin fitting pin and the sides of the pylon stop square hole.
Engine Examination
The engine examination revealed seized N1 and N2 systems, a sheen of oil beneath the engine, and ferrous debris on both upper and lower chip detectors. All lubrication lines between the airframe and engine were secure, as were external oil, fuel, and air lines. Engine control continuity was established between the cockpit and the fuel control system. The engine was removed and sent to the manufacturer for teardown, which disclosed multiple failed bearings, failed drives, and fractures within the turbine scavenge oil pump assembly.
Metallurgical Findings
Metallurgical examination by the manufacturer, reviewed by the NTSB Metallurgical Laboratory, indicated no significant heat distress on the failed No. 8 bearing. Debris from this bearing was found within the turbine scavenge area of the oil pump, consistent with bearing deterioration while oil flow was present. The failure of the turbine scavenge oil pump was consistent with debris ingestion from the No. 8 bearing during operation. Additional bearing failures were attributed to oil starvation, and damage to other engine components was consistent with rubbing and operation under oil-starvation conditions.
Maintenance History
Maintenance records showed the most recent annual inspection was completed on April 18, 2023. On November 16, 2023, the previous engine assembly was removed and the accident engine assembly was installed. The helicopter had flown 10.3 hours since that installation before the accident.