Md Helicopter 600 N745BW accident near San Andreas, California

Probable cause

The failure of the output shaft of the overrunning clutch inner race due to frictional overheating of the shaft, which resulted from the failure of the output bearing’s stainless-steel grease seal.

NTSB analysis

During low-altitude operations to visually assess power lines, the pilot and front seat passenger detected smoke in the cockpit when the helicopter was about 250 ft above ground level. The pilot elected to make a precautionary landing and rapidly lowered the collective. When the helicopter descended to an altitude of about 30 ft above ground level, the engine lost power. The helicopter then yawed to the left and landed hard, which caused the right landing gear to fail. A postaccident fire ensued and consumed most of the fuselage and tailboom.

Postaccident examination of the airframe revealed that most of the fuselage, tailboom, and cabin interior was destroyed due to extensive thermal damage. Flight control continuity could not be confirmed due to the thermal damage to the fuselage.

Examination of the engine revealed no mechanical malfunctions or failures that would have precluded normal operation. Data recovered from the engine control unit revealed primary exceedance messages for a main rotor droop, which was followed almost immediately by an engine surge then a flameout. Immediately after the main rotor droop, the torque and fuel flow dropped to zero, which was consistent with a complete loss of load. Additionally, the engine control unit data showed that the rotational speed of the engine rose above the rotational speed of the main rotor, which was consistent with a break in the main rotor drive system.

Examination of the main rotor drive system revealed the overrunning clutch subassembly remained attached to the power takeoff gear shaft on one end but had separated from the interconnecting driveshaft at the inner race output shaft. The output shaft of the overrunning clutch inner race fractured at the output bearing, which resulted in a loss of torque to the transmission and the main rotor system and a loss of load to the engine.

The output shaft deformed and fractured due to high-temperature overstress at the output bearing. The highest heat, which was localized to the area around the output bearing inner race, was not from the postaccident fire because the overall pattern of heating, scoring, and deformation was consistent with frictional heating from a failure of the output bearing. That failure caused the output bearing to seize and spin on the clutch inner race. The flakes of stainless steel found between the balls, cage, and outer race were likely remnants of the stainless-steel insert that was part of the grease seal, and the accumulation of the flakes between the balls indicated the seal failed before the bearing seized.

A review of maintenance documents revealed that the bearing was inspected and repacked with grease as required by the manufacturer.

Text from the NTSB. Open the NTSB docket for WPR20LA228 for the full record.