Bell 206B N111PM accident near El Centro, California

Probable cause

An in-flight failure of the engine-to-transmission driveshaft due to a fatigue fracture of the forward-most flex plate in the forward KAflex coupling that resulted from a loose bolt attachment between the flex plate and the forward driveshaft coupling. Contributing to the accident was maintenance personnel’s failure to identify the condition and remove the KAflex driveshaft for repair as specified by the manufacturer.

NTSB analysis

The pilot was flying in night visual flight rules (VFR) conditions about 100 ft above ground level (agl), toward an intended application site for agricultural pesticides, when the helicopter began to make a loud noise and vibrate. Unsure if the helicopter had lost rotor rpm or not, the pilot lowered the collective and initiated a run-on landing to a field.

Postaccident examination of the helicopter’s airframe and engine revealed that the forward-most flex plate of the forward flex coupling on the KAflex driveshaft assembly was fractured. No evidence of additional preaccident mechanical malfunction or failure that would have precluded normal operation was observed.

Fracture analysis revealed that the flex plate failed due to a fatigue fracture resulting from a loose attachment between the flex plate and the forward driveshaft fitting.

Multiple areas of missing or replacement torque paint were observed throughout the KAflex driveshaft assembly’s joints. It is likely that the joints had either slipped or been manually disturbed before the non-original torque paint was applied, which would have been cause for the driveshaft assembly to be removed and shipped to the manufacturer. However, no evidence of the driveshaft’s removal after visual confirmation of joint slippage or disturbance was recorded in the aircraft maintenance logs.

Text from the NTSB. Open the NTSB docket for WPR24LA067 for the full record.