Bell 47G-3B-1 N2618W accident near Garey, California

Probable cause

Maintenance personnel’s failure to properly check the torque paint on a fuel control unit B-nut, which resulted in the loosening of the B-nut, a loss of engine power at a low altitude, and a subsequent impact with terrain.

NTSB analysis

The pilot of the helicopter was applying a chemical to a field when the engine sustained a loss of power. The pilot entered an autorotation and the helicopter descended to the terrain, where it rolled onto its right side substantially damaging the main rotor system and severing the tailboom.

Postaccident examination of the pneumatic air lines revealed that there was a slight air leak at the accumulator and a large air leak at a pneumatic air line (Pc) between the power turbine governor (PTG) and the fuel control unit (FCU); the Pc line was not connected to the FCU. The B-nut connecting the Pc line to the FCU had backed off, allowing the Pc line to become disconnected. . This Pc line is one of several lines that use air pressure to communicate fuel scheduling commands between the PTG and the FCU. The security of this line is necessary for proper engine operation. Engine operation without the Pc line properly connected would result in the FCU reducing fuel to a sub-idle condition, preventing the engine from producing sufficient power for continued flight.

No other preaccident mechanical malfunctions or failures were found that would have precluded normal operation.

Maintenance was performed on the helicopter 15 days and 29.2 flight hours before the accident that included checking all B-nuts for torque paint to ensure adequate torque remained on the B-nuts. Maintenance personnel likely did not properly inspect the condition of the torque paint on the affected B-nut, which resulted in a disconnection of the B-nut from the FCU and a total loss of engine power.

Text from the NTSB. Open the NTSB docket for WPR24LA093 for the full record.