Bell Helicopter Textron 206B N169AA accident near Arcadia, Iowa

Probable cause

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

NTSB analysis

The pilot reported that he was completing an aerial application flight about 6 ft above the corn field when the engine sustained a loss of power. The helicopter descended into the corn and impacted the field. A small postcrash fire ensued near the engine and was extinguished.

Postaccident examination revealed all external air, fuel, and oil line connections were tight except for the pneumatic air line (Pc line) B-nut connecting the power turbine governor (PTG) to the fuel control unit (FCU). The Pc line was not connected to the FCU and a gap was noted between the B-nut and FCU union. Residual torque paint was identified; however, there was no new-effective torque paint identified on any B-Nut. It appeared the B-nut had backed off the FCU fitting. This Pc is one of several that use air pressure to communicate fuel scheduling demands between the PTG and FCU. The security of this line is necessary for proper engine operation. Operation of the engine without the B-nut properly connected would result in the FCU reducing fuel to a sub-idle condition, with the engine not capable of producing power for continued flight. There were no other mechanical functions or failures that would have precluded normal operation.

Maintenance was performed on the helicopter 23 days and 12 flight hours before the accident. It is likely that the maintenance personnel did not properly torque this B-but or apply torque stripe paint, which resulted in a disconnection of the B-nut and a loss of engine power.

Text from the NTSB. Open the NTSB docket for CEN22LA351 for the full record.