Bell 206-L4 N720RL accident near Abbeville, Louisiana

Probable cause

Failure of maintenance personnel to ensure adequate torque of a tailrotor drive shaft coupling bolt, which resulted in the partial disconnection of the drive shaft.

NTSB analysis

The helicopter was on its first flight after extensive maintenance. As the pilot approached a canal at an altitude of about 20 feet, he heard a noise and then a loud "pop." The helicopter then started to spin to the right and the pilot was unable to stop the spin. After about two revolutions, the pilot reduced engine power to idle, and the helicopter collided with terrain and water.

Examination of the helicopter revealed that the connection between the tail rotor drive shaft and the tail rotor gear box was partially separated. Of the two bolts that connected the drive shaft to the coupling on the gear box, one remained secured by its associated nut; however, the other bolt had backed out of the drive shaft, but remained in the coupling, and its corresponding nut was missing. The helicopter had accrued about 0.7 flight hours since its most recent maintenance, which included removing and reinstalling the tail rotor gear box. It is likely that the maintenance personnel did not ensure adequate torque of a tailrotor drive shaft coupling bolt, which allowed it to be back out of the drive shaft.

Text from the NTSB. Open the NTSB docket for CEN13LA321 for the full record.