Bell Helicopter Textron 206L-1 N242BH accident near Banning, California

Probable cause

The improper attachment of the tail rotor drive shaft to the air conditioner drive pulley without a drive ring, which resulted in the failure of the hardware, a loss of tail rotor drive, and the loss of control of the helicopter followed by a hard landing and rollover.

NTSB analysis

The pilot and two passengers were returning to the base airport following an aerial firefighting mission. During the approach to the airport, the helicopter began an uncontrolled right rotation that could not be arrested despite full use of the left pedal. The pilot attempted to land in the back yard of a private residence; however, the helicopter landed hard and rolled onto its left side. The helicopter sustained substantial damage to the main rotor system, fuselage, and tailboom.

An on-scene examination revealed the tail rotor drive shaft had separated at a flex coupling and components critical to the assembly were missing. Postaccident metallurgical examination of tail rotor drive shaft assembly, the pulley, and the recovered hardware revealed that tail rotor drive shaft flange was likely directly attached to the air conditioning pulley flange without a drive ring. The bolt length and recovered hardware would have been adequate to secure the drive shaft flange and the pulley flange together to a drive ring; however, the drive ring, two bolts, six washers and one nut were not found amongst the wreckage after a thorough search. The examination also revealed that the two bolts that were recovered failed from fatigue, likely from the cyclic stress due to the absence of a drive ring.

According to the helicopter maintenance records, there had been no recent work on the tail rotor drive shaft or air conditioning unit that would have required disassembly and reassembly of the failed flex coupling.

Text from the NTSB. Open the NTSB docket for WPR22LA340 for the full record.