Helicopter Entered An Uncommanded, Continuous Right Rotation About The Vertical Axis

Location: Stockton, California Accident Number: WPR22LA221
Date & Time: June 17, 2022, 06:15 Local Registration: N90326
Aircraft: Bell 206B Aircraft Damage: Substantial
Defining Event: Unknown or undetermined Injuries: 1 None
Flight Conducted Under: Part 91: General aviation - Positioning

Analysis: The pilot had departed his home base to relocate the helicopter for an upcoming aerial application job when he felt a vibration and heard a bang, followed immediately by an uncommanded right yaw. While performing a precautionary landing to an orchard, the helicopter entered an uncommanded, continuous right rotation about the vertical axis. The pilot rolled off the throttle and increased collective to cushion the landing, but the helicopter continued the right rotation as it touched down, which resulted in the helicopter rolling over onto its left side.

The helicopter sustained substantial damage to the fuselage, transmission, main rotor, and tail rotor assemblies. Fragments of one tail rotor blade was recovered about 2,000 ft from the main wreckage. The pilot stated that he was unsure if a bird had struck the tail rotor during flight. Postaccident examination of the tail rotor assembly revealed both blades were damaged, and one blade fractured and separated from the hub.

The tail rotor blades were shipped to the National Transportation Safety Board (NTSB) materials laboratory, and subsequently to the manufacturer’s laboratory for further examination. The combined examinations revealed translaminar overstress fractures of both tail rotor blades, with one blade exhibiting a leading-edge high-energy impact. Both blades exhibited an unidentified foreign material transfer onto their leading edges. A small amount of nitrile rubber was found lodged into the end of one blade; however, its origin was undetermined.

Probable Cause and Findings: The National Transportation Safety Board determines the probable cause(s) of this accident to be -- Tail rotor blade impact with an unidentified object, which resulted in the loss of control and subsequent rollover of the helicopter. 

FMI: www.ntsb.gov