Agusta A109 N109EX accident near Los Angeles, California

Probable cause

The loss of tail rotor control due to the separation of the tail rotor duplex bearing ring nut from the thrust sleeve, which resulted in the loss of helicopter control.

NTSB analysis

The pilot established the helicopter in an approach to land on a rooftop helipad. During the approach, as the helicopter was about 40 ft above the helipad, the helicopter began to yaw to the right despite the pilot’s control inputs. As the pilot continued the approach, he experienced a loss of tail rotor control and attempted to land on the helipad as the helicopter’s right yaw increased. A witness videoed the accident sequence which showed the helicopter rotating clockwise about the vertical axis and rolling to the left before it impacted the helipad. Postaccident examination of the helicopter revealed that the ring nut for the tail rotor duplex bearing was backed out of the sleeve, which resulted in the loss of tail rotor control.

During the investigation, the manufacturer provided historical records of four previous cases that involved ring nut failures. These cases resulted in the manufacturer developing changes to maintenance practices, issuance of European Union Aviation Safety Agency Emergency Airworthiness Directive 2012-0195E in September 2012, and the issuance of Federal Aviation Administration Airworthiness Directive (AD) 2014-02-08, in March 2014, which was superseded by AD 2015-11-08, in June 2015, as a result of a fourth occurrence. The AD added, in part, a daily pilot check to enhance detection of an impending failure of a tail rotor duplex bearing ring nut installation.

The accident pilot and the previous pilot who was assigned to the helicopter reported visually inspecting the two locking wires and signing off on the inspection.

Text from the NTSB. Open the NTSB docket for WPR21LA039 for the full record.