A catastrophic tail rotor system failure caused the loss of control in the fatal October 27 crash of an AW169 in Leicester, UK. This morning the UK’s Air Accidents Investigation Branch (AAIB) issued a special bulletin that enumerated the failure sequence that led to the pilot’s loss of control.
“The evidence gathered to date shows that the loss of control of the helicopter resulted from the tail rotor actuator control shaft becoming disconnected from the actuator lever mechanism,” the bulletin stated. This prevented the feedback mechanism from the tail rotor actuator from operating and the tail rotor from responding to yaw control inputs. The yaw stops were ineffective, the actuator continued to change pitch of the tail rotor blades, and an uncontrollable right yaw resulted.
On November 30, Leonardo published Emergency Alert Service Bulletins ASB 169-126 and ASB 189-217 calling for repetitive inspections of the castellated nut that secures the tail rotor actuator control shaft to the actuator lever mechanism and the tail rotor duplex bearing. EASA issued AD 2018-0261-E on November 30 mandating the repetitive inspections.
On the accident helicopter, the AAIB found that the castellated nut on the actuator end of the control shaft had been torqued to the extent that it friction welded it to the pin carrier and sheared the installed split pin. While the shaft was rotating under yaw control inputs it unscrewed from the nut, disconnected the shaft from the actuator lever, and caused the nut to become welded to the pin carrier. The AAIB also found that the tail rotor slider/spider duplex bearing showed fractures to the bearing cages and surface of the inner bearing races. The extent of the duplex bearing’s contribution to the accident remains under investigation.