Accident Summary
On November 6, 2020, about 1500 Pacific daylight time, an Agusta (Leonardo) A109S helicopter, registration N109EX, was substantially damaged when it was involved in an accident near the University of Southern California, Keck Medical Center, Los Angeles, California. The helicopter was operated as a Title 14 Code of Federal Regulations Part 135 helicopter air ambulance flight.
Injuries and Damage
The pilot and one passenger sustained serious injuries; one passenger was uninjured. The helicopter came to rest on its left side on the helipad. The fuselage, main rotor system, and tailboom were substantially damaged. The tail rotor assembly along with the 90° gearbox separated from the end of the tailboom.
Operational Details
The pilot reported that during a steep approach to the rooftop helipad, he decelerated to about 40 to 50 knots and changed the variable engine rpm switch from 100% to 102%. As the helicopter was about 40 ft above the helipad, the pilot noticed a slight yaw to the right that continued despite full left pedal application. The pilot stated that as the helicopter nose cleared the building, it “suddenly and very violently experienced a dramatic increase in right yaw.” The pilot realized the helicopter was going to be uncontrollable, and “dumped the collective” to stay over the helipad. The helicopter impacted the helipad, rolled left, and came to rest on top of the helipad.
Video captured by a witness in an adjacent building showed the helicopter approaching the rooftop helipad while slowly rotating clockwise about the vertical axis. The helicopter stopped descending momentarily while it continued to rotate. The helicopter then descended while continuing to rotate an additional 180° as it rolled to the left before impact. The main rotor blades contacted the helipad, followed by the left main landing gear and fuselage.
Tail Rotor Examination
Examination of the tail rotor assembly rotating controls revealed that the ring nut was separated from the thrust sleeve assembly. No other mechanical malfunctions or failures were found with the helicopter that would have precluded normal operations. Examination by the NTSB Materials Laboratory revealed a step, or worn footprint, in the bottom surface of the ring nut at the outside diameter edge. The threads of the outer diameter of the ring nut and the inner diameter of the thrust sleeve appeared intact. Examination of the grease for trace amounts of required Loctite were inconclusive.
According to training documents provided by the manufacturer, the rotating controls translate the output from the tail rotor control servo actuator into pitch changes of the tail rotor.
Previous Incidents and Maintenance Actions
During the investigation, the manufacturer advised the NTSB that there were four previous cases associated with loss of tail rotor control and the associated ring nut. Previous internal investigations that involved ring nut failures resulting from installation issues were notified, investigated, and managed in the past. Three occurrences involved the ring nut between December 2010 and October 2014; none were investigated by the NTSB. After the first three occurrences, the manufacturer changed maintenance procedures, including increasing ring nut torque value, dual lockwire application, application of Loctite to the ring nut, adding a note to ensure the duplex bearing is properly seated, and procedures to improve cleaning and greasing of the duplex bearing.
The European Union Aviation Safety Agency (EASA) issued emergency airworthiness directive (EAD) 2012-1095E dated September 25, 2012, citing cases of loosening of the tail rotor duplex bearing locking nut and identified absence or damage of one or both locking wires. The helicopter manufacturer issued five Technician Bulletins in September 2012 requiring visual inspection of the lock wire(s) and periodic inspection every 25 hours until compliance with disassembling and reassembling of the housing and slider group. The FAA issued Airworthiness Directive (AD) 2014-02-08 on March 7, 2014, based on the EASA EAD. A fourth failure occurred on October 21, 2014, after reassembly, leading the FAA to supersede with AD 2015-11-08 dated June 24, 2015, which removed disassembly and reassembly as a terminating action and included a daily check by the pilot.
Maintenance History
Review of the helicopter maintenance logbooks revealed that the helicopter was maintained in accordance with the manufacturer's extended maintenance program. The most recent 400-hour extended maintenance check, which included disassembly and inspection of the rotating controls, was completed on May 17, 2019, at a Hobbs time of 1,149.2 hours. Total times for both engines and the airframe at the time of the accident were 1,449.1 hours. The accident pilot signed off the tail rotor lockwire inspection on November 6, 2020, at 1120 hours; the previous pilot had signed off the same inspection at 0400 hours that day. Postaccident photos show that part of one locking wire remained attached to the thrust sleeve.
The manufacturer has developed a new tail rotor dual lock duplex bearing support with a more robust locking system, compatible with the current duplex bearing. Certification is targeted for the fourth quarter of 2022, with retrofit through service bulletins.