On November 6, 2020, about 1500 Pacific daylight time, an Agusta (Leonardo) A109S helicopter, N109EX, was substantially damaged when it was involved in an accident near the University of Southern California, Keck Medical Center, Los Angeles, California. The pilot and one passenger sustained serious injuries, and one passenger were uninjured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 135 helicopter air ambulance flight.

The pilot reported that during a steep approach to the rooftop helipad on the medical center, 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. The events were consistent with a video captured by a witness located in an adjacent building, which showed the helicopter as it approached 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 it impacted the helipad. The main rotor blades contacted the helipad, followed by the left main landing gear and fuselage. The helicopter came to rest on its left side while still on the helipad. All major components of the helicopter were located on the rooftop. The fuselage, main rotor system and the tailboom were substantially damaged. The tail rotor assembly along with the 90° gearbox was separated from the end of the tailboom. Examination of the tail rotor assembly rotating controls revealed that the ring nut was separated from the thrust sleeve assembly (figure 1.). No other mechanical malfunctions or failures were found with the helicopter that would have precluded normal operations.

Figure 1. Photo of the tail rotor rotating controls, showing the ring nut, and thrust sleeve. Photo courtesy of the Federal Aviation Administration (FAA). According to training documents provided by the helicopter manufacturer, the rotating controls are the mechanisms that translate the output from the tail rotor control servo actuator into pitch changes of the tail rotor. Examination of the tail rotor assembly and 90° gearbox by the National Transportation Safety Board (NTSB) Materials Laboratory revealed a step, or worn footprint, in the bottom surface of the ring nut at the outside diameter edge (figure 2). 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 locktite were inconclusive.

Figure 2. Image showing the step or worn footprint on the accident ring nut, and an exemplar ring nut. Photo of the exemplar ring nut courtesy of Leonardo Helicopters. During the investigation the manufacturer advised the NTSB that there were four previous cases that were associated with loss of tail rotor control and the associated ring nut. According to the manufacturer, previous internal investigations that involved ring nut failures resulting from installation issues of the ring nut were notified, investigated, and managed in the past. There were 3 occurrences found that involved the ring nut between December 2010 and October 2014. None of these occurrences were investigated by the NTSB. After the first three occurrences, the manufacturer made changes to maintenance procedures that included increasing the ring nut torque value, dual lockwire application, the application of Loctite to the ring nut, adding a note to the maintenance manual to ensure the duplex bearing is properly seated, and procedures to improve the cleaning and greasing operation of the duplex bearing.

The European Union Aviation Safety Agency (EASA) issued an emergency airworthiness directive (EAD), 2012-1095E, dated September 25, 2012. The reason for the EAD was “Cases of loosening of the tail rotor duplex bearing locking nut Part Number (P/N) 109-0130-97 have been reported on A109 helicopters. The investigation identified the absence or the damage of one or both of the locking wires securing the tail rotor duplex bearing locking nut P/N 109-0130-97.”

The helicopter manufacturer issued five Technician Bulletins (Bollettinos Tecnico) in September 2012, for all affected models. The bulletins required, in part, a visual inspection of the lock wire (s) securing the tail rotor duplex locking nut to the housing, periodic inspection (every 25 hours) to verify the condition of the two locking wires until compliance with disassembling and reassembling of the housing and slider group of the tail rotor rotating controls. The FAA issued Airworthiness Directive (AD) 2014-02-08 on March 7, 2014, based upon the EASA EAD. Following the issuance of the FAA AD 2014-02-08, a fourth failure occurred on October 21, 2014, after the housing and slider group of the tail rotor rotating controls had been reassembled. Consequently, the FAA superseded the AD 2014-02-08 with AD 2015-11-08, dated June 24, 2015, which included, in part, the removal of the disassembly and reassembly as a terminating action. The AD also included a daily check by the pilot to enhance detection of an impending failure of a tail rotor duplex bearing ring nut installation. Review of the helicopter maintenance logbooks revealed that the helicopter was maintained in accordance with the manufacturers extended maintenance program. The most recent 400-hour extended maintenance check, which included the disassembly, and inspection of the rotating controls was completed on May 17, 2019, at a Hobbs time of 1,149.2 hours. According to information provided by the operator on the NTSB Form 1620.1, the total times for both engines, about the time of the accident was 1449.1 hours. According to maintenance records, the airframe total time matched the engines total times. The accident pilot had signed off the tail rotor lockwire inspection on November 6, 2020, at 1120 hours. Prior to that, the previous pilot had signed off the tail rotor lockwire inspection on November 6, 2020, at 0400 hours. Postaccident photos show that part of one locking wire remained attached to the thrust sleeve (Figure 3).

Figure 3. The left photo shows part of one locking wire attached to the accident sleeve, and the right photo shows an exemplar locking wire from a different helicopter. (Photo of the accident ring and sleeve, courtesy of the FAA) According to the manufacturer a new tail rotor dual lock duplex bearing support has been developed that incorporates a more robust locking system and is compatible with the current duplex bearing. The target for certification is the fourth quarter of 2022 and will be retrofit through service bulletins.