# NTSB WPR21LA039: Agusta A109, N109EX, Los Angeles, CA, November 6, 2020 - Type: NTSB accident investigation - Classification: Accident · Serious injuries · Class 3 · Limited · Final report - Date: November 6, 2020, 15:00 local (23:00 UTC) - Place: Los Angeles, CA - Coordinates: 34.058037, -118.20811 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=34.0580&lon=-118.2081&mi=10 - Phase of flight: [Landing](https://openairdocs.org/ntsb-phases/550) (OpenAirDocs' grouping) - Defining event: [Landing: Flight control sys malf/fail](https://openairdocs.org/ntsb-events/333) - Highest injury: Serious injuries - Injuries: 2 serious, 1 minor - Status: Completed - Report: Approved by Office Director - Investigation class: Class 3 - NTSB office: Western Pacific Region - Parties: Federal Aviation Administration, Director of Maintenance Helinet Avn Services - ADs cited in the narratives: [AD 2014-02-08](https://openairdocs.org/ad/2014-02-08), [AD 2015-11-08](https://openairdocs.org/ad/2015-11-08) (OpenAirDocs) - NTSB case key (mKey): 102246 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=WPR21LA039 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/102246/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=102246 - Page: https://openairdocs.org/ntsb/WPR21LA039 ## 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. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Rotorcraft flight control: tail rotor control system](https://openairdocs.org/ntsb-findings/01046720): Aircraft › Aircraft propeller/rotor · Failure (probable cause); cases with it: [Agusta A109S, 1 case](https://openairdocs.org/ntsb-findings/01046720/cases.md?model=0260103), [All aircraft, 13 cases](https://openairdocs.org/ntsb-findings/01046720/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Landing: Flight control sys malf/fail](https://openairdocs.org/ntsb-events/333) (defining event); cases with it: [Agusta A109S, 1 case](https://openairdocs.org/ntsb-events/333/sequence.md?model=0260103), [All aircraft, 410 cases](https://openairdocs.org/ntsb-events/333/sequence.md) - [Emergency descent: Collision with terr/obj (non-CFIT) ](https://openairdocs.org/ntsb-events/470); cases with it: [Agusta A109S, 2 cases](https://openairdocs.org/ntsb-events/470/sequence.md?model=0260103), [All aircraft, 10,189 cases](https://openairdocs.org/ntsb-events/470/sequence.md) ## 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. ## Factual narrative 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. Examinationof 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 wascompleted 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. ## Aircraft - Registration: [N109EX](https://openairdocs.org/deregistered/N109EX-2021-11-30), registration cancelled 2021-11-30, S/N 22145, built 2009 (OpenAirDocs) - Reported make and model: AGUSTA A109 (S) - Registry model: [Agusta A109S](https://openairdocs.org/model/agusta-a109s) (registry code 0260103) (OpenAirDocs) - Registry engine: [Pratt & Whitney Canada PW207C](https://openairdocs.org/engine/pratt-whitney-canada-pw207c) (registry code 52203, turbo-shaft) (OpenAirDocs) - Category: Helicopter - Serial number: 22145 - Year built: 2009 - Airframe hours: 1446.7 - Engines: Pratt & Whitney PW207C 572 horsepower turbo shaft; Pratt & Whitney PW207C 572 horsepower turbo shaft - Damage: Substantial damage - Flight: Part 135 - Operator: Helinet Aviation Services - Registered owner: Prime Healthcare Air Transportation LLC - Airworthiness certificate: Standard, Normal - Last inspection: Unknown, October 22, 2020, 1,439.5 hours before - Departed: 03CA, La Mesa, CA - Destination: Same as departure - Flight plan: Company VFR - Airport: CL00, USC University Hospital (KECK USC) - Equipment: Video recording device, Onboard weather, Auto-pilot, ADS-B ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 45; injuries: minor; flight instructor, commercial; instrument ratings: helicopter; medical: class 1, with waivers/limitations; last flight review April 17, 2020; seat: right; restraint: 4-point; flight hours: 4,579 total, 81 in this make and model, 4,530 as pilot in command, 20 in the last 90 days, 6 in the last 30 days, 2 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 190° at 10 knots - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 72 °F, dew point 57 °F - Altimeter: 29.65 inHg - Observed at: EMT, 10 nm away ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Accident 20201106015399A](https://openairdocs.org/incident/20201106015399A), Nov 6, 2020: Agusta Spa A109S, Los Angeles, CA, Substantial damage. While on approach to the helipad at usc keck medical center, the aircraft lost control before touchdown and came to rest on it's side. The pilot indicated that… ## Docket (21 files) The files NTSB released with the investigation on January 25, 2023, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=102246. 26.5 MB in all. - Materials laboratory - factual report - rotating controls examination (item 1, PDF, 22 pages, 5.9 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14631334&FileExtension=pdf&FileName=21-071%20Materials%20Lab%20-%20Factual%20Report%20(WPR21LA039)-Rel.pdf - Materials laboratory - push pull tube examination (item 2, PDF, 3 pages, 182 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14714387&FileExtension=pdf&FileName=Materials%20Laboratory%20Factual%20Report%2020-060%20PP%20Tube-Rel.pdf - Helicopter data recorder specialist's factual report (item 3, PDF, 4 pages, 669 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=13188453&FileExtension=pdf&FileName=WPR21LA039_Vision1000Data_Final-Rel.pdf - Attachment 1 to helicopter data recorder specialist's factual report (item 4, CSV, 1.1 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=13189073&FileExtension=csv&FileName=Attachment%201-Rel.csv - Pilot/operator aircraft accident report, NTSB form 6120.1 (item 5, PDF, 9 pages, 1.9 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14697225&FileExtension=pdf&FileName=NTSB%20Form%206120_1%20WPR21LA039_Redacted-Rel.pdf - Pilot's statement (item 6, PDF, 5 pages, 45 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=12310060&FileExtension=pdf&FileName=N109EX%2011_06_2020_Redacted-Rel.pdf - Airframe and engine examination report (item 7, PDF, 21 pages, 3.2 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14702382&FileExtension=pdf&FileName=Airframe%20and%20Engine%20Examination%20WPR21LA039-Rel.pdf - Ring nut installation NTSB rev a (extract) (item 8, PDF, 17 pages, 1.6 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14677234&FileExtension=pdf&FileName=A109S%20N109EX%20-%20RING%20NUT%20INSTALLATION%20NTSB%20-%20REV%20A%20(Extract)-Rel.pdf - Pilots airworthiness directive daily sign off (item 9, PDF, 1 page, 96 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14552070&FileExtension=pdf&FileName=Ferris%20TR%20AD%20Compliance-Rel.pdf - ADS-B data (item 10, KMZ, 131 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14552862&FileExtension=kmz&FileName=20201106_A0269D_439743733-Rel.kmz - ADS-B raw data (item 11, XLSX, 5.7 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14553008&FileExtension=xlsx&FileName=20201106_A0269D_439743733_ADSB-Rel.xlsx - Easa ead 2012-0195E_1 (item 12, PDF, 3 pages, 150 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14552675&FileExtension=pdf&FileName=EASA_EAD_2012-0195-E_1-Rel.pdf - FAA airworthiness directives (item 13, PDF, 14 pages, 157 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14552340&FileExtension=pdf&FileName=FAA%20Airworthiness%20DIrectives-Rel.pdf - Maintenance records, 400-hour inspection (item 14, PDF, 1 page, 171 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14625415&FileExtension=pdf&FileName=109EX%20DUPLEX%20BEARING%20400HR%20LUBE-Rel.pdf - Dom report on lock wire inspection (item 15, PDF, 1 page, 441 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14702517&FileExtension=pdf&FileName=DoM%20Report%20on%20Lock%20WIre%20Inspections_Redacted-Rel.pdf - Rotating controls explained (item 16, PDF, 1 page, 108 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14545249&FileExtension=pdf&FileName=Rotating%20controls%20explained-Rel.pdf - T_r dual lock duplex bearing support (item 17, PDF, 1 page, 139 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14620917&FileExtension=pdf&FileName=T_R%20Dual%20Lock%20Duplex%20Bearing%20Support-Rel.pdf - Technical bulletins (item 18, PDF, 35 pages, 605 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14749174&FileExtension=pdf&FileName=Technical%20Bulletins-Rel.pdf - Statement of party representatives to NTSB investigation (item 19, PDF, 1 page, 769 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14553156&FileExtension=pdf&FileName=SIGNED%20NTSB%20FORM%20helinet_Redacted-Rel.pdf - Photos (item 20, PDF, 6 pages, 2.0 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14699478&FileExtension=pdf&FileName=Photos%20for%20WPR21LA039-Rel.pdf - Video file (item 21, MP4, 1.3 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14552312&FileExtension=mp4&FileName=Medical%20Helicopter-Rel.mp4 From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.