# NTSB CEN12LA120: Continental Copters 47G2, N9021R, Lohn, TX, January 1, 2012 - Type: NTSB accident investigation - Classification: Accident · Serious injuries · Limited investigation · Final report - Date: January 1, 2012, 14:00 local (20:00 UTC) - Place: Lohn, TX - Coordinates: 31.325, -99.408332 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=31.3250&lon=-99.4083&mi=10 - Phase of flight: [Maneuvering](https://openairdocs.org/ntsb-phases/450) (OpenAirDocs' grouping) - Defining event: [Maneuvering-low-alt flying: Flight control sys malf/fail](https://openairdocs.org/ntsb-events/333) - Highest injury: Serious injuries - Injuries: 2 serious - Status: Completed, report September 5, 2013 - Report: Approved by Office Director - NTSB office: Central Region - Parties: Federal Aviation Administration, Scott's - Bell 47, Inc. - NTSB case key (mKey): 82607 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=CEN12LA120 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/82607/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=82607 - Page: https://openairdocs.org/ntsb/CEN12LA120 ## Probable cause The improper installation of the tail rotor gearbox by maintenance personnel, which led to accelerated wear of the ring and pinion gears, resulting in the loss of tail rotor effectiveness and subsequent forced landing. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Tail rotor drive system: tail rotor gearbox](https://openairdocs.org/ntsb-findings/01046520): Aircraft › Aircraft propeller/rotor · Fatigue/wear/corrosion (probable cause); cases with it: [Bell-Continental Copters 47G2, 1 case](https://openairdocs.org/ntsb-findings/01046520/cases.md?model=1181069), [All aircraft, 12 cases](https://openairdocs.org/ntsb-findings/01046520/cases.md) - [Tail rotor drive system: tail rotor gearbox](https://openairdocs.org/ntsb-findings/01046520): Aircraft › Aircraft propeller/rotor · Incorrect service/maintenance (probable cause); cases with it: [Bell-Continental Copters 47G2, 1 case](https://openairdocs.org/ntsb-findings/01046520/cases.md?model=1181069), [All aircraft, 12 cases](https://openairdocs.org/ntsb-findings/01046520/cases.md) - [Maintenance: repair](https://openairdocs.org/ntsb-findings/02062015): Personnel issues › Task performance · Maintenance personnel (probable cause); cases with it: [Bell-Continental Copters 47G2, 1 case](https://openairdocs.org/ntsb-findings/02062015/cases.md?model=1181069), [All aircraft, 99 cases](https://openairdocs.org/ntsb-findings/02062015/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Prior to flight: Aircraft maintenance event](https://openairdocs.org/ntsb-events/040); cases with it: [Bell-Continental Copters 47G2, 1 case](https://openairdocs.org/ntsb-events/040/sequence.md?model=1181069), [All aircraft, 484 cases](https://openairdocs.org/ntsb-events/040/sequence.md) - [Maneuvering-low-alt flying: Flight control sys malf/fail](https://openairdocs.org/ntsb-events/333) (defining event); cases with it: [Bell-Continental Copters 47G2, 1 case](https://openairdocs.org/ntsb-events/333/sequence.md?model=1181069), [All aircraft, 410 cases](https://openairdocs.org/ntsb-events/333/sequence.md) - [Emergency descent: Loss of tail rotor effectiveness](https://openairdocs.org/ntsb-events/242); cases with it: [Bell-Continental Copters 47G2, 1 case](https://openairdocs.org/ntsb-events/242/sequence.md?model=1181069), [All aircraft, 100 cases](https://openairdocs.org/ntsb-events/242/sequence.md) - [Uncontrolled descent: Collision with terr/obj (non-CFIT) ](https://openairdocs.org/ntsb-events/470); cases with it: [Bell-Continental Copters 47G2, 1 case](https://openairdocs.org/ntsb-events/470/sequence.md?model=1181069), [All aircraft, 10,189 cases](https://openairdocs.org/ntsb-events/470/sequence.md) ## Analysis The pilot performed a preflight inspection of the helicopter to include checking the tail rotor oil level through its sight glass. He had flown the helicopter for more than an hour when the tail yawed to the left, slowly at first then rapidly. The pilot “cut” the throttle, flew the helicopter “to the ground as fast as possible,”and impacted terrain. A postaccident examination of the wreckage revealed that the tail rotor gearbox did not operate when it was rotated. A teardown revealed that the interior surfaces of the gearbox were covered with a black oily substance consistent with residual lubricating oil contaminated with liberated metal particles. The oil level sight glass exhibited no discoloration or other hindrance to normal oil level detection in service. Examination of the ring and pinion gears revealed that the teeth on the pinion gear were deformed and worn almost down to the tooth root in some locations. Wear was observed on the top land and drive faces of the ring gear. Wear patterns were noted on the coast faces of the ring and pinion gear teeth. The observation of wear on the coast faces of the gear teeth is most probably an indication that the spacing between the gears was too tight as a result of improper alignment during installation, causing accelerated wear. A logbook entry showed that the helicopter’s last annual inspection was completed about 6 months before the accident during which the tail rotor gearbox was overhauled and installed. ## Factual narrative On January 1, 2012, about 1400 central standard time, a Continental Copters Inc. 47G2 helicopter, N9021R, impacted terrain during a forced landing following a loss of tail rotor effectiveness near Lohn, Texas. The commercial pilot and passenger sustained serious injuries. The helicopter’s tailboom was substantially damaged during the forced landing. The helicopter was registered to and operated by Sky Horse Helicopters Inc. under the provisions of 14 Code of Federal Regulations Part 91 as an aerial observation flight. Day visual flight rules (VFR) conditions prevailed for the flight, which did not operate on a VFR flight plan. The helicopter departed on the local flight about 1300. The pilot stated in his accident report that he performed a preflight inspection of the helicopter to include checking the tail rotor oil level through its sight glass. He flew the helicopter for “over” an hour when the tail yawed to the left. The yaw was slow at first and then the helicopter yawed rapidly. The pilot “cut” the throttle and flew the helicopter “to the ground as fast as possible.” A logbook endorsement showed that the helicopter’s last annual inspection was completed on July 13, 2011, and that the helicopter had accumulated 8,989 hours of total flight time. The pilot reported that the helicopter had accumulated 9,057 hours of total flight time at the time of the accident and that he did not have any records in reference to the tail rotor gearbox. A Federal Aviation Administration (FAA) inspector examined the wreckage on-scene. The tail rotor gearbox did not operate when it was rotated. The FAA inspector measured the gearboxes' shim and he reported its thickness measurement as 0.015 inches, plus or minus 0.001 inches. The accident gearbox, marked with part no. 47-640-044-39 and serial no. A13-3661, was retained and was shipped to Scott’s – Bell 47 Inc. for detailed examination under FAA supervision. The examination, in part, revealed that the interior surfaces of the gearbox were covered with a black oily substance consistent with residual lubricating oil contaminated with liberated metal particles. The oil level sight glass exhibited no discoloration or other hindrance to normal oil level detection in service. Damage was noted on the upper aft portion of the main case assembly where the aft casting accommodated the tail rotor guard. This area of displaced or previously removed material was filled with a media consistent with an epoxy mix. The gearbox assembly was dismantled in accordance with procedures listed in its service instruction and no significant difficulties or abnormal conditions were encountered. Scott’s – Bell 47 Inc. report is appended to the docket material associated with this case. Parts, to include the ring and pinion gear, were removed and shipped to the National Transportation Safety Board (NTSB) Materials Laboratory. A NTSB Senior Materials Engineer examined the parts and produced Materials Laboratory Factual Report No. 13-028. Theexamination revealed that the teeth on the pinion gear were deformed and worn almost down to the tooth root in some locations. Wear was observed on the top land and drive faces of the ring gear. Wear patterns were noted on the coast faces of the ring and pinion gear teeth. Hardness testing revealed that both gears exhibited a trend of increasing hardness with decreasing distance from the bottom land surface. The NTSB Materials Laboratory report is appended to the docket material associated with this case. ## Aircraft - Registration: [N9021R](https://openairdocs.org/aircraft/N9021R), registered now, S/N CCI-165 (OpenAirDocs) - Reported make and model: CONTINENTAL COPTERS INC. 47G2 - Registry model: [Bell-Continental Copters 47G2](https://openairdocs.org/model/bell-continental-copters-47g2) (registry code 1181069) (OpenAirDocs) - Registry engine: [Lycoming VO-435-A1F](https://openairdocs.org/engine/lycoming-vo-435-a1f) (registry code 42200, reciprocating) (OpenAirDocs) - Category: Helicopter - Serial number: CCI-165 - Airframe hours: 9057 - Engines: LYCOMING VO-435-A1D 250 horsepower reciprocating - Damage: Substantial damage - Flight: Aerial observation, Part 91 - Operator: Sky Horse Helicopters Inc - Registered owner: Sky Horse Helicopters Inc - Airworthiness certificate: Standard, Normal - Last inspection: Annual, July 13, 2011, 68 hours before - Departed: Lohn, TX - Destination: Lohn, TX ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 54; injuries: serious; commercial; airplane ratings: single-engine land; instrument ratings: airplane; medical: class 2, without waivers/limitations; last flight review May 3, 2011; seat: left; flight hours: 14,000 total, 14,000 as pilot in command, 35 in the last 90 days ## Weather - Conditions: Visual (VMC), Day - Wind: 340° at 12 knots - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 55 °F, dew point 21 °F - Altimeter: 30.52 inHg - Observed at: BBD, 12 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 20120101000549A](https://openairdocs.org/incident/20120101000549A), Jan 1, 2012: Bell-Continental Copters Inc. 47G2, Lohn, TX, Substantial damage. Pilot stated he did a pre flight inspection per the operator's manual, loaded his passenger for the purpose of pasture count of deer on hunting leases. On the… ## Other investigations on N9021R (3) 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. - [NTSB LAX99LA047](https://openairdocs.org/ntsb/LAX99LA047), Dec 12, 1998: Bell 47G2 (N9021R), Helendale, CA, substantial damage. Probable cause: The pilot's failure to follow proper procedures to recover from main rotor blade aerodynamic retreating blade stall. - [NTSB LAX97LA285](https://openairdocs.org/ntsb/LAX97LA285), Aug 12, 1997: Bell 47G2 (N9021R), Mojave, CA, substantial damage. Probable cause: The student's improper use of the throttle and collective controls and the flight instructor's inadequate supervision of the flight. - [NTSB NYC72FHJ01](https://openairdocs.org/ntsb/NYC72FHJ01), Jul 11, 1971: Bell-Cci 47G2 (N9021R), Amityville, NY, substantial damage ## Docket (10 files) The files NTSB released with the investigation on July 2, 2013, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=82607. 5.0 MB in all. - Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 1, .PDF, 547 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40361272&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Redacted.PDF - Evidence Control Form (item 2, .PDF, 183 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40393944&FileExtension=.PDF&FileName=Evidence%20Control%20Form-Redacted.PDF - Retention / Release of Wreckage Form (item 3, .PDF, 39 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40393956&FileExtension=.PDF&FileName=Retention%20%2F%20Release%20of%20Wreckage%20Form-Redacted.PDF - Miscellaneous - Bell 47 Walk-around Inspection Checklist Excerpt (item 4, .PDF, 47 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40360868&FileExtension=.PDF&FileName=Miscellaneous%20-%20Bell%2047%20Walk-around%20Inspection%20Checklist%20Excerpt-Master.PDF - Materials Laboratory 15 - Factual Report 13-028 (with 8 embedded images) (item 5, .PDF, 1.3 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40392884&FileExtension=.PDF&FileName=Materials%20Laboratory%2015%20-%20Factual%20Report%2013-028%20(with%208%20embedded%20images)-Master.PDF - Other Pertinent Forms and Reports - Scott’s – Bell 47 Inc. Party Report (item 6, .PDF, 213 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40393945&FileExtension=.PDF&FileName=Other%20Pertinent%20Forms%20and%20Reports%20-%20Scott%E2%80%99s%20%E2%80%93%20Bell%2047%20Inc.%20Party%20Report-Master.PDF - View of a gear in the tailrotor gearbox supplied by the FAA (item 7, .PDF, 2.3 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40393922&FileExtension=.PDF&FileName=View%20of%20a%20gear%20in%20the%20tailrotor%20gearbox%20supplied%20by%20the%20FAA-Master.PDF - Miscellaneous - Shim measurement from the tailrotor gearbox supplied by the FAA (item 8, .PDF, 1 photo, 128 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40393923&FileExtension=.PDF&FileName=Miscellaneous%20-%20Shim%20measurement%20from%20the%20tailrotor%20gearbox%20supplied%20by%20the%20FAA-Redacted.PDF - Miscellaneous - No Records In Reference To Tail Rotor Gearbox (item 9, .PDF, 46 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40394092&FileExtension=.PDF&FileName=Miscellaneous%20-%20No%20Records%20In%20Reference%20To%20Tail%20Rotor%20Gearbox-Redacted.PDF - Statement of Party Representatives to NTSB Investigation (item 10, .PDF, 212 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40393924&FileExtension=.PDF&FileName=Statement%20of%20Party%20Representatives%20to%20NTSB%20Investigation-Redacted.PDF 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.