# NTSB WPR14LA062: Hiller UH 12E, N36MV, Chowchilla, CA, December 9, 2013 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: December 9, 2013, 06:30 local (14:30 UTC) - Place: Chowchilla, CA - Coordinates: 37.098056, -120.110832 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=37.0981&lon=-120.1108&mi=10 - Phase of flight: [Maneuvering](https://openairdocs.org/ntsb-phases/450) (OpenAirDocs' grouping) - Defining event: [Maneuvering-low-alt flying: Sys/Comp malf/fail (non-power)](https://openairdocs.org/ntsb-events/330) - Highest injury: No injuries - Injuries: 1 uninjured - Status: Completed, report January 12, 2015 - Report: Approved by Office Director - NTSB office: Western Pacific Region - Parties: Federal Aviation Administration FSDO - NTSB case key (mKey): 88531 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=WPR14LA062 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/88531/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=88531 - Page: https://openairdocs.org/ntsb/WPR14LA062 ## Probable cause The failure of the torsional drive coupling, which resulted in a loss of drive to the main rotor system during low-altitude flight. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Main rotor drive: engine/transmission coupling](https://openairdocs.org/ntsb-findings/01046310): Aircraft › Aircraft propeller/rotor · Failure (probable cause); cases with it: [Hiller UH-12E, 1 case](https://openairdocs.org/ntsb-findings/01046310/cases.md?model=4360122), [All aircraft, 24 cases](https://openairdocs.org/ntsb-findings/01046310/cases.md) - [Maintenance: scheduled/routine maintenance](https://openairdocs.org/ntsb-findings/02062010): Personnel issues › Task performance · Maintenance personnel; cases with it: [Hiller UH-12E, 3 cases](https://openairdocs.org/ntsb-findings/02062010/cases.md?model=4360122), [All aircraft, 320 cases](https://openairdocs.org/ntsb-findings/02062010/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: [Hiller UH-12E, 1 case](https://openairdocs.org/ntsb-events/040/sequence.md?model=4360122), [All aircraft, 484 cases](https://openairdocs.org/ntsb-events/040/sequence.md) - [Maneuvering-low-alt flying: Sys/Comp malf/fail (non-power)](https://openairdocs.org/ntsb-events/330) (defining event); cases with it: [Hiller UH-12E, 5 cases](https://openairdocs.org/ntsb-events/330/sequence.md?model=4360122), [All aircraft, 994 cases](https://openairdocs.org/ntsb-events/330/sequence.md) - [Autorotation: Collision with terr/obj (non-CFIT) ](https://openairdocs.org/ntsb-events/470); cases with it: [Hiller UH-12E, 14 cases](https://openairdocs.org/ntsb-events/470/sequence.md?model=4360122), [All aircraft, 10,189 cases](https://openairdocs.org/ntsb-events/470/sequence.md) ## Analysis The pilot, who also owned the helicopter, reported that he was performing frost control operations over a citrus grove at low altitude when the helicopter's engine began to overspeed with an accompanying decrease in rotor speed. He initiated an autorotation, and the helicopter subsequently landed hard in an adjoining orchard. The pilot removed the helicopter from the accident site immediately following the accident and began disassembling it and sending components out for repair before it was examined. During the subsequent examination of the helicopter, both the elastomeric ring and lower drive lugs (engine side) of the main transmission torsional drive coupling were found sheared off, which would have resulted in the loss of drive to the main rotor system described by the pilot. The pilot did not make the torsional drive coupling available for followup examination and reported that he had misplaced the airplane's maintenance logbooks. The torsional drive coupling incorporated an internal set of lugs designed to provide positive drive to the transmission if the elastomeric portion of the coupling failed. The lugs had sheared off, which indicated that the elastomeric coupling had failed before the accident flight. The coupling had a finite service life but, because no maintenance records were provided for examination, it could not be determined if it was within its life limit at the time of the accident. Almost 4 years before the accident flight, the helicopter had been involved in another accident, which destroyed the main rotor blades. The pilot could not provide evidence that the coupling was replaced after that accident, and, without access to the airplane's maintenance logbooks, it could not be determined if a routine inspection of the coupling had been performed. It is possible that the coupling sustained damage during the original accident, which resulted in its ultimate failure. ## Factual narrative On December 9, 2013, about 0630 Pacific standard time, a Hiller UH 12E, N36MV, landed hard near Chowchilla, California, following a loss of drive to the rotor system. The helicopter was owned and operated by S and S Helicopters under the provisions of 14 Code of Federal Regulations Part 91, as a frost control flight. The commercial pilot was not injured, and the helicopter sustained substantial damage during the accident sequence. The local flight departed from the operator's facility in Chowchilla, about 0400. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot/owner stated that he was performing frost control over a citrus grove at about 100 feet above ground level when the helicopter's engine began to overspeed, with an accompanying decrease in rotor speed. He initiated an autorotation and the helicopter stuck trees in an adjoining orchard. The owner removed the helicopter immediately following the accident, and began both disassembling it, and sending components out for repair prior to the arrival of a Federal Aviation Administration (FAA) inspector 2 days following the accident. The FAA inspector subsequently examined the helicopter and observed that both the elastomeric ring and lower drive lugs (engine side) of the main transmission torsional coupling had sheared. Despite multiple correspondences, the owner did not make the torsional coupling available for detailed examination by the NTSB investigator-in-charge. He additionally stated to the FAA inspector that the helicopter's maintenance logbooks had been misplaced. The helicopter was involved in another accident on February 2, 2010. The NTSB determined the probable cause to be, "A partial loss of engine power due to a failure of the throttle interconnect hardware, which resulted in a hard landing." During that accident the helicopter sustained substantial damage to the forward fuselage and tailboom, which had become crushed and separated from the aft bulkhead. Additionally, both main rotor blades were destroyed after making contact with the ground while rotating. Despite the significant nature of the damage, no FAA records were located indicating that an FAA Form 337 (Major Repair and Alteration - Airframe, Powerplant, or Appliance) had been submitted. According to maintenance records submitted during that investigation, as of February 25, 2009, the helicopter had accrued 6,815 total flight hours. The owner reported that during the period between that inspection and the last, dated September 20, 2013, the helicopter had accrued an additional 1,154 hours of flight time. The torsional coupling and clutch consisted of upper and lower flanged plates bonded to either side of a disk shaped section of elastomeric material. The elastomeric component provided a torsionally elastic cushion between the engine and transmission. The assembly also contained secondary drive lugs, which during normal operation do not contact each other. However, in the event of afailure of the elastomeric material, the secondary drive lugs would continue to drive the main transmission. The coupling was manufactured by Lord Mechanical Products, who defined a finite service life for the part. In 1998 Hiller Aviation issued Service Bulletin 21-4 Revision 1, specifying a series of 300 and 1,200 hours inspection intervals, along with adherence to the components finite service life in accordance with Lord service data as follows: "Prior to installation in the aircraft, parts should be stored in accordance with MIL-STD-1523 for a maximum of sixty (60) months. Couplings stored for more than twenty-four (24) months shall be inspected for ozone/flex cracks in accordance with Section I.A.2 (b) of this Service Data. After installation in the aircraft, the J-10114 series coupling operates on a FINITE SERVICE LIFE basis: 5,000 hours maximum or five (5) years maximum from date of installation or until the limits of the maintenance manual are met, whichever occurs first." The FAA inspector who examined the helicopter reported that the coupling was stamped with a manufacture date of 2007. The helicopter's owner held an FAA airframe and powerplant owner mechanics certificate with inspection authorization. Based on maintenance records from the previous accident, he appeared to be the primary mechanic for the helicopter. He initially reported to the FAA inspector that he inspected the coupling about 6-8 months prior to the accident, and that he had most likely replaced it with a spare component from another helicopter. In a follow-up conversation he reported that he had no recollection of ever replacing the coupling. ## Aircraft - Registration: [N36MV](https://openairdocs.org/deregistered/N36MV-2018-03-01), registration cancelled 2018-03-01, S/N 5060, built 1978 (OpenAirDocs) - Reported make and model: HILLER UH 12E - Registry model: [Hiller UH-12E](https://openairdocs.org/model/hiller-uh-12e) (registry code 4360122) (OpenAirDocs) - Registry engine: [Lycoming VO-540 series](https://openairdocs.org/engine/lycoming-vo-540-series) (registry code 41534, reciprocating) (OpenAirDocs) - Category: Helicopter - Serial number: 5060 - Year built: 1978 - Airframe hours: 7969 - Engines: Lycoming O540 305 horsepower reciprocating - Damage: Substantial damage - Flight: Other work use, Part 91 - Operator: Jeff Streeter Motor Sports Inc, doing business as S and S Helicopters - Registered owner: Jeff Streeter Motor Sports Inc - Airworthiness certificate: Special, Restricted (Special) - Last inspection: 100 hour, September 20, 2013 - Departed: N/A, Chowchilla, CA - Destination: N/A, Chowchilla, CA ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 67; injuries: minor; commercial; airplane ratings: multi-engine land, single-engine sea, single-engine land; instrument ratings: airplane; medical: class 2, with waivers/limitations; last flight review May 1, 2012; seat: right; flight hours: 30,000 total, 4,000 in this make and model, 100 in the last 90 days, 50 in the last 30 days, 18 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 230° at 4 knots - Visibility: 7 statute miles - Lowest ceiling: none - Temperature: 21 °F, dew point 18 °F - Altimeter: 30.37 inHg - Observed at: KMAE, 5 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 20131209023649A](https://openairdocs.org/incident/20131209023649A), Dec 9, 2013: Hiller UH-12E, Madera, CA, Substantial damage. The aircraft was performing agricultural flight operations, frost control, when the torsional coupling failed. Being at low altitude, the RPM went quickly… ## Other investigations on N36MV (1) 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 WPR10LA127](https://openairdocs.org/ntsb/WPR10LA127), Feb 2, 2010: Hiller UH-12E (N36MV), Los Banos, CA, substantial damage. Probable cause: A partial loss of engine power due to a failure of the throttle interconnect hardware, which resulted in a hard landing. ## Docket (7 files) The files NTSB released with the investigation on November 25, 2014, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=88531. 7.1 MB in all. - Pilot-Operator Aircraft Accident Report, NTSB Form 6120.1 (item 1, .PDF, 783 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40419460&FileExtension=.PDF&FileName=Pilot-Operator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Redacted.PDF - Photo 1 - Torsional Coupler Lower Plate (FAA Photo) (item 2, .PDF, 671 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40419461&FileExtension=.PDF&FileName=Photo%201%20-%20Torsional%20Coupler%20Lower%20Plate%20(FAA%20Photo)-Master.PDF - Photo 2 - Torsional Coupler Lower Plate (FAA Photo) (item 3, .PDF, 673 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40419462&FileExtension=.PDF&FileName=Photo%202%20-%20Torsional%20Coupler%20Lower%20Plate%20(FAA%20Photo)-Master.PDF - Photo 3 - Damage Sustained on Previous Accident (FAA Photo) (item 4, .PDF, 1.2 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40419463&FileExtension=.PDF&FileName=Photo%203%20-%20Damage%20Sustained%20on%20Previous%20Accident%20(FAA%20Photo)-Master.PDF - Photo 4 - Damage Sustained on Previous Accident (FAA Photo) (item 5, .PDF, 948 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40419464&FileExtension=.PDF&FileName=Photo%204%20-%20Damage%20Sustained%20on%20Previous%20Accident%20(FAA%20Photo)%0D%0A-Master.PDF - Logbook Excerpts from 2009 (item 6, .PDF, 1.6 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40419465&FileExtension=.PDF&FileName=Logbook%20Excerpts%20from%202009-Redacted.PDF - Hiller Service Instructions (item 7, .PDF, 1.2 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40419466&FileExtension=.PDF&FileName=Hiller%20Service%20Instructions-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.