# NTSB NYC97LA101: Hiller UH-12E, N4029S, Granby, NY, May 29, 1997 - Type: NTSB accident investigation - Classification: Accident · Serious injuries · Limited investigation · Final report - Date: May 29, 1997, 09:20 local (13:20 UTC) - Place: Granby, NY - Coordinates: 43.310565, -76.410659 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=43.3106&lon=-76.4107&mi=10 - Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping) - Highest injury: Serious injuries - Injuries: 1 serious - Status: Completed, report December 31, 1998 - Report: Approved by Office Director - NTSB case key (mKey): 39364 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=NYC97LA101 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/39364/pdf - Page: https://openairdocs.org/ntsb/NYC97LA101 ## Probable cause failure of the pilot/owner to replace the torsional clutch as specified by the manufacturer, and subsequent failure of the helicopter's clutch, which resulted in a forced landing (autorotation). A related factor was: the encounter with trees in the emergency landing area. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Airframe/component/system failure/malfunction, cruise - rotor drive system,clutch assembly - failure (cause) - maintenance,replacement - not performed, company/operator management (cause) - Forced landing, emergency descent/landing - autorotation - performed - In flight collision with object, emergency landing - object - tree(s) (contributing factor) ## Analysis The pilot/owner had completed an uneventful aerial application flight, and he began to reposition the helicopter to another site. En route, at 500 feet AGL, the helicopter's engine began to overspeed. The pilot entered autorotation, and the helicopter struck trees and landed hard. Examination revealed the elastomeric torsional coupling inside the transmission, and the secondary steel drive flanges were sheared. The manufacturer's service data for the coupling stated that mandatory replacement of the coupling should occur at 5,000 hours or 5 years from the date embossed on the rubber, or in accordance with maintenance requirements for the coupling, which ever occurred first. The raised numbers on the coupling indicated a date of May 1988, 9 years before the accident. ## Factual narrative On May 29, 1997, at 0920 eastern daylight time, a Hiller UH-12E, N4029S, was substantially damaged during a forced landing to a field in Granby, New York. The certificated commercial pilot was seriously injured. Visual meteorological conditions prevailed for the agriculture positioning flight that originated at Fulton, New York, about 0815. No flight plan had been filed for the flight conducted under 14 CFR Part 137. According to the pilot/owner, he had completed an uneventful application flight, and without landing, began to reposition to another site. While en route, at 500 feet above the ground, the helicopter's engine began to overspeed, and the pilot entered autorotation. During the descent, the helicopter struck trees and landed hard in a horse paddock. The pilot stated that he suspected a clutch failure in the transmission. A Federal Aviation Administration (FAA) Inspector stated that about 90 gallons of diluted chemicals had spilled, which included guthion, soyginex and SM-17. Examination of the helicopter after decontamination revealed no preimpact failure of the helicopter's flight controls. The transmission was disassembled and inspected at a helicopter maintenance facility in Canastota, New York. A letter to the FAA Inspector stated that the mercury clutch assembly was intact. The clutch shoes were within serviceable limits and no mercury leakage was evident. The letter also stated: "...The loss of main rotor drive was due to the failure of the torsional coupling. The elastomeric section of the torsional coupling was sheared horizontally, which resulted in failure of the coupling and both lower steel drive flanges..." 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 components are inseparable once bonded. The elastomeric component provided a torsionally elastic cushion between the engine and transmission. The assembly also contained secondary drive lugs. During normal operation, the lugs on each plate do not contact each other; however, in the event of a failure of the elastomeric material, the secondary drive lugs would continue to drive the main transmission. The torsional coupling, with clutch assembly, was shipped to the NTSB Materials Laboratory for examination. The metallurgist's factual report stated that the torsional coupling had separated through the elastomeric material just above the lower coupling plate. The clutch assembly appeared intact and undamaged. The secondary drive lugs on the lower plate were fractured. The fracture surfaces had no visible corrosion. The report further stated: "...visual examinations found fracture features consistence with fatigue propagation on both lugs. On both fractures the fatigue initiated at multiple origins near the lower end of the drive face of the respective lug...The fatigue had propagated almost entirely through the cross section of each lug before final bending overstress separation... The torsional coupling was manufactured by Lord Corporation, Mechanical Products Division, Erie, Pennsylvania. A review of their service data revealed that the service life of the coupling was a maximum of 5,000 hours or 5 years after installation. A warning in the service data sheet stated: "Mandatory replacement of coupling at 5,000 service hours or 5 years from date embossed on rubber, or in accordance with maintenance manual requirements for the couplings, whichever occurs first." The raised numbers on the torsional coupling indicated a date of May 1988. A Hiller Aircraft Corporation, Service Letter 21-7, stated: "To prevent damage to the mercury clutch during startup, it is important to operate the engine at 1,700 to 1,800 RPM. This will ensure timely engagement without excessive heat buildup...prolonged idle at low RPM's will cause overheating of the torsional coupling..." ## Aircraft - Registration: [N4029S](https://openairdocs.org/aircraft/N4029S), registered now, S/N 5129, built 1980 (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: 5129 - Airframe hours: 6440 - Engines: Lycoming VO-540-C2A 305 horsepower reciprocating - Damage: Substantial damage - Flight: Positioning, Part 137 - Registered owner: Helicair Ag Inc. - Airworthiness certificate: Special, Restricted (Special); Standard, Normal - Last inspection: 100 hour, May 13, 1997, 2 hours before - Departed: NONE, FULTON, NY ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 55; commercial; airplane ratings: single-engine land, multi-engine land; medical: class 2, valid medical--no waivers/lim.; seat: center; flight hours: 10,000 total, 3,600 in this make and model, 10,000 as pilot in command ## Weather - Conditions: Visual (VMC), Day - Visibility: 10 statute miles - Lowest ceiling: 25,000 ft broken - Temperature: 68 °F, dew point 50 °F - Altimeter: 30.00 inHg - Observed at: SYR, 19 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 19970529012149A](https://openairdocs.org/incident/19970529012149A), May 29, 1997: Hiller UH12E, Granby, NY, No damage. On aerial application, crashed in field, clutch failed. Chemical spill. 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.