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..."