On May 31, 2015, about 1126 Pacific daylight time, a Garlick Helicopters Inc. UH-1H, N462CC, sustained substantial damage during an emergency landing, about 10 miles east of Cove, Oregon. The commercial pilot, sole occupant of the helicopter, was not injured. The helicopter was registered to and operated by Elkhorn Aviation Inc.,under the provisions of 14 Code of Federal Regulations Part 133, as an external load flight. Visual meteorological conditions prevailed and a company visual flight rules (VFR) flight plan was filed for the local flight that departed from a field near Cove, Oregon about 1100, with a planned destination of Minam Lodge Airport, Cove, Oregon. The pilot reported that while maneuvering at low altitude, during logging operations, with an external load of about 24 logs, he felt a vibration and heard a "howling sound" coming from the transmission area of the helicopter. He immediately dropped the logs and initiated a precautionary landing to an open grass field. The pilot further reported that the helicopter's controls became stiff during the descent and he heard the rotor RPM decreasing. About 5 ft above the ground, he pulled up on the collective as hard as he could to cushion the landing, but the helicopter experienced a hard landing. Upon landing, the pilot noticed that the rotor RPM had decreased and was continuing to drop, while the engine rpm remained constant, at a high setting. Postaccident examination of the helicopter revealed flight control continuity. The main rotortransmission, that is mounted forward of the engine, was observed to be titled slightly forward, and the transmission input quill turned freely within the transmission, when rotated manually. The engine-to-transmission drive shaft assembly, manufactured by Kamatics, (a division of Kaman), transmits engine output power to the main rotor transmission and was installed between the engine and the transmission. The drive shaft assembly is composed of the engine-to-transmission drive shaft, two flexible Kaflex couplings, a transmission shaft flange, and an engine shaft flange. The Kaflex coupling utilizes metal flexure elements to accommodate for misalignment in the engine-to-transmission drive shaft assembly during operation (i.e. relative movement between the engine and main rotor transmission). On the forward end of the drive shaft assembly, a Kaflex coupling is installed between the forward flange of the drive shaft to the transmission shaft flange. On the aft end of the drive shaft assembly, a second Kaflex coupling is installed between the flange of the drive shaft to the engine shaft flange. The flex coupling metal flexture elements at each end of the drive shaft were fractured into multiple pieces. The remnant flex coupling remained installed on the engine shaft flange via attaching hardware (nuts and bolts). No remnants of the Kaflex coupling was found to be attached to the transmission shaft flange; the fractured attaching hardware was recovered. The engine-to-transmission drive shaft, the Kaflex coupling and their fragments, and an electronic RPM detector were sent to the NTSB Materials laboratory for further examination. Further examination of the components revealed that the fracture surfaces of the Kaflex coupling metal flexture elements were consistent with overload and that no evidence of fatigue cracking was present. All the attachment bolts for the Kaflex coupling on both the transmission shaft flange and the engine shaft flange, had the appropriate markings for their specified bolt part number, for this application. The transmission shaft flange, that was separated from the transmission, contained two through holes for the attachment bolts. One of the attachment bolts was observed to be fractured, at the threaded portion of the bolt. Microscopic examination of the fractured bolt surfaces revealed arrest marks that were consistent with fatigue cracking, that emanated from the shank area. The area where the fracture surface originated, contained fretting and deformation damage. Progressive fatigue fracture features were present through about 80% of the bolt's cross thread section. The attachment bolts were checked with an alloy analyzer and their content was consistent with the manufacturer specified composition. According to the bolt manufacture representative, the tensile strength of the attachment bolts is supposed to be a minimum of 160,000 pounds per square inch (psi). The fractured bolt was removed from the transmission shaft flange and a hardness test was accomplished. Testing revealed that the hardness value of the bolt was about 166,000 psi, which was greater than the minimum specified tensile strength. The transmission shaft flange contained an inner tube portion that was located inside the engine-to-transmission drive shaft. The tube portion had deformation damage all around the circumference. The sides of the tube portion were deformed inward. The transmission end of the engine-to-transmission drive shaft had severe deformation damage all around the hollow shaft. The hollow shaft showed evidence of outward deformation. The outward deformation was consistent with the transmission shaft flange breaking out and away from the drive shaft. The engine shaft flange coupling assembly remained attached. The two attachment bolts and corresponding nuts were relatively intact. The shank portion of one of the bolts showed evidence of lateral deformation that was in the same location of the fatigue origin of the fractured bolt found in the transmission shaft flange. Further, the bolts showed evidence of deformation and material loss all around the threads. The bolts were also fractured and showed evidence of ductile fractures in the shear mode. According to the helicopter operator's manual, a main drive shaft failure will result in a complete loss of power to the rotor and a possible engine overspeed. An examination of the electronic RPM detector revealed no evidence of mechanical damage.