Incident Overview
On May 31, 2015, at about 1126 Pacific daylight time, a Garlick Helicopters Inc. UH-1H helicopter, registration N462CC, made an emergency landing approximately 10 miles east of Cove, Oregon. The commercial pilot, who was the sole occupant, was not injured. The helicopter was registered to and operated by Elkhorn Aviation Inc., under 14 Code of Federal Regulations Part 133 for an external load flight. Visual meteorological conditions prevailed, and a company VFR flight plan had been filed for the local flight, which departed from a field near Cove about 1100, with a planned destination of Minam Lodge Airport, Cove, Oregon.
Pilot Report
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. He immediately dropped the logs and initiated a precautionary landing to an open grass field. During the descent, the helicopter's controls became stiff, and the pilot heard the rotor RPM decreasing. About 5 feet 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. After 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
Postaccident examination of the helicopter revealed flight control continuity. The main rotor transmission, mounted forward of the engine, was observed to be tilted 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. The assembly consists of the drive shaft, two flexible Kaflex couplings, a transmission shaft flange, and an engine shaft flange. The Kaflex coupling uses metal flexure elements to accommodate misalignment during operation. On the forward end, a Kaflex coupling connects the drive shaft to the transmission shaft flange; on the aft end, another connects the drive shaft to the engine shaft flange.
Examination of the drive shaft assembly showed that the metal flexure 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. No remnants of the Kaflex coupling were found attached to the transmission shaft flange; the fractured attaching hardware was recovered. The drive shaft, Kaflex coupling fragments, and an electronic RPM detector were sent to the NTSB Materials laboratory for further examination.
Component Analysis
Further examination revealed that the fracture surfaces of the Kaflex coupling metal flexure elements were consistent with overload, with no evidence of fatigue cracking. All attachment bolts for the Kaflex coupling on both flanges had appropriate markings for the specified bolt part number. The transmission shaft flange, separated from the transmission, contained two through holes for attachment bolts. One attachment bolt was fractured at the threaded portion. Microscopic examination of the fractured bolt surfaces revealed arrest marks consistent with fatigue cracking, emanating from the shank area. The fracture origin area 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 manufacturer's representative, the tensile strength should be a minimum of 160,000 psi. The fractured bolt was removed from the transmission shaft flange and subjected to a hardness test, which revealed a value of about 166,000 psi, exceeding the minimum specified tensile strength.
The transmission shaft flange contained an inner tube portion located inside the engine-to-transmission drive shaft. The tube portion had deformation damage all around its circumference, with the sides deformed inward. The transmission end of the drive shaft had severe deformation damage all around the hollow shaft, showing evidence of outward deformation 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 bolt showed lateral deformation at the same location as the fatigue origin of the fractured bolt from the transmission shaft flange. The bolts also showed deformation and material loss around the threads, and were fractured with evidence of ductile fractures in shear mode.
According to the helicopter operator's manual, a main drive shaft failure results in a complete loss of power to the rotor and a possible engine overspeed. Examination of the electronic RPM detector revealed no evidence of mechanical damage.