On April 7, 2008, at approximately 1230 central daylight time, a Hiller UH-12E helicopter, N34SD, was returning to a landing zone after an agricultural application when a loud noise was heard with associated vibrations. The pilot elected to perform an autorotation into a nearby field. The commercial pilot, the sole occupant of the helicopter, was not injured. The helicopter was owned and operated by Texas Air Cargo Inc. The local flight originated from private property located approximately 5 miles north of Quinton, Oklahoma. Visual meteorological conditions prevailed and no flight plan was filed for the 14 Code of Federal Regulations Part 137 agricultural application flight. According to the pilot, the helicopter was cruising at approximately 1,000 feet above ground level when a loud noise was heard by the pilot. The pilot reported that he quickly reduced engine power and began an autorotation. Furthermore, the pilot stated there was no difficulty controlling the helicopter through the emergency landing and he was able to perform a normal flare touching down with no associated side angle. The pilot stated that the gearbox had failed resulting in the damage to the helicopter. On-scene photos were provided by the pilot. The photos provided revealed a fracture of the tail rotor gearbox yoke, twisting of the drive shaft, and damage to the tail rotor blades. The pilot and his employees reported no evidence of a ground strike existed at the accident scene. Furthermore, another individual reported recovering a tail rotor blade "some distance" from the site of the autorotation. On a NTSB Form 6120, the pilot reported having 33,000 total hours with 30 hours in make and model. Parts from the helicopter were recovered for examination. A metallurgic examination was conducted by the NTSB laboratory and revealed all components failed as a result of overload. Additionally, no fatigue failures were detected. An automated weather reporting station located at McAlister Regional Airport (MLC), approximately 28 nautical miles to the south-east of the accident site, reported winds from 110 degrees at 11 knots gusting to 14 knots, visibility 10 statute miles, ceiling broken at 6,000 feet, temperature 75 degrees Fahrenheit, dew point 50 degrees Fahrenheit, and a barometric pressure of 29.93 inches of Mercury.