On October 10, 2000, about 0830 mountain daylight time, an Enstrom F-28F, N223RS, registered to and operated by Reeder & Haye Aircraft as a 14 CFR Part 91 maintenance test flight, landed hard in a field located about five miles southeast of Twin Falls, Idaho. Visual meteorological conditions prevailed at the time and no flight plan was filed for the local flight. The commercial pilot and passenger were not injured. The flight originated from Twin Falls about 30 minutes prior to the accident. During a telephone interview and subsequent written statement, the pilot reported that this was a maintenance test flight to check controls, collective pitch position, throttle positioning, and rpm correlation control entering an autorotation. The pilot stated that during the first autorotation, the rpm was high and the correlator wanted to overspeed. During the second autorotation, the rpm was low and when the pilot corrected with collective pitch, the correlator over responded, resulting in an rpm overspeed. The pilot closed the throttle and increased the collective, however, the rpm dropped and the pilot had to complete the autorotation to touch down on rough terrain. So as not to have any forward speed at touch down, and fearing that the helicopter might roll over, the pilot anticipated a hard flare to stop the forward speed. Just prior to touch down, the tailrotor contacted the ground, and the tailrotor gearbox separated from the tail boom. The helicopter yawed to the right about 150 degrees before the pilot was able to level the helicopter, stop the yaw and land without further incident.