On April 3, 2004, about 0745 Pacific standard time, a Bell 47G-4A, N7NB, collided with terrain following a loss of tail rotor control near Bakersfield, California. Inland Crop Dusters, Inc., was operating the helicopter under the provisions of 14 CFR Part 137. The commercial pilot, the sole occupant, sustained serious injuries; the helicopter sustained substantial damage. The local aerial application flight departed Shafter-Minter Field Airport, Shafter, California, about 0700. Visual meteorological conditions prevailed, and no flight plan had been filed. In a written statement, the pilot reported that while turning to reposition himself on the spray run he was making, he heard a rattling noise from the aft end of the helicopter. He then lost tail rotor authority and entered a flat spin. The helicopter was only about 150-175 feet above ground level (agl). He rolled off the throttle and lowered the collective. The pilot attempted to make a run-on landing, but the helicopter continued to spin until it impacted terrain. The helicopter incurred damage to the cabin, skids, tail rotor drive system, tail boom, and center section. A Federal Aviation Administration inspector examined the helicopter after the accident. He stated that there was a crack on the left side support tube area of the tail boom, prior to it connecting to the tail rotor assembly. The inboard side of the crack had significant deposits of rust. It was also noted that the cross-over support tube in that area of the frame was missing. The inspector stated that this would have added excessive stress to the structure during turning maneuvers and caused the last spline coupling shaft to disconnect from the tail rotor gear box. A 50-hour maintenance inspection was completed the day before the accident.