On March 7, 1998, at 0956 hours Pacific standard time, a Bell UH-1B, N204DR, crashed near Hollister, California, following a separation of the tail rotor gear box. The aircraft was destroyed and the commercial pilot, the sole occupant, suffered serious injuries. Visual meteorological conditions prevailed and no flight plan was filed. The flight was operated by Cantrell Helicopters under 14 CFR Part 137. The pilot reported that he was spraying a field approximately 1-mile northeast of Hollister. At the end of his third pass over the field, he pulled up to go over power lines and was starting a right turn when he felt and heard a thump. He lowered the collective and attempted to straighten out from the right turn. The pilot stated that a strong vibration started and the nose of the aircraft pitched down. The helicopter started a right turn, which began increasing in momentum. The pilot's memory of the event ceases at this point. There was a ground crewman in the field who witnessed the accident. The witness stated that while the pilot was making a pass down the field about 35 feet agl, he saw a large piece of the tail rotor blade fly off the ship, followed by the tail rotor hub assembly, the remaining blade, and the 90-degree gearbox. The helicopter started into a 360-degree spiral to the right, made 2 1/2 rotations, then impacted the ground in a 30-degree nose-down attitude. The witness helped the pilot exit the helicopter and noted that the engine was still producing about 60 percent power. A Federal Aviation Administration inspector from the San Jose Flight Standards District Office responded to the accident site and examined the wreckage. The tail fin and tail rotor gearbox was found approximately 125 feet from the fuselage. The tail rotor gearbox and tail rotor blades were examined at the Bell Helicopter Textron Materials Laboratory in Fort Worth, Texas, under the supervision of the Safety Board. A copy of their report is appended to this file. Their report was forwarded to the Safety Board's Materials Laboratory for review. The Bell metallurgist reported that examination of the tail vertical fin disclosed an area of chafing which wore through the spar and initiated a fatigue crack. The fatigue crack initiated in an area where the vertical fin driveshaft cover fastened on the left side of the aircraft near the upper end. Chafing had occurred between the cover and the spar, wearing away the spar. The primary fatigue crack initiated in the worn area, but the origin of the crack had been destroyed. The fatigue crack propagated from the spar downward through eight rivet holes in the skin a distance of 7 inches, then turned aft a distance of 3.8 inches. The fatigue crack propagated through the entire width of channel a distance of 6.5 inches and through the five areas of the stiffener. The fatigue cracking in the vertical fin covered 65 to 75 percent of the structure's cross section. According to Bell, the UH-1B helicopter requires a daily inspection to visually check the area of the upper vertical fin, per the Aviation Unit Technical Manual (55-1520-219-PMS), where the fatigue cracking occurred. The operator reported that he did not have the aircraft logbooks in his possession, therefore the Safety Board conducted no review.