HISTORY OF FLIGHTOn September 11, 2019, about 1300 Pacific daylight time, a Sikorsky S-58ET helicopter, N1168U, was substantially damaged when it was involved in an accident near Azusa, California. The pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 ferry flight. The pilot reported that he was ferrying the helicopter from Shafter Airport-Minter Field (MIT), Shafter, California, to Corona Municipal Airport (AJO), Corona, California. During cruise flight, when the helicopter was about 1,000 ft above ground level and at an airspeed of about 110 knots, the pilot “heard and felt a loud bang without the onset of noise or vibration.” Subsequently, the helicopter began to yaw to the right, and the pilot conducted an immediate autorotation and landed the helicopter in a baseball field. A postflight inspection revealed that one tail rotor blade had separated from the tail rotor assembly and that the remaining three tail rotor blades, which remained attached to the tail rotor assembly, exhibited dents and chordwise bends, The 90° gearbox had fractured, exposing the internal gears, and the vertical stabilizer was impact damaged. (See figure 1.) Figure 1. Left-side view of the tail rotor assembly, 90° degree gear box, and the vertical stabilizer (Source: Pilot). AIRCRAFT INFORMATIONA review of the maintenance records revealed that, on April 20, 2007, the blade that had separated from the tail rotor assembly was repaired. According to documents, details of the repairs were on file at the repair station that performed the work. The maintenance records also indicated that the paint on the blade was removed and that the blade was refinished and static balanced. After completion of the work, the blade was deemed to be in an airworthy condition. The company that performed the last repair stated that it retained no documents related to the work performed on the blade and could provide no additional information. The tail rotor blade was installed on the accident helicopter on May 13, 2013, when the helicopter had accumulated a total time of 18,897 hours. The blade had a total time of 885 hours and an “on condition” retirement life and was in compliance with all applicable airworthiness directives and service bulletins. According to the mechanic who performed the daily inspection of the helicopter on the morning of the accident flight, he wiped down the tail rotor blades and saw no working metal, cracks in the paint, or evidence of a crack. AIRPORT INFORMATIONA review of the maintenance records revealed that, on April 20, 2007, the blade that had separated from the tail rotor assembly was repaired. According to documents, details of the repairs were on file at the repair station that performed the work. The maintenance records also indicated that the paint on the blade was removed and that the blade was refinished and static balanced. After completion of the work, the blade was deemed to be in an airworthy condition. The company that performed the last repair stated that it retained no documents related to the work performed on the blade and could provide no additional information. The tail rotor blade was installed on the accident helicopter on May 13, 2013, when the helicopter had accumulated a total time of 18,897 hours. The blade had a total time of 885 hours and an “on condition” retirement life and was in compliance with all applicable airworthiness directives and service bulletins. According to the mechanic who performed the daily inspection of the helicopter on the morning of the accident flight, he wiped down the tail rotor blades and saw no working metal, cracks in the paint, or evidence of a crack. WRECKAGE AND IMPACT INFORMATIONThe separated section of the tail rotor blade was not recovered. The shank and the other three blades of the tail rotor assembly were shipped to the National Transportation Safety Board’s Materials Laboratory for examination. Fractographic examination of the shank revealed the blade had separated perpendicularly to the blade length along a chord plane located about 3 inches from the bottom of the blade. (See figure 2.) Figure 2. Side view of tail rotor blade shank. Examination of the tail rotor shank revealed a thumbnail-shaped pattern emanating from one side of the blade that was consistent with fatigue that had propagated through about three-quarters of the blade. No nicks, gouges, damage, or contamination were observed at the fatigue origin. Further examination of the tail rotor blade shank and the other three tail rotor blades revealed that they met the design specifications identified in the engineering drawings.