On August 7, 2012, about 0845 central daylight time, a Bell 214ST helicopter, N409SB, experienced a loss of tail rotor authority near Avalon, Texas. The commercial and airline transport pilots were not injured during the precautionary landing, and the helicopter was substantially damaged. The helicopter was owned and operated by Bell Helicopter, Textron, Inc. Fort Worth, Texas, under the provisions of 14 Code of Federal Regulations Part 91 as a research and development flight. Day visual meteorological conditions prevailed for the local flight which operated without a flight plan. The local flight originated from the Arlington Municipal Airport (KGKY), Arlington, Texas at 0815. According to the pilots the purpose of the flight was to collect data on the helicopter's rotor system. During the flight test, the helicopter completed a right turn and as they were setting up for another test condition, the pilot's heard a "bang," and felt the helicopter yaw. The pilot of the chase aircraft reported that something had departed the tail of the helicopter. The accident pilots then initiated an autorotation to a nearby field; however, helicopter yawed, rolled right, and came to rest on its side. The helicopter, the tail rotor gear box, and tail rotor blades were recovered and transported back to a Bell facility. Representatives from the NTSB, Federal Aviation Administration (FAA), and Bell Helicopter, then examined the helicopter and components at the manufacturer's facility. The tail rotor's 90-degree gear box was broken in two; one of the mast's two counterweight bellcranks and associated hardware were missing (and not located/recovered from the accident site); the remaining counterweight bellcrank remained attached. Examination of the remaining counterweight bellcrank, revealed that the assembly was in place, but "loose". The dustcap/grease cover for the remaining counterweight bellcrank was removed, exposing the castellated nut. The examination found that the nut's cotterpin was missing. Both tail rotor blades' attachments remained in place; both blades had separated outboard of their respective attachments. On the side with the missing counterweight bellcrank, impact marks consistent with the size and shape of a counterweight bellcrank, were found on the edge and base of its associated tail rotor blade, as well as on the right side of the helicopter. The rotor blade was torn in two, with the separation starting near the impact marks at the base of the blade. Since the accident helicopter was used to collect data on a new rotor system, the tail rotor blades and components had been removed to install test instrumentation. A review of the helicopters' maintenance workbook revealed the temporary re-installation of the instrumented tail rotor section. However, the maintenance entry was not specific as to whether the reinstallation of the instrumented tail rotor assembly was conducted as an assembly or as individual components. The mechanics installing the instrumented tail rotor treated the unit as a complete sub-assembly and not as individual components. The difference meant that the individual components were not assembled or inspected, prior to or after installation onto the helicopter. The helicopter was on its third flight after the reinstallation of the instrumented tail rotor system and had accumulated about 2.8 flight hours, before the accident.