Accident Summary
On August 7, 2012, about 0845 central daylight time, a Bell 214ST helicopter, registration N409SB, experienced a loss of tail rotor authority near Avalon, Texas. The helicopter, operated by Bell Helicopter Textron Inc. of Fort Worth, Texas, was conducting a research and development flight under 14 Code of Federal Regulations Part 91. The two pilots, holding commercial and airline transport certificates, were not injured during the subsequent precautionary landing. The helicopter sustained substantial damage.
Flight Details
The flight originated from Arlington Municipal Airport (KGKY) at 0815 for a local flight in day visual meteorological conditions without a flight plan. According to the pilots, the purpose was to collect data on the helicopter's rotor system. During the flight test, after completing a right turn and while setting up for another test condition, the pilots heard a "bang" and felt the helicopter yaw. The pilot of a chase aircraft reported that something had departed the tail of the helicopter. The accident pilots initiated an autorotation to a nearby field, but the helicopter yawed, rolled right, and came to rest on its side.
Examination and Findings
The helicopter, tail rotor gear box, and tail rotor blades were recovered and transported to a Bell facility. Representatives from the National Transportation Safety Board (NTSB), Federal Aviation Administration (FAA), and Bell Helicopter examined the components. 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; the remaining counterweight bellcrank remained attached but was found to be loose. The dustcap/grease cover for that bellcrank was removed, exposing a castellated nut; the nut's cotter pin was missing.
Both tail rotor blades' attachments remained in place, but 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.
Maintenance Background
Because 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 helicopter's maintenance workbook revealed the temporary reinstallation of the instrumented tail rotor section. However, the maintenance entry was not specific as to whether the reinstallation was conducted as an assembly or as individual components. The mechanics installing the instrumented tail rotor treated the unit as a complete sub-assembly, not as individual components. This 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.