Accident Overview
On June 17, 2019, about 1950 eastern standard time, a Robinson R44 helicopter, registration N1241W, was substantially damaged in an accident near Key West, Florida. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 revenue sightseeing flight. The pilot and two passengers were not injured.
According to the pilot, he flew the helicopter earlier that day without any unusual indications. After washing and drying the helicopter and inspecting it during the process, he departed from the airport and entered the right crosswind pattern. As the helicopter climbed through 300 ft above ground level, the pilot detected rapid tail vibrations. He radioed the tower and requested a return to the airport. While proceeding back, he heard a loud pop and felt a hard right yaw. Attempts to regain control using the anti-torque pedals were ineffective. He initiated an autorotation, and the helicopter began to yaw to the left. Again, control attempts were ineffective. As the helicopter descended toward the water, the pilot deployed the floats and landed upright on the water without further incident.
Investigation Findings
Examination of the helicopter revealed that the tail rotor assembly had separated from the tail boom in flight, resulting in substantial damage. The separation occurred where the tail rotor gearbox input cartridge and tail boom flange mate. Examination at the National Transportation Safety Board Materials Laboratory showed that all input cartridge lugs and flange pieces were accounted for. The top left side mounting lug had the fractured mating tail boom flange still secured in place via a bolt; the fracture surface was consistent with overstress. The bottom port side of the tail boom flange had the fractured portion of the mating input cartridge still secured via a bolt; the fracture surface was also consistent with overstress.
The tail boom casting that attaches the tail rotor gearbox to the tail boom had all four locking inserts threaded into the casting bolt holes. New, unused bolts were obtained and tested for locking capability. Hand-tightened, a torque wrench showed each bolt reached 35 to 40 in-lbs when halfway threaded, within the expected range for drag effect once the locking insert engaged. After removal and re-assembly, the bolts were fully inserted and torqued to the manufacturer-required 240 in-lbs. All bolt bottom surfaces aligned with the edge of the locking insert, about 3 to 4 threads from the inboard surface. All bolts locked inside the inserts as designed.
The input cartridge bolt holes were examined and found deformed in the holes of the two right side mounting lugs, which were missing their respective bolts. These holes were out of round with a ridge of material circumferentially around the edge of the outboard face. All bolt holes had damage around the inner diameter appearing as imprints from threads of mating bolts; the deepest imprints were on the top right side lug hole. All tail boom flange bolt holes had threading present and not stripped, though some damage was observed.
Maintenance History
The helicopter was last inspected on May 10, 2019, at a total aircraft time of 1,523.75 hours. At that time, the tail boom was replaced. Replacement would have involved removal of the tail rotor gearbox and reattaching it to the new tail boom. Following replacement, the pilot reported rapid tail rotor vibrations, which required multiple attempts to successfully balance the tail rotor as part of post-maintenance checks. The helicopter had flown about 60 hours since that inspection without any discrepancies.