Robinson R44 accident near Key West, Florida, June 17, 2019

The NTSB's narrative final · quoted from the NTSB record

Shortly after departure, the pilot detected rapid vibrations from the tail of the helicopter. While returning to the airport, the mounting bolts that secured the tail rotor gearbox to the tail boom failed, which resulted in the separation of the tail rotor. The pilot lost yaw control of the helicopter, but successfully performed an autorotation to the water below. A postaccident examination of the mounting hardware between the tail rotor gearbox input cartridge and the tail boom mating flange found that two of the four bolts were missing; both missing bolts came from the right side of the connection. The left side bolts remained secured in place. The missing bolts’ holes in the input cartridge were deformed and were out of round. All of the 4 bolt holes on the input cartridge displayed imprints from the bolt threads, with the deepest imprints on the right sides of the holes, consistent with movement between the lug holes and the bolt shanks. All tail boom flange bolt holes were examined and found that the threading was present, not stripped, and locking inserts present. About 60 hours before the accident, the tail boom assembly was removed and replaced. As part of this maintenance, the tail rotor input cartridge and gearbox would have been removed from the old tailboom and placed on the new tailboom. It is likely that, during this procedure, the right-side mounting bolts were not completely secured to the tail boom casting locking inserts and properly torqued, which resulted in the failure of the right side mounting bolts. The helicopter was flown earlier that day without incident or indication of impending failure.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.