Bell Helicopter Textron Canada 407 N98ZA

The pilot of the helicopter was conducting a positioning flight. About 5 minutes after departure, the onboard video recorder captured him saying, “what is going on here?” The pilot subsequently contacted air traffic control and requested to return to the departure airport, but he did not declare an emergency or state that he needed assistance. Upon initial contact with the tower controller at the destination airport, the pilot stated that he “might need the runway”; several minutes later, the controller cleared the pilot to land on the runway numbers. As the helicopter approached the airport and its indicated airspeed began to decay below about 30 knots, the helicopter entered a right yaw and completed several 360° rotations around the main rotor mast before impacting terrain next to the runway, resulting in substantial damage. Postaccident examination of the helicopter revealed that the tail rotor crosshead drive plate, which was positioned behind the pitch change rod attachment nut, was not bolted to the tail rotor crosshead. The two attachment bolts were not present, and no remnants of any bolts were found in the threaded receptacles in the crosshead. The threads were undamaged and showed no signs of corrosion, deformation, smearing, or cross-threading, indicating that the attachment bolts were likely not installed. The tail rotor was installed on the day before the accident after the replacement of four feathering bearings. The operator’s director of maintenance (DOM) performed the installation and had a mechanic verify that the mast nut torque was correctly applied. After the DOM completed the installation, another mechanic verified the work. A company maintenance pilot then completed a preflight inspection of the helicopter, ground functional checks, and three consecutive maintenance runs. The accident flight was the first flight after the completion of this work. According to the DOM, between the mast nut torque application and completion of the installation, he was “called out” to consult on two different aircraft repairs. He did not recall the amount of time that had elapsed before he resumed the installation work. At some point during the installation, the DOM failed to properly secure the tail rotor crosshead drive plate. This error was subsequently not detected by the mechanic during his check of the DOM’s work, the maintenance pilot while balancing the tail rotor, or the accident pilot during the preflight check. The helicopter experienced a loss of tail rotor antitorque control due to the separation of the crosshead drive plate, but the helicopter was still controllable at speeds at or above effective translational lift. It is likely that the increased efficiency of the main and tail rotors, the streamlining effect of the fuselage, and the increased effectiveness of the vertical stabilizer at cruise speed all prevented the helicopter from entering an uncontrolled yaw while the pilot was returning to the airport. However, the increased engine power required to slow the helicopter to perform a normal approach to a hover to land on the runway numbers resulted in a torque moment that could not be overcome given the loss of tail rotor antitorque control. A run-on landing, during which the pilot would have maintained a forward speed above effective translational lift, would have afforded greater yaw stability, and thus have increased the chance for a successful landing.

Quoted verbatim from the NTSB record.