Accident Overview
A helicopter pilot departed from a chemical loading site at approximately 0415 hours, initiating a left turn before proceeding to spray a sunflower field. The wreckage was subsequently located half a mile west of the intended spray field. The helicopter had collided with level farm terrain while in a descent.
Postaccident Examination
A postaccident examination of the helicopter was conducted. The horizontal and vertical stabilizers, as well as the tail rotor assembly, appeared to be undamaged. The main rotor mast had rotated approximately 45 degrees forward, and symmetrical upward crushing of the cabin area was observed.
Tail Rotor Control System Findings
Examination of the anti-torque control system revealed that the tail rotor pitch change shaft's AN320 castellated shear nut had been stripped from the rotor end of the shaft. The threads on the shaft remained intact, but the threads on the nut were missing. The shaft's cotter pin hole retained the sheared portions of the cotter pin. The shearing force that caused the nut failure originated from an unknown source. Additionally, a laboratory report noted an ongoing wear imprint from the shaft end into the castle nut. Damage to the interior of the pitch change cap was also found during the wreckage examination.