What happened
The pilot reported that while departing on an agricultural spraying operation, the helicopter experienced an uncommanded yaw to the right. Application of the left pedal failed to correct the yawing motion. To manage the situation, the pilot initiated an autorotative landing and subsequently pulled the collective lever to cushion the impact with nearby trees.
The investigation
A post-accident examination of the helicopter revealed that the tail rotor drive shaft had become bent between bearing number 1 and bearing number 2. This deformation allowed the forward section of the shaft to pull away from its coupling, effectively disconnecting the control input from the tail rotor system.
Findings
The loss of directional control was directly caused by the mechanical failure of the tail rotor drive shaft. The bending of the shaft between the bearings resulted in the separation of the forward shaft from the coupling, preventing the pilot from maintaining heading during the critical departure phase. The subsequent impact with trees was a result of the inability to control the aircraft's yaw.