Robinson R44 N7508Z accident near Blair, Nebraska

Probable cause

The failure of the lower actuator bearing support bracket, which resulted in the misalignment of the clutch actuator, the loss of belt tension on the main rotor drive system, and the subsequent forced landing on sloped terrain.

NTSB analysis

After completing a spray pass during a helicopter aerial application flight, the pilot initiated a climb over trees to head south. The pilot reported that, at the top of the climb, he noticed that the engine rpm was beyond "max limitations" and that the main rotor rpm was in the green range. The pilot initiated a right pedal turn to the north and decreased the engine throttle to lower the engine rpm. The engine rpm remained at the upper limit, but the main rotor rpm decreased. The pilot then attempted to correct the main rotor rpm by increasing the throttle; however, the main rotor rpm fluctuated and then continued to decrease. Unable to control and maintain the main rotor rpm, the pilot performed a forced landing to a field with sloped terrain, during which the main rotor blades impacted terrain. The helicopter sustained substantial damage to the main rotor blades and tailboom.

Postaccident examination of the upper and lower actuator support bearing brackets revealed they were separated from the scroll, and evidence of fretting and rub marks was noted between the brackets and scroll walls. In addition, elongated rivet holes were observed on the scroll wall, and the associated rivets were sheared. Torque paint was noted on the outboard rivets.

It is likely that the bracket supporting the lower actuator bearing became loose and sheared the rivets, which allowed the actuator bearing to shift and led to a subsequent misalignment of the clutch actuator. The misalignment of the clutch actuator resulted in the loss of belt tension between the engine and main rotor drive system. The reason for the loose bracket and sheared rivets could not be determined due to damage.

A manufacturer service bulletin required an inspection of the brackets for evidence of fretting or looseness, and that was accomplished about 169 flight hours before the accident. The compliance procedure, which was to occur at the next 100-hour or annual inspection, was to visually inspect the brackets and attaching rivets for evidence of fretting or looseness and, on the lower bracket, to apply torque seal in a vertical stripe across both outboard rivets to lower scroll to facilitate future inspections. It is likely that the wear was not apparent at the time of the inspection.

Text from the NTSB. Open the NTSB docket for CEN19LA240 for the full record.