On July 27, 2019, at 1628 central daylight time, a Robinson R44 II helicopter, N7508Z, impacted terrain near Blair, Nebraska. The pilot sustained minor injuries, and the helicopter sustained substantial damage to the main rotor blades and tailboom. The helicopter was registered to and operated by NE IA Helicopters, LLC, Tekamah, Nebraska, as a Title 14 Code of Federal Regulations Part 137 aerial application flight. Day visual meteorological conditions prevailed at the time of the accident, and a flight plan was not filed. The local flight departed an off-airport location for aerial application operations. According to the pilot, after completing the last spray pass, he initiated a climb over a tree ridge and began to head towards the south. At the top of the climb, he noticed the engine RPM was beyond "max limitations," and the main rotor RPM was in the green range. The pilot initiated a right pedal turn to the north and decreased the engine throttle in an attempt to lower the engine RPM. After adjusting the engine throttle, 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 up and down, and continued to decrease. Unable to control and maintain the main rotor RPM, the pilot performed a forced landing to a corn field. During the forced landing to sloped terrain, the main rotor blades impacted the terrain, and the helicopter came to rest upright. A review of the aircraft records revealed Robinson Helicopter Company service bulletin (SB) 94, Subject: Actuator Lower Bearing A185 Brackets, issued June 8, 2017, was completed on May 5, 2018, at an aircraft total time of 2,181.3 hours. The compliance procedure was to visually inspect the A185 brackets and attaching rivets for evidence of fretting or looseness, and on the lower A185 bracket, apply torque seal in a vertical stripe across both outboard rivets to lower scroll to facilitate future inspections. The records also revealed the most recent 100-hour/annual inspection was completed on May 3, 2019, at a total airframe time of 2,244.0 hours. The total airframe time at the accident was 2,350.5 hours. A postaccident examination included the removal of the clutch assembly, freewheel clutch, belt tension actuator, time delay assembly, cooling fan, sections of the scroll, and actuator bearing support brackets. The components were shipped to Robinson Helicopter Company for further examination. The components were examined by a National Transportation Safety Board engineer and a technical representative of the Robinson Helicopter Company. The lower actuator support bearing bracket had separated from the scroll. Torque paint had been applied on the outer rivets, in accordance with SB-94. Fretting and rub marks were also noted between the bracket and scroll wall. Elongated rivet holes were observed on the scroll wall, and associated rivets were sheared. The upper actuator bearing support brackets were separated from the scroll and displayed fretting and rub marks between the brackets and scroll wall. All rivet holes were elongated and the rivets were sheared. The clutch actuator upper housing had fractured from housing to slider tube. The fracture surface was jagged and angular. The actuator screw was fractured at the worm gear interface in a thread grove, and the fracture surface was jagged and angular. As received, the cooling fan was tested and found to be out of balance per the manufacturer specifications; however, the fan is dynamically balanced once installed on the airframe which may be different as received from the manufacturer.