Accident Narrative
On July 27, 2019, at 1628 central daylight time, a Robinson R44 II helicopter (registration N7508Z) impacted terrain near Blair, Nebraska. The helicopter was registered to NE IA Helicopters, LLC, of Tekamah, Nebraska, and was conducting a Title 14 Code of Federal Regulations Part 137 aerial application flight. Day visual meteorological conditions prevailed, and no flight plan was filed. The local flight had departed from 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 turned south. At the top of the climb, the engine RPM exceeded maximum limitations while main rotor RPM remained in the green range. The pilot executed a right pedal turn to the north and reduced the throttle to lower engine RPM. After the throttle adjustment, engine RPM stayed at the upper limit, but main rotor RPM decreased. The pilot then attempted to increase the throttle to correct main rotor RPM, but it fluctuated and continued to decline. Unable to maintain main rotor RPM, the pilot performed a forced landing into a corn field. During the landing on sloped terrain, the main rotor blades struck the ground, and the helicopter came to rest upright. The pilot sustained minor injuries, and the helicopter sustained substantial damage to the main rotor blades and tailboom.
Aircraft Records and Service Bulletins
A review of aircraft records indicated that Robinson Helicopter Company service bulletin (SB) 94, issued June 8, 2017, was completed on May 5, 2018, at an aircraft total time of 2,181.3 hours. The bulletin required visual inspection of the A185 brackets and attaching rivets for fretting or looseness, and application of torque seal on the lower A185 bracket to facilitate future inspections. The most recent 100-hour/annual inspection was performed on May 3, 2019, at a total airframe time of 2,244.0 hours. Total airframe time at the time of the accident was 2,350.5 hours.
Postaccident Examination
Postaccident examination involved removal of the clutch assembly, freewheel clutch, belt tension actuator, time delay assembly, cooling fan, scroll sections, and actuator bearing support brackets. These components were sent to Robinson Helicopter Company for detailed analysis.
An engineer from the National Transportation Safety Board and a technical representative from Robinson Helicopter Company examined the components. The lower actuator support bearing bracket had separated from the scroll. Torque paint had been applied on the outer rivets as per SB-94. Fretting and rub marks were present between the bracket and scroll wall. Elongated rivet holes were found on the scroll wall, and the associated rivets were sheared. The upper actuator bearing support brackets were also separated from the scroll, displaying fretting and rub marks, with elongated rivet holes and sheared rivets.
The clutch actuator upper housing had fractured from housing to slider tube, exhibiting a jagged and angular fracture surface. The actuator screw was fractured at the worm gear interface in a thread groove, also with a jagged and angular surface. The cooling fan, as received, tested out of balance per manufacturer specifications, but the fan is dynamically balanced when installed on the airframe, which may differ from its as-received condition.