What happened
On October 26, 2023, a Robinson R44 helicopter, registration N141CD, was involved in an accident near Rowlett, Texas, during a Part 91 instructional flight. The flight was intended to provide a commercial pilot with instruction in basic IFR maneuvers, including RNAV approaches.
While flying in level flight, the flight instructor observed a sudden increase in engine and rotor rpm, accompanied by the high rotor rpm horn. The instructor took control of the aircraft and attempted to reduce the throttle, but the rpm continued to increase and fluctuate between 95 and 109 percent. Suspecting a governor failure, the instructor initiated an autorotation to prevent a potential engine failure.
The crew selected a recently plowed, muddy dirt field for landing. During the touchdown, the helicopter's skids dug into the mud, causing the aircraft to nose forward. This movement resulted in the main rotor striking the ground, causing the helicopter to spin violently before coming to a rest. The flight instructor and the student pilot both sustained minor injuries.
The investigation
Investigators examined the helicopter's Engine Monitoring Unit (EMU)/Governor Controller. While the unit was capable of maintaining engine rpm within the governing range, operational testing at the manufacturer's facility could not replicate the malfunction experienced by the crew. An examination of the circuit board revealed no anomalies that would have prevented normal operation.
Data from an onboard video and data recorder confirmed that engine manifold pressure increased rapidly at 1649:24, followed by rpm fluctuations. The recorder also captured the instructor'1s transition to manual control and the subsequent mayday call for an emergency engine out.