What happened
During a solo instructional flight, a helicopter experienced a split in rotor and engine tachometer readings accompanied by an audible low rotor RPM alarm while in cruise flight. Recognizing a potential mechanical emergency, the pilot executed a precautionary landing. During the maneuver, the aircraft nosed over in a rocky, uneven field, resulting in substantial damage.
The investigation
Investigators examined the control box, tachometer cable, and the tachometer itself. Upon initial inspection, the warning circuitry within the control box was found to be non-operational; however, it functioned correctly after technicians repaired an open circuit discovered on the circuit board. While no anomalies were found within the tachometer, a visual inspection of the cable revealed signs of excessive wear and potential binding caused by the windup of the outer strands. This cable binding is consistent with the observed bouncing of the tachometer needle.