Accident Summary
A single-engine helicopter experienced a hard ground impact while conducting a precautionary autorotational landing. The event occurred after the pilot received an engine overspeed indication.
Pilot Actions
The pilot reported observing an increase in rotor rpm and attempted to decrease it without success. He then reduced the throttle and entered autorotation. The pilot did not recall hearing any audible increase in engine or rotor rpm during the event.
Post-Accident Examination
Examination of the wreckage revealed that the dual tachometer indicator had a faulty turbine rpm indication. When functionally tested, it depicted an overspeed situation. The rotor tachometer generator and its associated indication were found to operate satisfactorily.
Maintenance History
Review of the helicopter's maintenance records showed that the dual tachometer indicator had been overhauled 39 days prior to the accident and installed on the accident helicopter 37 days prior. No other anomalies were noted with the engine or helicopter structure.
Engine Data Analysis
Examination of the engine control unit's nonvolatile memory revealed no rpm exceedence. Instead, an underspeed situation was recorded, attributed to a high engine rpm indication. The pilot more than likely noted an exceedence in engine rpm on the dual tachometer and interpreted it as a rotor rpm exceedence. He then failed to confirm the indication and proceeded with an autorotation while the engine and rotor system were operating normally.