Incident Overview
During a flight at 100 mph, the pilot of a Bell 47G2 helicopter experienced a retreating blade stall. The pilot reported that the helicopter began to shake, describing the sensation as an "out of balance feeling, like when the washing machine gets unbalanced." The rotor rpm subsequently began to decay.
Pilot Actions
In response, the pilot reduced power and stabilized the airspeed to approximately 60 to 65 mph, at which point the shaking ceased. However, the pilot stated that he was unable to maintain lift and therefore entered an autorotation over hilly terrain. During the autorotation, the tail rotor blades impacted an embankment, and the main rotor blades severed the tail boom.
Relevant Manual and Handbook Information
The pilot's manual for the Bell 47G2 indicates that the never-exceed speed (Vne) for the helicopter is 100 mph. According to the FAA's Basic Helicopter Handbook, the recommended recovery procedure for retreating blade stall is to reduce collective pitch, increase rotor rpm, reduce forward airspeed, and minimize maneuvering.
Conclusion
The accident sequence involved a retreating blade stall at the Vne speed, leading to a loss of lift and an autorotation that resulted in ground contact and structural damage. No further causal factors are stated in the source.