ASRS 2008727

I am writing to share that I experienced a malfunction of a Robinson R44 automatic drive belt tensioner component, while conducting helicopter patrols flying over non-populated terrain. I remember hearing a banging sound, followed by an immediate and uncommanded rise in engine RPM, and the illumination of warning / hazard lights with a loss of perceived power. I immediately initiated an autorotational descent, while Person B reported the situation to Dispatch. The autorotation was concluded successfully with the aircraft landing upright on its skids in a soft field of mud. I then immediately shut down the engine, electrical switches, stopped the rotor blades from rotating, and determined at that point there was no further danger to the occupants of the aircraft. The two souls on board, myself and Person B, both walked away without need for aid, and with both reporting no injuries. I initiated a call to the Chief Pilot to report the mechanical failure, while Person B contacted other personnel for assistance. After an initial inspection of the aircraft and surrounding area, there was no damage to persons or property on the surface and no further damage to the aircraft outside what caused the initial loss of power. After further inspection no damage to flight controls or structural components, to include sheet- metal, skids or blades, was found. The aircraft was later removed from the private party's field via helicopter and transported via trailer to the department’s contracted maintenance facility.