NYC08FA134 · Robinson R22 Beta II

While the helicopter was en route at an estimated 200 hundred feet, a witness on the ground heard the engine "spitting and sputtering." Another witness saw parts “shedding” from the helicopter, then saw it reverse direction and descend until she lost sight of it behind a building. The helicopter subsequently impacted the ground in a vacant lot and erupted into flames. A postflight examination of the engine revealed no mechanical anomalies. The carburetor air temperature gauge was reported to be inoperative and interpolation of a carburetor icing probability chart revealed atmospheric conditions conducive to "serious icing" at glide power. However, the position of the carburetor heat could not be determined and the investigation could not establish the role that carburetor icing may have played in the accident. An examination of the two main rotor blades revealed that each had a section missing, beginning about equidistant from the rotor hub. Both sections were subsequently located away from the main wreckage. Both main rotor blades were also coned upwards from the hub, which, in conjunction with the missing blade sections, was consistent with a low rotor rpm condition. An examination of the tail rotor driveshaft, and the push-pull tube running along it, revealed that they had become entwined, consistent with a “whirl” condition. According to a representative of the manufacturer, such a whirl mode could occur if an overspeed occurs; the manufacturer's maximum operating main rotor rpm limitation is 102%. The event likely began with a partial loss of engine power, followed by the pilot’s delayed response to a low rotor rpm condition, which then resulted in the coning of the main rotor blades and a loss of main rotor blade material.

Verbatim NTSB analysis and probable cause from the NTSB dataset