On January 27, 2016, about 1157 eastern standard time, a Robinson R22 Beta, N404LE, was substantially damaged when it was involved in an accident near Lindenhurst, New York. The flight instructor and student pilot were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. According to the flight instructor, the helicopter had been airborne for about 2 hours and was returning to Republic Airport (FRG), Farmingdale, New York, when it began to experience vibrations, which increased in intensity. He further stated that, as he began to reduce power, the vibrations became “very violent” and were soon followed by the low rotor rpm horn and light warning system. The flight instructor performed an autorotation to a street in a residential neighborhood. The helicopter touched down and bounced, the main rotor blades struck a sign, and the helicopter rolled over onto its right side. The flight instructor stated that he was using full carburetor heat throughout the entire flight. On-site examination of the helicopter by a Federal Aviation Administration (FAA) inspector revealed that the tailboom separated just before the vertical fin, and the tail rotor gear box and tail rotor blades displayed damage consistent with ground contact. An unquantified amount of fuel was observed in the helicopter’s fuel tanks. The carburetor heat control was observed in the “ON” position. A subsequent engine test run revealed that after the engine was started, it operated smoothly at idle and when the rpm was increased; however, during a magneto check, when the left magneto was selected, the engine shut down. The engine continued to operate if the right or both magneto position was selected. Further examination of the left and right magnetos revealed that, when placed on a test stand, each respective magneto’s terminals produced a blue spark across a 7-millimeter gap throughout all magneto rpms. The torque putty on the left magneto’s case half mating screw was compromised. Subsequent disassembly of the left magneto revealed that the internal magneto timing was off by two teeth, which would have increased the gap between the distributor gear finger and the distributor block’s stationary electrodes.