ERA16LA094

According to the flight instructor, the helicopter had been airborne for about 2 hours when it began to experience vibrations, which increased in intensity. He stated that the vibrations became “very violent” as he began to reduce engine power and were soon followed by the activation of the low rotor rpm horn and light warning system. The flight instructor performed an autorotation to a street in a residential area. The helicopter touched down and bounced, the main rotor blades struck a sign, and the helicopter rolled over onto its right side, which resulted in substantial damage to the airframe. A postaccident engine test run revealed that after the engine was started, it operated smoothly at idle and when the rpm was increased; however, when the left magneto was selected, the engine shut down. Both magnetos were removed and functionally checked on a test stand with no malfunctions noted. 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; but not precluded the magneto’s ability to function. The flight instructor was using full carburetor heat throughout the accident flight. It is likely that the cumulative effect of carburetor heat, which would have reduced engine rpm, and an intermittent failure of the left magneto for reasons that could not be determined resulted in the engine roughness that preceded the autorotation.

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.