Robinson Helicopter R22 N404LE accident near Lindenhurst, New York

Probable cause

A rough running engine as a result of an intermittent failure of the left magneto for reasons that could not be determined, which resulted in a subsequent autorotation, impact with a sign, and rollover.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for ERA16LA094 for the full record.