Robinson R44 N440SH accident near Westminster, Maryland

Probable cause

The pilot's attempt to move the helicopter from a sloped surface after an overspeed event, which resulted in a loss of helicopter control and impact with a car. The overspeed event resulted from misrouted and damaged wiring in the magneto, which subsequently resulted in errant and random signals to the governor.

NTSB analysis

The commercial pilot stated that, during takeoff, the helicopter experienced an engine and main-rotor overspeed and that he then conducted a precautionary landing on a down-sloping portion of a parking deck. Skid marks indicate that only the toes and tails of the landing gear's skids contacted the deck upon landing rather than the full length of each skid tube. The pilot disembarked the passengers and then "attempted to reposition the helicopter to its original spot." He "increased the throttle to return to normal engine rpm," but the helicopter "started spinning uncontrollably." As the helicopter rotated around its mast, the owner/operator, who was standing beside the helicopter, ducked to avoid the tailboom before it struck a parked car. The pilot said that he stopped the engine and the helicopter's rotation by pulling the mixture control. The tail rotor blades and vertical stabilizer sustained substantial damage.

During a test run of the helicopter, when the engine was operating in the governed range, an engine overspeed occurred, similar to that described by the pilot. When the engine was operating with the governor in the "off" position, the engine rpm was controllable with no anomalies noted. Disassembly of the magneto that sent the signal to the governor controller module revealed that the wiring was misrouted and damaged, which resulted in errant, random signals to the governor and the subsequent loss of helicopter control.

Text from the NTSB. Open the NTSB docket for ERA18LA269 for the full record.