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FORT WORTH, Texas (AP) — Federal investigations have concluded that vibration caused by high speed and low rotor rotation speed both led to a Bell Helicopter aircraft crash that killed two people.

The crash happened July 6, 2016, about 40 miles south of Dallas as the crew was flight testing a Bell 525 Relentless helicopter. Both people aboard the aircraft died.

A National Transportation Safety Board report released Tuesday says the crew was testing the aircraft’s characteristics when an engine ceases operating at high speed. As the main rotor slowed, its blades began to flap wildly, causing a vibration that shook the pilot so violently his involuntary control inputs made the vibrations worse. The wildly oscillating main rotor blades cut the helicopter’s tail boom, causing the aircraft to break up in midflight.

Bell officials said in a statement they have modified the flight controls so a pilot’s involuntary movements won’t be passed on to the rotors.

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