DESTIN — A National Transportation Safety Board investigation has determined that the separation of the tail rotor drive shaft on a Robinson helicopter used by Timberview Helicopter for tours lacked a washer when it malfunctioned more than three years ago.

In its factual report released Friday on the accident that occurred just before noon Nov. 26, 2015, in Destin, the NTSB reported that “the hardware was in the correct sequence with the exception of the omission of a washer between the flex plate and the clutch shaft yoke.”

That caused a loud pop just as the helicopter pilot had begun to power up for liftoff. The flight was canceled and the pilot and three passengers on board were not injured.

Examination of the helicopter found the steel tail rotor drive shaft was fractured all the way around where it meets the forward flange. Also, a portion of the intermediate flex plate remained bolted to the flange while another part of it remained bolted to the clutch shaft yoke. In addition, the tailcone attachment frame and the damper assembly mount angle had fractures.

There was no report of any damage to any frame or to the interior of the tailcone behind the damper. The NTSB Materials Laboratory has kept parts of the flex plate and the clutch shaft yoke for further examination.

The helicopter had accumulated 1,770 of flight time when the accident happened.

NTSB said during its inspection that the helicopter “exhibited features consistent with over-stress.” The agency also questioned in its factual report whether measurements to the intermediate flex plate were correct.

A mechanic who performed repairs on the helicopter said the dimensions were the same during and after the clutch removal and installation. They also matched when he installed new hardware.

Meanwhile, a representative of the manufacturer of the Robinson helicopter told the NTSB that a missing washer could cause fatigue and failure of the tailrotor drive-line. He added that the company had never seen a failure like the one in the Destin accident.