An investigation into a serious helicopter accident in north Queensland earlier this year has been unable to determine the cause.

An instructor and student were injured in April this year when their helicopter slammed into the ground at the Proserpine airport during a flying lesson.

The instructor suffered serious injuries, needing 60 stitches in the back of his head after he was badly cut by the rotor blades.

An investigation by the Australian Transport Safety Bureau (ATSB) has found neither the student or instructor could recall what happened immediately before the incident and there were no witnesses.

It ruled out weather and mechanical faults as factors.

The investigation also found the aircraft was about 11 kilograms overweight on take-off, but concluded that did not contribute to the accident.