Transport safety investigators found a QantasLink crew took all possible precautions on a flight to Canberra in which the captain and a passenger hit their heads during severe turbulence.
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The Australian Transport Safety Bureau's report, released last week, found the crotch strap of the captain's safety harness was probably insufficiently tightened, but otherwise said the crew's actions had minimised the risk of injury or damage to the plane.
The Bombadier Dash-8 "dropped suddenly" when it hit turbulence east of the Brindabella Ranges during the Melbourne to Canberra flight on October 10 last year.
The report said "everything in the flight deck became airborne", the autopilot disengaged and the captain, who was also the pilot flying, hit their head on the overhead air vent and light.
The impact knocked off the captain's headset, but they handed control over to the first officer as they refitted it and reset the pilot.
The flight continued to Canberra without further incidents.
One passenger hit their head on the overheard locker during the aircraft's drop, but they declined offers of medical treatment after the plane landed.
QantasLink advised the captain to visit a doctor, who diagnosed him with minor injuries.
The plane returned to service two days after the incident when no defects were found.
The ATSB report said the flight crew noted there was severe turbulence near the Brindabellas on the forecast during the pre-flight briefing.
The captain slowed the plane to 210 knots (389km/h), the safest speed to pass through turbulence, as the aircraft passed the eastern side of the range.
The seatbelt sign had already been activated and the cabin crew had been told at the start of the flight that it would be switched on early due to the conditions.
The report found the turbulence was probably related to "mountain wave activity", which can be generated on the leeward side of mountains as wind passes over a range.
It said the captain's injury was probably due to "some slackness of their crotch strap", the part of the safety harness that holds someone in place if an aircraft unexpectedly drops.
"The captain felt the aircraft drop from underneath them," the report said.
"The captain reported that the crotch strap can loosen with occupant movement... the aircraft was subject to minor fluctuations in G before the turbulence incident."
The report said the flight crew were prepared for the risk of turbulence during the approach and descent into Canberra.
There were no recommendations from the report, but it carried a safety message and noted the pilot's actions prior to hitting the turbulence " minimised the risk of injury to personnel and damage to the aircraft".
"Their incident highlights the importance of flight crew preparation for entry into an area of forecast turbulence and the importance of ensuring the correct adjustment of all harness straps," the report said.
