Investigators will examine whether a pilot's lack of oxygen led to a firefighting plane crash which killed three people in outback Queensland.
A pilot and two camera operators, including 22-year-old American William Jennings, were on the the twin-engine charter plane that was conducting aerial fire surveillance operations for bushfires in Queensland's northwest late last year.
An Australian Transport Safety Bureau (ATSB) preliminary report said there were radio communications issues with the pilot before the plane crashed near Cloncurry in early November.
The ATSB said the pilot's speech before the crash would be further analysed, with investigators examining the possibility he was suffering from of a lack of oxygen in the body known as hypoxia.
"Radio recordings during this period indicate that the pilot's rate and volume of speech had been substantially lowered from earlier communications and was worsening," ATSB chief commissioner Angus Mitchell said.
Investigations so far have found communications with the plane were lost for over an hour after an air controller requested the pilot change radio frequency.
Once communications were re-established the pilot told the controller they were to conduct work near Mount Gordon.
They were provided clearance for this work but the pilot, who ATSB said seemed uncertain, repeated the clearance four times in four minutes.
About 20 minutes after the pilot's last transmission the plane began to descend rapidly toward the ground.
It crashed in open bushland and was destroyed by fire post-impact.
"During their on-site examinations investigators were able to account for all major aircraft components and determined that both engines and propellers had indications that the engines were running at impact," Mr Mitchell said.
"However, it was not possible to determine the operability of the aircraft's pressurisation and oxygen systems."
The ATSB will release a final report when investigations into the crash conclude.
