Investigators say a fatal plane crash off the Gold Coast may have been caused by the pilot attempting low level aerobatics.
Pilot Marcel van Hattem, 52, and passenger Trista Applebee were killed when the Soviet-era Yak 52 plane crashed into the water near South Stradbroke Island in June 2019.
Mr van Hattem had taken Ms Applebee on a scenic flight that had been an early birthday present for the 31-year-old.
Ms Applebee's partner was waiting at the Southport air strip and alerted authorities when they did not return.
The Australian Transport Safety Board said it was a "possibility" low level aerobatics had caused the crash.
It said witnesses and recorded data indicated the plane had been conducting a number of manoeuvres at less than 500 feet before it crashed into the water at high speed.
ATSB director transport safety Stuart Godley said Mr van Hattem had only been trained to conduct aerobatics "at no less than 3000 feet" in the two-seat ex-military trainer aircraft.
"While...we could not determine with certainty that the pilot was conducting an aerobatic manoeuvre immediately prior to the aircraft's impact with the water, the ATSB considered it a possibility," Dr Godley said.
"The pilot conducted low-level aerobatics without having completed the required training or having the appropriate endorsement to do so.
"This would have potentially limited the pilot's appreciation of the inherent risks associated with low level aerobatics."
Dr Godley said the pilot had been warned about conducting similar low-level moves after previous flights but it had not been officially reported.
He encouraged people to report their concerns to ATSB's confidential REPCON service or the Civil Aviation Safety Authority online.
"This accident highlights the inherent risks associated with performing low level aerobatics where there is a reduced safety margin for recovery," Dr Godley said.
"Even more so, it demonstrates the importance of being suitably trained and qualified to conduct these operations."
The ATSB investigation also found a lever - or bellcrank - attached to the plane's hinged rear flap - or elevator - had cracked under fatigue.
They said it had not contributed to the accident but did warrant a safety advisory notice to Yak 52 owners.
