An aircraft "ran rough" and stopped completely just after take-off before ditching in mangroves in southeast Queensland, a preliminary investigation into the fatal accident has found.
The Australian Transport Safety Bureau released the report on Monday into the crash that killed two men and two children at Scarborough, north of Brisbane, on December 19.
The single-engine Rockwell International 114 left Redcliffe aerodrome for the private scenic flight in fine weather just after 9am.
The pilot, Lincom Group chairman and founder Roy Watterson, together with the company's national hire fleet coordinator Chris Mocanu and his two children died in the crash.
Witnesses saw the aircraft take off and retract its landing gear, but reported the engine "ran rough briefly before stopping completely" a short time later, ATSB director transport safety Michael Walker said.
The pilot was heard on the radio saying they were returning to the aerodrome and the plane was seen making two left turns consistent with manoeuvring back to the runway, and extending its landing gear.
"As the aircraft neared the mangrove tree line to the north of the aerodrome, it was observed to descend and ditch into the water of a tidal mud flat, about 170 m from the shoreline," Dr Walker said.
"During the ditching, the aircraft flipped over, coming to rest inverted in about 2 m of water."
The wreckage was recovered and has been examined, but further investigations will include testing engine components, analysis of data recorded from onboard systems, and more analysis of available footage.
"Should a critical safety issue be identified at any time during the course of the investigation, the ATSB will immediately notify relevant parties so appropriate and timely safety action can be taken," Dr Walker said.
The final report is expected to be completed this year.
