The wreckage of the seaplane is lifted from the river on January 4. Credit:Australian Transport Safety Bureau Sydney Seaplanes, the owner of the 1963 model, said the company remained bewildered by the crash as a preliminary investigation report released on Wednesday found no mechanical faults or safety deficiencies. The De Havilland Canada DHC-2 Beaver aircraft took off from Cottage Point north of Sydney about 3pm before plunging in "a near vertical position" into nearby Jerusalem Bay. Ten minutes later, it was almost entirely submerged in 13 metres of water. The crash claimed the lives of all on board: British chief executive Richard Cousins, 58, his sons William, 25, and Edward, 23, his fiancee Emma Bowden, 48, her daughter Heather, 11, and pilot Gareth Morgan, 44.

The plane took a marked detour from its approved route before crashing. Mr Cousins had planned to retire in March from his position as head of the world's largest catering company, Compass, and wed Ms Bowden midyear. "The key question arising from the report is why the plane crashed approximately halfway down Jerusalem Bay, which is surrounded by steep terrain and has no exit," Sydney Seaplanes managing director Aaron Shaw said. The victims of the New Year's Eve crash. "It is not a route we authorise in our landing and take-off area register and the plane simply should not have been where it was."

Flying over Jerusalem Bay, the plane piloted by Mr Morgan took a steep 80- to 90-degree banking turn to the right before falling. The wreckage from the de Havilland Canada DHC-2 Beaver transported to land for investigation. Credit:Wolter Peeters "A turn of this nature at low altitude by a pilot with Gareth's skills, experience and intimate knowledge of the location is totally inexplicable," Mr Shaw said. Witnesses have told investigators the seaplane was flying over Jerusalem Bay at an altitude below the height of the surrounding terrain. Several people reported the sound from the aircraft's engine was constant and appeared normal.

Nat Nagy, an executive director of the Australian Transport Safety Bureau, appealed for more witnesses to come forward, especially any who saw the plane taxi and then turn into the bay. "One of the main lines of inquiry for us now is to try to work out exactly what was happening throughout that time," Mr Nagy said. "Firstly, why the pilot turned that way and then whether it was an attempt to turn around." The ATSB's investigators will not comment on what caused the plane to plunge into Jerusalem Bay until they complete their investigation and release a final report in about 12 months. Their preliminary report provides few clues but confirms the bureau investigated a fatal accident involving the same aircraft in 1996. It was later repaired before re-entering service in 2000. Sydney Seaplanes bought the aircraft six years later and has flown it thousands of times since.

As recently as last November, an inspection of the seaplane was carried out and a new maintenance release issued. A scheduled engine change was carried out at the time. Police divers helped retrieve almost all of the wreckage and while the bureau has kept the engine and propeller for further analysis, it was yet to find any problem with the plane. The ATSB found no evidence that a collision with a bird or other object might have led to the crash. Mr Morgan had spent years working for Sydney Seaplanes with total flying experience of more than 10,000 hours, of which about 9000 were on float planes. He was "an extremely experienced float plane pilot who had all the required qualifications and licences", Mr Nagy said.

Asked whether mental illness might have been a factor in the crash, Mr Nagy said "we have received no information or facts that there was any concern in that area". The pilot was reported to have "a high standard of health" but investigators are set to examine autopsy results when they become available from the NSW coroner. Mr Nagy said he did not know which of the Cousins-Bowden family had been sitting in the passenger seat at the time of the crash but investigations routinely look at possible passenger interference. He said investigators were trying to retrieve information from personal electronic items found amidst the wreckage, which may shed light on what was happening in the cockpit. The plane was not required to have any voice or flight data recorder.

Sydney Seaplanes, which resumed flights in mid-January, has audited all planes and put pilots through extra training.