Darcy McCarter, 23 and Daniel Burrill, 33, were repatriating an indigenous man's body from Darwin to Elcho Island in October when their Cessna 210 lost its wings, ripped through bushland and crashed on a rural road 30km from the airport.

"Witnesses in the vicinity of Howard Springs saw the aircraft descend rapidly in a relatively flat attitude with a portion of each wing missing," a preliminary Australian Transport Safety Bureau report found.

Both pilots were secured in their seats prior to the aircraft impacting terrain from a vertical descent.

Witnesses spotted clouds form over the rural area and to avoid bad weather, the pilot diverted the plane right of the flight path shortly after take off.

"Some reported that the cloud went 'very black' at the time of the accident, and that starting about 10 minutes after the accident, it rained heavily for about an hour," the ATSB report said.

"The developing cumulus clouds may have produced strong updrafts or downdrafts."

About 20 minutes after take off at 1.29pm on October 23, air traffic controllers noticed the aircraft turn abruptly to the left while its speed fluctuated and it descended and climbed in altitude.

The charter plane accelerated to 150 knots in the final minute of the flight, above the manoeuvring speed of 118 knots, and when this occurs turbulence can produce damaging wing loading.

"At airspeeds above about 145 knots, this loading can result in failure of the aircraft structure," the report noted.

"The wing spars had fractured in over-stress, and exhibited bending deformation consistent with forces acting upwards and rearwards on the wings. "

Wreckage from the Air Frontier plane, which was more than 40 years old, was strewn either side of Gunn Point Road, with its wings and "severely impact-damaged" body found 740 metres apart.

"The propeller did not exhibit any evidence of rotation at impact, consistent with fuel exhaustion resulting from the ruptured integral wing-fuel tanks," the ATSB report said.

The single-engine, six-seater aircraft showed no pre-existing defects that could have contributed to the accident, maintenance inspections were up to date and there was no evidence of a fire.

The investigation will further examine electronic data, the effect of weather conditions on the flight and pilot experience before a final report is released.

All seven in-flight breakups of Cessna 210 aircraft in the United States since 2000 have involved flight into thunderstorms or associated turbulence, a loss of control following inadvertent flight into meteorological conditions or a combination of both.