This occurrence was not the subject of an on-site investigation by the ATSB.
The Dromader aircraft was engaged in fire bombing operations in rugged terrain. Retardant carried in the Dromader's hopper was to be dropped on a fire, burning about half way up the eastern slope of a steep valley, oriented northwest to southeast. Before releasing the retardant, the pilot made a dummy run from the southeast to the northwest at a height of about 50 ft above the tree canopy. He then manoeuvred the aircraft onto a reciprocal heading for the drop run. Although that was in the direction of the head of the valley, the pilot's intention was to turn west into the valley after releasing the retardant.
The pilot reported that after releasing the retardant he applied full power and attempted to climb and turn, but found that the aircraft performance was less than that expected. He reported that when he attempted to bank the aircraft away from the side of the valley, the aircraft's performance diminished, depriving him of manoeuvrability. The pilot believed that he had encountered adverse windshear conditions in the lee of the ridge, associated with an active thunderstorm to the east of the ridge. Unable to turn away from terrain, the pilot maintained control of the aircraft but was flying toward the head of the valley. When he saw that a collision with trees was inevitable he transmitted on the dedicated communication frequency `I'm going in' and slowed the aircraft, allowing it to settle into the tree canopy. The aircraft collided with the foliage, pitched nose down and dropped to the ground in a near vertical attitude.
After ground impact, a fire started in the engine compartment and the pilot quickly egressed from the burning wreckage. The aircraft was destroyed by impact forces and the post impact fire. The plot reported that he had not detected any abnormality with the aircraft immediately prior to the accident.
Witnesses on the ground reported the presence of a thunderstorm on the other side of the ridge and occasional strong gusts of wind from the northeast. One witness reported seeing the aircraft wings roll to a "knife-edge" (90 degrees) attitude, then return to level just before the aircraft struck the tree canopy. The crew of an observation aircraft operating overhead the fire bombing activity, heard the pilot's transmission and watched as the Dromader impacted heavily timbered terrain below the top of the ridge. They also reported the presence of the nearby thunderstorm, northeast of the area. The crew of the observation aircraft reported that they did not encounter any significant turbulence or windshear at their altitude, about 1,500 ft above the Dromader.
The ATSB was unable to determine the exact circumstances of the accident. It was possible that the outflow of air from the thunderstorm, spilled over the ridge, creating down draughts that were in excess of the aircraft's climb performance, depriving the pilot of the manoeuvrability necessary for executing the intended flight path.