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.