Collision with terrain - Air Tractor Inc AT-502A , VH-MVS, near Nyngan NSW, 18 September 1997

Aircraft information

The aircraft was fitted with a Pratt and Whitney PT6A-45R
turbine engine driving a five-bladed metal Hartzell
reversible-pitch propeller. This engine-propeller combination
provided the aircraft with a significantly higher performance than
other models of the aircraft type that were powered by piston
engines.

The pilot

The pilot was appropriately qualified to undertake the flight.
Of his total flying experience, about 9,500 hours was in
agricultural flying. However, he had flown only about 80 hours on
turbine-powered Air Tractor aircraft, the bulk of his flying
experience being on piston engine models.

Wreckage examination

Examination at the accident site revealed that the aircraft had
struck the ground inverted and in a level attitude. Ground marks
indicated that the aircraft was yawing right and at low forward
speed at impact.

The fire destroyed the airframe. The destruction of some
components of the flight control system, particularly the aileron
system, precluded determination of their serviceability prior to
impact.

Three blades of the propeller had failed in overload against the
direction of engine rotation. The position of failure was about 200
mm from the propeller hub. The failure surfaces indicated that the
blades were at a coarse pitch angle at the time of failure.

Survival aspects

The metal end fittings of the pilot's safety harness were found
in the wreckage. Each was separated from the other with the latch
on the locking buckle stowed in its housing, as it would be if the
harness was locked. Examination of the end fittings revealed no
discernible deformation on any load carrying part of the fittings,
suggesting that the harness was not secured at impact.

The witness description of the aircraft's behaviour, along with the evidence at the impact site, indicated that the aircraft probably stalled aerodynamically as the pilot reversed the direction of the procedure turn. It is likely that the aircraft was in a nose-high attitude at the time. This would explain the apparent low forward speed of the aircraft at impact. Assuming that the pilot was conducting the turns at typical altitudes, it is unlikely that there would have been sufficient height available for the pilot to recover the aircraft to normal flight. The pilot's low experience level on turbine powered Air Tractor aircraft compared with piston engine powered models may have contributed to the loss of control.

Details obtained during the investigation did not allow any conclusions to be drawn concerning the security of the pilot's safety harness at impact.

After arriving at the property strip, the pilot was briefed on
the spray area by the property owner. In the meantime, the aircraft
was loaded with 1,500 litres of spray solution. The pilot
subsequently boarded the aircraft and was observed to secure his
safety harness, and don gloves and a helmet, before departing for
the task.

The spray area was about 800 m from the strip. Witnesses at the
strip observed the aircraft as it began spraying and noted that the
procedure turns at the end of each run were conducted at a
consistent height. The turns were flown initially to the left to
offset the aircraft from the previous run, and then reversed to
align the aircraft for the next run. About 30 minutes after
spraying had commenced, one of the witnesses observed the aircraft
rolling right during a turn reversal in a procedure turn. The
aircraft continued rolling until it was in an inverted attitude,
and then descended into the ground where it immediately caught
fire.