Accident Summary
An aircraft involved in superphosphate spreading operations crashed after striking powerlines. The pilot had conducted an aerial survey of the property with the property owner, during which power poles in the valley ahead and to the left of the airstrip were marked. At the start of operations, wind was light north-easterly and ambient temperature was 16° Celsius. After refueling, before the thirty-sixth flight of the day, ambient temperature had increased to 28° Celsius and wind direction changed to south-westerly.
Shortly after takeoff on a previous flight, the aircraft was observed to sink after overflying high-voltage power lines between the marked poles. On the next flight, the aircraft made a tight left turn and flew down the valley adjacent to the left marked power pole. On the final flight, the aircraft attempted to follow the track of the previous flight. While crossing power lines south-west of the marked power pole, the landing gear and left wing tip struck the powerlines. The broken powerline became jammed behind the left aileron washout plate, and the aircraft impacted the ground 100 metres beyond the power pole. Ground impact forces destroyed the aircraft and reduced the cockpit area to non-survivable dimensions.
Investigation Findings
On-site examination of the aircraft and subsequent laboratory testing of components did not reveal any pre-existing mechanical defects or abnormalities that could be considered factors in or contributory to the accident. Powerline impact marks on the aircraft were consistent with the aircraft being in a left banked attitude when it struck the wire. The investigation revealed that the loader driver's truck bucket load gauge had no conversion/calibration chart, and that the aircraft was being operated in excess of the maximum allowable weight for takeoff.
The absence of a superphosphate trail before wire impact indicated that the pilot did not dump any of the load and was either unaware of, or had forgotten about, the existence of powerlines to the south-west of the marked power pole.
Probable Cause
The investigation considered it probable that the pilot had elected to fly down the valley, taking advantage of the downslope, to compensate for a degradation of aircraft performance while operating overweight in the changed ambient conditions. The following factors were considered relevant: the aircraft was operated overweight for takeoff; the pilot did not adjust takeoff weight to achieve acceptable climb performance; the pilot was unaware of or forgot about the powerlines to the south-west, or misjudged clearance while trying to overfly them.