An investigation report into a fatal plane crash on the New South Wales south coast has found the left wing separated from the plane mid-flight due to an undetected fatigue crack.
The M18 Dromader was carrying out water bombing operations for the Rural Fire Service when it crashed near Ulladulla in October 2013.
The crash resulted in the death of experienced pilot David Black, 43, from Trangie in central-west NSW.
The final report from the Australian Transport Safety Bureau (ATSB) found the wing separated from the plane due to a fatigue crack in the lower attachment fitting, which originated in small corrosion pits.
The corrosion pits were not successfully removed during maintenance, and the unapproved inspection method may not have been effective in detecting the crack.
"The result of weathering, interaction with the metal and so on could lead to corrosion of the metal and that leads to pitting of the metal, and the way of dealing with that is to strip back the metal and remove the pitting," ATSB chief commissioner Martin Dolan said.
"And that wasn't done fully effectively in this case."
The report also noted the harder and faster an aircraft was flown, the more rapidly the structure would fatigue.
Mr Dolan said despite approval to carry heavier loads, the stresses to the plane could have been more accurately calculated.
"There was a range of tests and approvals that were done to allow it to fly with a heavier load," he said.
"But looking back over the various calculations done about that, we think that the risk of parts of the aircraft being subject to stress and fatigue risk was probably under calculated."
Since the accident, Commissioner Dolan said procedures for inspections and recommendations for carrying additional weight were close to being fully implemented.
"They have actually been implemented or are in the process of being implemented, and we'll just keep an eye on them until the work has been complete," he said.
"But we are satisfied with the action that has been taken or the action that's proposed."
The M18 Dromaders are no longer approved for water bombing operations.
In-flight breakup involving PZL Mielec M18A Dromader aircraft, VH-TZJ, 37 km west of Ulladulla, NSW on 24 October 2013
Investigation number: AO-2013-187
Investigation status: Completed
On 24 October 2013, the pilot of a modified PZL Mielec M18A Dromader, registered VH-TZJ, was conducting a firebombing mission about 37 km west of Ulladulla, New South Wales. On approach to the target point, the left wing separated. The aircraft immediately rolled left and descended, impacting terrain. The aircraft was destroyed and the pilot was fatally injured.
What the ATSB found
The ATSB found that the left wing separated because it had been weakened by a fatigue crack in the left wing lower attachment fitting. The fatigue crack originated at small corrosion pits in the attachment fitting. These pits formed stress concentrations that accelerated the initiation of fatigue cracks.
The ATSB also found that, although required to be removed by the aircraft manufacturer’s instructions, the corrosion pits were not completely removed during previous maintenance. During that maintenance, the wing fittings were inspected using an eddy current inspection method. This inspection method was not approved for that particular inspection and may not have been effective at detecting the crack.
Data from a series of previous flights indicated that the manner in which the aircraft was flown during its life probably accelerated the initiation and growth of the fatigue crack.
Finally, the ATSB also found a number of other factors which, although they did not contribute to the accident, had potential to reduce the safety of operation of PZL M18 and other aircraft. These included the incorrect calculation of the flight time of M18 aircraft and a lack of robust procedures for the approval of non-destructive inspection procedures.