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.
What's been done as a result
The Civil Aviation Safety Authority (CASA) revised the airworthiness directive for inspection of the wing attachment fittings to ensure that they were inspected using the magnetic particle inspection method. CASA also made, or plans to make, a number of changes to their systems and procedures to address issues identified in this report.
Separately, the ATSB reminded operators of M18 aircraft of the importance of the correct application of service life factors when operating at weights above the original maximum take-off weight. In addition, PZL Mielec plans to release additional maintenance documentation clarifying the need for removal of the wings for proper inspection of the wing attachment fittings. Finally, at the request of the owner, the supplemental type certificate for operation of the modified M18 Dromader at take-off weights up to 6,600 kg has been suspended by CASA.
This accident shows that even when flying within operational limits, the ‘harder’ and faster an aircraft is flown the more rapidly the structure will fatigue.
To help ensure that maintenance objectives are consistently met, the ATSB reminds aircraft maintenance personnel of the importance of only using properly-approved maintenance instructions. This accident confirms the importance of referring directly to those maintenance instructions when conducting maintenance.