Fatigue cracks in wing mounting lugs caused the crash of an M18A Dromader firebomber in 2013, according to the ATSB report released yesterday.
Dromader VH-TZJ was in operation as a single-engine aerial tanker (SEAT) over a bushfire near Ulladulla, NSW, on 24 October 2013, when the port wing separated in flight. The pilot died in the accident.
" ... the left wing separated because it had been weakened by a fatigue crack in the left wing lower attachment fitting," the ATSB report states.
"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 discovered during the investigation that manufacturer–PZL Mielec–issued instructions to remove the pits, but the pits were not completely removed during previous maintenance. Also, the components were tested for cracks using an eddy current inspection, which was not approved for that particular inspection and may not have detected the crack.
The ATSB report also stated that the way VH-TZJ had been flown prior to the accident could have accelerated the crack growth rate.
"The aircraft had been operated at higher speeds and subjected to a more severe flight load spectrum than assumed by the manufacturer when it determined the aircraft's service life limitation. This likely increased the rate of fatigue damage, increasing the rate of formation and growth of the micro-cracks in the left outboard wing lower attachment fittings."
CASA has since revised the Airworthiness Directive for inspecting the wing attachments to specify magnetic particle inspection.
The full investigation report is on the ATSB website.