Identification of Cracks
On 5 October 2016, during routine maintenance, four strut ribs from the No. 2 engine pylon were found to have developed fatigue cracks. The parts conformed to the manufacturer’s engineering drawing and material specifications, with no obvious defects detected. The fatigue cracking is likely attributable to exposure to vibratory loading that occurred within the pylon during normal engine operation.
Potential Contributing Factors
A factor that may have contributed to the crack growth was the number of hours and flight cycles that the airframe had accumulated within its service life. Despite the crack damage, the manufacturer’s analysis indicated that the pylon retained sufficient residual strength for all load cases, suggesting that the aircraft remained safe to operate.
Previous Engine Seizure
Approximately six months prior to the detection of the strut rib cracking, the No. 2 engine sustained a seizure that resulted in the transmission of significant vibratory and torque loads into the pylon structure. However, metallurgical analysis of the fracture at the crack origins did not show any evidence of plastic deformation or tearing of the alloy that might otherwise suggest that the seizure was a contributing factor to the cracking.
Inspection Interval
Compared to the 48-month inspection interval recommended by the manufacturer, the 24-month inspection interval set by Qantas provided greater opportunity for the timely detection of cracking and other defects within the pylon. The reduced inspection interval proved effective in this case, detecting cracks before they reached a level considered by the manufacturer to be a safety risk.