Failure Investigation of Nose Jack Mounting Bracket of AVRO Aircraft H-1517

Abstract

During the servicing of Avro aircraft H-1571, AOG build up and check-I i nspection were going on at 19 BRD, Guwahati. On completion of the same when the aircraft was lowered, a metal shearing sound was heard. Visual examin-ation revealed that, the vertical stiffener of the front face of pressure bulk head had buckled causing the jacking bracket mounting to shear. Chemical composition of nose jack mounting bracket is close to INDAL IS grade or Indian specification 24345 IS 3436 for Al base alloys. The total impurity content exceeds the specified value for sample C and titanium content in both the sample is slightly higher than the specification in the present investigation. Microstructure of the alloy consists of single phase a-Al having disper-sion of second phases. In transverse direction the grains are elongated owing to mechanical working of the alloy with the aspect ratio of 1:8 and 1:6 for sample B and C respectively. The grain size of sample B is 70um and the same for sample C is -85um. Sample B and C consist of Al-Cu-Mn-Mg and Cu-Al second phases respectively. No micro-structural heterogeneity has been found and component failure owing to structural degradation during service exploitation can be ruled out. The hardness in both the direction of the alloy lies within the range of 128-139BHN. The minor deviation in hardness occurs owing to difference in number density of second phases and varia-tion in processing parameters. The failure of the NJMB is caused by the action of fretting corrosion and fatigue. In service exploitation, few of the rivet holes experienc-ed fretting and got loosen. Air containing Ca and S as extraneous elements entered through the gap and promote pitting. These pits act as fatigue crack initiation sites. During service exposure the fatigue crack grew along the thickness direction of the component. Fretting corrosion results in decrement in section thickness and enhances the gap at the riveted joints. More gap leads to severe fretting and rubbing of loosen part one against other; hence most of the fracture surface lost the signature of fatigue crack propagation. When the crack length becomes sufficient, the load bearing ability of the component drops and the rest of the part failed under overload mani-festing micro void coalescence.