Fatigue fracture of a aircraft canopy lever reverse

A forged 7075-T6 aluminium alloy lever reverse of the canopy balancing system of a MB339 CD aircraft was found to be broken in two pieces during a pre-flight inspection. Visual observation revealed failure occurred close to fixing hole of the torsion bar and lugs of the actuator were significantly to be found longer than those traditionally used. In addition, the mark on the piston due to the action of the cylinder flange was to be found very close to the bottom travel. Electron microscopy showed failure origin was located at the internal edges of the lever reverse in correspondence of the most stressed areas, as confirmed by FEA, and due to relatively high cycles fatigue. The fatigue started by the application of an abnormal force due to an over length of the actuator lugs and was also aggravated by the stress concentration effect of pitting at the edge, as confirmed by SEM and metallographic examination. Recommendations emphasized the need for a prompt inspection of all lugs and replacement of actuators if necessary.