Fatigue and Fracture of a Cast Flight Control Linkage

A fractured flight control linkage from a trainer aircraft was submitted for evaluation. Analysis showed fracture of the cast aluminum linkage was preceded by reversed bending fatigue. In addition to fatigue striations, Wallner lines through brittle phases were observed on the fracture surface. Examination of field returns showed cracking in multiple instances. Separate root causes were identified, each showed different fracture surface morphologies. Vibration measurements confirmed the likely source of lateral excitation, and quantified the extent of tail to tail variability. A redesigned linkage (from wrought aluminum) demonstrated much greater life. There has been no subsequent distress reported.