In modern aircraft, polymer matrix composites (PMC) are becoming more prevalent in both primary and secondary structures due to their high strength and stiffness to weight ratios. The scope of this thesis is to predict the service life of the Diamond DA-20 Katana based on the viscoelastic properties of the MSG L285/H287 epoxy matrix. The prediction methodology is applied to the composite wing spar because the spar bears the majority of the load during flight. The methodology requires the experimental characterization of the epoxy matrix using the time-temperature superposition principle. The results were then applied to the spar to determine its long term response using the Classical Lamination Theory applied to complex geometries. Failure was determined based on the Tsai-Hill and maximum stain failure theory or if the compressive flange buckles under the load.