This paper presents an aerodynamic degradation study of an iced airfoil, using the Lattice Boltzmann approach with the commercial software PowerFLOW. Three-dimensional numerical simulations were performed with an extruded constant section of the GLC-305 airfoil with a leading-edge double-horn ice shape using periodic boundary conditions. The freestream Reynolds number, based on the chord, is 3.5 million and the Mach number is 0.12. An extensive comparison of the main flow features with experimental data is performed, including aerodynamic coefficients, pressure coefficient distributions, velocity and turbulence contours along with its profiles at several positions, and stagnation streamlines. The drag coefficient agrees well with experiments, in spite of a small shift. Two different wind tunnel measurements, using different measurement techniques, were compared to the CFD results, which mostly stayed in between the experimental data. Velocity and turbulence intensity contours as well as stagnation streamlines enabled a more detailed comparison of the flow field, which showed great accuracy of the simulations to predict the reattachment location. Overall, very good agreement is obtained with the available reference data. The numerical tool used to calculate the aerodynamic performance was able to deal with very complex flows, which in this case is highly unsteady, turbulent and characterized by large recirculation zones downstream of the ice. Such flow unsteadiness is caused by the flow separation and adverse pressure gradients. A mesh resolution analysis indicated grid convergence using a medium resolution setup, which provided good accuracy with fast turnaround times for the simulations. This enabled a complete angle of attack polar sweep, including post-stall angles.