This paper presents the results of recent investigations into the aerodynamics of simulated runback ice accretion on airfoils. Aerodynamic testing was performed on a fullscale, 72-inch (1828.8-mm) chord, NACA 23012 airfoil model over a Reynolds number range of 4.7×10⁶ to 16.0×10⁶ and a Mach number range of 0.10 to 0.28. A high-fidelity, ice casting simulation of a runback ice accretion was attached to the model leading edge. For Re = 16.0×10⁶ and M = 0.20, the artificial ice shape decreased the maximum lift coefficient from 1.82 to 1.51 and decreased the stalling angle of attack from 18.1 deg. to 15.0 deg. In general, the performance effects were insensitive to Reynolds and Mach number changes over the range tested. Aerodynamic testing was also conducted on a quarter-scale, NACA 23012 model (18-inch (457.2-mm) chord) at Re = 1.8×10⁶ and M = 0.18, using low-fidelity, geometrically-scaled simulations of the full-scale casting. It was found that simple, twodimensional simulations of the upper and lower surface runback ridges provided the best representation of the full-scale, high-Reynolds number iced-airfoil aerodynamics. Higher fidelity simulations of the runback ice accretion that included geometrically scaled threedimensional features resulted in larger performance degradations than what was measured on the full-scale model. Based upon this research, a new subclassification of spanwise-ridge ice is proposed that distinguishes between short and tall ridges. This distinction is made in terms of the fundamental aerodynamic characteristics as described in this paper.