This paper focuses on robustness analysis for wind turbine control systems. The dynamics of a wind turbine are nonlinear and time-varying due to several effects including blade rotation, wind shear, tower shadowing, and varying wind conditions. Thus classical gain/phase/delay margins, computed using frequency domain concepts, are insufficient for turbine control systems. The robustness analysis in this paper is instead performed in two steps. First, the turbine dynamics are linearized at either constant wind conditions or along a fixed (hub height) wind speed trajectory. This yields a linear time-varying (LTV) model for the turbine dynamics. Next, disk margins are computed using existing results for finite-horizon LTV systems. These disk margins account for uncertainty at the blade pitch and/or generator torque inputs. This method is applied to assess the margins for a 2.5 MW Clipper Liberty turbine operated by the University of Minnesota. The turbine model and source control law used in the analysis was provided by Clipper. These results provide additional insight into the robustness of the existing turbine control law.