Operating airfoils under mechanical stress in combination with oxidation and corrosion, abrasion wear, and subsequent regeneration results in complex surface structures that influence the performance of aircraft engines. For predicting this impact on performance the authors propose a reduced order model capable of assessing the effect of surface roughness as a basis for making decisions before or during the regeneration process. The accuracy of this model is increased by using improved roughness sensitive transition and turbulence models created for Reynolds-Averaged Navier–Stokes simulations. Experimental studies are carried out to determine the local and integral effect of complex surface structures on blades and on a flat plate. Direct numerical simulations are also performed to study the effect of complex surface structures on a turbulent boundary layer and contribute to improving the accuracy of prediction, which is achieved by using re-calibrating models and taking into account the effect of skewness and anisotropy of complex surface structures on turbine blade losses.