analytical and numerical methods to the prediction of parasitic drag due to excrescences introduced by wing repairs. Various approaches for calculating drag coecient increments and associated drag magnification factors for wing repair plates are presented. The practical consequences of the dierent modelling constraints have been investigated for typical repair locations encountered during aircraft maintenance. It has been found that drag magnification factors predicted by analytic methods based on the boundary layer integral equations are to be treated with caution in regions of relatively thin boundary layers and in supercritical flows. More moderate, yet similar, constraints apply to viscous/inviscid interaction numerical methods. RANS CFD simulations have been shown to be the most promising approach to-date for calculating the full extent of the drag penalty introduced by wing repairs. However, special care needs to be taken during grid generation, simulation, post-processing and analysis in order to minimise the eect of numerical uncertainties on the result.