Demand for higher reliability@ robustness and performance in epicyclical gearboxes have led SKF to develop Design for Six Sigma (DfSS) based simulation tools and methods. This paper will illustrate the advantages of using simulation driven design in the development of planetary gearboxes for multi megawatt wind turbines. The simulation example will show the influence of the housing flexibility and of the non?linear bearing and gear stiffness on the gearbox performance under transient load. In particular the load distribution and deformation of the planetary gears and bearings will be analyzed. The flexibility and accurate stiffness description led to non?intuitive results. The gear deformation and load distribution led to significantly different results compared to results obtained by using traditional calculation tools and methods. A comparison between advanced and standard calculation methods is given as evidence that advanced analyses should be used to design reliable@ robust and high performing gearboxes. A virtual design of experiments was used to determine the most influential parameters affecting the gearbox performance. This paper will highlight the results of this DfSS study.