The paper describes a method for the calculation of viscous flows that includes the prediction of the transition onset location. It is an engineering approach aimed at providing a useful and necessary tool for air vehicle design and assessment. Some background that led to the adoption of the present approach is first given and the method is then described. Particular problems associated with the approach are identified and solutions are provided. Results from the initial validation of the method for both two- and three-dimensional flows are presented. The results illustrate the effects on flow-field predictions of modelling transition and the need to incorporate transition modelling in CFD methods is clearly demonstrated. The results also illustrate that the improved modelling can lead to a better understanding of the complex flows associated with some recent air vehicle designs, and the sensitivity of those flows to transition.