The validity of current prediction methods for the formation of contrails from modern turbofan engines is investigated. A modelling framework has been developed for estimating the impact of engine configurations on contrail formation by analyzing the physics of the mixing between core and bypass flow. Within this framework, Engine Performance Modelling yields the flow conditions at exhaust nozzle exits. A 2D axisymmetric CFD Flow Field Solver then determines the flow parameters behind the engine. Water vapour dispersion is simulated using a species transport model, after which contrail locations are determined by a set of criteria for homogeneous ice crystal formation. Finally, particles are tracked to determine mixing lines, which are compared to those from current prediction methods. From a Leap-1A engine case study using this framework, it can be concluded that assumptions of instantaneous mixing underlying current methods are not valid, resulting in their overestimation of the mixing line gradient.