The need for assembly parts and structures where the perfect fitting is not guaranteed due to manufacturing/assembly tolerances and/or the influence of several physical effects, i.e. gravity, is increasing constantly in different industrial sectors and in particular in the aerospace industry.Some techniques have been developed to deal with it. In the field of large aero structures, custom-made parts using reverse engineering techniques are used to machine parts whose geometry requires to be customized for each produced aircraft. In the field of aircraft shells, a technique based on the characteristics of non-rigid components that can be slightly deformed to clear geometrical conditions by controlled forces to strain within its stress limits.FITFLEX project exploit this last technique and was carried out by Airbus Group Innovations and Airbus. The objective is supporting the Airbus A350 XWB ramp-up and the current manual positioning process of the shell by force control, defining a measurement based assembly including a flexible best-fit system.The case subject of study on FITFLEX project is the A350 XWB rear fuselage, a 14 m by 5 m side shell positioning process.