Abstract:Assembly lines are used for a large variety of products in different industrial sectors. In this paper the focus is placed on complex assembly systems and workstations used for the final assembly of large and bulk products, such as trucks, aircrafts, buses, tool machines. An high number of tasks to be performed at a single assembly station, several workers involved in parallel in the assembly process and long Takt times make such systems different from the models intensively studied in the literature (e.g. the traditional Simple Assembly Line Balancing Problem). This study firstly presents a new balancing model to address the problem of the total cost minimization when different operator skills are involved at the same time and then it applies the model to a real industrial case.