The ergonomic aspect in the assembly lines has become very important to consider nowadays. Its study helps improving operator's environment and life quality at work. This ergonomic study in aircraft assembly lines showed that some tasks increase the risks of musculoskeletal disorders. In order to help industrial managers to reduce these risks, a novel ergonomic index for task scheduling has been developed. This index has the objective of improving the solution quality in terms of ergonomic fairness by balancing difficult tasks between each operator. This criterion has been used in a model that has then been tested on real-life instances of aircraft assembly line to witness its efficiency.