This thesis work was developed in collaboration with Avio Aero-A GE Aviation Business and it is focused on the study and analysis of the performance of an advanced turboprop in windmill conditions. The windmilling phenomenon is among sub-idle conditions, which occurs when the air flow entering the stopped engine causes spool rotation. Knowledge of windmilling performance is essential in the design phase of an aeronautical compressor. However, the phenomenon is difficult to understand and predict. In this thesis work a physical model capable of predicting engine performance in windmill conditions was developed, thanks to the use of NPSS (Numerical Propulsion System Simulation, a component-based object-oriented engine cycle simulator software).