Aim of the paper is to present a validated numerical methodology for the optimization of a multilayer armour. The optimization is intended to warrant the same ballistic capability (of a monolithic ballistic steel plate) with respect to a small caliber AP bullet but with a reduction in weight. The use of aluminium and composite layers has been considered. An advanced but efficient numerical methodology is proposed and validated against experimental data from the literature and from tests carried out by the authors. Although the methodology is not directly applied to a specific helicopter frame, the results and the discussion are potentially beneficial for the design of lightweight armour thus fitted for aeronautic purposes.