The object of this Master Thesis is to conduct a CFD simulation of Fire Extinguishing agent Halon 1301 (CBrF(3)) spreading into a medium-size helicopter engine nacelle in order to predict the efficiency of the Fire Extinguishing System (FES). CAD models of FES, engine and nacelle, the softwares licenses and technical support were all provided by Leonardo - Helicopters Division (LHD) (Cascina Costa) and it is the result of one year of cooperative effort due to the implicit difficulty of the case. The reason that lead the company to commission such a long trip to me are explained by the need of finding an alternative certification process to this type of systems installed on board of aircrafts and helicopters.