Because of the recent economic crisis, both the regulations to lower down emissions and fuel prices have remarkably increased. For this reason, the main goal of many aircraft companies is to make an airplane project cost efficient. In other words, the Life-Cycle-Cost (LCC) of it needs to be analyzed. In particular, considerations concerning cost savings are especially important during the preliminary design phase, for it is during this phase that fundamental decisions are taken. The main purpose of this thesis is to deeply describe a cost estimation method: the Roskam method. As a matter of fact, this method takes into account 4 different cost categories: the RDTE phase, the Manufacturing & Acquisition phase, the Operating phase and the Disposal phase.