A method of designing an aircraft component formed by a number of elements that includes a phase to estimate the effect of a variation of a design variable on the component weight which method includes the following steps: a) providing the main primary structure basic data of the aircraft component and the Reserve Factors associated to its design criteria; b) obtaining a breakdown of the component weight by such design criteria using a fictitious weight calculated taking into account the relative importance of their critical design criteria; c) obtaining the weight effect of a design variable variation, recalculating firstly the new Reserve Factors using suitable functions for the variation of the Reserve Factors vs. the variation of the design variable and, secondly, recalculating the component weight using suitable functions for the variation of the element dimensions vs. the variation of the Reserve Factors.