the present invention relates to a method of manufacture of a group of transverse internal stiffening ribs for an aerodynamic fairing of an engine mounting device , including : production , for each rib , of a given rib preform by superplastic forming ; and in respect of at least one of the ribs of the group having a smaller size than that of the preform , division of the latter into four preform parts each incorporating one of the four corners of the quadrilateral formed by the preform , followed by the joining of the four parts to one another so as to obtain the rib .