The paper concerns an application of lightness factors in comparative analysis of strength properties of basic materials being applied in aeronautical structures – in a historical perspective. The use of lightness factors enables effective estimation how lighter will be the structural elements (of the same strength or stiffness) made from different kind of materials : traditional as well as advanced composites. It is quite easy to find the solution to the inverse problem, i.e. to estimate how differ will be stiffness or strength for the same mass of the structural elements. Very particular application of the lightness factors are noted in engineers calculations of composite gliders wing spars, where they appears as the materials constants and as structure loading factors as well. The paper presents some examples of application of the lightness factors in strength analysis of the composite shells applied in the shear webs of the wing spars, and refers to the design recommendations issued by German aviation authority (LBA).