between its opposite side faces 110 a and 110 b 24 layers of warp yarns c 1 to c 24 and as many layers of weft yarns t 1 to t 24 in this portion 112 the weave is of the interlock type with a satin type weave for the warp yarns c 1 and c 24 in the portions adjacent to the opposite side faces 110 a and 110 b 3d weaving of a fiber structure with an interlock type weave in its core and a 2d or 3d satin type weave in its skin is itself known reference may be made to document wo 2006 136755 in the portion 114 that is to form an airfoil preform the thickness of the fiber blank between its side faces 110 a and 110 b varies thus in order to take account of this thinning of the profile of the blade that is to be fabricated layers of warp yarns and a corresponding number of weft yarns are withdrawn progressively from the fiber blank in the example of fig3 it is the warp yarns c 2 c 3 and c 4 that are successively withdrawn from beside the side face 110 a and warp yarns c 23 c 22 and c 21 that are.