A method of stiffening a rib (19, 20) during the assembly of an aircraft wing is described. A stiffener panel (40, 41) is attached to a face of the rib (19, 20). The rib is aligned with one or more wing components with the stiffener panel (40, 41) attached. An assembly operation such as drilling (16) is performed with the rib aligned and the stiffener panel attached. The stiffener panel (40, 41) is removed from the rib after the assembly operation. The stiffener panel has a resilient external sealing ring which is mounted on a face of the panel and forms an external perimeter of a vacuum cavity; and at least one resilient internal sealing ring which is mounted on the panel (20, 21) within the perimeter defined by the external sealing ring and forms an internal perimeter of the vacuum cavity. The stiffener can be attached to the rib by a vacuum clamping force, the vacuum being sealed by the external and internal sealing rings. The internal sealing ring(s) seal any holes which are present in the rib. The stiffener (40, 41) can support the rib (20, 21) during alignment to give it the required flatness.