A duct wall of a fan casing of a gas turbine engine comprises an intake section and a containment casing, which are interconnected by bolts at flanges. An acoustic flutter damper is provided between the flanges to reduce or eliminate flutter arising in blades of the fan at certain important operating conditions. The damper provides flexibility at the connection between the intake section and the containment casing so that, in the event of detachment of a blade or a bladed fragment, the resulting deflection wave in the containment casing can be accommodated by displacement and/or deformation of the acoustic flutter damper, reducing the risk that the bolts will shear to allow the intake section and the containment casing to become detached from each other. The acoustic flutter damper may comprise a circumferential array of separate segments.