A duct wall (2) of a fan casing of a gas turbine engine comprises an intake section (4) and a containment casing (6), provided respectively with flanges (20,22). An acoustic flutter damper (28) is secured between the flanges (20,22). The acoustic flutter damper (28) has a skin (34,36) which accommodates an internal structure which serves to provide damping of flutter arising in fan blades (18). In the event of detachment of a blade (18) or a blade fragment, the resulting deflection wave in the containment case (6) causes the internal structure to rupture, and the load path between the intake section (4) and the containment casing (6) passes along the skin (34,36), which consequently maintains the connection between the intake duct (4) and the containment casing (6), while permitting substantial radial deflection of the containment casing (6) relative to the intake section (4).