The fan duct of a ducted fan gas turbine engine has a fan case (20) lined with a honeycomb structure (22) that acts to absorb the energy of a separated part of a blade (16). A layer of composite material (24) lining honeycomb structure (22) delaminates/breaks when a separated blade part passes through a further, inner honeycomb liner (26) and hits it.The resulting free end of composite liner (24) wraps round the striking end of the blade part, thus blunting the cutting action of the blade part and spreading the generated forces to the extent that the blade part is de-energised sufficiently to prevent it penetrating the fan case (20).