with reference to fig1 , a ducted fan gas turbine engine generally indicated at 10 comprises , in axial flow series , an air intake 1 , a propulsive fan 2 , an intermediate pressure compressor 3 , a high pressure compressor 4 , combustion equipment 5 , a high pressure turbine 6 , an intermediate pressure turbine 7 , a low pressure turbine 8 and an exhaust nozzle 9 . air entering the air intake 1 is accelerated by the fan 2 to produce two air flows , a first air flow into the intermediate pressure compressor 3 and a second air flow that passes over the outer surface of the engine casing 12 and which provides propulsive thrust . the intermediate pressure compressor 3 compresses the air flow directed into it before delivering the air to the high pressure compressor 4 where further compression takes place . compressed air exhausted from the high pressure compressor 4 is directed into the combustion equipment 5 , where it is mixed with fuel that is injected from a fuel injector 14 and the mixture combusted . the resultant hot combustion products expand through and thereby drive the high 6 , intermediate 7 and low pressure 8 turbines before being exhausted through the nozzle 9 to provide additional propulsive thrust . the high , intermediate and low pressure turbines respectively drive the high and intermediate pressure compressors and the fan by suitable interconnecting shafts . fig2 depicts a section of the fan casing in more detail and shows the fan casing 1 which is made up of an outer casing 20 with a thin skin 22 of a harder material . within the inner skin is an acoustic liner 24 that is provided by an aluminium honeycomb core filled with a low - density epoxy filler . the acoustic liner provides a first layer of energy absorption for a released blade but also damps noise . other known forms of acoustic liner may be used . the inner surface of the acoustic liner may be provided with a septum layer which is sufficiently robust to inhibit damage to the acoustic liner from ice shedding from the aerofoil but is not so robust that a blade is prevented from passing through it . the outer casing 20 is preferably formed from a lightweight material such as aluminium , aluminium alloy or a composite which is made up from repeated layers of carbon or glass fibres embedded in a suitable resin , e . g epoxy . the thin skin 22 is preferably steel and may be a precipitation hardened stainless alloy to reduce the likelihood of corrosion . a skin thickness of between 1 mm and 2 mm has been found to be acceptable for most applications . so as to prevent galvanic corrosion between the skin 22 and the casing 20 a coating ( 30 , fig3 ) such as polyurethane or a suitable elastomer is applied to the skin so as provide appropriate galvanic insulation . preferably the polyurethane or elastomer can be cured following location of the skin and the casing so as to bond the components together . the coating can also be provided to fully encase the skin so as to prevent galvanic corrosion between the skin and the casing as well as between the sin and the acoustic liner . the thickness of the coating between the skin and the casing is of sufficient thickness , typically greater than 0 . 5 mm , to allow differential thermal expansion across the joint over the expected temperature range . one of the advantages of using a galvanic insulation layer 30 such as polyurethane is that it is significantly more flexible than the material of both the skin and the casing . the flexibility inhibits the transmission of any generated shear force between the skin and casing . although the steel skin may be manufactured as a cylinder it is desirable to provide it as a plurality of sheet sections with a lap joint between adjacent sheets . because a detached blade has a circumferential component it is desirable that the radially inner portion of the lap joint is ahead of the radially outer portion of the lap joint in the direction of the circumferential component as such an arrangement prevents a release blade from travelling under the skin . the size of the lap joint is such that differential thermal expansion over the expected temperature range can be accommodated . in one embodiment there are two sheets sections with each sheet section providing 180 degrees of the skin circumference . in alternative embodiments 3 , 4 , 6 or 8 sheet sections are provided with each sheet section providing 120 degrees , 90 degrees , 60 degrees or 45 degrees of the skin circumference as appropriate . the skin may be further stiffened to provide greater protection under impact through the provision of at least one rib . preferably two or more parallel ribs are provided at an appropriate spacing that can be around quarter of the blade tip true chord . such a spacing is beneficial as it permits the blade leading edge to engage the ribs and assist in the fracture of the blade on the hard skin . as depicted in fig3 , the ribs are arranged in a circumferentially extending array with each rib bounded at its radially outer end by the casing skin . each rib has a radial component that is preferably along the true radius from the engine axis but could lean towards the direction of blade rotation i . e . towards a tangent to provide further fracture assistance . each rib is also inclined relative to the engine axis and preferably at angle that is orthogonal to the tip stagger angle of the fan blade 2 . this has been found to be particularly beneficial for catching the blade leading edge in the ribs . in an alternative embodiment the ribs are at an angle which facilitates their deflection upon impact of the blade . the deflection may cause a domino effect of deflections along the array of ribs as the blade moves in the circumferential direction . the deflections may also result in a crushing effect to the ribs . each deflection or crushing movement absorbs energy from the blade and reduces the risk of any possibility of unwanted escape of material from the engine . in a further alternative embodiment each rib has a wavy profile to further improve the stiffness of the skin . each rib 32 may be of the solid type as shown in fig3 a or , more preferably , of a hollow type as shown in fig3 b that is provided by a fold of the skin 22 . each fold may be formed from a stamping of the skin which , by impact from the reverse side , causes an elongate depression within the localised region of the passing fan . alternatively , the folds may extend across the whole of the skin section but it will be appreciated that adds additional material which increases the weight of the component . one of the advantages of the hollow type of rib is that the force of any impact to it from the blade is pushed sideways and further absorbed which reduces the possibility of the blade being pushed through the casing skin . in an alternative manufacturing method the skin or skin section is passed through a set of hot or cold rollers set at an angle to roll the hollow rib features . alternatively the ring rolling roller can be set with features in its surface such that the ribs are formed during the ring rolling process — this will also work where the skin is haded . if the skin is designed to fit a con - di casing a preferred method is to make the skin by a fabrication type method . this can be arranged such that the rib also becomes the point where the plates overlap and features in the rib are then used to lock together the skin sections . some freedom of movement is required in this joint as it is may be difficult to get the last interlocking joint to mate correctly . when assembled an even pressure is applied on the inside , for instance using a hydraulic bag arrangement to ensure good contact between the liner plates and the pu whilst the pu is then cured . using an induction heater to heat the liner plates the temperature can be controlled by measuring the resistance change of the plates and the pu cured to adhere to the case and the liner plates . this method will also work for a parallel liner . alternative methods of manufacture could include additive manufacturing methods where selective layers are built up using powder bed or wire fed technologies . to improve hoop strength of the arrangement , the fore and aft edges can be rolled to turn over the edge to create a mini flange . alternatively small weld at the overlaps can ensure a hoop continuous structure even if the abradable and acoustic liners were to collapse . fig4 depicts a skin manufactured from a plurality of skin segments . the skin itself may be located in a suitably formed channel within the outer casing 20 such that the skin radially inside diameter is flush with the radially inside diameter of the outer casing either side of the skin . such an arrangement may assist assembly . each of the lap joints 35 is at an angle that is parallel to the angle of the ribs to avoid the necessity of the lap joint cutting across one or more of the ribs . the relationship between the ribs and angled lap joint helps to prevent unwanted axial movement along the axis 40 of the skin , outer casing and casing . the skin 22 or casing 20 may be provided with dimples that increase the stiffness of the component . preferably the dimples are positioned away from the ribs . it will be appreciated that the provision of a harder skin to a more lightweight casing material mitigates against potential puncture problems inherent when the lightweight material contains a blade formed of harder material . the use of a galvanic corrosion layer between the skin and the casing not only solves the problem of galvanic corrosion between the skin and the material of the casing , which may be a composite , but also addresses the problem of differential thermal expansion which may occur between the different materials . the use of a plurality of skin sections improves the ability to manufacture the casing with lap joints providing further capability to absorb differential expansion . the use of a polyurethane , epoxy or other suitable polymer with adhesive properties as the galvanic corrosion layer further offers advantages in the joining of the skin to the outer case . the skin can become a replaceable item which is replaceable in addition to , or separately from , the acoustic liners between the skin and the fan . beneficially , the skin provides further protection to the casing which becomes less likely to be damaged during replacement of the liners which may become damaged in normal use from the impact of ice or other foreign objects . the ribs provide benefits in that both the stiffness of the skin and the ability to better break up a released blade is improved as well as limiting the axial extent of the blade and casing interaction . where different embodiments are shown or described it will be appreciated that where appropriate features may be interchangeably moved between embodiments .