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A cooling system for a gas turbine engine (FIG. 1) comprises a pre-swirl arrangement, preferably including a pre-swirl chamber, for providing cooling air to a turbine blade disc, and a ventilation arrangement for providing ventilation air to a rotating component of the gas turbine engine. The coolin

A cooling system for a gas turbine engine (FIG. 1) comprises a pre-swirl arrangement, preferably including a pre-swirl chamber, for providing cooling air to a turbine blade disc, and a ventilation arrangement for providing ventilation air to a rotating component of the gas turbine engine. The cooling system includes an air bypass arrangement, which preferably includes first, second and third air bypass duct portions, for conveying ventilation air away from the pre-swirl arrangement.

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We claim: 1. A cooling system for a gas turbine engine, the cooling system comprising a pre-swirl chamber, a pre-swirl arrangement for providing cooling air to a turbine blade disc, and a ventilation arrangement for providing ventilation air to a rotating component of the gas turbine engine, said c

We claim: 1. A cooling system for a gas turbine engine, the cooling system comprising a pre-swirl chamber, a pre-swirl arrangement for providing cooling air to a turbine blade disc, and a ventilation arrangement for providing ventilation air to a rotating component of the gas turbine engine, said cooling system including an air bypass arrangement for conveying ventilation air away from the pre-swirl arrangement, wherein the ventilation air is at a lower pressure than the cooling air and a portion of cooling air is conveyed into said air bypass arrangement from said pre-swirl chamber, said air bypass arrangement including a first air bypass duct portion, a second air bypass duct portion and a third air bypass duct portion wherein said third air bypass duct portion is defined in part by a static component of the gas turbine engine structure and wherein the third air bypass duct portion is further defined in part by a movable sealing component of the gas turbine engine. 2. A cooling system according to claim 1, wherein the air bypass arrangement is arranged to convey ventilation air to a further rotating component of the gas turbine engine to provide cooling to the further rotating component. 3. A cooling system according to claim 1, wherein the air bypass arrangement includes an air bypass duct for conveying ventilation air to a further rotating component of the gas turbine engine to provide cooling to the further rotating component. 4. A cooling system according to claim 3, wherein said first air bypass duct portion extends circumferentially about a rotational axis of the gas turbine engine. 5. A cooling system according to claim 4, wherein the first air bypass duct portion is generally annular. 6. A cooling system according to claim 4, wherein the first air bypass duct portion defines an air inlet for receiving, in use, ventilation air from the ventilation arrangement. 7. A cooling system according to claim 6, wherein the air inlet is also arranged to receive, in use, a proportion of cooling air from the pre-swirl arrangement. 8. A cooling system according to claim 4, wherein said second air bypass duct portion intersects the pre-swirl arrangement. 9. A cooling system according to claim 8, wherein the second air bypass duct portion is arranged to receive air from the first air bypass duct portion and to convey the air into the third air bypass duct portion. 10. A cooling system according to claim 1, wherein said second air bypass duct portion intersects the pre-swirl arrangement. 11. A cooling system according to claim 10, wherein the second air bypass duct portion intersects an air supply arrangement of the pre-swirl arrangement. 12. A cooling system according to claim 11, wherein the second air bypass duct portion extends generally radially through the air supply arrangement. 13. A cooling system according to claim 1, wherein the third air bypass duct portion is defined in part by a combustion rear inner casing of the gas turbine engine. 14. A cooling system according to claim 1, wherein the movable sealing component includes a pre-swirl means and the third air bypass duct portion is arranged to convey the air from the third air bypass duct portion through the pre-swirl means. 15. A cooling system according to claim 14, wherein the pre-swirl means is arranged to convey air from the third air bypass duct portion towards a rotating seal component of the gas turbine engine to cool the rotating seal component. 16. A cooling system according to claim 14, wherein the pre-swirl means is arranged to convey air from the third air bypass duct portion towards a rim of the turbine blade mounting disc to cool the disc rim. 17. A cooling system according to claim 14, wherein the pre-swirl means is arranged to convey air from the third air bypass duct portion into a cooling passage of a turbine blade to cool the blade. 18. A gas turbine engine including a cooling system according to claim 1. 19. A cooling system for a gas turbine engine, the cooling system comprising a pre-swirl arrangement for providing cooling air to a turbine blade disc, and a ventilation arrangement for providing ventilation air to a rotating component of the gas turbine engine, said cooling system includes an air bypass arrangement for conveying ventilation air away from the pre-swirl arrangement wherein the air bypass arrangement includes a first air bypass duct portion, a second air bypass duct portion and a third air bypass duct portion wherein said third air bypass duct portion is defined in part by a static component of the gas turbine engine structure wherein said third air bypass duct portion is further defined in part by a movable sealing component of the gas turbine engine and wherein the third air bypass duct portion is further defined in part by an expandable sealing member provided between the movable sealing component and the pre-swirl arrangement. 20. A cooling system according to claim 19, wherein the expandable sealing member is operable in use to prevent the passage of cooling air from the pre-swirl arrangement into the third air bypass duct portion. 21. A cooling system according to claim 19, wherein the expandable sealing member comprises a spiral sealing ring arrangement.