A method of controlling the temperature of a turbine casing (160; 260; 360; 460) of a gas turbine engine, the engine including an array of circumferentially spaced turbine blades (130; 230) disposed radially inwardly of the casing (160; 260; 360; 460) and circumscribed by a carrier section comprising a plurality of carrier segments (100; 200; 300; 400), each carrier segment (100; 200; 300; 400) including a carrier wall (140; 250; 350; 450) disposed radially inwardly of the casing (160; 260; 360; 460) and radially outwardly of the turbine blades (130; 230), and the carrier wall (140; 250; 350; 450) comprising one or more portions facing the casing (160; 260; 360; 460), wherein at least one of the one or more portions of the carrier wall (140; 250; 350; 450) is provided with one or more impingement apertures (152; 252; 352; 452) therein for passage therethrough of air of a predetermined temperature from a feed source into impingement onto the turbine casing (160; 260; 360; 460), and wherein the carrier segments (100; 200; 300; 400) are arranged radially inwardly of the turbine casing (160; 260; 360; 460) and radially outwardly of the turbine blades (130; 230), with the said one or more portions of their respective carrier walls (140; 250; 350; 450) facing the turbine casing (160; 260; 360; 460), wherein, the one or more portions provided with the impingement aperture(s) (152; 252; 352; 452) therein is a radially outer one of a pair of carrier walls (240,250; 340,350; 440,450), each carrier wall (240,250; 340,350; 440,450) extending between front and rear carrier ends, wherein the pair of carrier walls (240,250; 340,350; 440,450) define therebetween one or more chambers (280; 380; 480) for receiving therein heating or cooling air from a feed source via said front end, and the one or more portions provided with the impingement aperture(s) (152; 252; 352; 452) therein comprises one or more extension sections (250E; 350E; 450E) extending axially, relative to the engine's longitudinal axis, from a main carrier wall section (250M; 350M; 450M) via which the radially outer carrier wall (250; 350; 450) is united with the remainder of the carrier segment (100; 200; 300; 400), and wherein the or each axial extension section (250E; 350E; 450E) is provided with an extension set of impingement aperture(s) therein, in addition to the main section (250M; 350M; 450M) impingement apertures, wherein the method comprises: in a first mode passing air of a predetermined temperature from a feed source through the extension set of impingement apertures and main section impingement apertures (152; 252; 352; 452) in the one or more portions of the carrier wall (140; 250; 350; 450) of the or each carrier segment (100; 200; 300; 400) and into impingement on the casing (160; 260; 360; 460), so that the temperature of the casing (160; 260; 360; 460) is controlled in dependence on the predetermined temperature of the impinging airflow thereon, wherein the air passing through the extension set of impingement apertures travels to the main carrier wall section; and in a second mode exhausting the air radially outboard of the extension wall section once it has impinged onto the casing (160; 260; 360; 460), from a space between the carrier segment (100; 200; 300; 400) and the casing (160; 260; 360; 460) and the extension section, wherein the air passing through the main section impingement apertures travels to the extension section and exhaust. in a first mode passing air of a predetermined temperature from a feed source through the extension set of impingement apertures and main section impingement apertures (152; 252; 352; 452) in the one or more portions of the carrier wall (140; 250; 350; 450) of the or each carrier segment (100; 200; 300; 400) and into impingement on the casing (160; 260; 360; 460), so that the temperature of the casing (160; 260; 360; 460) is controlled in dependence on the predetermined temperature of the impinging airflow thereon, wherein the air passing through the extension set of impingement apertures travels to the main carrier wall section; and in a second mode exhausting the air radially outboard of the extension wall section once it has impinged onto the casing (160; 260; 360; 460), from a space between the carrier segment (100; 200; 300; 400) and the casing (160; 260; 360; 460) and the extension section, wherein the air passing through the main section impingement apertures travels to the extension section and exhaust.