A gas turbine engine comprising first and second axially spaced turbine rotor stages (46, 48) and a turbine casing (56) radially outside the rotor stages. A first seal segment arrangement (58) forms a cavity (64) radially between the first turbine rotor stage (46) and the turbine casing (56). A first air source (82) is coupled to the first seal segment arrangement (58). A second seal segment arrangement (70) forms a cavity (74) radially between the second turbine rotor stage (48) and the turbine casing (56). A heating chamber (84) is provided radially between the second seal segment arrangement (70) and the turbine casing (56). A duct (86) is coupled between the first air source (82) and the heating chamber (84).