Abstract: The invention is aimed at avoiding excessive wear of the tips of the turbine blades in deceleration of a gas turbine engine, and provides a control ring which is loosely supported by fixed structure. The control ring is made from a material which reacts more slowly to thermal changes than the support structure and firstly being loosely supported thereby, avoids the stresses which would be otherwise experienced by the fixed joining of differing materials for operation in varying temperatures. Secondly, on deceleration of the associated engine with consequent fall in operating temperature, the control ring contracts more slowly than the turbine assembly and so avoids engaging the shrouds abradable linings with the tips of the blades.