ROTARY ASSEMBLY

There is provided a rotary assembly 400 for driving spool rotation. The rotary assembly 400 comprises a rotor 410 and a flow modifier 51 0, 520. The rotor 410 is configured to be mechanically coupled to a spool 440 of a gas turbine engine 10. The flow modifier 510, 520 is configured to receive flow from and/or direct flow to the rotor 410. The rotary assembly 440 is configured to permit relative movement between the rotor 410 and the flow modifier 510, 520 to move between: a turbine configuration in which the rotor 410 is configured to receive air from an external air source to drive the spool 440 to rotate; and a compressor configuration in which the rotor 410 is configured to be driven to rotate by the spool 440 and to receive and compress air from the gas turbine engine 10, and discharge the compressed air for supply to the airframe system. The rotary assembly 440 also comprises controller 590 configured to control relative movement between the rotor 410 and the flow modifier 510, 520 through a range of turbine positions of the turbine configuration to vary a torque applied to the rotor 410 for driving the spool 440.