An aircraft cabin blower system comprising: a cabin blower compressor having an air bearing or magnetic bearing arrangement; a transmission having: a transmission output arranged to drive the cabin blower compressor via the air bearing or magnetic bearing arrangement; a first transmission input arranged to receive mechanical power from a gas turbine engine; and a second transmission input; a reversible variator arranged to receive power from the gas turbine engine and to output mechanical power to the second transmission input, the reversible variator operable to output in both forward and reverse directions of rotation; and a controller configured to control an output speed and direction of rotation of the reversible variator, wherein: a speed of the transmission output is a function of a speed of the first transmission input, a speed of the second transmission input and the direction of rotation of the variator output, the second transmission input increasing the speed of the transmission output when the variator output rotates in the forward direction and reducing the speed of the transmission output when the variator output rotates in the reverse direction; and the controller is configured so that, during a start-up process of the cabin blower system, the variator output is controlled to rotate in the reverse direction until a start-up threshold condition is satisfied and then reversed to rotate in the forward direction, the threshold condition defined so that, when the variator output reverses and rotates in the forward direction, the speed of the transmission output is accelerated to a speed greater than or equal to a lift-off speed of the air bearing or magnetic bearing arrangement. a cabin blower compressor having an air bearing or magnetic bearing arrangement; a transmission having: a transmission output arranged to drive the cabin blower compressor via the air bearing or magnetic bearing arrangement; a first transmission input arranged to receive mechanical power from a gas turbine engine; and a second transmission input; a reversible variator arranged to receive power from the gas turbine engine and to output mechanical power to the second transmission input, the reversible variator operable to output in both forward and reverse directions of rotation; and a controller configured to control an output speed and direction of rotation of the reversible variator, wherein: a speed of the transmission output is a function of a speed of the first transmission input, a speed of the second transmission input and the direction of rotation of the variator output, the second transmission input increasing the speed of the transmission output when the variator output rotates in the forward direction and reducing the speed of the transmission output when the variator output rotates in the reverse direction; and the controller is configured so that, during a start-up process of the cabin blower system, the variator output is controlled to rotate in the reverse direction until a start-up threshold condition is satisfied and then reversed to rotate in the forward direction, the threshold condition defined so that, when the variator output reverses and rotates in the forward direction, the speed of the transmission output is accelerated to a speed greater than or equal to a lift-off speed of the air bearing or magnetic bearing arrangement. a speed of the transmission output is a function of a speed of the first transmission input, a speed of the second transmission input and the direction of rotation of the variator output, the second transmission input increasing the speed of the transmission output when the variator output rotates in the forward direction and reducing the speed of the transmission output when the variator output rotates in the reverse direction; and the controller is configured so that, during a start-up process of the cabin blower system, the variator output is controlled to rotate in the reverse direction until a start-up threshold condition is satisfied and then reversed to rotate in the forward direction, the threshold condition defined so that, when the variator output reverses and rotates in the forward direction, the speed of the transmission output is accelerated to a speed greater than or equal to a lift-off speed of the air bearing or magnetic bearing arrangement.