A control system operable includes a modified primary control parameter for an aircraft, the control system includes: a primary control parameter leg configured to output a demand in an aircraft primary control parameter; a primary control parameter compensation leg configured to receive a change in absolute levels and/or spatial distributions of swirl angle and/or fan pressure at a primary control parameter relative to a reference and convert the change into the primary control parameter; a processor adapted to receive the demand in the primary control parameter output from the primary control parameter leg and the change to the primary control parameter output from the primary control parameter compensation leg; compare the demand in the primary control parameter output from the primary control parameter leg and the change to the primary control parameter output from the primary control parameter compensation leg; and generate a modified primary control parameter for the aircraft.