The invention concerns an aircraft propulsion control system for an aircraft with a plurality of gas turbine engines mounted at different spaced positions relative to an airframe. One or more sensor is arranged to sense a condition indicative of vapor trail formation by an exhaust flow from one or more of the plurality of engines. A controller is arranged to be responsive to thrust demand for the aircraft and selectively to control each of the engines so as to modify the exhaust flow of at least one engine relative to at least one further engine according to the positions of the engines on the airframe. The proximity of an engine to a vortical wake flow created by the airframe may be used to determine suitable exhaust flow modifications. The controller may output a separate throttle control signal to each engine and/or may adjust the fuel composition supplied to each engine.