The invention concerns an aircraft propulsion control system in which multiple gas turbine engines (10) are under the control of a controller (30). 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 engines. The controller (30) is arranged to be responsive to a thrust demand (51) for the aircraft and to control the thrust produced by each of the engines (10) concurrently so as to alter the efficiency of the engines upon sensing of the vapor trail formation condition, while satisfying the aircraft thrust demand. The controller (30) may output a separate throttle control signal (35) to each engine (10).
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