This invention concerns a method of delivering fuel to an aircraft engine 60, which involves providing a plurality of distinct fuel sources 20, 22, a first fuel source 20 comprising a first fuel having a first aromatic content and a second fuel source 22 comprising a second fuel having a second aromatic content. One or more ambient atmospheric condition is determined for at least a portion of a flight path of the aircraft, said condition being indicative of a likelihood of contrail 135 formation by the engine 60. A desirous fuel composition for combustion by the engine is determined based upon the one or more ambient atmospheric condition and a ratio of the first and second fuels from said respective fuel sources is selected according to said desirous fuel composition. The selected ratio of the first and second fuels is delivered to the aircraft engine 60.