METHOD OF CONTRAIL MITIGATION AND AIRCRAFT HAVING CONTRAIL MITIGATION FUNCTIONALITY

A method of mitigating contrails produced by an aircraft having a set of gas turbine engines (220, 230) comprises the steps of (i) for each engine in a first subset (220) of the engines, reducing the operating efficiency of the engine to produce a reduction in thrust provided by that engine and (ii) for each engine in a second subset (230), increasing the fuel flow to the engine to increase the thrust provided by that engine, the set of at least two gas turbine engines consisting of the first and second subsets. The method provides for contrail mitigation action by means of engine operating efficiency reduction to be directed to a first subset of engines for which contrail mitigation per unit engine operating efficiency reduction is greatest, the resulting reduction in thrust provided by such engines being at least partially compensated by increasing fuel flow to engines of the second subset.