A gas
turbine engine for an aircraft comprises an engine core comprising a
turbine, a
combustor, a compressor, and a core shaft connecting the
turbine to the compressor. The engine comprises a fan located upstream of the engine core and arranged to be driven by the core shaft. The engine comprises a
nacelle surrounding the fan and the engine core and defining a bypass duct located radially outside of the engine core, where the
bypass ratio, defined as the ratio of the
mass flow rate of the flow through the bypass duct to the
mass flow rate of the flow through the core at
cruise conditions, is at least 4. The engine comprises a plurality of actuators and a
fuel supply system arranged to supply fuel to fueldraulically drive at least one
actuator of the plurality of actuators. The engine comprises two fuel-oil heat exchangers arranged to have oil and the fuel flow therethrough, the heat exchangers comprising a primary fuel-oil
heat exchanger arranged to heat at least the majority of the fuel, and a secondary fuel-oil
heat exchanger arranged to provide additional heat to the fuel to be supplied to fueldraulically drive the at least one fueldraulic
actuator. The heat exchangers are controlled such that, under
cruise conditions, a
heat transfer ratio of: rateofheattransferfromoiltofuelinthesecondaryfuel−oilheatexchangerrateofheattransferfromoiltofuelintheprimaryandsecondaryfuel−oilheatexchangers has a maximum value of at least 0.35.