| CPC F02C 9/26 (2013.01) [F02C 7/224 (2013.01); F02C 7/232 (2013.01); F05D 2260/20 (2013.01); F05D 2270/303 (2013.01)] | 17 Claims |

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1. A gas turbine engine for an aircraft, comprising:
a controller;
a combustor;
a fuel-oil heat exchanger arranged to receive fuel from a fuel tank onboard the aircraft and transfer heat from the oil to the fuel;
a fuel return line arranged to return at least some fuel that has passed through the heat exchanger to the fuel tank; and
a modulator valve arranged to modulate the flow of fuel along the fuel return line, wherein
the controller is configured to control the modulator valve to initiate return of fuel to the fuel tank when
i) an indication of an operating condition is provided, and
ii) fuel having passed through the heat exchanger is at a temperature of at least 120° C.; and
the operating condition is one or more of a proportion of sustainable aviation fuel (SAF) in the fuel, a thermal stability of the fuel, an oxygen content of the fuel and a sulphur content of the fuel.
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