Gas turbine fuel return line

A method of operating a gas turbine engine, the gas turbine engine including a combustor;

• a fuel-oil heat exchanger arranged to receive fuel from a fuel tank on board an aircraft and transfer heat from oil to the fuel;
• a fuel return line arranged to return at least some fuel that has passed through the fuel-oil heat exchanger to the fuel tank;
• and a modulator valve arranged to modulate a flow of fuel along the fuel return line;
• wherein the method comprises modulating the flow of fuel along the fuel return line using the modulator valve such that a ratio of a temperature in Kelvin of fuel in the fuel tank to a temperature in Kelvin of fuel being delivered to the combustor is less than 0.56.