Gas turbine engine heat exchange

A method of operating a gas turbine engine, the gas turbine engine comprising:

• a combustor arranged to combust a fuel;
• and
• a fuel management system arranged to provide the fuel to the combustor, wherein the fuel management system comprises:
• two fuel-oil heat exchangers through which oil and the fuel flow, the heat exchangers arranged to transfer heat between the oil and the fuel and comprising a primary fuel-oil heat exchanger and a secondary fuel-oil heat exchanger;
• a fuel pump arranged to deliver the fuel to the combustor, wherein the fuel pump is located between the two fuel-oil heat exchangers;
• and
• a recirculation valve located downstream of the primary fuel-oil heat exchanger with respect to the flow of the fuel, the recirculation valve arranged to allow a controlled amount of fuel which has passed through the primary fuel-oil heat exchanger to be returned to an inlet of the primary fuel-oil heat exchanger;
• wherein the primary fuel-oil heat exchanger is downstream of the secondary fuel-oil heat exchanger with respect to the flow of the oil;
• wherein the method comprises selecting fuel such that the calorific value of the fuel provided to the gas turbine engine at cruise conditions is at least 43.5 MJ/kg;
• and
• wherein a ratio of heat transfer from the oil to the fuel for the primary and secondary fuel-oil heat exchangers is between 70:30 and 90:10, and the heat transfer is measured in energy per unit of volume.