Aircraft heat management

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

• an engine core comprising a turbine, a compressor, a combustor arranged to combust a fuel, and a core shaft connecting the turbine to the compressor;
• a fan located upstream of the engine core;
• a fan shaft;
• a main gearbox that receives an input from the core shaft and outputs drive to the fan via the fan shaft, the gearbox comprising gears and journal bearings;
• a recirculating lubrication system arranged to supply oil to lubricate the gears and journal bearings, the lubrication system comprising a first oil tank arranged to supply oil to the gears and journal bearings a second oil tank arranged to supply oil to the journal bearings only;
• and
• a heat exchange system arranged to transfer heat between the oil and the fuel,
• wherein the method comprises, at cruise conditions:
• controlling the heat exchange system so as to transfer heat to the fuel from the oil;
• transferring at least 40% of the heat lost from the oil to the fuel, with the remainder of the heat transferred from the oil to air;
• and providing cooler oil to the journal bearings than to the gears.