Combustion of fuel

A gas turbine engine for an aircraft, the gas turbine engine comprising:

• an engine core comprising a turbine, a compressor, a combustor configured 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 configured to receive an input from the core shaft and to output drive to the fan via the fan shaft;
• a primary oil loop system configured to supply oil to lubricate the main gearbox;
• and
• a heat exchange system configured to transfer heat between the oil and the fuel, the primary oil loop system being configured such that the oil has an average temperature of at least 180° ° C. on entry to the heat exchange system at cruise conditions, wherein
• the heat exchange system is configured to transfer heat from the oil to the fuel to lower viscosity of the fuel to a maximum of 0.58 mm2/s on entry to the combustor at cruise conditions, and
• the fuel comprises sustainable aviation fuel, the sustainable aviation fuel is up to 100% of the fuel, and the sustainable aviation fuel has density of between 90% and 98% that of kerosene and a calorific value of between 101% and 105% that of kerosene.