Operating a gas turbine engine

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;
• at least one bearing arranged to support the fan shaft;
• an oil loop system arranged to supply oil to the at least one bearing;
• and
• a heat exchange system comprising:
• an air-oil heat exchanger through which the oil in the oil loop system flows;
• a fuel-oil heat exchanger through which the oil in the oil loop system and the fuel flow such that heat is transferred between the oil and the fuel;
• and
• a temperature sensor arranged to provide an indication of fuel temperature downstream of the fuel-oil heat exchanger;
• and
• at least one valve arranged to allow at least one of oil flow rate and air flow rate through at least one of the fuel-oil heat exchanger and the air-oil heat exchanger to be varied,
• the method comprising:
• determining if the fuel temperature has increased above a set threshold at cruise conditions, based on an output from the temperature sensor;
• and
• in response to determining that the fuel temperature has increased above the set threshold at cruise conditions, controlling the at least one valve so as to change the at least one flow rate through the at least one heat exchanger.