Contrail parameters

A fuel characteristic determination system for determining one or more fuel characteristic of an aviation fuel suitable for powering a gas turbine engine of an aircraft, the system comprising:

• a contrail sensor arranged to determine one or more contrail parameters related to contrail formation by the gas turbine engine, the contrail sensor being arranged to perform a sensor measurement on a region behind the gas turbine engine in which a contrail is or can be formed;
• and
• a fuel characteristic determination module arranged to determine one or more fuel characteristics of the fuel based on the one or more contrail parameters,
• wherein the one or more fuel characteristics determined include any one or more of:
• (i) a hydrocarbon distribution of the fuel;
• (ii) a percentage of sustainable aviation fuel in the fuel;
• (iii) an aromatic hydrocarbon content of the fuel;
• and/or
• (iv) an indication that the fuel is a fossil fuel.

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

• determining, during use of the gas turbine engine, one or more contrail parameters related to contrail formation by the gas turbine engine, wherein determining the one or more contrail parameters comprises performing a sensor measurement on a region behind the gas turbine engine in which a contrail is or can be formed, and wherein the one or more control contrail parameters are determined during a varying operation of the aircraft and correspond to a value of a varying parameter at which a contrail begins to form;
• and
• controlling an operating parameter of the aircraft or gas turbine engine according to the one or more contrail parameters and the value of the varying parameter at which the contrail begins to form, the varying parameter being a varying engine operating parameter and/or an ambient condition parameter, and the controlling including:
• i) changing an operating parameter of a heat management system of the gas turbine engine;
• ii) changing a fuel temperature of a fuel within the gas turbine engine;
• and/or iii) changing a flight characteristic of the aircraft.