A method of determining one or more fuel characteristic of an aviation fuel suitable for powering a gas turbine engine of an aircraft is disclosed. The method includes: determining, during use of the gas turbine engine, one or more contrail parameters related to contrail formation by the gas turbine engine. The determining of the one or more contrail parameters includes performing a sensor measurement on a region behind the gas turbine engine in which a contrail is or can be formed. The method also includes determining one or more fuel characteristics of the fuel based on the one or more contrail parameters. A fuel characteristic determination system, a method of operating an aircraft and an aircraft are also disclosed.