one or more main aircraft fuel tanks are provided with one or more fuel gauges arranged to provide a measure of the amount of fuel in said one or more main fuel tanks . one or more auxiliary fuel tanks are also provided . the aircraft uses fuel during a first phase of fuel consumption , upon commencement of which phase the auxiliary fuel tanks hold fuel . after the first phase said one or more auxiliary fuel tanks are empty and a second phase commences . during the first phase , an indication of the amount of fuel carried by the aircraft is provided by means of summing the amount of fuel in the main fuel tanks as measured by said one or more fuel gauges and the amount of fuel remaining in said one or more auxiliary fuel tanks as calculated using a measure relating to the fuel consumption rate . during the second phase of fuel consumption , an indication of the amount of fuel carried by the aircraft is provided on the basis of the amount of fuel measured by said one or more fuel gauges .