Abstract: An aircraft fuel system comprises a fuselage tank and wing fuel tanks each connected to the fuselage fuel tank by a respective fuel line. A first fuel line connects the fuselage fuel tank to a collector fuel tank, which is connected by second fuel lines to aircraft engines in use of the system. Unspent fuel output from the aircraft engines is returned to the collector fuel tank via third fuel lines. A fuel line arrangement is arranged to draw fuel from, and return that fuel to, the wing fuel tanks. Fuel within the third fuel lines is thermally coupled to fuel within the fuel line arrangement by a heat exchanger. Heat energy within unspent fuel output from the aircraft engines is transferred to the wing fuel tanks, reducing the likelihood of fuel freezing within these tanks during extended flying in cold environments.