An aircraft fuel system, comprising: a fuel tank; a fuel line; and an engine; wherein the fuel line includes a first conduit configured to carry fuel from the tank towards the engine, and a second conduit configured to carry fuel from the engine towards the tank, wherein one of the first conduit or the second conduit is disposed annularly around the other of the first conduit or the second conduit. Also, a method of operating the system. The system may be used to suppress ice formation in the first conduit and/or provide leakage detection.