The fuel transfer system is a series of plumbing and valves  that permits movement of fuel from one tank to another on  board the aircraft. In-tank fuel boost pumps move the fuel  into a manifold and, by opening the fuel valve (or refueling  valve) for the desired tank, the fuel is transferred. Not all jet  transports have such fuel transfer capability. Through the use  of a fuel feed manifold and crossfeed valves, some aircraft  simply allow engines to be run off fuel from any tank as a  means for managing fuel location.  Occasionally, defueling the aircraft is required for an  inspection or repair. The same fueling station is used, and  the hose from the fuel truck is connected to same receptacle  used to fuel the aircraft. To allow fuel to exit the aircraft,  a defueling valve is opened. Fuel can either be pumped out  of the aircraft using the boost pumps located in the tanks  that need to be emptied, or the pump in the refueling truck  can be used to draw the fuel out of the tanks. Control over  the operation is maintained by positioning various shutoff  and crossfeed valves, as well as the defuel valve so that fuel  travels from the tank to the fueling station and into the truck. 14-15 Figure 14-18.  DC-3 fuel system. CODE Supply Supply flow Pressure Pressure flow Fuel vapor return Vent line Electric cable Left hand main tank Right hand main tank Left hand auxiliary tank Right hand auxiliary tank Tank float Tank float Tank selector for left hand engine Tank selector for left hand engine Left hand  crossfeed valve Right hand  crossfeed valve Right hand fuel strainer Left hand fuel strainer Crossfeed line (closed) Right hand engine carburetor Left hand engine carburetor Fuel pressure gauges 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 22 21 Poppet valve Vacuum Diaphragm Pressire Venturi tube Throttle Nozzle Fuel vapor to manifold Metered fuel Propane line (winterization) Prime line Left hand engine driven pump Right hand engine-driven pump Restrictor fitting Fuel pressure warning switch Left hand firewall Right hand firewall Wobble pump for left hand engine Wobble pump for right hand engine Solenoid for oil dilution (closed) Solenoid primer (closed) Pressure gauge line