An aircraft fuel system allows the crew to pump, manage, and deliver fuel to an aircraft’s propulsion system and Auxilary Power Unit (APU). Fuel systems differ significantly due to the different performances of the plane in which they are installed.
A single-engine piston aircraft has a simple fuel system; a tanker (such as the KC-135), in addition to managing its fuel, can also provide energy to other aircraft. Fuel is piped through fuel lines to a fuel control valve (usually known as the fuel selector). This valve serves several functions. The first function is to act as a fuel shut-off valve. This is required to provide the crew with a means to prevent fuel from reaching the engine in case of an engine fire. The second function allows the pilot to choose which tank feeds the engine.
Many aircraft have left and proper tank selections available to the pilot. Some Cessna airplanes provide only from both tanks, and many have the option to feed from left, right, or both tanks. The reason to have left-only and right-only options are to allow pilots to balance fuel load to reduce the banking moment. In some aircraft, the shut-off function is a different valve located after the fuel selector valve.
After the selector valve, there usually is a calculator – a fuel filter that can be drained. Other drainage points are in each tank (often more than one contaminant collection sump per tank) and at the injection pump. Each tank is vented (or pressurized) to allow air into the tank to take the place of burned fuel; otherwise, the tank would be under negative pressure, resulting in engine fuel starvation.
A vent also allows for changes in atmospheric pressure and temperature.