Aircraft fuel system

An aircraft fuel system allows the crew to pump, manage, and deliver aviation- or jet fuel to the propulsion system and Auxiliary Power Unit (APU) of an aircraft. Fuel systems differ greatly due to different performance of the aircraft 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 own fuel, can also provide fuel 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 reaching the engine in case of an engine fire. The second function is to allow the pilot to choose which tank feeds the engine. Many aircraft have the left tank and right tank selections available to the pilot. Some Cessna airplanes feed 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 is 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 gascolator - 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 pressurised) to allow air into the tank to take the place of burned fuel; otherwise, the tank would be in negative pressure which would result in engine fuel starvation. A vent also allows for changes in atmospheric pressure and temperature.

The fuel level indication system in the simplest form is a transparent window on the tank side and in its usual application a float-driven potentiometer installed in the tank. After the TWA Flight 800 disaster, a revision was made to aircraft fuel systems to address the potential explosion hazard of electrical components located in the fuel tank. Single-engine piston aircraft fuel level systems moved to utilize float level gauges from the CNG and LPG industries which had the float drive a magnetic coupling and relocated the potentiometer outside the fuel tank.

...
Wikipedia