Within each flight cycle, an aircraft fuel system must deliver clean fuel to the engine at a proper flow rate to sustain flight— regardless of the operating conditions. A fuel system is composed of boost pumps, tanks, strainers, selector valves, pressure gauges, engine-driven pumps, and more.
Typically, there are several tanks to store the required amount of fuel. The location of these depends on the system design and structure of the aircraft. In a standard assembly, a line leads to the selector valve from each one of these tanks, which is operated from the cockpit. The aircraft fuel pump boosts then forces fuel through the selector valve to the main line strainer. The strainer acts as a filtering unit, which removes water and dirt from the fuel. It then travels through a bypass in the engine-driven pump, causing it to rotate. Once it is operating at an adequate speed, it delivers fuel to the metering device to measure, allowing the aircraft to start up. These aircraft fuel systems can be categorized into two variations: a gravity feed system or a pump feed system.
It is not uncommon for aircraft to come equipped with a fuel tank in each wing. In this design, gravity is used to deliver the fuel to the engine; hence, a gravity feed system. The area above the fuel is vented to sustain atmospheric pressure on the fuel as the tank is emptied. The tanks are also vented to each other to maintain equal pressure between the two of them. An outlet on each tank feeds lines that connect to a fuel shutoff valve or multi position selector valve. As the fuel travels it passes through a main system strainer which removes sediment and/or water. It then flows to the carburetor or the primer pump for engine starting and operating. The gravity feed system doesn’t require a fuel pump and is simplistic in design.
Pump feed systems involve one or more pumps that are used to move fuel from the fuel tanks to the engine. Each fuel tank has a line from the outlet to a selector valve. In this system, fuel cannot be drawn from both tanks simultaneously. If the fuel in one tank is depleted, the pump would draw air. This negates the need to connect the tank vent spaces together. The fuel then passes through the selector valve, making its way through the main strainer, where it can then supply the engine primer. It then travels downward to the fuel pumps; one electric pump and one engine-driven fuel pump are arranged next to each other. These draw fuel from the tank and transport it to the carburetor. The two pumps facilitate each other as the engine-driven pump is the primary operator while the electric pump provides a supportive option.
Some high-performance aircraft are equipped with a fuel system that features fuel injection as opposed to a carburetor. This design combines gravity flow with the use of a fuel pump, operating in unison; but we’ll save that for next time.