The aircraft fuel system stores fuel and delivers the proper amount of clean fuel at the right pressure to meet the demands of the engine. A well designed fuel system ensures positive and reliable fuel flow throughout all phases of flight, which include changes in altitude, violent maneuvers and sudden acceleration and deceleration. Furthermore, the system must be reasonably free from tendency to vapor lock, which can result from changes in ground and inflight climatic conditions. Such indicators as fuel pressure

gauges, warning signals, and tank quantity gauges are provided to give continuous indications of how the system is functioning. The simplest type of fuel system is the gravity feed, which is still in use on many low powered airplanes. A gravity feed system is shown in figure 4-7. The fuel tanks are mounted above the carburetor, with gravity causing the fuel to flow from the tanks to the carburetor. A selector valve is provided to stop the fuel flow or to select a particular tank in the system from which to draw fuel. A strainer filters the fuel before it reaches the carburetor. A drain is provided for removing water and sediment trapped at the strainer. A primer furnishes the additional fuel required for engine starting.

Airplanes equipped with a high output engine require a fuel system that supplies fuel to the carburetor at a positive pressure. The  basic source for this pressure is an engine driven fuel pump, but auxiliary fuel pumps or booster pumps are required in every pressure feed system to: (1) supply fuel pressure for starting the engine; (2) supply fuel to the primer system; and (3) to serve as an emergency pump in case the engine driven pump fails.