Powered aircraft obviously require fuel to achieve flight, and these systems must be both safe and efficient in how they transfer fuel from the tanks to the engines. The design and complexity of these systems, however, largely depends on the aircraft itself, with the size and layout being the most critical factors. A high-wing aircraft will use a different fuel system from a low-wing aircraft, for instance, and engines with carburetors will have different requirements compared to those with fuel injection.
High-wing aircraft with fuel tanks in each wing are a very common design. In such configurations, a simple gravity-feed system can be used to deliver fuel. In a gravity-feed system, the space above the liquid fuel within the tank is vented to maintain atmospheric pressure as the tank empties, and the two tanks are connected via valves to a selector. This selector can change the flow of fuel, shutting off from one tank or the other, closing both, or opening both.
Low and mid-wing aircraft cannot use gravity-feed systems, as the fuel tanks are located at the same level or below the engines. Therefore, pumps are used to move fuel from the tanks to the engine or engines. In a pump-feed system, fuel tanks are not connected to each other.
Some high-wing high-performance aircraft will use fuel injection systems rather than carburetors. Fuel injection systems spray pressurized fuel into the engine intake or directly into the cylinders.
On larger commercial aircraft like the Boeing 777 and Airbus A320, fuel systems are far more complex. These aircraft incorporate multiple redundancy systems and more options for how fuel is drawn from various tanks throughout the fuselage and wings. They also connect to components like the auxiliary power unit (or APU), single point pressure refueling, and fuel jettison systems that are not found in smaller aircraft. These fuel tanks can carry thousands of pounds of fuel, and require venting similar to those that fuel reciprocating engines. A series of vent tubing and channels are built, connected the fuel tanks to the surge tanks, or to vent overboard in emergency circumstances.
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