The aircraft fuel system is highly advanced, ensuring that engines are always supplied with a constant flow of fuel during a flight operation regardless of changes in altitude, speed, pressure, and more. To achieve this, a series of fuel tanks, selector valves, pumps, carburetors, and other components work together to efficiently transport fuel from the tanks to the engine for combustion. In this blog, we will discuss the main components of the aircraft fuel system, allowing you to better understand their functionalities.
Depending on the type of aircraft and the positioning of its structures and components, a gravity-feed or fuel-pump system may be present. For aircraft that utilize high-wing construction where fuel tanks are placed within the wings, a gravity-system may be used. With such types, the forces of gravity are often enough to allow for fuel to travel from each wing tank to the engine through fuel lines. With the carburetor placed below the fuel selector valve, fuel flows through the strainer and into the carburetor to be mixed with air before combustion. In some cases, a gravity feed pump may also be present if needed.
In other aircraft, such as those with middle to low-wing configurations, the carburetor is placed above the fuel tanks, and thus a pump system must be present in order to efficiently move fuel. With two boost pumps, pressure can be increased in order to force fuel towards the carburetor. Aircraft with pumping often feature an additional auxiliary pump, ensuring that functionality can continue in the case of a failure of an engine pump.
While the installation of fuel tanks may vary by aircraft model, most are typically located within the wings. With a filler cap located on the top of the wing, aircraft can be refilled with AVGAS or other fuel types with ease. To ensure that the pressure of fuel tanks remains within safe values across the duration of the flight, venting is provided to maintain atmospheric pressure. With overflow fuel drains, fuel can also expand in response to changing temperatures without damaging the tank. To prevent contamination from reaching the engine, fuel strainers and fuel sumps are also put in place to collect and remove any debris or sediment.
In both types of aircraft fuel systems, fuel primers are implemented in order to draw fuel and vaporize it for the cylinders before combustion. The fuel primer is very important for the optimal functionality of the aircraft, as it ensures that there is enough heat for the carburetor even when operating in colder conditions. During flight, it is important that the fuel primer is locked in place when not being used, as it may become loose due to vibrational forces which could result in overly rich fuel and air mixtures.
To properly draw equal amounts of fuel from each tank to maintain a balance of weight, fuel selectors and gauges may be used. With the fuel selector valve, pilots can choose to draw from the left, right, or both tanks. By monitoring gauges during the flight, pilots can ensure that tanks are being drawn from at equal rates as well as check for any unusual signs of fuel activity through the use of a sensing unit placed within the tanks. With a fuel pressure gauge, pilots can also ensure optimal combustion.
For refueling, one should always make sure to utilize the correct fuel grade and type for their aircraft, as each will present various characteristics, benefits, and drawbacks for different applications. On the flight deck of an aircraft or near the filler cap of a wing, operators can reference the fuel placard to ensure that the correct fuel grade is used. In general, AVGAS can be identified by its octane or performance number as well.
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