An aircraft’s fuel system is responsible for providing a consistent delivery of fuel at a predetermined flow rate and pressure established by the manufacturer to ensure proper engine functioning under normal operating conditions. Additionally, the type and grade of fuel utilized must meet the requirements set forth by the engine manufacturer with regard to chemical composition and octane rating. To better understand the complexities associated with aircraft fuel systems, this blog will outline the fuel and duel delivery system of typical light aircraft.
The fuel delivery system consists of an array of items, including fuel tanks, fuel lines, fuel pumps, fuel strainers, valves, vents, fuel level sensors, and cockpit gauges. When in operation, the fuel pumps move the necessary amount of fuel from the tanks to the engine, while the fuel strainers prevent contaminated fuel from making its way into the engine. During this interaction, the valves and vents control the movement and pressure in the fuel system. Apart from these basic components, there are two primary fuel delivery methods used on light aircraft: the gravity fed system and the pumped system.
Gravity Fed Systems
As the name suggests, gravity fed systems rely on gravitational force to feed the engine with fuel from the tanks. This is only possible if the tanks are situated higher than the engine, so gravity fed systems are usually found on high wind aircraft like the Cessna 172. With such systems, regulations dictate that they must provide a flow rate of at least 150% of the takeoff fuel consumption of the engine throughout the certified flight envelope. If the engine is fuel injected and not running on a carburetor-based system, a booster pump may be installed to augment the gravity fed system.
Pump Fed System
Low-wing aircraft with wing-mounted tanks necessitate a pump to transfer fuel from the tanks to the carburetor or injectors. A system that relies on a pump to feed the engine requires a certain level of redundancy in the form of a primary main pump and a backup auxiliary pump. It is important to note that both pumps must have the individual capacity to supply the engine with fuel at a rate of 125% of the maximum requirement throughout the certified flight envelope. When both pumps are working simultaneously, the fuel pressure may not surpass the inlet pressure limits of the engine. In multi-engine aircraft, each engine must be capable of operating independently of one another, and neither one of the fuel pumps should draw more fuel from more than one tank at a time.
Fuel Tanks
As aircraft fuel tanks experience excessive vibration, aerodynamic forces, varying temperatures, inertial loads, and even lightning strikes during normal operation, they must be designed to handle all these stressors and resist deformation under any loading in the design envelope. More than that, they cannot corrode from fuel storage, and measures should be established to minimize the possibility of foreign contaminants like dirt, dust, and water from entering the tank. They cannot be installed on the engine side of the firewall, and they must be sealed off from personnel compartments by a fume and fire-proof enclosure. Furthermore, the fuel tanks should be ventilated at all times so that dangerous fumes or vapors do not accumulate while the aircraft is in the air or grounded. There are three common types of fuel tanks, those of which are integral tanks, rigid tanks, and bladder tanks.
Different Types of Aviation Fuels
Every aircraft engine is designed to be compatible with a certain fuel, meaning that it cannot use mixed fuels or a fuel other than the one specified by the manufacturer. For instance, reciprocating engines burn aviation gasoline (Avgas), whereas gas turbine engines burn jet fuel. Avgas is a leaded fuel that contains tetraethyllead (TEL), which is a toxic substance utilized to raise the octane rating of the fuel. This inhibits engine knocking in high compression aircraft engines. Meanwhile, jet fuel is a kerosene-based substance, with the two most widely used variations being Jet-A and Jet-A1.
Conclusion
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