Aircraft Fuel Systems & Their Operation
An aircraft fuel system is a configuration that allows a crew to pump, manage, and deliver fuel to the propulsion system, engine, or other auxiliary power unit of an aircraft. A common aircraft fuel system consists of fuel tanks, fuel lines, fuel valves, fuel pumps, filters, heaters, and fuel system indicators. This blog will look at each of these engine fuel system components and shed light on their functions and role in the system as a whole.
The first major component of any aircraft fuel system is the fuel tank. There are three common types of fuel tanks: rigid removable tanks, bladder tanks, and integral fuel tanks. Each of the three types are used depending on the aircraft’s design, intended use, and age. These tanks are constructed of corrosion-resistant materials and feature a sump at the base of the tank for water or other contaminants that could damage the tank. This sump is fitted with a drainage valve to remove debris during the pre-flight inspection. Fuel tanks also feature a part called a baffling, which prevents fuel from moving around rapidly during sudden flight maneuvers.
The next part of the system is the fuel lines, which take the fuel from the tank and funnel it into the engine. Depending on their location and application, fuel lines are either rigid or flexible. Rigid fuel lines are commonly made of aluminum alloy, although in areas that are subject to damage from heat, debris, and abrasion, stainless steel lines are implemented. Flexible fuel lines are made from synthetic rubber and feature a fiber braid wrap to strengthen the exterior. These fuel lines are used in areas of vibration between components. The diameters of the fuel line, whether rigid or flexible, is determined by the fuel flow requirements of the system.
The aircraft fuel valves are used to shut off fuel flow or direct fuel to a certain direction. Apart from the valves on the aforementioned sump drain, smaller aircraft fuel systems include only a selector valve. This valve carries out the shutoff and direction functions in a single valve. Large aircraft have an array of valves to shut off, transfer, and direct fuel flow. These valves can be controlled manually, electronically, or electromagnetically. Another important component, the fuel pump, is present in any aircraft fuel system other than gravity-feed configurations. This pump uses pressure to deliver fuel to the metering device of the engine. The most common type of fuel pump is engine-driven, though supplementary auxiliary pumps are used in many cases as well.
In high altitudes where the temperature is very low, keeping the fuel warm to prevent the formation of ice crystals is critical. Ice on the tank or filter can hinder or completely block the flow of fuel, causing the engine to fail. Fuel heaters are used to warm the fuel so ice does not form and melt any that has already formed. The most common fuel heaters are known as air/fuel heaters which use warm bleed air from the compressor to heat the fuel. Fuel heaters are controlled by a switch in the cockpit and operate intermittently, only as needed.
The final part of the aircraft fuel system is the array of fuel system indicators. All aircraft fuel systems must be equipped with a fuel quantity indicator and other indicators to monitor the flow, temperature and flow of fuel. Other indicators include valve position indicators and standard warning lights.
