Aircraft fuel systems consist of numerous components that work together to safely store, control, filter, monitor, and deliver fuel to the engine. These components must operate reliably under varying flight conditions while maintaining proper fuel pressure, flow, and contamination control. Understanding the construction and function of each component is essential for aircraft operation, inspection, and maintenance.
Fuel Tanks
There are three basic types of aircraft fuel tanks: rigid removable tanks, bladder tanks, and integral fuel tanks. The type, design, intended use, and age of the aircraft determine which fuel tank configuration is installed. Most tanks are constructed of noncorrosive materials. They are typically made to be vented either through a vent cap or a vent line. Aircraft fuel tanks include a low point called a sump where contaminants and water can settle. The sump is equipped with a drain valve used to remove the impurities during preflight walk-around inspection. [Figure 1]
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| Figure 1. Sumping a fuel tank with a fuel strainer that is designed to collect the sump drain material in the clear cylinder to be examined for the presence of contaminants |
Most aircraft fuel tanks contain internal baffles to reduce rapid fuel movement during flight maneuvers. Use of a scupper constructed around the fuel fill opening to drain away any spilled fuel is also common. Refer to the Types of Fuel Tanks section for more detail.
Fuel Lines and Fittings
Aircraft fuel lines can be rigid or flexible depending on location and application. Rigid lines are often made of aluminum alloy and are connected with Army/Navy (AN) or military standard (MS) fittings. However, in the engine compartment, wheel wells, and other areas, subject to damage from debris, abrasion, and heat, stainless steel lines are often used.
Flexible fuel hose has a synthetic rubber interior with a reinforcing fiber braid wrap covered by a synthetic exterior. [Figure 2]
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| Figure 2. A typical flexible aircraft fuel line with braided reinforcement |
The hose is approved for fuel and no other hose should be substituted. Some flexible fuel hoses have a braided stainless-steel exterior. [Figure 3]
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| Figure 3. A braided stainless steel exterior fuel line with fittings |
The diameters of all fuel hoses and lines are determined by the fuel flow requirements of the aircraft fuel system. Flexible hoses are used in areas where vibration exists between components, such as between the engine and the aircraft structure.
Sometimes manufacturers wrap either flexible or rigid fuel lines to provide even further protection from abrasion and especially from fire. A firesleeve is secured over the line with steel clamps at the end fittings. [Figure 4]
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| Figure 4. Exterior fuel hose wrap that protects from fire, as well as abrasion, shown with the clamps and pliers used to install it |
As mentioned, aircraft fuel line fittings are usually either AN or MS fittings. Both flared and flareless fittings are used. Problems with leaks at fittings can occur. Technicians should avoid overtightening a leaking fitting. If the proper torque does not stop a leak, depressurize the line, disconnect the fitting and visually inspect it to determine the cause. The fitting or line should be replaced if needed. Replace all aircraft fuel lines and fittings with approved replacement parts from the manufacturer. If a line is manufactured in the shop, approved components must be used.
Several installation procedures for fuel hoses and rigid fuel lines exist. Hoses should be installed without twisting. The printed lay line on the outside of the hose is used to monitor fuel hose twist. Separation should be maintained between all fuel hoses and electrical wiring. Never clamp wires to a fuel line. When separation is not possible, always route the fuel line below any wiring. If a fuel leak develops, it does not drip onto the wires.
Metal fuel lines and all aircraft fuel system components need to be electrically bonded and grounded to the aircraft structure. This is important because fuel flowing through the system generates static electricity that must be dissipated to ground rather than build up. Special bonded cushion clamps are used to secure rigid fuel lines in place. They are supported at intervals shown in Figure 5.
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| Figure 5. Rigid metallic fuel lines are clamped to the airframe with electrically bonded cushion clamps at specified intervals |
All fuel lines should be supported so that there is no strain on the fittings. Clamp lines so that fittings are aligned. Never force two fittings into alignment by tightening the threads. They should thread easily, and a wrench should be used only for tightening. Additionally, a straight length of rigid fuel line should not be made between two components or fittings rigidly mounted to the airframe. A small bend is needed to absorb any strain from vibration or expansion and contraction due to temperature changes.
Fuel Valves
There are many fuel valve uses in aircraft fuel systems. They are used to shut off fuel flow or to route the fuel to a desired location. Other than sump drain valves, light aircraft fuel systems may include only one valve, the selector valve. It incorporates the shutoff and selection features into a single valve. Large aircraft fuel systems contain numerous valves. Most simply open and close and are known by different names related to their location and function in the fuel system (e.g., shutoff valve, transfer valve, crossfeed valve). Fuel valves can be manually operated, solenoid-operated, or operated by electric motor.
A feature of all aircraft fuel valves is a means for positively identifying the position of the valve at all times. Hand-operated valves accomplish this through detents engaged by a spring-loaded pin or similar mechanism when the valve is set in each position. Combined with labels and a directional handle, this makes it easy to identify by feel and by sight that the valve is in the desired position. [Figure 6]
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| Figure 6. Detents for each position, an indicating handle, and labeling aid the pilot in knowing the position of the fuel valve |
Motor- and solenoid-operated valves use position annunciator lights to indicate valve position in addition to the switch position. Flight management system (FMS) fuel pages also display the position of the fuel valves graphically in diagrams called up on the flat-screen displays. [Figure 7]
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| Figure 7. The graphic depiction of the fuel system on this electronic centralized aircraft monitor (ECAM) fuel page includes valve position information |
Many valves have an exterior position handle, or lever, that indicates valve position. When maintenance personnel directly observe the valve, it can be manually positioned by the technician using this same lever. [Figure 8]
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| Figure 8. This motor-operated gate valve has a red position indicating lever that can be used by maintenance personnel to identify the position of the valve. The lever can be moved by the technician to position the valve |
Refer to the Types of Fuel Valves section for more detail.
Fuel Pumps
Except for aircraft equipped with gravity-feed fuel systems, all aircraft have at least one fuel pump to deliver clean fuel under pressure to the fuel metering device for each engine. Engine-driven pumps are the primary delivery device. Auxiliary pumps are used on many aircraft as well. Sometimes known as booster pumps or boost pumps, auxiliary pumps are used to provide fuel under positive pressure to the engine-driven pump and during starting when the engine-driven pump is not yet up to speed for sufficient fuel delivery. They are also used to back up the engine-driven pump during takeoff and at high altitude to guard against vapor lock. On many large aircraft, boost pumps are used to move fuel from one tank to another.
There are many different types of auxiliary fuel pumps in use. Most are electrically operated, but some hand-operated pumps are found on older aircraft. The various fuel pump types used in aircraft are discussed in the Types of Fuel Pumps section.
Fuel Filters
Two main types of fuel cleaning devices are utilized on aircraft. Fuel strainers are usually constructed of relatively coarse wire mesh. They are designed to trap large pieces of debris and prevent their passage through the fuel system. Fuel strainers do not inhibit the flow of water. Fuel filters are generally constructed of fine mesh. In various applications, they can trap fine sediment that may be only thousandths of an inch in diameter and also help trap water. The technician should be aware that the terms “strainer” and “filter” are sometimes used interchangeably. Micronic filters are commonly used on turbine-powered aircraft. This is a type of filter that captures extremely fine particles in the range of 10–25 microns. A micron is one-thousandth (1/1,000) of a millimeter. [Figure 9]
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| Figure 9. Size comparison of 1-micron dust particle and pin head |
Refer to the Filters and Strainers section for more details.
Fuel Heaters and Ice Prevention
Turbine-powered aircraft operate at high altitude where the temperature is very low. As the fuel in the fuel tanks cools, water in the fuel condenses and freezes. It may form ice crystals in the tank or as the fuel/water mixture slows and contacts the cold filter element on its way through the fuel filter to the engine(s). The formation of ice on the filter element blocks the flow of fuel through the filter. A valve in the filter unit bypasses unfiltered fuel when this occurs. Fuel heaters are used to warm the fuel so that ice does not form. These heat exchanger units also heat the fuel sufficiently to melt any ice that has already formed.
The most common types of fuel heaters are air-fuel heaters and oil/fuel heaters. An air-fuel heater uses warm compressor bleed air to heat the fuel. An oil/fuel exchanger heats the fuel with hot engine oil. This latter type is often referred to as a fuel-cooled oil cooler (FCOC). [Figure 10]
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| Figure 10. Jet transport aircraft fly at high altitudes where temperatures can reach –50 °F. Most have fuel heaters somewhere in the fuel system to help prevent fuel icing. This fuel-cooled oil cooler on an RB211 turbofan engine simultaneously heats the fuel while cooling the oil |
Fuel heaters often operate intermittently as needed. A switch in the flight deck can direct the hot air or oil through the unit or block it. The flight crew uses the information supplied by the filter bypass indicator lights and fuel temperature gauge [Figure 11] to know when to heat the fuel.
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| Figure 11. A Boeing 737 cockpit fuel panel showing illuminated valve position indicators and fuel filter bypass lights. The fuel temperature in tank No.1 is also indicated |
Fuel heaters can also be automatic. A built-in thermostatic device opens or closes a valve that permits the hot air or hot oil to flow into the unit to warm the fuel. [Figure 12]
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| Figure 12. An air-fuel heat exchanger uses engine compressor bleed air to warm the fuel on many turbine engine powered aircraft |
Some aircraft have a hydraulic fluid cooler in one of the aircraft fuel tanks. The fluid helps warm the fuel as it cools in this type of full-time heat exchanger.
Fuel System Indicators
Aircraft fuel systems utilize various indicators. All systems are required to have some sort of fuel quantity indicator. Fuel flow, pressure, and temperature are monitored on many aircraft. Valve position indicators and various warning lights and annunciations are also used. Refer to the Fuel System Indicators section for more details.