Fuel System Types and Selectors

Fuel cooled oil cooler (FCOC) Gascolator* Strainer Vent tube Filler cap Tank On-off valve Engine Carburetor*fuel filter with sediment trap and drain Drain Sump drain helicopter. 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. Typically, a helicopter has only one or two fuel tanks located near the center of gravity (CG) of the aircraft, which is near the main rotor mast. Thus, the tank, or tanks, are usually located in or near the aft fuselage. Some helicopter fuel tanks are mounted above the engine allowing for gravity fuel feed.

Others use fuel pumps and pressure feed systems. Fundamentally, helicopter fuel systems differ little from those on fixed-wing aircraft. Gravity-feed systems have vented fuel tanks with an outlet strainer and shutoff valve. Fuel flows from the tank through a main filter to the carburetor. [Figure 14-24] A slightly more complex system for a light turbine-powered helicopter is shown in Figure 14-25. Two in-tank electric boost pumps send fuel through a shutoff valve rather than a selector valve, since there is only one fuel tank. It flows through an airframe filter to an engine filter and then to the engine-driven fuel pump. The fuel tank is vented and contains an electrically operated sump drain valve. A pressure gauge is used to monitor boost pump output pressure and differential pressure switches warn of fuel filter restrictions. Fuel quantity is derived through the use of two in-tank fuel probes with transmitters.

Larger, heavy, multiengine transport helicopters have complex fuel systems similar to jet transport fixed-wing aircraft. They may feature multiple fuel tanks, crossfeed systems, and pressure refueling. Fuel System Components To better understand aircraft fuel systems and their operation, the following discussion of various components of aircraft fuel systems is included. Fuel Tanks There are three types of aircraft fuel tanks: rigid removable tanks, bladder tanks, and integral fuel tanks. The type of aircraft, its design and intended use, as well as the age of the aircraft determine which fuel tank is installed in an aircraft.

Most tanks are constructed of noncorrosive material(s). They are typically made to be vented either through a vent cap or a vent line. Aircraft fuel tanks have a low area called a sump that is designed as a place for contaminants and water to settle. The sump is equipped with a drain valve used to remove the impurities during preflight walk-around inspection. [Figure 14-26] Most aircraft fuel tanks contain some sort of baffling to subdue the fuel from shifting rapidly during flight maneuvers. Use of a scupper constructed around the fuel fill opening to drain away any spilled fuel is also common.

Rigid Removable Fuel Tanks Many aircraft, especially older ones, utilize an obvious choice for fuel tank construction. A rigid tank is made from various materials, and it is strapped into the airframe structure. The tanks are often riveted or welded together and can include baffles, as well as the other fuel tank features described above. They typically are made from 3003 or 5052 aluminum alloy or stainless steel and are riveted and seam welded to prevent leaks. Many early tanks were made of a thin sheet steel coated with a lead/tin alloy called terneplate. The terneplate tanks have folded and soldered seams. Figure 14-27 shows the parts of a typical rigid removable fuel tank.

14-18 Fuel boost caution light Boost pump (electric) Pressure switch Check valve Filler cap Upper fuel probe Fuel supply Pump delivery Purged fuel Lower fuel probe Sump drain (electric) Thermal relief valve Vent Shut-off valve Fuel pressure gauge To fuel control Airframe filter (caution light is on flight deck) Fuel quantity Engine driven fuel pump Filter Transducer Pressure differential switch Fuel filter caution light is on flight deck Restrictor Regardless of the actual construction of removable metal tanks, they must be supported by the airframe and held in place with some sort of padded strap arrangement to resist shifting in flight. The wings are the most popular location for fuel tanks. Figure 14-28 shows a fuel tank bay in a wing root with the tank straps. Some tanks are formed to be part of the leading edge of the wing. These are assembled using electric resistance welding and are sealed with a compound that is poured into the tank and allowed to cure. Many fuselage tanks also exist. [Figure 14-29] In all cases, the structural integrity of the airframe does not rely on the tank(s) being installed, so the tanks are not considered integral.

As new materials are tested and used in aircraft, fuel tanks are being constructed out of materials other than aluminum, steel, and stainless steel. Figure 14-30 shows a rigid removable fuel tank from an ultralight category aircraft that is designed to collect the sump drain material in the clear cylinder to be examined for the presence of contaminants. 14-19 Vents Overboard drain line Sump Standpipe Finger screen Filler cap Baffle plates Scupper Drain valve Corrosion inhibitor parts. Fuel tank straps a stand in a paint booth. Padded straps hold the fuel tank securely in the structure. constructed from Vipel® isophthalic polyester UL 1316/UL 1746 resin and composite. Its seamless, lightweight construction may lead to the use of this type of tank in other aircraft categories in the future.

Being able to remove and repair, or replace, a fuel tank can be a great convenience if a leak or malfunction with the tank exists. Repairs to fuel tanks must be done in accordance with manufacturers’ specifications. It is especially critical to follow all safety procedures when welding repairs are performed. Fuel vapors must be removed from the tank to prevent explosion. This typically involves washing out the tank with water and detergent, followed by a steam or water 14-20 flush, as determined by the manufacturer. Once repaired, fuel tanks need to be pressure checked, usually while installed in the airframe, to prevent distortion while under pressure.

Bladder Fuel Tanks A fuel tank made out of a reinforced flexible material called a bladder tank can be used instead of a rigid tank. A bladder tank contains most of the features and components of a rigid tank but does not require as large an opening in the aircraft skin to install. The tank, or fuel cell as it is sometimes called, can be rolled up and put into a specially prepared structural bay or cavity through a small opening, such as an inspection opening. Once inside, it can be unfurled to its full size. Bladder tanks must be attached to the structure with clips or other fastening devices. They should lie smooth and unwrinkled in the bay. It is especially important that no wrinkles exist on the bottom surface so that fuel contaminants are not blocked from settling into the tank sump. [Figure 14-31] Bladder fuel tanks are used on aircraft of all sizes. They are strong and have a long life with seams only around installed features, such as the tank vents, sump drain, filler spout, etc.

When a bladder tank develops a leak, the technician can patch it following manufacturer’s instructions. The cell can also be removed and sent to a fuel tank repair station familiar with and equipped to perform such repairs. The soft flexible nature of bladder fuel tanks requires that they remain wet. Should it become necessary to store a bladder tank without fuel in it for an extended period of time, it is common to wipe the inside of the tank with a coating of clean engine oil. Follow the manufacturer’s instructions for the dry storage procedures for fuel cells. Integral Fuel Tanks On many aircraft, especially transport category and high- performance aircraft, part of the structure of the wings or fuselage is sealed with a fuel resistant two-part sealant to form a fuel tank. The sealed skin and structural members provide the highest volume of space available with the lowest weight.

This type of tank is called an integral fuel tank since it forms a tank as a unit within the airframe structure. Integral fuel tanks in the otherwise unused space inside the wings are most common. Aircraft with integral fuel tanks in the wings are said to have wet wings. For fuel management purposes, sometimes a wing is sealed into separate tanks and may include a surge tank or an overflow tank, which is normally empty but sealed to hold fuel when needed. When an aircraft maneuvers, the long horizontal nature of an integral wing tank requires baffling to keep the fuel from sloshing. The wing ribs and box beam structural members INBOARD Surge tank 5 valves (WBL 157.0) 6 valves (WBL 227.0) 1 valve (WBL 557.0) Body Valve shown in the integral tank rib structure of a Boeing 737 airliner.

Fuel is prevented from flowing outboard during maneuvers. The tank boost pumps are located inboard of WBL 157. serve as baffles and others may be added specifically for that purpose. Baffle check valves are commonly used. These valves allow fuel to move to the low, inboard sections of the tank but prevent it from moving outboard. They ensure that the fuel boost pumps located in the bottom of the tanks at the lowest points above the sumps always have fuel to pump regardless of aircraft attitude. [Figure 14-32] Integral fuel tanks must have access panels for inspection and repairs of the tanks and other fuel system components.

On large aircraft, technicians physically enter the tank for maintenance. Transport category aircraft often have more than a dozen oval access panels or tank plates on the bottom surface of the wing for this purpose. [Figure 14-33A] These aluminum panels are each sealed into place with an O-ring and an aluminum gasket for electrostatic bonding. An outer clamp ring is tightened to the inner panel with screws, as shown in Figure 14-33B. When entering and performing maintenance on an integral fuel tank, all fuel must be emptied from the tank and strict safety procedures must be followed. Fuel vapors must be purged from the tank and respiratory equipment must be used 14-21 Tank access panel No. 1 Boost pump access panel No. 1 Boost pump access panel No. 2 No. 2 No. 4 No. 6 No. 8 No. 10 No. 12 No. 14 No. 3 No. 5 No. 7 No. 9 No. 11 No. 13 APL CL INBOARD FORWARD A Access door Molded rubber seal Lower wing surface Knitted aluminum gasket Clamp ring Mounting bolt (TYP) B 14-22 Synthetic cover Fiber braid Synthetic tube by the technician. A full-time spotter must be positioned just outside of the tank to assist if needed.

Aircraft using integral fuel tanks normally have sophisticated fuel systems that include in-tank boost pumps. There are usually at least two pumps in each tank that deliver fuel to the engine(s) under positive pressure. On various aircraft, these in-tank boost pumps are also used to transfer fuel to other tanks, jettison fuel, and defuel the aircraft. Fuel Lines & 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 14-34] The hose is approved for fuel and no other hose should be substituted. Some flexible fuel hoses have a braided stainless steel exterior. [Figure 14-35] 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 fire sleeve cover is held over the line with steel clamps at the end fittings. [Figure 14-36] 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 are cautioned to not overtighten a leaky fitting. If the proper torque does not stop a leak, depressurize the line, disconnect the fitting and visually inspect it for a 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 writing printed on the outside of the hose is used as a lay line 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 fuel system generates static electricity that must have a place to flow to ground rather than build up. Special bonded 14-23 cushion clamps are used to secure rigid fuel lines in place.

They are supported at intervals shown in Figure 14-37. All fuel lines should be supported so that there is no strain on the fittings. Clamp lines so that fittings are aligned. Never draw two fittings together by threading. 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 Approximate distance between supports (inches)Tubing OD (inch) 9 12 16 22 30 40 3/16 5/16 1/2 3/4 11/4 2 1/8 1/4 3/8 5/8 7 11/2 to Grommet Support tube at least 1/4 inch from edge of hole Bonded cushion clamp with electrically bonded cushion clamps at specified intervals.

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 have 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 the use of detents into which a spring-loaded pin or similar protrusion locates and labeling aid the pilot in knowing the position of the fuel valve. 14-24 electronic centralized aircraft monitor (ECAM) fuel page includes valve position information. 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 14-38] 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 monitors. [Figure 14-39] 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 14-40] Hand-Operated Valves There are three basic types of hand-operated valves used in aircraft fuel systems. The cone-type valve and the poppet- Detent plate Valve housing Inlet port Bored passageway Brass cone Outlet port inlet and the outlet ports. It shuts off the flow when the un-bored portion of the cone is aligned with the inlet port(s).

type valve are commonly used in light general aviation aircraft as fuel selector valves. Gate valves are used on transport category aircraft as shutoff valves. While many are motor operated, there are several applications in which gate valves are hand operated. Cone Valves A cone valve, also called a plug valve, consists of a machined valve housing into which a rotatable brass or nylon cone is set. The cone is manually rotated by the pilot with an attached handle. Passageways are machined through the cone so that, as it is rotated, fuel can flow from the selected source to the engine. This occurs when the passageway 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.

Cam moved by selector valve handle From reserve tank From tank 1 From tank 2To main strainer selector valve. 14-25 aligns with the desired fuel input port machined into the housing. Figure 14-41 shows a cross sectional view of a cone valve. The cone can also be rotated to a position so that the passageway(s) does not align with any fuel input port. This is the fuel OFF position of the valve. Poppet Valves Selector valves are also commonly the poppet type. As the handle is rotated in this valve, a cam on the attached shaft lifts the poppet off the seat of the desired port being selected. At the same time, spring-assisted poppets close off the ports that are not selected. Detents lock the valve into position when the cam pushes a poppet fully off of its seat. There is also a positive detent when the cam engages none of the poppets, which is the OFF position of the valve. [Figure 14-42] A similar mechanism is used in some selector valves, but balls are used instead of poppets.

Manually-Operated Gate Valves A single selector valve is not used in complex fuel systems of transport category aircraft. Fuel flow is controlled with a series of ON/OFF, or shutoff, type valves that are plumbed between system components. Hand-operated gate valves can be used, especially as fire control valves, requiring no electrical power to shutoff fuel flow when the emergency fire handle is pulled. The valves are typically positioned in the fuel feed line to each engine. Hand-operated gate valves are also featured as ground-operated defuel valves and boost pump isolation valves, which shut off the fuel to the inlet of the boost pump, allowing it to be changed without emptying the tank.

Gate valves utilize a sealed gate or blade that slides into the path of the fuel, blocking its flow when closed. When the handle is rotated, the actuating arm inside the valve moves the gate blade down between seals and into the fuel flow path. A thermal relief bypass valve is incorporated to relieve excess pressure buildup against the closed gate due to temperature increases. Motor-Operated Valves The use of electric motors to operate fuel system valves is common on large aircraft due to the remote location from the flight deck of fuel system components. The types of valves used are basically the same as the manually operated valves, but electric motors are used to actuate the units. The two most common electric motor operated fuel valves are the gate valve and the plug-type valve.

The motor-operated gate valve uses a geared, reversible electric motor to turn the actuating arm of the valve that moves the fuel gate into or out of the path of the fuel. As with the manually operated gate valve, the gate or blade is sealed. A manual override lever allows the technician to observe the Roller Actutating arm Blade Mounting plate Index pin O-Ring Seals Thermal relief valve Valve handle attached here Housing Gear train actuator Motor Manual override lever Housing Thermal relief valve O-ring Roller Arm Blade Seals aircraft fuel systems. in large aircraft fuel systems. 14-26 position of the valve or manually position it. [Figure 14-44] Less common is the use of a motorized plug-type fuel valve; an electric motor is used to rotate the plug or drum rather than it being rotated manually. Regardless of the type of valve used, large aircraft fuel system valves either allow fuel to flow or shut off flow.

Solenoid-Operated Valves An additional way to operate a remotely located fuel valve is through the use of electric solenoids. A poppet-type valve is opened via the magnetic pull developed when an opening solenoid is energized. A spring forces a locking stem into a notch in the stem of the poppet to lock the valve in the open position. Fuel then flows through the opening vacated by the poppet. To close the poppet and shut off fuel flow, a closing solenoid is energized. Its magnetic pull overcomes the force of the locking stem spring and pulls the locking stem out of the notch in the poppet stem. A spring behind the poppet forces it back onto its seat. A characteristic of solenoid-operated fuel valves is that they open and close very quickly. [Figure 14-45] Fuel Pumps Other than aircraft 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. A discussion of the various pump types found in the aviation fleet follows. Hand-Operated Fuel Pumps Some older reciprocating engine aircraft have been equipped with hand-operated fuel pumps. They are used to back up the engine-driven pump and to transfer fuel from tank to tank. The wobble pumps, as they are known, are double-acting pumps that deliver fuel with each stroke of the pump handle. They are essentially vane-type pumps that have bored passages in the center of the pump, allowing a back-and-forth motion to pump the fuel rather than a full revolution of the vanes as is common in electrically driven or engine-driven vane-type pumps.

pump. As the handle is moved down from where it is shown, the vane on the left side of the pump moves up, and the vane on the right side of the pump moves down. As the left vane moves up, it draws fuel into chamber A. Because chambers A and D are connected through the bored center, fuel is also drawn into chamber D. At the same time, the right vane forces fuel out of chamber B, through the bored passage in the center of the pump, into chamber C and out the fuel outlet through the check valve at the outlet of chamber C. When the handle is moved up again, the left vane moves down, forcing fuel Fuel in Fuel out D B C A Closing solenoid Spring Fuel inFuel out Opening solenoid control wires Opening solenoid Spring force developed by energized solenoids to open and close a poppet. starting and fuel transfer on older transport category aircraft.

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