Fuel Tanks and System Components

The third basic type of turbine engine fuel available is Jet B. It is a wide-cut fuel that is basically a blend of kerosene and gasoline. Its volatility and vapor pressure reflect this and fall between Jet A and A VGAS. Jet B is primarily available in Alaska and Canada due to its low freezing point of approximately –58 °F, and its higher volatility yields better cold weather performance. Turbine Engine Fuel Issues Purity issues related to turbine engine fuels are unique. While A VGAS experiences similar issues of solid particle contamination and icing concerns, the presence of water and fuel-consuming microbes is more prominent in jet fuel, which has different molecular structure and retains water in two principal ways. Some water is dissolved into the fuel.

Other water also is entrained in the fuel, which is more viscous than A VGAS. The greater presence of water in jet fuel allows microbes to assemble, grow, and live on the fuel. Since turbine engine fuels always contain water, microbial contamination is always a threat. The large tanks of many turbine engine aircraft have numerous areas where water can settle, and microbes can flourish. Areas between the fuel tank and any water that may come to rest in the bottom of the tanks is where the microbes thrive. These microorganisms form a bio-film that can clog filters, corrode tank coatings, and degrade the fuel. They can be controlled somewhat with the addition of biocides to the fuel. [Figure 14-11] Anti-ice additives are also known to inhibit bacterial growth.

Since the microbes are sustained by fuel and water, best practices must be followed to keep the water in fuel to a minimum. Avoid having fuel in a storage tank for a prolonged period of time on or off the aircraft. Drain sumps and monitor the fuel for settled water. Investigate all incidents of water kill microbes that live on hydrocarbons. discovered in the fuel. In addition to water in jet fuel supporting the growth of microorganisms, it also poses a threat of icing. Follow the manufacturer’s instructions for fuel handling procedures and fuel system maintenance. Aircraft Fuel Systems While each manufacturer designs its own fuel system, the basic fuel system requirements referenced at the beginning of this chapter yield fuel systems of similar design and function in the field. In the following sections are representative examples of various fuel systems in each class of aircraft discussed. Others are similar but not identical. Each aircraft fuel system must store and deliver clean fuel to the engine(s) at a pressure and flow rate able to sustain operations regardless of the operating conditions of the aircraft.

Small Single-Engine Aircraft Fuel Systems Small single-engine aircraft fuel systems vary depending on factors, such as tank location and method of metering fuel to the engine. A high-wing aircraft fuel system can be designed differently from one on a low-wing aircraft. An aircraft engine with a carburetor has a different fuel system than one with fuel injection. Gravity Feed Systems High-wing aircraft with a fuel tank in each wing are common. With the tanks above the engine, gravity is used to deliver the fuel. A simple gravity feed fuel system is shown in Figure 14-12. The space above the liquid fuel is vented to maintain atmospheric pressure on the fuel as the tank empties. The two tanks are also vented to each other to ensure equal pressure when both tanks feed the Right tank Left tank BOTH LEFT RIGHT OFF Fuel supply Vent Fuel selector valve Strainer Primer Carburetor wing aircraft is the simplest aircraft fuel system.

14-9 engine. A single screened outlet on each tank feeds lines that connect to either a fuel shutoff valve or multiposition selector valve. The shutoff valve has two positions: fuel ON and fuel OFF. If installed, the selector valve provides four options: fuel shutoff to the engine; fuel feed from the right-wing tank only; fuel feed from the left fuel tank only; fuel feed to the engine from both tanks simultaneously. Downstream of the shutoff valve or selector valve, the fuel passes through a main system strainer. This often has a drain function to remove sediment and water. From there, it flows to the carburetor or to the primer pump for engine starting.

Having no fuel pump, the gravity feed system is the simplest aircraft fuel system. Pump Feed Systems Low- and mid-wing single reciprocating engine aircraft cannot utilize gravity-feed fuel systems because the fuel tanks are not located above the engine. Instead, one or more pumps are used to move the fuel from the tanks to the engine. A common fuel system of this type is shown in to a selector valve. However, fuel cannot be drawn from both tanks simultaneously; if the fuel is depleted in one tank, the pump would draw air from that tank instead of fuel from the full tank. Since fuel is not drawn from both tanks at the same time, there is no need to connect the tank vent spaces together.

From the selector valve (LEFT, RIGHT, or OFF), fuel Fuel supply Pump delivery Right tank Left tank BOTH LEFT RIGHT OFF Primer Fuel selector valve Strainer Carburetor Electric pump plunger type Engine-driven pump diaphragm type located in wings below the engine uses pumps to draw fuel from the tanks and deliver it to the engine. flows through the main strainer where it can supply the engine primer. Then, it flows downstream to the fuel pumps. Typically, one electric and one engine-driven fuel pump are arranged in parallel. They draw the fuel from the tank(s) and deliver it to the carburetor. The two pumps provide redundancy. The engine-driven fuel pump acts as the primary pump. The electric pump can supply fuel should the other fail.

The electric pump also supplies fuel pressure while starting and is used to prevent vapor lock during flight at high altitude. High-Wing Aircraft with Fuel Injection System Some high-wing, high-performance, single-engine general aviation aircraft are equipped with a fuel system that features fuel injection rather than a carburetor. It combines gravity flow with the use of a fuel pump(s). The Teledyne- Continental system is an example. [Figure 14-14] Note: Fuel injection systems spray pressurized fuel into the engine intake or directly into the cylinders. Fuel without any air mixed in is required to provide a measured, continuous spray and smooth engine operation.

Fuel pressurized by an engine-driven pump is metered as a function of engine rpm on the Teledyne-Continental system. It is first delivered from the fuel tanks by gravity to two smaller accumulator or reservoir tanks. These tanks, one for each wing tank, consolidate the liquid fuel and have a relatively small airspace. They deliver fuel through a three- way selector valve (LEFT, RIGHT, or OFF). The selector valve also acts simultaneously as a diverter of air that has been separated out of the fuel in the engine-driven fuel pump and returned to the valve. It routes the air to the vent space above the fuel in the selected reservoir tank.

An electric auxiliary fuel pump draws fuel through the selector valve. It forces the fuel through the strainer, making it available for the primer pump and the engine-driven fuel pump. This pump is typically used for starting and as a backup should the engine-driven pump fail. It is controlled by a switch in the flight deck and does not need to be operating to allow the engine-driven fuel pump access to the fuel. The engine-driven fuel pump intakes the pressurized fuel from the electrically driven pump or from the reservoir tanks if the electric pump is not operating. It supplies a higher-than- needed volume of fuel under pressure to the fuel control.

Excess fuel is returned to the pump, which pumps it through the selector valve into the appropriate reservoir tank. Fuel vapor is also returned to tanks by the pump. The fuel control unit meters the fuel according to engine rpm and mixture control inputs from the flight deck. 14-10 Fuelreservoir tank Right tank Aft Forward OFF Fuel reservoirtank Left tank Aft Forward Engine primer Selector valve Auxiliary fuel pump Engine-driven fuel pump Fuel flow indicator Fuel injection distributor manifold Fuel injection control unit Fuel strainer To intake manifold Vent check valveVent check valve Vented filler caps injection used on high-wing, high-performance single-engine aircraft.

The fuel control delivers the fuel to the distribution manifold, which divides it and provides equal, consistent fuel flow for individual fuel injector in each cylinder. [Figure 14-15] A fuel flow indicator tapped off of the distribution manifold provides feedback in flight deck. It senses fuel pressure but is displayed on a dial calibrated in gallons per hour. Small Multiengine (Reciprocating) Aircraft Fuel Systems Low-Wing Twin The fuel system on a small, multiengine aircraft is more complicated than a single-engine aircraft but contains many of the same elements. An example system used on a low- wing aircraft is illustrated in Figure 14-16. It features the main fuel tanks in the wing tips and auxiliary tanks in the wing structure. A boost pump is located at the outlet of each main tank. This pressurizes the entire fuel system from the tank to the injectors eliminating the possibility of vapor lock. An engine can operate with just its boost pump running in the event the engine-driven injection pump fails. Typically, the boost pumps are used to prime and start the engine.

Two selector valves are required on twin-engine aircraft, one for each engine. The right selector valve receives fuel from a main tank on either side of the aircraft and directs it to the right engine. The left selector valve also receives fuel from either main tank and directs it to the left engine. This allows fuel to crossfeed from one side of the aircraft to the opposite engine if desired. The selector valves can also direct fuel from the auxiliary tank to the engine on the same side. Crossfeed of fuel from auxiliary tanks is not possible. From the outlet of the selector valve, fuel flows to the strainer. On some aircraft, the strainer is built into the selector valve unit.

From the strainer, fuel flows to the engine-driven fuel pump. The engine-driven fuel pump is an assembly that also contains a vapor separator and a pressure regulating valve with an adjustment screw. The vapor separator helps eliminate air from the fuel. It returns a small amount of fuel and any vapor present back to the main fuel tank. The pump supplies pressurized fuel to the fuel control. The fuel control, one for each engine, responds to throttle and mixture control settings from the flight deck and supplies the proper amount of fuel to the fuel manifold. The manifold divides the fuel and sends it to an injector in each cylinder. A fuel pressure gauge is placed between the fuel control unit outlet and the manifold to monitor the injector-applied pressure that indicates engine power.

14-11 Left aux. fuel tank (opt.) Right aux. fuel tank (opt.) Left fuel tank Right fuel tank AUX LEFT RIGHT OFF AUX LEFT RIGHT OFF Selector valve Right engine fuel pump to heater Fuel pressure adjusting screw Drain controlStrainer Right oil dilution solenoid To right engine oil pump On/off prime Vapor return lineDrain valve Vapor separator Right engine fuel control unit To right aux. pump Fuel pressure switch for aux. pumps Aux. fuel pumpDrain valve Aux pump Vent Vent in filler cap Drain valve Filler cap Fuel injection nozzleRight engine fuel manifold To cylinders To cylinders Dual fuel flow gauge Ignition switches Drain plugs Fuel quantity indicator Aux. tank fuel quantity indicator Drain plugs ThrottleMixture control Vent in filler cap Filler cap Drain valve Vent Drain valve Aux fuel pump Selector valve Drain control Vapor return line Drain valve Strainer To left engine oil pump Left oil dilution solenoid Vapor separator Left engine fuel pump Fuel pressure adjusting screw On/off prime Throttle Mixture control To left aux. pump Fuel pressure switch for aux.

pumps Left engine fuel control unit Left engine fuel manifold To cylindersTo cylinders Fuel injection nozzle Filter screen Filter screen CODE Fuel Vapor return line Mechaninal actuation Electrical actuation Check valve Pressure relief valve High-Wing Twin A simplified system on a high-wing, twin-engine aircraft that combines gravity feed with an electric fuel pump is illustrated in Figure 14-17. Directly downstream of the selector valves are the fuel strainers and then an electric fuel pump for each engine. This pump draws fuel from the selected tank and sends it under pressure to the inlet side of the fuel injection metering unit. The metering unit for each engine provides the proper flow of fuel to the distribution manifold which feeds the injectors.

Large Reciprocating-Engine Aircraft Fuel Systems Large, multiengine transport aircraft powered by reciprocating radial engines are no longer produced. However, many are still in operation. They are mostly carbureted and share many features with the light aircraft systems previously discussed. valve for each engine allows an engine-driven pump to pull fuel from the main tank or an auxiliary tank. The fuel passes through a strainer before reaching the pump where it is delivered to the engine. The outlet of the pump can feed either engine through the use of a crossfeed line with valves controlled in the flight deck. A hand-operated wobble pump located upstream of the strainer is used to prime the system for starting. Fuel vapor lines run from the pressure carburetor to the vent space in the main and auxiliary tanks. Fuel pressure gauges are tapped off of the carburetor for power indication.

14-12 Left inboard Left outboard OFF Right inboard Right outboard OFF Selector valves Electric fuel pump Injection nozzles Fuel injection distributor manifold Fuel injection metering unit StrainersFuel supply Pump delivery Metered fuel pressure The hand-operated wobble pumps were replaced by electric pumps on later model aircraft. A fuel pressure warning light tapped in downstream of the engine-driven fuel pump alerts the crew should fuel pressure decline. Not all large, old aircraft have this fuel system. This is merely an example. Other aircraft share similar features and possess unique features of their own. The same is true for small reciprocating-engine aircraft. There are many systems that share features with those described above, but they also differ in some ways. Always consult the manufacturer’s data when working on aircraft fuel systems and follow all instructions for service and repair. The fuel system of an aircraft provides the life blood for engine operation and must be maintained with the highest discretion.

Jet Transport Aircraft Fuel Systems Fuel systems on large transport category jet aircraft are complex with some features and components not found in reciprocating-engine aircraft fuel systems. They typically contain more redundancy and facilitate numerous options from which the crew can choose while managing the aircraft’s fuel load. Features like an onboard APU, single point pressure refueling, and fuel jettison systems, which are not needed on smaller aircraft, add to the complexity of an airliner fuel system. Jet transport fuel systems can be regarded as a handful of fuel subsystems as follows: 1. Storage.

2. Vent. 3. Distribution. 4. Feed. 5. Indicating. Most transport category aircraft fuel systems are very much alike. Integral fuel tanks are the norm with much of each wing’s structure sealed to enable its use as a fuel tank. Center wing section or fuselage tanks are also common. These may be sealed structure or bladder type. Jet transport aircraft carry tens of thousands of pounds of fuel on board. Figure 14-19 shows a diagram of a Boeing 777 fuel tank configuration with tank capacities. There are optional fuel storage configurations available on the same model airliner. For example, airlines expecting to use an aircraft on transoceanic flights may order the aircraft with long-range auxiliary tanks. These additional tanks, usually located in the fuselage section of the aircraft, can alter fuel management logistics in addition to complicating the fuel system.

In addition to main and auxiliary fuel tanks, surge tanks may also be found on jet transports. These normally empty tanks located in the wing structure outboard of the main wing tanks are used for fuel overflow. A check valve allows the one-way drainage of fuel back into the main tanks. Surge tanks are also used for fuel system venting. 14-13 CODE Supply Supply flow

Pressure

Pressure flow Fuel vapor return Vent line Electric cable Left hand main tank Right hand main tank Left hand auxiliary tank Right hand auxiliary tank Tank float Tank float Tank selector for left hand engine Tank selector for left hand engine Left hand crossfeed valve Right hand crossfeed valve Right hand fuel strainer Left hand fuel strainer Crossfeed line (closed) Right hand engine carburetor Left hand engine carburetor Fuel pressure gauges 1 2 3 4 5 6 7 8 9 10 1112 13 14 15 16 17 18 19 20 21 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 22 21 Poppet valve Vacuum Diaphragm Pressire Venturi tube Throttle Nozzle Fuel vapor to manifold Metered fuel Propane line (winterization) Prime line Left hand engine driven pump Right hand engine-driven pump Restrictor fitting Fuel pressure warning switch Left hand firewall Right hand firewall Wobble pump for left hand engine Wobble pump for right hand engine Solenoid for oil dilution (closed) Solenoid primer (closed) Pressure gauge line Transport category fuel systems require venting similar to reciprocating engine aircraft fuel systems. A series of vent tubing and channels exists that connects all tanks to vent space in the surge tanks (if present) or vent overboard. Venting must be configured to ensure the fuel is vented regardless of the attitude of the aircraft or the quantity of fuel on board. This sometimes requires the installation of various check valves, float valves, and multiple vent locations in the same tank.

A transport category aircraft fuel distribution subsystem consists of the pressure fueling components, defueling components, transfer system, and fuel jettison or dump system. Single-point pressure fueling at a fueling station accessible by ramp refueling trucks allows all aircraft fuel tanks to be filled with one connection of the fuel hose. Leading and trailing edge wing locations are common for these stations. Figure 14-21 shows an airliner fueling station with the fueling rig attached. To fuel with pressure refueling, a hose nozzle is attached at the fueling station and valves to the tanks required to be filled are opened. These valves are called fueling valves 14-14 Float APU fuel line shroud drain mast Frame arrester Drain Check valve Surge tank Tank 1 Surge tank Tank 2 Tank Gallons Pounds* 9,560 26,100 45,200 64,000 174,900 302,900 Left main tank Right main tank Center tank Total * Usable fuel at level attitude Fuel density = 6.7 pounds per U.S. gallon.

Main tank Surge tank Center tank Vent duct (upper wing stringer) Relieves trapped air during fueling Wing tank float valve Float Hinge Check CheckFloat valve valvesValve stop valve(normal vent) (Typ 2 places) Valve stop(Typ 2 PLS) O-ring Wing lower skin Moulded seal phenolic gasket Climb vents Vent lineNormal & climb vent Drain hole Float valve Upper surface (sealed stringer 9) Vent scoop & surge tank Crossover tube access panel Upper surface (sealed stringer 11) Surge tank gravity drain line Deflector Center tank WS 583 Sump drain (5 places) Check valve attitude. 14-15 or refueling valves depending upon the manufacturer’s preference. Various automatic shutoff systems have been designed to close tank fueling valves before the tanks overfill or are damaged. Gauges on the refueling panel allow refueling personnel to monitor progress.

Occasionally, defueling the aircraft is required for an inspection or repair. The same fueling station is used, and the hose from the fuel truck is connected to same receptacle used to fuel the aircraft. To allow fuel to exit the aircraft, a defueling valve is opened. Fuel can either be pumped out of the aircraft using the boost pumps located in the tanks that need to be emptied, or the pump in the refueling truck can be used to draw the fuel out of the tanks. Control over the operation is maintained by positioning various shutoff and crossfeed valves, as well as the defuel valve so that fuel travels from the tank to the fueling station and into the truck.

The fuel transfer system is a series of plumbing and valves that permits movement of fuel from one tank to another on board the aircraft. In-tank fuel boost pumps move the fuel into a manifold and, by opening the fuel valve (or refueling valve) for the desired tank, the fuel is transferred. Not all jet transports have such fuel transfer capability. Through the use of a fuel feed manifold and crossfeed valves, some aircraft simply allow engines to be run off fuel from any tank as a means for managing fuel location. Dedicated transfer boost pumps move fuel into a transfer manifold. Opening the fuel valve on one of the tanks transfers the fuel into that tank. The transfer manifold and boost pumps are also used to jettison fuel overboard by opening the proper dump valves with a transfer boost pump(s) operating.

Additionally, the transfer system can function to supply the engines if the normal engine fuel feed malfunctions. category aircraft allows all fuel tanks to be filled from one position. The fuel feed subsystem is sometimes considered part of the fuel distribution system. It is the heart of the fuel system since it delivers fuel to the engines. Jet transport aircraft supply fuel to the engines via in-tank fuel boost pumps, usually two per tank. They pump fuel under pressure through a shutoff valve for each engine. A manifold or connecting tubing typically allows any tank to supply any engine through the use of crossfeed valves. Boost pump bypasses allow fuel flow should a pump fail. The engines are designed to be able to run without any fuel boost pumps operating. But, each engine’s shutoff valve must be open to allow flow to the engines from the tanks.

Most jet transport fuel feed systems, or the engine fuel systems, have some means for heating the fuel usually through an exchange with hot air or hot oil taken from the engine. Figure 14-23 shows the fuel cooled oil cooler (FCOC) on a Rolls Royce RB211 engine, which not only heats the fuel but also cools the engine oil. Fuel indicating systems on jet transport aircraft monitor a variety of parameters, some not normally found on general aviation aircraft. Business jet aircraft share many of these features. True fuel flow indicators for each engine are used as the primary means for monitoring fuel delivery to the engines. A fuel temperature gauge is common as are fuel filter bypass warning lights. The temperature sensor is usually located in a main fuel tank. The indicator is located on the instrument panel or is displayed on a multifunction display (MFD). These allow the crew to monitor the fuel temperature during high altitude flight in extremely frigid conditions. The fuel filters have bypasses that permit fuel flow around the filters if clogged. Indicator light(s) illuminate in the flight deck when this occurs.

Low fuel pressure warning lights are also common on jet transport aircraft. The sensors for these are located in the boost pump outlet line. They give an indication of possible boost pump failure. Fuel quantity gauges are important features on all aircraft. Indications exist for all tanks on a transport category aircraft. Often, these use a capacitance type fuel quantity indication system and a fuel totalizer as is discussed later in this chapter. The location of fuel instrumentation varies depending on the type of flight deck displays utilized on the aircraft. Helicopter Fuel Systems Helicopter fuel systems vary. They can be simple or complex depending on the aircraft. Always consult the manufacturer’s manuals for fuel system description, operation, and maintenance instructions.

14-16 APU master start Tank pump Tank pump No. 2 Tank Main MainRightLeft compartment compartmentcompartmentcompartment Manifold drainFuel quantity test & out fill Tank 1 Tank 3 Drain/Fill Fuel quantity indicator power normalFire control To engine No. 1 To engine No. 2 To APU To engine No. 3APU off Flight Norm agent arm engineer’s TOTAL FUEL QTY Eng 1 fire Eng 2 fire APU fuel Eng 3 fire lower press lo GROSS WT panel Alt Master caution Upper mainFuel Norm L fuel pump X-Feed discharge valve openpress to reset CB panelOverhead APU panel master start Fuel pump Flight 20 engineer’s 0 40 Run X-feed lower panel Fuel Master caution typical 3 TEMP Flight open dump –20 FUEL engineer’s °C OpenFlight engineer’s places – 40 – 60 Glareshield (2) upper panelupper panel Close Tank pump Tank pumpOver Overpress lo press loboard board Tank pump Tank pump Trans Fwd L. Aft R. Aft Trans Fwd Aft On On On On On Off Off Off Off Off No. 1 Tank No. 2 Tank No. 3 Tank Outboard Main Left Right Main Outboard compartment compartment compartment compartment compartment compartment Fuel manifold drain valve (typical 2 places) APT boost Flight engineer’s pump lower panel (typical Manifold drainNo. 1 & No. 3 tank) Fuel quantity Refueling/defueling test & out fill adapter (typical 4 places) Tank 1 Tank 3 To No. 1 Fuel tank To No. 2 Fuel tank Fuel valvefuel quantity indicator fuel quantity indicator Fuel valve Drain/FillFlight engineer’s openopen Normlower panel (typical 3 places) close Flight engineer’s Fuel Flight engineer’s lower Fuel lower panelsched panel (typical 2 places) sched Fuel storage Fuel dump Pressure switch On-Off valve (air) Float shutoff valve Crossfeed/Transfer On-Off valve (fluid) Float control switch Pressure operated shutoff Boast pump pressure Quantity transmitter Float control valve valve (normally closed) Electrically actuated Refuel/Defuel Temperature sensor Boost pump Check valve Mechanically actuated and lines are used to complete the fuel dump system, the refuel/defuel system, back-up fuel delivery system, and the fuel storage system.

LEGEND 14-17