HI OFF LOW FIGURE 4-11. Centrifugal fuel booster pump.
a large screening surface encased in a compact housing. Reinforcing the screen is a coarse, heavywire mesh.
Auxiliary Fuel Pumps The electrically driven centrifugal booster pump, shown in figure 4-I1, supplies fuel under pressure to the inlet of the engine-driven fuel pump. This type of pump is an essential part of the fuel system, particularly at high altitudes, to keep the pressure on the suction side of the engine-driven pump from becoming low enough to permit the fuel to boil. This booster pump is also used to transfer fuel from one tank to another, to supply fuel under pressure for priming when starting the engine, and, as an emergency unit, to supply fuel to the carburetor in case the enginedriven pump fails. To increase the capacity of the pump under emergency conditions, many pumps are equipped with a two-speed or variable-speed control so that the recommended fuel inlet pressure to the carburetor can be maintained. As a precautionary measure, the booster pump is always turned on during takeoffs and landings to ensure a positive supply of fuel.
The booster pump is mounted at the tank outlet within a detachable sump or is submerged in fuel at the bottom of the fuel tank. The seals between the impeller and the power section of the pump prevent leakage of fuel or fumes into the motor.
If any liquid or vapor should leak past the seal, it is vented overboard through a drain. As an added precaution in nonsubmerged-type pumps, air is allowed to circulate around the motor to remove dangerous fuel vapor.
As fuel enters the pump from the tank, a highspeed impeller throws the fuel outward in all directions at high velocity. The high rotational speed swirls the fuel and produces a centrifuge action that separates air and vapor from the fuel before it enters the fuel line to the carburetor.
This results in practically vapor-free fuel delivery to the carburetor and permits the separated vapors to rise through the fuel tank and escape through the tank vents. Since a centrifugal-type pump is not a positive-displacement pump, no relief valve is necessary.
Although the centrifugal type is the most common type of booster pump, there are still a few sliding-vane-type booster pumps in service.
This type, too, is driven by an electric motor.
Unlike the centrifugal type, it does not have the advantage of the centrifuge action to separate the vapor from the fuel. Since it is a positivedisplacement-type pump, it must have a reliet valve to prevent excessive pressure. Its construction and operation are identical to the enginedriven pump.
Hand Pump The hand, or wobble, pump is frequently used on light aircraft. It is generally located near other fuel system components and operated from the cockpit by suitable controls. A diagram of a wobble pump is shown in figure 4-12. When the handle attached to the central blade is operated, the low pressure created on the chamber below the upward moving blade, permits the incoming fuel pressure to lift the lower flapper and allows fuel to flow into this chamber. At the same time fuel flows through a drilled passageway to fill the chamber above the downward moving blade. As the blade moves downward, the lower flapper closes, preventing fuel from escaping back into the inlet line. The fuel below the downward moving blade flows through a passageway into another chamber and is discharged through an outlet flapper valve to the carburetor. The cycle is re- 85
Fuel outlet Flapper Blade Inlet fuel Discharge fuel Fuel inlet Booster pump pressure A. Balance line C. Pump inlet FICURE 4-12. Schematic diagram of a wobble B. Pump outlet D. Bypass valve pump.
FIGURE 4-14. Engine-driven fuel pump peated each time the handle is moved in either (bypass flow).
direction.
Engine-Driven Fuel Pump The purpose of the engine-driven fuel pump is to deliver a continuous supply of fuel at the proper pressure at all times during engine operation. The pump widely used at the present time is the positive-displacement, rotary-vane-type A schematic diagram of a typical engine-driven pump (vane-type) is shown in figure 4-13. Regardless of variations in design, the operating principle of all vane-type fuel pumps is the same.
= Booster pump pressure Eta Engine-driven pump pressure A. Balance line B. Pump outlet C. Pump inlet FIGURE 4-13. Engine-driven fuel pump (pressure delivery).
The engine-driven pump is usually mounted on the accessory section of the engine. The rotor, with its sliding vanes, is driven by the crankshaft through the accessory gearing. Note how the vanes carry fuel from the inlet to the outlet as the rotor turns in the direction indicated. A seal prevents leakage at the point where the drive shaft enters the pump body, and a drain carries away any fuel that leaks past the seal. Since the fuel provides enough lubrication for the pump, no special lubrication is necessary.
Since the engine-driven fuel pump normally discharges more fuel than the engine requires, there must be some way of relieving excess fuel to prevent excessive fuel pressures at the fuel inlet of the carburetor. This is accomplished through the use of a spring-loaded relief valve that can be adjusted to deliver fuel at the recommended pressure for a particular carburetor. Figure 4-13, shows the pressure relief valve in operation, bypassing excess fuel back to the inlet side of the pump. Adjustment is made by increasing or decreasing the tension of the spring The relief valve of the engine-driven pump is designed to open at the set pressure regardless of the pressure of the fuel entering the pump. To maintain the proper relation between fuel pressure and carburetor inlet air pressure, the chamber above the fuel pump relief valve is vented either to the atmosphere or through a balance line to carburetor air inlet pressure. The combined pressures of spring tension and either atmospheric or 86
carburetor inlet air pressure determine the absolute pressure at which the relief valve opens. This balanced-type relief valve has certain objectionable features that must be investigated when encountering fuel system troubles. A sylphon or diaphragm failure will allow air to enter the fuel on the inlet side of the pump if the pump inlet pressure is less than atmospheric. Conversely, if the pump inlet pressure is above atmospheric pressure, fuel will be discharged from the vent.
For proper altitude compensation the vent must be open. If it should become clogged by ice or foreign matter while at altitude, the fuel pressure will decrease during descent. If the vent becomes clogged during ascent, the fuel pressure will increase as the altitude is increased.
In addition to the relief valve, the fuel pump has a bypass valve that permits fuel to flow around the pump rotor whenever the pump is inoperative. This valve, shown in figure 4-14, consists of a disk that is lightly spring-loaded against a series of ports in the relief valve head. When fuel is needed for starting the engine, or in the event of engine-driven pump failure, fuel at booster-pump pressure is delivered to the fuel pump inlet. When the pressure is great enough to move the bypass disk from its seat, fuel is allowed to enter the carburetor for priming or metering.
When the engine-driven pump is in operation, the pressure built up on the outlet side of the pump, together with the pressure of the bypass spring, holds the disk on its seat and prevents fuel flow through the ports.
Valves Selector valves are installed in the fuel system to provide a means for shutting off the fuel flow, for tank and engine selection, for crossfeed, and for fuel transfer. The size and number of ports (openings) vary with the type of installation, For example, a single-engine aircraft with two fuel tanks and a reserve fuel supply requires a valve with four ports--three inlets from the tanks and a common outlet. The valve must accommodate the full flow capacity of the fuel line, must not leak, and must operate freely with a definite "feel" or "click" when it is in the correct position.
Selector valves may be operated either manually or electrically. A tube, rod, or cable is attached to a manually operated valve so that it can be operated from the cockpit. Electrically operated valves have an actuator, or motor. The three main types of selector valves are the poppet, cone, and disk.
The poppet-type selector valve has an individual poppet valve at each inlet port. A cam and yoke on the same shaft act to open the selected poppet valve as the yoke is turned. Figure 4-15 shows how the cam lifts the upper poppet valve from its seat when the control handle is set to the "number 2" tank. This opens the passage from the "number 2" tank to the engine. At the same time, a raised portion of the index plate drops into a notch in the side of the cam. (See the detail of the index mechanism.) This produces the "feel" that indicates the valve is in the wide open position. The control handle should always be set by "feel" rather than by the marking on the indicator dial. The index mechanism also keeps the valve in the desired position and prevents creeping caused by vibration. Some valves have more than one raised portion on the cam to allow two or more ports to be opened at the same time.
The cone-type selector valve has either an allmetal or a cork-faced aluminum housing. The cone, which fits into the housing, is rotated by means of a cockpit control. To supply fuel from the desired tank, the cockpit control is turned until the passages in the cone align with the correct ports in the housing. An indexing mechanism aids in obtaining the desired setting and also holds the cone in the selected position. Some cone-type valves have a friction release mechanism that reduces the amount of turning torque required to make a tank selection and that can be adjusted to prevent leakage.
The rotor of the disk-type selector valve fits into a cylindrical hole in the valve body. A disktype valve is shown in figure 4-16. Note that the rotor has one open port and several sealing disksone for each port in the housing. To select a tank, the rotor is turned until the open port aligns with the port from which fuel flow is desired. At this time, all other ports are closed by the sealing disks. In this position, fuel will flow from the desired tank to the selector valve and out through the engine-feed port at the bottom of the valve.
To ensure positive port alignment for full fuel flow, the indexing mechanism (shown in the center of figure 1-16 forces a spring-loaded ball into a ratchet ring. When the selector valve is placed in the closed position, the open port in the rotor is opposite a blank in the valve body, while each sealing disk covers a tank port.
Reserve tank From tank 1 Jam Index plate 'Tank No. 2 From reserve To main strainer tank From tank 2 To main strainer The index mechanism Tank No. 1 FIGURE 4-15. Poppet-type selector valve.
Fuel tank shutoff valves have two positions, open and closed. They are installed in the system to prevent fuel loss when a fuel system component is being removed or when a part of the system is damaged. In some installations they are used to control the fuel flow during fuel transfer. They are operated either manually or electrically. An electrically operated fuel shutoff valve includes a reversible electric motor linked to a sliding-valve assembly. The motor moves the valve gate in and out of the passage through which the fuel flows, thus, shutting off or turning on the fuel flow.
FUEL SYSTEM INDICATORS Fuel Quantity Gages Fuel quantity gages are necessary so that the operator may know the quantity of fuel remaining in the tanks during operation of the aircraft. The four general types of fuel gages are: (1) Sight glass, (2) mechanical, (3) electrical, and (4) electronic. The type of fuel gage installation depends on the size of the aircraft and the number and location of the fuel tanks. Since the sight glass and mechanical fuel gages are not suitable for aircraft where tanks are located an appreciable distance from the cockpit, larger aircraft use either electrical or electronic fuel quantity gages. On some aircraft, one fuel gage, called a totalizer, indicates the total amount of fuel remaining in all the fuel tanks.
The sight glass is the simplest form of fuel quantity gage. The indicator is a glass or plastic tube placed on the same level as the tank. It operates on the principle that a liquid seeks its Valve body mechanism Rotor Sealing disk Index mechanism FIGURE 4-16. Disk-type selector valve.
Float FIGURE 4-17. Float-and-lever type fuel level gage.
own level. The tube is calibrated in gallons or has a metal scale near it. The sight glass may have a shutoff valve so that the fuel can be shut off for cleaning and for preventing loss of fuel if the tube is broken.
The mechanical-type fuel quantity gage is usually located in the tank and is known as a direct reading gage. It has an indicator connected to a float resting on the surface of the fuel. As the fuel level changes, the float mechanically operates the indicator, thus showing the level of fuel in the tank. One type of mechanical fuel gage is illustrated in figure 4-17.
The electrical-type quantity gage consists of an indicator in the cockpit and a float-operated transmitter installed in the tank. As the fuel level changes, the transmitter sends an electric signal to the indicator, which shows the changing fuel level. Two important advantages of this fuel quantity gage (and the electronic type discussed in the next paragraph) are that the indicator can be located any distance from the tank and the fuel levels of several tanks can be read on one indicator.
The electronic-type (capacitance) fuel quantity gage differs from the other types in that it has no movable devices in the fuel tank. Instead of floats and their attendant mechanical units, the dielectric qualities of fuel and air furnish a measurement of fuel quantity. Essentially, the tank transmitter is a simple electric condenser. The dielectric (or nonconducting material) of the condenser is fuel and air (vapor) above the fuel.
The capacitance of the tank unit at any one time will depend on the existing proportion of fuel and vapors in the tank. The capacitance of the transmitter is compared to a reference capacitor in a rebalance-type bridge circuit. The unbalanced signal is amplified by the voltage amplifiers that drive a phase discriminating power stage. The output stage supplies power to one phase of a two-phase a.c. motor that mechanically drives a rebalancing potentiometer and indicator pointer.
The electronic type system of measuring fuel quantity is more accurate in measuring fuel level, as it measures the fuel by weight instead of in gallons. Fuel volume will vary with temperature (a gallon of gasoline weighs more when it is cold than when it is hot); thus, if it is measured in pounds instead of gallons, the measurement will be more accurate.
In addition to the cockpit fuel quantity indicating system, some aircraft are provided with a means to determine the fuel quantity in each tank when the aircraft is on the ground. This is accomplished in several different ways. Some manufacturers use float-operated, direct-reading fuel gages mounted in the lower surface of the wing. Another means is to use under-wing bayonet gages. There are two types in use, the drip gage and the sight gage.
When using the drip gage it is necessary to proceed slowly, using the trial-and-error method to find the exact fuel level. In large area tanks a proportionately large amount of fuel is represented by a fraction of an inch variation in fuel level.
The long, hollow drip tubes require some time to drain once they are filled with fuel, and a substantial error in reading will be made if the diminishing drainage drip is mistaken for the steady drip that signifies that the tube is properly po- When the cap and hollow drip tube are drawn out from the lower wing surface, the fuel enters the open top of the tube when it reaches the level of the fuel. As stated previously steady drip from a drip hole signifies that the tube is properly positioned with a tiny head of fuel above the opening. The drip gage tube may be calibrated in pounds or inches. When calibrated in inches, the reading is compared with a special chart to give a reading of fuel quantity in gallons.
The sight gage is somewhat simpler in construction than the drip gage, and offers unmistakable visual evidence when it is properly positioned for reading. As shown in figure 4-18, the sight gage is basically a long lucite rod, protected by a calibrated tube, which terminates at the top in an exposed quartz tip. When the tip is above the fuel it acts as a reflector. Light rays traveling up the lucite rod are deflected at right angles by 89
Fuel Flowmeter Quartz tip (May be either conical The fuel flowmeter is normally used only in or chisel point) multiengine aircraft. The system consists of a transmitter and an indicator. The transmitter is Lucite rod installed in the fuel inlet line to the engine, where Permanently it measures the rate of fuel flow. The transmitter mounted is electrically connected to the indicator located Protective tube housing in the cockpit. This gage shows the rate of fuel (Has embossed consumption in pounds per hour.
calibration marks) The transmitter signal may be developed by a Removable guide movable vane mounted in the fuel flow path.
housing The impact of fuel causes the vane to swing and move against the restraining force of a calibrated spring. The final position assumed by the vane represents a measure of the rate at which fuel is Latch spring passing through the flowmeter and the corresponding signal to be sent to the indicator. A vane-type fuel flowmeter system is illustrated in figure 4-19.
The transmitter used with turbine engines is the mass-flow type having a range of 500 to 2,500 Read tank quantity at this point Latching lug and pin, Lower surface of wing typical three places (Retains bayonet Sight in stowed window position) FIGURE 1-18. Under-wing sight gage.
the 45° surface at one side of the tip and deflected 90° again by the 45° surface at the opposite side and returned down the lucite rod.
Any portion of the tip submerged in fuel will not act as a reflector. Consequently, when the fuel level is part way up the taper, a light pattern is created that is visible at the lower end of the lucite rod and that has the dimension and shape described by the intersection of the tip and the fuel. When the reflected light is reduced to the smallest perceptible point in the case of conetipped gages, or hairline in the case of chisel-tipped gages, the rod is properly positioned. The fuel tank quantity can be read on the tube where it emerges from the recessed guide housing. Drip gage readings are taken at this location also.
pounds per hour. It consists of two cylinders placed in the fuel stream so that the direction of fuel flow is parallel to the axes of the cylinders.
(See figure 4-20.) The cylinders have small vanes in the outer periphery. The upstream cylinder, called the impeller, is driven at a constant angular velocity by the power supply. This velocity imparts an angular momentum to the fuel. The fuel then transmits this angular velocity to the turbine (the downstream cylinder), causing the turbine to rotate until a restraining spring force balances the force due to the angular momentum of the fuel. The deflection of the turbine positions a magnet in the second harmonic transmitter to a position corresponding to the fuel flow. The turbine position is transmitted to the flight station indicator by means of a selsyn system.
Fuel Pressure Gage The fuel pressure gage indicates the pressure of the fuel entering the carburetor. This gage may be included with the oil pressure gage and the oil temperature gage in one casing, called the engine gage unit. Most aircraft today have separate gages for these functions. An engine gage unit is shown in figure 4-21.
The fuel pressure gage is a differential pressure indicator with two connections on the back of the indicator housing. The air connection (see figure 4-22) is vented to the carburetor air inlet, and the fuel connection is attached to the fuel 90
26 V 400 - AC Transmitter Fuel flow — Restraining spring FIGURE 4-19. Vane-type fuel flowmeter system.
inlet chamber of the carburetor. In this way the gage indicates the difference between the fuel pressure entering the carburetor and the air pressure at the carburetor air inlet. In some installations, the air fitting on the gage is left open to the air pressure of the cockpit, which is generally the same as the pressure of the atmosphere. When this venting arrangement is used, the relief valve of the engine-driven fuel pump is also vented to the atmosphere, and the gage indicates the fuel pressure resulting from the adjusted spring pressure only. In order to dampen pressure pulsations that cause pointer fluctuation, a restrictor fitting (A) is installed at the carburetor end of the fuel gage line. (See the Y connection shown in figure
4-22.) The second restrictor (B) meters fuel to the oil system during oil dilution. The arrangement of these restrictors provides an indicated drop in fuel pressure when the oil dilution system is used. The oil-dilution system will be discussed Decoupling disk Fuel flow Second harmonic transmitter Impeller motor Calibrated restraining Fluid passage Fluid passage springs | 115VAC Timing contacts B 00 28VDC Indicator Power supply FIGURE 4-20. Schematic of a turbine engine fuel flow indicating system.
200 200 20 FUEL / 11 FIGURE 4-21. Engine gage unit.
thoroughly in the Powerplant Handbook, and is mentioned at this time only because the fuel pressure indicator provides a means for a check on the operation of other fuel system units.
In small aircraft the fuel pressure gage may be actuated by a Bourdon tube (an instrument that converts changes in pressure to mechanical motion), or an aneroid and bellows type, installed with a pressure line leading directly from the carburetor to the indicator. On larger aircraft, where the fuel pressure gage is located some distance from the carburetor, a transmitter is usually installed. The pressure transmitter may be a simple cast metal cell that is divided into two chambers by a flexible diaphragm. Pressure applied by the fuel source to the transmitter inlet pushes against the diaphragm and builds up an equal pressure to a thin fluid (highly refined kerosene), which transfers the pressure to the indicator mechanism. Some installations, however, use electrical transmitters to register fuel pressure on the gage. In this electrical arrangement, the pressure-indicating unit is contained in the transmitter. Fuel pressure, acting upon the aneroid and bellows portion of the unit, causes motion of one part of an electrical unit (the synchro transmitter). As the unit turns, it causes a similar movement of a corresponding unit (the synchro motor). This receiving unit actuates the indicator on the instrument panel. These pressure and electrical arrangements make it unnecessary for combustible fuel to enter the cockpit or flight deck, thereby reducing fire risk.
A fuel pressure gage often used with fuel injection systems on light aircraft engines is illustrated in figure 4-23. A gage of this type registers metered fuel pressure at the fuel injection unit distributor valve and is a direct indication of engine power output when installed in a fuel injection system for light aircraft engines. The dial of the gage is marked to indicate percent of power. The gage does not indicate either the engine-driven pump or the boost pump pressure.
Pressure Warning Signal In an aircraft with several tanks, there is always the possible danger of allowing the fuel supply in one tank to become exhausted before the selector valve is switched to another. To prevent this, pressure-warning signals are installed in some aircraft. The complete installation, shown in figure 4-22, consists of a pressure-sensitive mechanism and a warning light. The warning mechanism has both a fuel and an air connection.
The connection marked "fuel is connected to the fuel pressure line of the carburetor. The air connection is vented to either atmospheric or carburetor air inlet pressure. This arrangement prevents the warning mechanism from acting in response to changes in the absolute pressure of the fuel. If, for example, the absolute pressure of the fuel decreases because of a change in atmospheric or carburetor air inlet pressure, the change is also reflected at the warning mechanism, which then cancels the effects of the change. Normal fuel pressure against the power surface of the diaphragm holds the electrical contacts apart. When the fuel pressure drops below specifed limits, the contacts close and the warning light is turned on.
This alerts the operator to take whatever action is necessary to boost the fuel pressure.
Valve-In-Transit Indicator Lights On large multiengine aircraft, each of the fuel crossfeed and line valves may be provided with a valve-in-transit indicator light. This light is on only during the time the valve is in motion and is off when movement is complete.
Fuel Temperature Indicator A means for checking the temperature of the fuel in the tanks and at the engine is provided on some turbine-powered aircraft. During extreme cold, especially at altitude, the gage can 92
Air scoop › Warning light Electrical connections to warning light Pressure gage Warning mechanism Contacts Adjustment screw To oil Y connection Carburetor dilution Diaphragm Inlet from valve Air Fuel tuel pump Location of units in the system Pressure waming mechanism FIGURE 4-22. Fuel pressure-indicating system.
be checked to determine when fuel temperatures are approaching those at which there may be danger of ice crystals forming in the fuel.
MULTIENGINE FUEL SYSTEMS The design of the fuel system for an aucraft having two or more engines presents problems not normally encountered in single-engine fuel systems. A large number of tanks are often required to carry the necessary fuel. These tanks may be located in widely separated parts of the aircraft, such as the fuselage and the inboard and outboard sections of the wings. The individual engine fuel systems must be interconnected so that fuel can be fed from the various tanks to any engine. In case of engine failure, the fuel normally supplied to the inoperative engine must be made available to the others.
Crossfeed System The twin-engine fuel system illustrated in figure 4-24 is the simple crossfeed type. As shown, the tank selector valves are set to supply fuel from the main tanks to the engines. These valves can also be positioned to supply fuel from the auxiliary tanks. The crossfeed valve is shown in the off position. It can also be set to supply fuel from the fuselage tank to either or both engines and to crossfeed. A few of the numerous combinations in which the three valves can be set are also illustrated.
Manifold System The main feature of the four-engine system % 45 55 shown in figure 4-25 is the fuel manifold. This FUEL CRUISE POWER 65 fuel manifold system is actually a variation of the crossfeed. As shown, fuel is being supplied from PRESSURE the main tanks directly to the engines. The mani- PSE fold valves can also be set so that all tanks feed 7000' into the manifold and each engine receives its fuel 5000 supply from this line. The auxiliary fuel supply SEA 3000' can be delivered to the engines only through the LEVEL manifold. The main advantage of this system is its flexibility. Should an engine fail, its fuel is 16 immediately available to the other engines. If a tank is damaged, the corresponding engine can be supplied with fuel from the manifold.
Another advantage of this system is that all FIGURE 4-23. Fuel pressure gage for fuel-injection fuel tanks can be serviced at the same time system. through a single line manifold connection. This 93
Left engine Right engine Fuselage tank Engine-driven pump Filter Crossfeed valve Main tank Auxiliary Auxiliary Main tank tank tank Boost pump Tank selector valve Main Aux. Main Aux. Main Aux sam Aux. Main Auxiliary tanks Right main tank Fuselage tank Fuselage tank to engines to both engines to left engine to both engines FIGURE 4-24. Twin-engine crossfeed system schematic.
method of fuel servicing has greatly reduced servicing time on large aircraft because fuel can be introduced into the fueling manifold under high pressure.
FUEL JETTISON SYSTEMS A fuel jettison system is required for transport category and general aviation aircraft if the maximum take-off weight exceeds the maximum landing weight. The maximum take-off and landing weights are design specifications and may be found in the Aircraft Type Certificate data sheets.
A fuel jettison system must be able to jettison enough fuel within 10 minutes for general aviation, or 15 minutes for transport category aircraft, to meet the requirements of the specifications and Federal Air Regulations. It must be operable under the conditions encountered during all operations of the aircraft.
Design requirements are that fuel jettisoning must be stopped with a minimum of fuel for 45 minutes of cruise at maximum continuous power for reciprocating engines. Turbine powered aircraft require enough fuel for take-off and landing and 45 minutes cruising time.
The fuel jettisoning system is usually divided into two separate, independent systems, one for each wing, so that lateral stability can be maintained by jettisoning fuel from the "heavy" wing if it is necessary to do so. Normally, if an unbalanced fuel load exists, fuel will be used from the "heavy" wing by supplying fuel to engines on the opposite wing.
The system consists of lines, valves, dump chutes and chute-operating mechanisms. Each wing contains either a fixed or an extendable dump chute depending upon system design. In either case the fuel must discharge clear of the airplane.
TROUBLESHOOTING THE FUEL SYSTEM In order to become proficient at the art of troubleshooting, one must be familiar with the complete system. To do this, one can become familiar with the schematics of various portions of the system, the nomenclature of the units, and their particular function within the system by studying aircraft and engine maintenance manuals.
Engine Engine Engine Engine No. 1 No. 2 No. 3 No. 4
• Filter Auxiliary tank Engine-driven Firewall Manifold Refueling pump shut-off valve • valves receptacle Tank Fuel manifold shut-off valve No. 1 tank No. 2 tank Auxiliary tank No. 3 tank No. 4 tank Boost pump Flow to engine Flow blocked FIGURE 4-25. A typical manifold crossfeed system.
Location of Leaks and Defects The location of leaks and defects within the internal portions of the fuel system is usually a matter of observation of the pressure gage and operation of the selector valves to determine where the trouble lies. Troubleshooting of the internal fuel system can be aided by visualizing the path of flow of the fuel from the fuel tank to the fuel-metering device, noting the location of the pump(s), selector valves, emergency shutoff valves, etc.
The location of leaks or defects in the external portions of the fuel system involves very little time in comparison to locating leaks within the internal system. Usually, fuel leaks are evidenced by stains or wet spots, if they are newly developed, and by the presence of fuel odor. The plumbing, clamps, gaskets, supports, etc., are to be examined carefully at each inspection period. Any defect or leak in the internal or external fuel system is a potential hazard.
Replacement of Gaskets, Seals, and Packings In order to prevent leakage of fuel, it is of utmost importance that all gaskets, seals, and packings be properly installed. Listed below are some of the general precautions that should always be observed.
When replacing units of the fuel system, it is necessary to check each part for cleanliness, ensure that all of the old gasket material is removed, and ensure that none of the old seal remains in the groove seat. Always replace old gaskets and seals with new ones, check the new gaskets and seals for cleanliness and integrity, and ensure that it is the right part for the job. Mating surfaces should be perfectly flat so that the gasket can do the job for which it is designed. Screws, nuts, and bolts that hold units together should be evenly tightened or torqued to prevent leakage past the gasket or seal.
FUEL TANK REPAIRS There are three basic type fuel cells used in aircraft. Welded sheet metal integral and fuel cell.
No fuel system is airworthy if it will not contain fuel. Inspection of the fuel tank bays or aircraft structure for evidence of fuel leaks is a very important part of the preflight inspection.
Welded Steel Tanks Welded tanks are most common in the smaller single and twin engine aircraft. If the access plates to the fuel tank compartment are discolored the tank should be inspected for leaks. When leaks are found, the tank must be drained and inerted. Fuel will be drained in accordance with 95
local instructions and the manufacturer's recommendations. Inerting the tank may be accomplished by slowly discharging a CO, fire extinguisher (5 lb. minimum size) into the tank.
Dry nitrogen may be used if it is available. If the tank is to be welded, removal is necessary.
Before welding, the tank must be steamed for a minimum of 8 hours. This is to remove all traces of fuel. Air pressure not over ½ psi may be used to detect the leaking area. Liquid soap or bubble solution brushed in the suspected area may identify the leak. Aluminum tanks are fabricated from weldable alloys. After riveting patches in place, the rivets may be welded to insure no leaks from that area. Pressure checks should be performed after repairs are completed to assure that all leaks were corrected.
Fuel Cells Fuel cell leaks will usually appear on the lower skin of the aircraft. A fuel stain in any area should be investigated immediately. Fuel cells suspected of leaking should be drained, removed from the aircraft and pressure checked. When performing a pressure check, ¾ to ½ psi air presure is adequate. All fuel cell maintenance must be accomplished in accordance with the manufacturer's specifications.
3/4 DIA 1-1/2 DI/
A - SLOW SEEP
B - SEEP Integral Fuel Tanks The integral tank is a nonremovable part of the aircraft. Because of the nature of an integral tank, some leaks allow fuel to escape directly to the atmosphere. This makes it completely feasible to disregard certain minute leaks that do not represent a fire hazard or too great a loss of fuel. In order to standardize the procedures for integral tank fuel storage maintenance, the various rates of fuel leakage are classified.
Fuel Leak Classification The size of the surface area that a fuel leak moistens in a 30-minute period is used as the classification standard. Wipe the leak area completely dry with clean cotton cloths. Compressed air may also be used to dry the leak areas that are difficult to wipe. Wear goggles when using compressed air to dry the leak area. Dust the leak area with dyed red talcum powder. The talcum powder turns red as the fuel wets it, making the wet area easier to see.
At the end of 30 minutes, each leak is classified into one of four classes cake The four classes of heavy seep, or running leak.
leaks are shown in figure 4-26. A slow seep is a leak in which the fuel wets an area around the
- AREA WHERE FUEL
APPEARS TO FLOW
OR RUN, FOLLOWING
CONTOUR OF SKIN
WHEN THIS AREA IS WIPED DRY.
C - HEAVY SEEP
D - RUNNING LEAK FIGURE 4-26. Fuel leak classification.