TM 1-407, Aircraft Induction, Fuel and Oil Systems, 1941: Section 4 - Fuel Systems

SECTION IV: FUEL SYSTEMS

21. General.-a. A well-designed fuel System must provide for the storage of the required amount of fuel in the available space within the airplane structure, and for the delivery of the fuel to the carburetor at the proper rate and pressure. The system must be positive and reliable under all conditions of flight and, if possible, simple in operation. Indicators, such as the fuel-pressure gage and tank-contents gages, are installed to give a continuous indication of the functioning of the system.

b. The continued development of military airplanes has been accompanied by many new problems relative to fuel systems. The practice of installing numerous small tanks rather than a few larger ones has resulted in increased plumbing and complication, but, has also permitted a more efficient use of the available fuel storage space. The use of external superchargers also increases the demands on the fuel system, especially in regard to pump capacity at high altitudes. The trend of late developments in carburetors is also toward increased pressures for the delivery and discharge of the fuel. It is obvious that the fuel system of a modern airplane is a very complicated arrangement requiring careful installation, adjustment, and inspection.

22. Basic fuel-system circuits.-Since each particular fuel system has certain special features, it is necessary to begin the discussion by considering first the simpler designs.

a. The gravity fuel system (fig. 20) though elementary in design is still in use on a number of training-type airplanes. The advantages

FIGURE 20-Gravity fuel-feed system.

are simplicity and reliability, but this system cannot be used on tactical airplanes because of structural arrangement and higher pressure requirements. The actual pressure available from a gravity system can be calculated as approximately 1 pound per square inch for each 40 inches head of fuel. Thus, it may be estimated that in order to produce a delivery pressure of 3 pounds per square inch, a vertical head of 120 inches of fuel is necessary.

b. Figure 21 is a schematic diagram of a complete fuel system of a relatively simple type. Certain additional units have been intentionally omitted in order that the basic principles can be more clearly indicated. In tracing the fuel flow, attention is called to the location of the main fuel strainer and the arrangement of the two fuel pumps with a single relief valve. The use of an air vent line from the air intake to the pressure gage and relief valve is also significant. The direction of fuel flow from supply tanks to the carburetor is clearly indicated by arrows near the fuel lines. The exact pressure generated by the fuel pump depends on the adjustment of the relief valve and may be from 3 to 15 pounds per square inch according to the type of carburetor used on the engine. In general, the proper fuel pressure for a float type carburetor or mechanical fuel injector is approximately 3 pounds per square inch; for a diaphragm type variable venturi carburetor, 5 or 6 pounds; and for a pressure injection carburetor, 13 to

FIGURE 21-Pressure fuel-feed system.

15 pounds. In all cases, these figures represent the desired differential fuel pressure, and the pressure gage is connected so as to indicate this difference. In many airplanes the carburetor air pressure is appreciably higher than the normal atmospheric pressure, the increase being delivered by a ramming air intake or by an external supercharger.

23. Fuel-system units.-In order to clarify the principles of operation of complex airplane fuel systems, the various units are discussed separately in the following paragraphs:

a. Tanks.-Fuel tanks are usually constructed of aluminum, aluminum alloy, or stainless steel. The general design is indicated in figure 22, but the shape is variable, depending on the airplane for which it is designed.

FIGURE 22-Sectional view of fuel tank.

Large tanks require internal baffles for increased rigidity and to prevent objectionable surging of the fuel in flight. In order to counteract corrosion in aluminum and aluminum alloy tanks, a small capsule, containing potassium dichromate is installed near the bottom of the tank.

(1) The fillernecks of fuel tanks are so installed that an expansion space is automatically provided when the tank is serviced. A vent line from the top of the tank leads overboard so as to reduce the danger of fire from fuel or vapors which may be discharged. It is highly important that this vent line be properly installed and free from any obstruction.

(2) Most fuel systems provide a reserve fuel supply, either by the use of a separate reserve tank or by the arrangement shown in figure 22. In either case, the reserve fuel supply is adequate to operate the engine for at least 20 minutes at full rated power.

(3) Some airplanes, especially long-range types, may be equipped with special fuel tanks in order to increase the cruising range for a particular mission. When such tanks are serviced, attention must be given to the total weight of the airplane in order to prevent overloading.

(4) Water may occasionally be found in fuel tanks, either as a result of condensation or careless servicing, therefore, regular drainage of the tank sumps is most important.

b. Lines and fittings.-Fuel lines are generally made from copper or annealed aluminum alloy tubing, the latter being more common on late-model airplanes. The tubing size is governed by the fuel-flow requirements of the engine. Except for the lines between flexible connections, tubing should be properly supported by clamping to structural members. A band of red paint near each end serves to identify a fuel line. Connections between tubing and fuel-system units are made by means of pipe fittings, solderless flared tube fittings, hose connections, or by a combination of these methods.

(1) Standard pipe thread fittings are shown in figure 23. It will be noted that all threads on these fittings are tapered (3/4 inch per foot), and therefore when screwed together make a leak proof connection by the sealing action of the threads. When installing these fittings, no attempt should be made to turn them until the threads are completely engaged. Permanent joint compounds are not required and are generally forbidden. Approved lubricants which do not harden will, however, be found beneficial in making pipe connections. It must be understood that these materials function purely as lubricants and not as sealing compounds for improperly installed fittings.

Many fuel-system units, such as fuel tanks, fuel cocks, strainers, pumps, and carburetors, are provided with female pipe threads for the installation of various line fittings.

FIGURE 28-Common pipe fittings used in airplane plumbing.

(2) For connecting thin-walled tubing to pipe fittings, the threepiece solderless fittings (fig. 24) are commonly employed. To make this type of connection, the tube is cut to proper length and a nut and sleeve are placed over the tube.

FIGURE 24.-Flared-tube union.

The tube is then flared at the open end by an appropriate tool, and the nut is screwed on a nipple, elbow, or similar fitting. In this case it must be noted that the seal is between the tube and fitting, at the flare, not in the threads. A small amount of lubricant on the threads will facilitate tightening, but the use of heavy sealing compounds is unnecessary and possibly dangerous.

(3) Wherever there is relative motion between two points with a connecting line, a flexible hose connection (fig. 25) is used at each point.

FIGURE 25-Hose connection,

(4) Cone union fittings which are attached to copper tubing by high melting point solder may be used in fuel systems, but the solderless type is generally preferable.

c. Cocks and dials.-Fuel cocks have a number of uses in airplane fuel systems. They perform such functions as tank selectors, engine selectors, cross feed valves, etc. The size and number of ports of fuel cocks naturally vary according to the type of installation for which they are intended. Fuel cocks must have the full flow capacity of the fuel lines, must be free from leakage, and at the same time operate with little effort. These requirements have produced considerable development on these units, with the result that satisfactory designs are now available.

(1) Fuel cocks having a cone-shaped cock rotor have been used quite extensively in fuel systems and have been fairly satisfactory. The principal disadvantage of this type is the high torque required for operation, resulting in a rather uncertain feel of the operating control. An improved model uses a cam arrangement for unseating the cone between index positions, thus improving the action. External clearance adjustments are provided on this type which must be maintained within proper limits.

(2) A metal cock with synthetic rubber inserts around the ports is also available. This type is simple, operates freely, and provides good sealing properties.

(3) The dial used in conjunction with a fuel cock has the proper number of positions for the particular installations. A diagram of a fuel cock and dial is shown in figure 26.

d. A.Strainers.-The main strainer installed in a fuel system is a most important item. Its function is to prevent dirt or other foreign matter from entering the fuel pump and carburetor, and by virtue of its low position to trap any small amount of water which may be present in the system.

FIGURE 26.-Fuel cock and indicator dial.

A drain is provided at the bottom for frequent draining, and the entire screen may be easily removed for cleaning Figure 27 shows a sectional view of a common fuel strainer

FIGURE 27.-Fuel strainer.

(1) Strainers of fairly coarse mesh are installed in the fuel tanks on the outlet connections as shown in figure 22. Their function is quite obvious and-requires no particular comment.

(2) The carburetor body generally incorporates a small strainer of sufficient capacity to stop foreign matter which passes through the main strainers as well as particles which are introduced when fuel lines are removed. The carburetor strainer usually requires less frequent cleaning than the main strainer.

e. Pumps and relief valves.-Fuel pumps are required to furnish fuel prior to starting an engine and to deliver a continuous supply at the proper pressure at all times during engine operation. The various pump types are listed separately as follows:

(1) The hand pump (often called "wobble" pump) is generally located fairly near other fuel-system units and is operated by suitable levers from the pilot's compartment. The common design is very reliable and quite simple in operation. By reference to figure 28, the path of fuel flow is easily followed. It will be noted that the

FIGURE 28.-Hand fuel pump (wobble pump).

chambers which are not delivering fuel are being filled and will discharge when the handle motion is reversed. These pumps must be installed in a vertical position in order to secure proper action of the check valves. The inlet and outlet connections are, in most cases marked on the pump body.

(2) The most common engine-driven fuel pumps used in airplane fuel systems are of the eccentric sliding-vane type. Figure 29 shows the principle of operation of this type of pump. When the pump rotor is turned in the direction indicated, the vanes move the fuel from the inlet to the outlet of the pump. Since the pump is symmetrical about a vertical axis it will pump in either direction with equal efficiency. Reversing the direction of rotation has the same effect as changing the pump on the mount 180°. Most pumps of

FIGURE 29.-Engine-driven fuel pump (vane type).

this type are plainly marked to indicate the discharge port for either direction of rotation. At the point where the drive member enters the pump body it is necessary to provide a seal to prevent fuel leakage. The seal is generally a hardened steel member held by

FIGURE 30-Combination fuel pump, relief valve, and by-pass valve.

end pressure against one pump bearing. Very little trouble is experienced with this type seal in service; however, a drain line is provided to drain fuel in case the unit becomes worn or distorted.

(3) The action of a relief valve is best explained by considering its operation in conjunction with a fuel pump (fig. 30). In order to furnish a constant fuel supply to the carburetor under all conditions, a fuel pump is designed to deliver much more fuel at any speed than the engine actually requires. Therefore, a spring-loaded relief valve is placed parallel with the pump in order to relieve the surplus fuel to the intake side of the pump. By adjusting the spring tension in the relief valve, the differential pressure generated by the pump is accurately controlled. The bellows or diaphragm. incorporated in the relief valve is essential for two reasons: It provides for air venting from a ramming air intake or external supercharger, and by its balancing action helps to maintain constant discharge pressure,

FIGURE 31.-Variable-volume fuel pump.

regardless of variations in pressure on the suction side of the pump. When not connected to air pressure, the interior of the bellows or diaphragm is left open to atmospheric pressure through a small restricted fitting. When properly connected, a single relief valve will accommodate both the hand pump and engine-driven pump. A bypass feature, which permits fuel flow through the valve assembly, is also provided so that the unit may be used in varied fuel-system applications.

(4) In certain installations the escape of fuel through the pressure relief valve has aggravated vapor formation in the system with resultant loss of pumping efficiency. This effect has been most noticeable in systems using high-pressure carburetors and/or external superchargers when operating at high altitudes. The escape of fuel from the high pressure side of the pump through the relief valve often causes vaporization of fuel in the passages. To remedy this condition, a variable-volume fuel pump having no relief valve may be used. In this type of pump a heavy diaphragm, actuated by fuel pressure, regulates the position of the pump sleeve with respect to the rotor. Figure 31 illustrates the principle of the variable-volume pump. Note that the actuating force on the diaphragm is fuel discharge pressure against an adjustable spring. By this arrangement, the pump sleeve assumes the proper position to deliver the required volume of fuel to the carburetor at the proper pressure. Any variation in pressure from the correct setting will produce an immediate readjustment of the sleeve position. The space above the diaphragm is normally vented to the carburetor air intake pressure in order to maintain correct differential pressure. The general installation and hook-up of the variable-volume pump are quite similar to pumps having relief valves.

FIGURE 32.-Remote flexible drive-shaft assembly.

(5) The handling of volatile aircraft gasoline requires special care in the location of the fuel pump in the plumbing circuit. In general, the pump should be below the fuel tanks and as near to them as possible in order to avoid vapor locking difficulties. This arrangement often places the pump several feet from the engine and necessitates a remote pump drive.

(a) The most common pump drive is illustrated in figure 32. The essential units, are a flexible drive shaft and the necessary adapters and couplings. Although the drive assembly requires periodic inspection and lubrication, it has contributed to definite improvement in fuel pump performance. This type of drive is suitable only for installations in which the pump is located a relatively short distance from the engine.

(b) In larger airplanes the desired pump position may be several feet from the engine, and so a hydraulic pump drive is often installed. By reference to figure 33 the operating principles are easily understood.

The hydraulic circuit is entirely independent of other hydraulic units on the airplane. This arrangement permits a most favorable location of the fuel pump and consequently greater reliability in engine operation. An interesting modification of the hydraulic drive is the addition of an oil regulator to vary the flow of hydraulic fluid according to the fuel requirements of the engine. This variable speed hydraulic drive has been used quite effectively on certain airplanes. It is important to distinguish clearly between the variable speed fuel pump and the variable volume pump.

(c) Electric fuel pump drives are also entirely practical and may be used in certain airplanes. This method of drive offers the same advantages as the hydraulic type in regard to optional pump location and is somewhat easier to install. Obviously, only an explosion proof motor of special design is satisfactory for driving fuel pumps.

FIGURE 34.-Fuel pressure warning signal.

Warning signals.-(1) In airplanes having a number of fuel tanks there is some danger involved in allowing the fuel supply in one tank to become exhausted before switching the selector to another tank. Quite often the pilot may be engaged in other duties and fail to notice the low fuel supply until the engine misfires from a lean mixture. Under such circumstances immediate action is required in order to prevent engine failure. To eliminate this danger, fuel pressure warning signals are employed.

(2) The actuating mechanism of the fuel pressure warning signal is a diaphragm (or bellows) vented to fuel pressure on one side and to carburetor air pressure on the other side. The diaphragm is mechanically connected to electrical-contact points (fig. 34). The contact points are in turn connected to a source of power and to a red light in the pilot's compartment. During normal engine operation the fuel pressure is sufficiently high to keep the contact points open so that the light remains off. But when a serious loss of pressure occurs the points close, and the red light warns the pilot of the low-fuel pressure. This device is most effective when used in conjunction with carburetors which store a small amount of fuel in the supply chambers. The signal switch is installed very close to the carburetor and on the same level to eliminate pressure errors. Restricted fittings, such as those used in the fuel-pressure gage line, must not be installed between the carburetor and the, warning signal. Carburetors which require continuous pressure to discharge the fuel will cut out very quickly after the initial drop in fuel pressure, and therefore on such systems the warning signal does not give adequate warning of fuel system failure.

g. Pressure gage.-The fuel-pressure gage is a, differential pressure indicator with two connections on the case (fig. 35). The air connection

FIGURE 35.-Fuel-pressure gage (sectional view).

is vented to the carburetor air intake and the fuel connection is connected to the, fuel-pressure chamber of the carburetor. Thus, the gage, indicates the difference between the fuel pressure entering the carburetor and the air pressure at the carburetor inlet. In some systems having no external supercharger or ramming air intake, the air fitting on the gage is open to normal atmospheric pressure. In order to dampen pressure impulses which cause pointer fluctuation, a restricted fitting is installed at the carburetor end of the fuel-gage line. The correct fuel pressure value depends upon the type carburetor, but the values mentioned in paragraph 22a(2) are approximately correct. In this connection it must be remembered that the fuel-pressure gage indicates the pressure existing at the carburetor only if it is installed at the same level as the carburetor. If the gage is located above the carburetor, the reading will be lower than the carburetor pressure, and if the gage is below the carburetor it will read higher than normal. In certain airplanes, correction must be made for this factor in order to determine the exact fuel pressure at the carburetor inlet.

h. Primer pumps.-The standard primer pump is essentially a manually operated piston pump with inlet and outlet check valves (fig.36). Fuel for the primer may be supplied from almost any point in the fuel system. The fuel discharged from the pump is generally injected into a number of intake pipes on the engine, in most cases into the upper cylinder pipes on the radial type of engine. The procedure to be followed in priming engines varies according to the type and the atmospheric temperature, a greater priming charge being required in cold weather. When not in use the plunger should he placed in the "off" position to prevent fuel from flowing into the induction system when the engine is in operation.

i. Vapor eliminators.-In many fuel systems an appreciable amount of fuel vapor is often present in the fuel pressure line connected to the carburetor. The vapors are due chiefly to the action of the

FIGURE 36.-Engine primer.

mechanical pump, especially when the pressure on the pump inlet is quite, low. A float type, carburetor is not disturbed to any great extent by a small amount of fuel vapor, because the float chamber vents permit the vapor to escape. However, many high-pressure carburetors and mechanical fuel injectors will malfunction due to vapor formation. To remedy this condition vapor eliminators are employed. The, general internal mechanism of a vapor eliminator is shown in figure 37. The unit is normally installed in the pressure line immediately preceding the carburetor or injector. In operation, the upper part of the housing contains a small amount of vapor and the lower part contains fuel. When the vapor content increases, the float drops slightly thus opening the needle valve and permitting some vapor to escape. As the vapor escapes, the liquid level rises and the float closes the needle valve to prevent a loss of fuel. The needle valve operation is generally rather intermittent but entirely

FIGURE 37.-Vapor eliminator.

effective in disposing of fuel vapors. The outlet connection returns the vapor to one of the fuel tanks. Some carburetors are built with a small vapor eliminator as an integral part of the carburetor assembly.

j. Liquidometers.-Tank contents gages or liquidometers are practically all of the float type but differ in methods employed in transmitting the indication to the instrument panel. Three methods are in common use: direct mechanical type, hydraulic type, and electrical type (figs. 38, 39, 40). Adjustments are normally provided

FIGURE 38-Mechanical liquidometer.

on all types for both range of operation and pointer position in order that the indicator will be accurate throughout its scale. Each type has particular methods of adjustment so a general system cannot be given. Some designs employ a single indicator with a selector switch for a number of individual tank units.

FIGURE 39.-Hydraulic liquidometer.

FIGURE 40.-Electric liquidometer.

FIGURE 41-Single-engine fuel system.

FIGURE 42-Twin-engine fuel system.

24. Typical fuel-system circuits.-a. The essential units and circuits of a typical single engine airplane fuel system are illustrated in figure 41. This diagram is not intended to apply to any specific airplane but represents standard principles of fuel-system arrangement. Attention is called to the installation of the pressure gage and warning light switch. The system as shown is adaptable to airplanes having ramming air intakes and external superchargers.

b. The twin-engine fuel system (fig. 42) is essentially a pair of single engine systems plus the arrangement for interconnecting the two systems by a suction or pressure cross feed line. With this circuit it is possible to operate both engines with only one engine pump and to supply fuel to one or both engines from either set of fuel tanks. Continued addition of units and lines, using these same principles, will give fuel-system circuits for airplanes having more than two engines.

c. In many airplanes having a large number of small fuel tanks, the pilot must give frequent attention to the fuel selector valve and tank contents' gage. To relieve the pilot of this duty, much development has been accomplished in the design of completely automatic fuel systems. Such systems require very little attention in flight and are quite reliable in operation. Provisions are generally retained for manual control if the automatic feature is not desired.

25. Operation.-In addition to the specific data pertaining to individual type airplanes, the following instructions regarding the general operation of fuel systems should be observed:

a. During ground operation, the tank selector cock should be placed in all tank positions with the engine operating as a check on the fuel flow from each tank. The fuel pressure gage reading should be observed and the pressure adjusted if necessary. The signal light is checked for proper operation, and the liquidometer indication on all fuel tanks should be noted. In multi-engine airplanes each system is checked separately with the cross feed valve in the "off" position.

b. Take-off is accomplished with the fuel tank selector cock turned to a tank which is known to contain an adequate fuel supply. When both main and reserve fuel supplies are carried in the same tank, the fuel cock will be placed in the reserve position prior to take-off. When switching fuel tanks in flight, the selector cock is moved to the proper position, and if any fluctuation in fuel pressure is observed the hand pump may be used. Under no circumstances should the hand pump be operated in such a manner as to create excessive fuel pressure. The same general instructions concerning fuel system operation apply when landing as in take-off.

c. During normal tactical maneuvers most fuel systems are entirely reliable and require no special considerations. However, no standard systems are designed to function properly in inverted flight or during other unauthorized maneuvers.

26. Maintenance.-a. The maintenance requirements of fuel systems and units may be itemized as follows:

(1) Fuel tanks are checked for security of mounting, dents, or leaks, and the sumps will be drained periodically.

(2) Lines and fittings must be inspected for cracks, proper support, and security of nuts and clamps.

(3) Fuel cock controls are rotated to check for free operation, backlash, and accuracy of pointer indication. If excessive backlash is noted, the entire operating mechanism will be checked for worn universal joints, loose pins, broken drive lugs, etc.

(4) Fuel strainers require periodic draining and cleaning of screens.

(5) Hand pumps should operate freely and develop the required fuel pressures. The engine pumps should be checked for security of mounting and proper adjustment of the relief valve. The pump drive mechanism requires periodic inspection and lubrication.

(6) Fuel pressure warning signals can be conveniently checked for proper adjustment by operating the hand pump and observing the pressure at which the light comes on. Adjustment is sometimes required in the contact mechanism.

(7) Engine primers are checked for free operation and for signs of fuel leakage at the packing. Adjustment or replacement of the packing may be necessary.

(8) Vapor eliminators are checked for leakage and for proper operation of the float mechanism.

(9) Liquidometers must be accurate at all tank levels from empty to full positions. Adjustments for range and position are frequently required.

b. The entire fuel system is carefully inspected for proper safetying, wear, or damage of any description. A final check is made by observing closely the fuel system action with the engine operating.