AIRCRAFT FUEL SYSTEMS
INTRODUCTION
Aircraft fuel systems vary in complexity from the extremely simple systems found in small, single-engine air-planes to the complex systems in large jet transports. Regardless of the type of aircraft, all fuel systems share many of the same common components. Every system has one or more fuel tanks, tubing to carry the fuel from the tank(s) to the engine(s), valves to control the flow of fuel, provisions for trapping water and contaminants, and a method for indicating the fuel quantity. Although fuel systems in modern aircraft are relatively simple, the safety and reliability of these systems is dependent upon proper inspection and maintenance.
All powered aircraft, whether rotary or fixed wing, depend upon a continuous, uninterrupted flow of uncontam-inated fuel under all operating conditions. The weight of the fuel constitutes a large percentage of the aircraft's total weight. This may range from about 10% of the gross weight of small personal airplanes, to more than 40% for jet aircraft used on long overseas flights.
The weight of the fuel requires that the structure be strong enough to carry it in all flight conditions. The aircraft designer locates the fuel tanks so that the decreasing weight from fuel consumption will not cause balance prob-lems. To reduce stresses on the airframe and improve structural life, many jet transports have fuel management procedures that specify how the fuel is to be used from the various tanks. For example, a Boeing 747 will first use the fuel in the center wing tank, followed by fuel in the inboard tanks until their quantities are equal to the out-board tanks.
Improper management of the fuel system has caused more aircraft accidents than failures of any other single sys-tem. Engine failure will occur if all of the fuel in the tanks has been burned, but engines will also stop if an empty tank is selected, even though there is fuel in the other tanks.
Contamination in the fuel may clog strainers or filters and shut off the flow of fuel to the engines. Contamination may take many forms, including solid particles, water, ice and bacterial growth. Water that condenses in partially filled tanks will stop the engine when it flows into the metering system. Water in turbine-powered aircraft is a special problem, as the more viscous jet fuel will hold water entrained in such tiny particles that it does not easily settle out. When the fuel temperature drops at high altitude, the water may form ice crystals that can freeze on the fuel filters and shut off the flow of fuel. Many jet engines have fuel heaters to prevent ice formation on the fuel filters and fuel metering system components.
The type or grade of aircraft fuel must be carefully matched to the engine. It is the responsibility of the pilot in command to verify before a flight is started that the aircraft is adequately supplied with the proper fuel. The per-son doing the refueling can assist by being vigilant for problems with fuel quality and type. A significant prob-lem is the introduction of jet fuel, which is designed for turbine engines, into the fuel tanks of turbocharged pis-ton engine aircraft. These engines are designed to operate on high-octane gasoline. At high power settings, jet fuel contamination causes severe detonation, which can lead to catastrophic engine failure, usually on takeoff. The potential for a disastrous accident is obvious.
AVIATION FUELS AND FUEL SYSTEM
REQUIREMENTS
Aviation fuel is a liquid containing chemical energy that, through combustion, is released as heat energy and then converted to mechanical energy by the engine. Gasoline and kerosene are the two most widely used aviation fuels.
CHARACTERISTICS OF
AVIATION FUELS
Weight is always a primary consideration in aircraft operation. Every extra pound (kilogram) used in the airframe and powerplant subtracts one pound (kilo-gram) from the aircraft's useful load. For this reason, aviation fuels must have the highest possible energy, or heat value per pound. Typical 100LL aviation gasoline (avgas) has 18,720 British Thermal Units (BTUs) per pound. Jet A turbine fuel has about 18,401 BTUs per pound. However, Jet A weighs 6.7 pounds per gallon while a gallon of 100LL weighs 6 pounds. In other words, jet fuel is denser than avgas, and as a result, Jet A supplies 123,287 BTUs per lon whereas 100LL supplies 112,320 BTUs per gal-lon. The density of jet fuel varies more widely than the density of avgas with variations in temperature. Jet fuel density becomes an important factor when fueling large jet transport aircraft.
The dynamics of the internal combustion cycle demand certain properties from gasolines. Aircraft engines compound these demands because they must operate under a wide range of atmospheric conditions. One of the most critical characteristics of aviation gasoline is its volatility, which is a measure of a fuel's ability to change from a liquid into a vapor. Volatility is usually expressed in terms of Reid vapor pressure. Vapor pressure represents the required pressure above the liquid to prevent vapors from escaping at a given temperature. The vapor pressure of 100LL aviation gasoline is approxi-mately seven pounds per square inch at 100 F. Jet A, on the other hand, has a vapor pressure of less than 0.1 psi at 100 F, and Jet B has a vapor pressure of between two and three pounds per square inch at 100 F. Automotive gasoline, by comparison, can produce vapor pressures as high as 14 psi at 100蚌. A fuel's volatility is critical to its performance in an aircraft engine. For example, in a piston engine, the
fuel must vaporize readily in the carburetor to burn evenly in the cylinder. Fuel that is only partially atomized leads to hard starting and rough running. On the other hand, fuel that vaporizes too readily can evaporate in the fuel lines and lead to vapor lock. Furthermore, in an aircraft carburetor, an excessively volatile fuel causes extreme cooling within the carburetor body when the fuel evapo-rates. This increases the chances for the formation of carburetor ice, which can cause a rough running engine or a complete loss of engine power. Fuel injection systems reduce the problems associated with partial atomization and icing caused by fuel cooling but are also susceptible to vapor lock. Therefore, the ideal aviation fuel has a high volatil-ity that is not excessive to the point of causing vapor lock.
RECIPROCATING ENGINE FUEL
Aviation fuels are distilled from crude oil by frac-tional distillation. Each different product to be extracted from the crude oil has a distinct boiling temperature. Each product is boiled off or separated from the crude oil as it is heated to increasingly higher temperatures. Gasoline boils at a relatively low temperature and is taken off first; then the heav-ier fractions are boiled off to become turbine engine fuel, diesel fuel, and furnace oil.Aviation gasoline consists almost entirely of hydro-gen and carbon compounds. Some impurities in the form of sulfur and dissolved water will be present. This water cannot be avoided since the gasoline is exposed to moisture in the atmosphere. A small amount of sulfur, always present in crude oil, is left in during manufacture.
The characteristics and properties of aviation gaso-line govern the following: the selection of the proper fuel; the installation of hoses, gaskets, and seals; and the ability to operate the engine reliably, efficiently, and without damage. Though the avia-tion maintenance technician does not normally choose the system components, the technician will benefit from a thorough understanding of the vari-ous factors that go into such selection.
VOLATILITY
Volatility is a measure of a liquid's tendency to vaporize under given conditions. Gasoline is a com-plex blend of volatile hydrocarbon compounds that have a wide range of boiling points and vapor pres-sures. It is blended in such a way that a straight chain of boiling points is obtained. This is necessary to obtain the required starting, acceleration, power, and fuel mixture characteristics for the engine. VAPOR LOCK
If the fuel does not vaporize readily enough, it can result in hard starting, slow warm-up, poor accelera-tion, uneven fuel distribution to the cylinders, and excessive crankcase dilution. If the gasoline vaporizes too readily, fuel lines may become filled with vapor and deliver a reduced supply of gasoline to the engine. In severe cases, this may result in engine stop-page. This phenomenon is referred to as vapor lock-ing. The Reid vapor pressure test gives a measure of a gasoline's tendency to vapor lock. In this test, a sam-ple of the fuel is sealed in a "bomb" equipped with a pressure gauge. The apparatus is then immersed in a constant-temperature bath and the indicated pressure is noted. The higher the corrected vapor pressure of the sample under test, the more susceptible it is to vapor locking. Aviation gasolines are limited to a maximum of 7 psi to minimize the tendency to vapor lock at high altitudes. [Figure 15-1]
CARBURETOR ICING
Carburetor icing is also related to volatility. When fuel changes from liquid to vapor, it extracts heat from its surroundings. The more volatile the fuel, the more rapid the heat extraction. As gasoline vaporizes leaving the discharge nozzle of a float-type carburetor, it can freeze the water vapor in the incoming air. This moisture may freeze on the walls of the induction system, the venturi throat, or the throttle valve. This type of ice formation restricts the fuel and air passages of the carburetor. It can cause loss of power, and if not eliminated, eventual engine stoppage. This icing condition is most severe in temperatures ranging from 30 to 40 F (-1 C to +4 C) outside air temperature, but may occur at much higher temperatures.
AROMATIC FUELS
Some fuels may contain considerable quantities of aromatic hydrocarbons, which are added to increase the rich mixture performance rating of the fuel. Such fuels, known as aromatic fuels, can swell some types of hoses and other rubber parts of the fuel sys-tem. For this reason, aromatic-resistant hoses and rubber parts have been developed for use with aro-matic fuels. The use of aroaro-matic fuels is associated with the high-horsepower, reciprocating engines used on military and large transport-category air-craft. These aircraft are disappearing from the active fleet, and this type of fuel is no longer available. DETONATION
Reciprocating engine aircraft require high-quality aviation gasolines to ensure reliable operation. These fuels are specially formulated to possess cer-tain characteristics that allow them to function reli-ably in aircraft. To understand the different num-bers used to designate fuel grades, the aircraft tech-nician must first be familiar with detonation in rec-iprocating engines.
When a fuel-air charge enters the cylinder of a pis-ton engine, it is ignited by the spark plugs. Ideally, the fuel burns at a rapid but uniform rate. The expanding gases then push the piston downward, turning the crankshaft and creating power.
Detonation is the explosive, uncontrolled burning of the fuel-air charge. It occurs when the fuel burns unevenly or explosively because of excessive tem-perature or pressure in the cylinder. Rather than smoothly pushing the piston down, detonation slams against the cylinder walls and the piston. The pressure wave hits the piston like a hammer, often damaging the piston, connecting rods, and bearings. Figure 15-1.The Reid vapor pressure tester is used for
mea-suring fuel samples. Vapor pressure is a major factor in the susceptibility of a fuel to vapor lock.
75-4 Aircraft Fuel Systems
Figure 15-2.This chart illustrates the pressure created in a cylinder as it passes through its various strokes. As you can see, when normal combustion occurs, cylinder pressure builds and dissipates evenly. However, when detonation occurs, cylinder pressure fluctuates dramatically.
This is often heard as a knock in the engine. Detonation also causes high cylinder head tempera-tures, and if allowed to continue, can melt engine components. [Figure 15-2]
Detonation can happen any time that an engine overheats. It can also occur if an improper fuel grade is used. The potential for engine overheating is greatest under the following conditions:
Use of fuel grade lower than recommended Takeoff with an engine that is above or very near the maximum allowable temperature
Operation at high rpm and low airspeed
Extended operations above 75 percent power with an extremely lean mixture
PREIGNITION
Combustion is precisely timed in a properly func-tioning ignition system. In contrast, preignition is when the fuel/air mixture ignites too soon.
Preignition is caused by hot spots in the cylinder. A hot spot may be caused by a small carbon deposit, a cracked ceramic spark plug insulator, or almost any damage within the combustion chamber. When preignition exists, an engine may continue to oper-ate even though the ignition has been turned off. In extreme cases, preignition can cause serious dam-age to the engine in a short period of time.
Preignition and detonation often occur simultane-ously, and one may cause the other. Inside the air-craft, it will be difficult to distinguish between the two since both are likely to cause engine roughness and high engine temperatures.
OCTANE AND PERFORMANCE NUMBERS Aviation gasoline is formulated to burn smoothly without detonating, or knocking, and fuels are numerically graded according to their ability to resist detonation. The higher the number, the more resistant the fuel is to detonation. The most
com-mon grading system is octane rating. The octane number assigned to a fuel compares the anti-knock properties of that fuel to a mixture of iso-octane and normal heptane. For example, grade 80 fuel has the same anti-knock properties as a mixture of 80 per-cent iso-octane and 20 perper-cent heptane. 100 Octane fuel has the same anti-knock properties as pure iso-octane gasoline. Engines having high compression ratios and/or high horsepower output require higher-octane fuel.
Some fuels have two performance numbers, such as 100/130. The first number is the lean-mixture rating, whereas the second number represents the rich-mixture rating. To avoid confusion and to minimize errors in handling different grades of avi-ation gasolines, it has become common practice to designate the different grades by the lean mixture performance numbers only. Therefore, aviation gasolines are identified as Avgas 80, 100, and 100LL. Although 100LL performs the same as grade 100 fuel, the "LL" indicates it has a low lead content.
Another way petroleum companies help prevent detonation is to mix tetraethyl lead into aviation fuels. However, it has the drawback of forming cor-rosive compounds in the combustion chamber. For this reason, additional chemicals such as ethylene bromide are added to the fuel. These bromides actively combine with lead oxides produced by the tetraethyl lead allowing the oxides to be discharged from the cylinder during engine operation.
PURITY
Aviation fuels must be free from impurities that would interfere with the operation of the engine or the components in the fuel and induction system. Even though many precautions are observed in stor-ing and handlstor-ing gasoline, it is not uncommon to find a small amount of water and sediment in an air-craft fuel system. A small amount of such contami-nation is usually retained in the strainers of the fuel system. Generally, this is not considered dangerous if the strainers are drained and cleaned at frequent intervals. However, the water can present a serious problem because it settles to the bottom of the fuel tank and can then be circulated through the fuel sys-tem. A small quantity of water flowing with the gasoline through the carburetor jets will not be espe-cially harmful. An excessive amount of water will displace the fuel passing through the jets, causing loss of power and possible engine stoppage.
Under certain conditions of temperature and humidity, condensation of moisture (from the air)
occurs on the inner surfaces of the fuel tanks. Since this occurs on the portion of the tank above the fuel level, it is obvious that servicing an airplane imme-diately after flight will do much to minimize this hazard.
FUEL IDENTIFICATION
In the past, there were four grades of aviation gaso-line, each identified by color. The only reason for mentioning the old ratings is because manuals on older airplanes may still contain references to these colors. The old color identifiers were:
80/87 Red 91/96 Blue 100/130 Green 115/145 Purple
The color code for the aviation gasoline currently available is as follows:
80 Red 100 Green 100LL Blue
A change in color of an aviation gasoline usually indicates contamination with another product or a loss of fuel quality. A color change can also be caused by a chemical reaction that has weakened the dye component. This color change itself may not affect the quality of the fuel, but if one has occurred, determine the cause before releasing the aircraft for flight.
The most positive methods of identifying the type and grade of fuel include the following:
1. Marking of the Hose. A color band not less than one foot wide is painted adjacent to the fitting on each end of the hose used to dispense fuel. The bands completely encircle the hose and the name and grade of the product is stenciled lon gitudinally in one-inch letters over the color band.
2. Fuel trucks and hydrant carts are marked with large fuel identification decals on each side of the tank or body and have a small decal on the dash board. These decals utilize the same color code. The fixed ring around both the dome covers and hydrant box lids are also painted in accordance with the color code. In short, all parts of the fuel ing facility and equipment are identified and
75-6 Aircraft Fuel Systems
delivery pipes of truck fill stands are banded with colors corresponding to those on the dis-pensing hose. [Figure 15-3]
3. In addition to coloring fuels, a marking and cod-ing system has been adopted to identify the vari-ous airport fuel handling facilities and equip-ment, according to the kind and grade of fuel they contain. For example, all aviation gasolines are identified by name, using white letters on a red background. In contrast, turbine fuels are identified by white letters on a black background.
TURBINE ENGINE FUELS
Aircraft gas turbine engines are designed to operate on a distillate fuel, commonly called jet fuel. Jet fuels are also composed of hydrocarbons with a lit-tle more carbon and a higher sulfur content than gasoline. Inhibitors may be added to reduce corro-sion, oxidation, and the growth of microbes or bac-teria. Anti-icing additives are also added. Turbine
engines can operate for limited periods on aviation gasoline. However, prolonged use of leaded avgas forms tetraethyl lead deposits on turbine blades and decreases engine efficiency. Turbine engine manu-facturers specify the conditions under which gaso-line can be used in their engines, and these instruc-tions should be strictly followed. Reciprocating engines will not operate on turbine fuel. Jet fuel should never be put into a piston engine aircraft. VOLATILITY
One of the most important characteristics of jet fuel is its volatility. It must, of necessity, be a compro-mise between several opposing factors. A highly volatile fuel is desirable to aid starting in cold weather and to make aerial restarts easier and surer. Low volatility is desirable to reduce the possibility of vapor lock and to reduce fuel loss to evaporation. At normal temperatures, gasoline in a closed con-tainer or tank can give off so much vapor that the
Figure 15-3. Labeling and color-coding of fuel carriers, hoses, and equipment helps to prevent filling the aircraft with the wrong fuel.
fuel/air mixture may be too rich to burn. Under the same conditions, the vapor given off by Jet B fuel can be in the flammable or explosive range. Jet A fuel has such a low volatility that at normal temperatures it gives off very little vapor and does not form flamma-ble or explosive fuel/air mixtures. [Figure 15-4]
Figure 15-4. The vaporization of avgas and jet fuels varies as the temperature changes. This chart shows that at normal temperatures, avgas is too rich to burn, Jet A is too lean to burn, and Jet B is very flammable, at least in the low-normal temperature range.
FUEL TYPES
Because jet fuels are not dyed, there is no color identification for them. They range from colorless to straw-colored (amber), depending on age or the crude petroleum source.
There are currently two types of turbine fuel in use: JET A and JET A-l, which are kerosene types, and JET B, which is a blend of gasoline and kerosene fractions. Jet A-l specifies a freeze point of -52.6 F (-47 C). Jet A specifies a freeze point of -40 F (-40 C). JP-4, similar to Jet B, is normally used by the military, particularly the Air Force. This fuel has an allowable freeze point of -50 C (-58 F). Jet fuel designations, unlike those for avgas, are merely numbers that label a particular fuel and do not describe any performance characteristics.
PROBLEMS WITH WATER IN TURBINE FUEL
Water has always been one of the major contamina-tion problems with aviacontamina-tion fuel. It condenses out of the air in storage tanks, fuel trucks, and even in air-craft fuel tanks. Water exists in aviation fuels in one of two forms, dissolved and free.
depending upon the fuel composition and tempera-ture. This can be likened to humidity in the air. Undissolved, excess water is called "free water." Lowering the fuel temperature causes dissolved water to precipitate out as free water, somewhat similar to the way fog is created. Typically, dis-solved water does not pose a problem to aircraft and cannot be removed by practical means.
Free water can appear as water slugs or as entrained water. A water slug is a relatively large amount of water appearing in one body or layer. A water slug can be as little as a pint or as much as several hun-dred gallons. Entrained water is suspended in tiny droplets. Individual droplets may or may not be vis-ible to the naked eye, but they can give the fuel a cloudy or hazy appearance, depending upon their size and number. Entrained water usually results when a water slug and fuel are violently agitated, as when they pass through a pump. Lowering the tem-perature of a fuel saturated with dissolved water. Because of its high viscosity, entrained water is often visible in turbine fuel as a haze. Entrained water usually settles out with time.
Most aircraft engines can tolerate dissolved water. However, large slugs of free water can cause engine failure, and ice from slugs and entrained water can severely restrict fuel flow by plugging aircraft fuel filters and other mechanisms.
Jet engine fuel control mechanisms contain many small parts that are susceptible even to small accu-mulations of ice. Fuel heaters protect fuel systems that are subject to ice crystals. These devices can sat-isfactorily deal with dissolved and even entrained water; however, there is little margin for handling large amounts of free water. Some fuel filters are equipped with a differential pressure sensor across the filter element. This sensor will illuminate a warn-ing light on the instrument panel if the filter ices up and the pressure drop across the element rises to the preset value. To further minimize the ice problem, most jet fuel is treated with an anti-icing additive that mixes with the water in the fuel and lowers its freez-ing point so it will remain in its liquid state.
One commonly used anti-icing additive is Prist, manufactured by PPG Industries. Prist is added to jet fuel during refueling. It has limited solubility in jet fuel but is completely soluble in water. When dissolved in water, Prist lowers the water's freezing point. The water/Prist mixture then stays in a liquid state and passes through fuel lines and filters. Water dissolves in aviation fuels in varying amounts High-flying jet aircraft are often equipped with a
15-8 Aircraft Fuel Systems
temperature sensor in one of the outboard fuel tanks. The Aircraft Flight Manual may contain a restriction for maintaining the fuel temperature at a specified value above the fuel freezing point. If the fuel temperature becomes too cold, the aircraft may have to descend to warmer air to avoid problems with ice formation in the fuel.
Water can enter an airport fuel system through leaks in the seals of equipment, or brought in when fuel is delivered. The best means of minimizing the amount of water entering a system is through inspection and maintenance of equipment, and by making certain that only dry fuel is received. Water can be detected in many ways. To find free water lying in the bottom of underground storage tanks, apply a water-finding paste to the end of a gauge stick and place it in the tank. Allow at least 30 seconds for the paste to react since other contami-nants can slow its reaction time. For above ground tanks and equipment, draw a sample into a glass container and simply look for free water. A small amount of liquid vegetable dye (cake coloring) is helpful to highlight free water in a sample. It mixes with and colors the water but is insoluble in fuel. Water is removed from fuel by providing adequate filtration or separation equipment will remove water from the fuel.
The other problem with water in turbine engine fuel is that it may serve as a home for microscopic-sized animal and plant life. Microbial growth, or contam-ination with bacteria, or "bugs," has become a criti-cal problem in some turbine fuel systems and some aircraft. Because microbes thrive in water, a simple and effective method to prevent or retard their growth is to eliminate the water.
Sometimes microbial growth occurs despite efforts to eliminate water from the fuel tanks. Microbiocides are introduced into fuel storage tanks to combat microbial growth. It should be introduced into the fuel when the tank is about half filled to ensure faster and more complete dispersion. Normally the micro-biocide is introduced initially at a high concentration to kill the growth. Once the initial treatment is completed, the concentration is cut in half for long-term maintenance of fungus-free fuel. FUEL CONTAMINATION
Contaminants can include either soluble or insolu-ble materials or both. The more common forms of aviation fuel contamination include solids, water, surfactants, and microorganisms. Fuel can be conta-minated by mixing with other grades or types of
fuels, by picking up compounds from concentra-tions in rust and sludge deposits, by additives, or by
any of a number of other soluble materials.
The greatest single danger to aircraft safety from contaminated fuels cannot be attributed to solids, exotic microorganisms, surfactants, or even water. It is contamination resulting from human error. Any human error that fills an aircraft with the wrong grade or type of fuel or mixes different types of fuel is cause for serious concern. An accident may be the end result. The possibility of human error can never be eliminated, but it can be minimized through careful design of fueling facilities, good operating procedures, and adequate training.
BASIC FUEL SYSTEM
REQUIREMENTS
Requirements for fuel system design are specified in detail in the parts of the Federal Aviation Regulations under which the aircraft was built. The vast majority of airplanes in the general aviation fleet are built under FAR Part 23, [Airworthiness Standards: Normal, Utility, and Acrobatic Category Airplanes). Awareness of the basic fuel system requirements for these airplanes will help the air-craft maintenance technician better understand the function of an aircraft fuel system.
1. No pump can draw fuel from more than one tank at a time, and provisions must be made to prevent air from being drawn into the fuel sup ply line. (23.951)
2. Turbine-powered aircraft must be capable of
sustained operation when there is at least 0.75 cc of free water per gallon of fuel, and the fuel is cooled to its most critical condition for icing. The system must incorporate provisions to pre vent the water that precipitates out of the fuel from freezing on the filters and stopping fuel flow to the engine. (23.951)
3. Each fuel system of a multi-engine aircraft must be arranged in such a way that the failure of any one component (except the fuel tank) will not cause more than one engine to lose power.
(23.953)
4. If multi-engine aircraft feed more than one
engine from a single tank or assembly of inter connected tanks, each engine must have an
independent tank outlet with a fuel shutoff
valve at the tank. (23.953)
5. Tanks used in multi-engine fuel systems must have two vents arranged so that they are not likely to both become plugged at the same time. (23.953)
6. All filler caps must be designed so that they are not likely to be installed incorrectly or be lost in-flight. (23.953)
7. The fuel systems must be designed to prevent the ignition of fuel vapors by lightning. (23.954) 8. A gravity feed syste m must be able to flow 150% of the takeoff fuel flow when the tank contains the minimum fuel allowable, and
when the airplane is positioned in the attitude that is most critical for fuel flow. (23.955) 9. A pump-feed fuel system must be able to flow
125% of the takeoff fuel flow required for a rec iprocating engine. (23.955)
10. If the aircraft is equipped with a selector valve that allo ws the engine to operate from more than one fuel tank, the system must not cause a loss of power for more than ten seconds for a single-engine or twenty seconds for a multi - engine airplane when switching from a dry
tank. (23.955)
11. Turbine-powered aircraft must have a fuel sys tem that will supply 100% of the fuel required for its operation in all flight attitudes, and the flow must not be interrupted, as the fuel system automatically cycles through all of the tanks or fuel cells in the system. (23.955)
12. If a gravity-feed system has interconnected tank outlets, it should not be possible for fuel feeding from one tank to flow into another tank and cause it to overflow. (23.957)
13. The amount of unusable fuel in an aircraft must be determined and this must be made known to the pilot. Unusable fuel is the amount of fuel in a tank when the first evidence of malfunction occurs. The aircraft must be in the attitude that is most adverse for fuel flow. (23.959)
14. The fuel system must be so designed that it is free from vapor lock when the fuel is at a tem perature of 110 F under the most critical oper ating conditions. (23.961)
15. Each fuel tank compartment must be adequately vented and drained so no explosive vapors or liquid can accumulate. (23.967)
16. No fuel tank can be on the engine side of the firewall, and it must be at least one -half inch away from the firewall. (23.967)
17. No fuel tank can be installed inside a personnel compartment of a multi-engine aircraft. (23.967)
18. Each fuel tank must have a 2% expansion space that cannot be filled with fuel, and it must also have a drainable sump where water and conta minants will normally accumulate when the aircraft is in its normal ground attitude. (23.969 and 23.971)
19. Provisions must be made to prevent fuel spilled during filling of the tank from entering the air craft structure. (23.973)
20. The filler opening of an aircraft fuel tank must be marked with the word "AVGAS" and the minimum grade of fuel for aircraft with recipro cating engines. For turbine-powered aircraft,
the tank must be marked with the word "JET FUEL" and the permissible fuel designation. If the filler opening is for pressure fueling, the maximum permissible fueling and defueling
pressure must be specified. (23.1557)
21. If more than one fuel tank has inte rconnected outlets, the airspace above the fuel must also be interconnected. (23.975)
22. If the carburetor or fuel injection system has a vapor elimination system that returns fuel to one of the tanks, the returned fuel must go to the tank that is required to be used first.
(23.975)
23. All fuel tanks are required to have a strainer at the fuel tank outlet or at the booster pump. For a reciprocating engine, the strainer should have an 8- to 16-mesh element, and for turbine
engines, the strainer should prevent the pas sage of any object that could restrict the flow or damage any of the fuel system components. (23.977)
24. For engines requiring fuel pumps, there must be one engine driven fuel pump for each engine. (23.991)
25. There must be at least one drain that will allow safe drainage of the entire fuel system when the airplane is in its normal ground attitude.
(23.999)
26. If the design landing weight of the aircraft is less than that permitted for takeoff, there must be provisions in the fuel system for jettisoning fuel to bring the maximum weight down to the design landing weight. (23.1001)
27. The fuel-jettisoning valve must be designed to allow personnel to close the valve during any part of the jettisoning operation. (23.1001)
FUEL SYSTEM OPERATION
The aircraft fuel system stores fuel and delivers the proper amount of clean fuel at the right pressure to meet the demands of the engine. The fuel system must be designed to provide positive and reliable fuel flow through all phases of flight. This must include changes in altitude, violent maneuvers, and sudden acceleration and deceleration. Furthermore, the system must be reasonably free from any ten-dency to vapor lock. Indicators such as tank quan-tity gauges, fuel pressure gauges, and warning sig-nals provide continuous monitoring of how the sys-tem is functioning.
SMALL SINGLE-ENGINE
AIRCRAFT FUEL SYSTEMS
Single-engine aircraft may utilize any of several types of fuel systems, depending upon whether a
car-buretor or fuel injection system is used, and whether the aircraft is a high-wing or low-wing design. GRAVITY-FEED SYSTEMS
The most simple aircraft fuel system is found on small, high-wing single-engine training-type air-planes. These systems normally use two fuel tanks, one in either wing. The two tank outlets are connected to the selector valve. Fuel can be drawn from either tank individually, or both tanks can feed the engine at the same time. A fourth position on the selector valve turns off all fuel to the engine. Since both tanks can feed the engine at the same time, the space above the fuel in both tanks must be interconnected and vented outside of the airplane. The vent line normally termi-nates on the underside of the wing where the possi-bility of fuel siphoning is minimized. [Figure 15-5]
After the fuel leaves the selector valve, it passes through the main strainer and on to the carburetor inlet. Fuel for the primer is taken from the main strainer.
PUMP-FEED SYSTEMS
Low-wing airplanes cannot use gravity to feed fuel to the carburetor. An engine-driven and/or electric pump must be used to provide adequate fuel pres-sure. The selector valve in these systems can nor-mally select either tank individually, or shut off all flow to the engine. The selector valve does NOT have a "Both" position, because the pump would pull air from an empty tank rather than fuel from a full tank. After leaving the fuel selector valve, the fuel flows through the main strainer and into the electric fuel pump. The engine-driven pump is in parallel with the electric pump, so the fuel can be moved by either. There is no need for a bypass fea-ture to allow one pump to force fuel through the other. To assure that both pumps are functioning, note the fuel pressure produced by the electric pump before starting the engine, and then, with the
engine running, turn the electric pump off and note the pressure that is produced by the engine-driven pump. [Figure 15-6]
The electric pump is used to supply fuel pressure for starting the engine and as a backup in case the engine-driven pump should fail. It also assures fuel flow when switching from one tank to the other. HIGH-WING AIRPLANE USING
A FUEL INJECTION SYSTEM
The Teledyne-Continental system returns part of the fuel from the engine-driven fuel pump back to the fuel tank. This fuel contains any vapors that could block the system, and by purging all of these vapors from the pump and returning them to the tank, they cannot cause any problems in the engine. Fuel flows by gravity from the wing tanks through two feed lines, one each at the front and rear of the inboard end of each tank, into two small accumulator (reser-voir) tanks, and from the bottom of these tanks to the selector valve. [Figure 15-7]
75-72 Aircraft Fuel Systems
Figure 15-7. A Teledyne-Continental fuel injection system used on some high-performance single-engine airplanes. This system uses a combination of gravity feed, an electric vane-type pump, and an engine-driven fuel pump to deliver fuel to the engine. Excess fuel is returned to the selector valve in this system.
The selector valve directs fuel from the desired accumulator tank to the engine, and at the same time directs the fuel vapor from the engine-driven pump back to the selected reservoir tank. This vapor then returns to the wing tank that supplies the reser-voir tank.
The electric auxiliary fuel pump picks up the fuel at the discharge of the selector valve and forces it through the strainer to the inlet of the engine-driven fuel pump. From the engine-driven fuel pump, the fuel flows to the fuel-air control unit where the fuel that is needed for engine operation goes to the cylin-ders, and all of the excess fuel returns to the inlet side of the pump. Some of this fuel has vapor in it and is returned to the selector valve through the fuel-return check valve.
SMALL MULTI-ENGINE
AIRCRAFT FUEL SYSTEMS
The RSA fuel injection system does not return fuel to the tank like the Teledyne-Continental system
just discussed. This system is used on both single and multi-engine aircraft.
When used on a multi-engine aircraft, each wing has two fuel tanks connected together, which serve as a single tank, and the selector valves allow any engine to operate from the tanks in either wing. The term "cross-feed" indicates that an engine is drawing fuel from the opposite wing. Fuel flows from the selector valve to the fuel filter, then to the electric fuel pump, on to the engine-driven pump, into the fuel injection system, and to the cylinders. [Figure 15-8]
Instrumentation for this system consists of fuel quan-tity, fuel pressure, and fuel flow gauges. The fuel quantity gauges show the total amount of fuel in the two tanks in each wing. The fuel pressure gauges show the pressure produced by each fuel pump. This pressure is measured at the inlet of the fuel metering unit. The fuel flow indicator is a differential pressure gauge that reads the pressure drop across the fuel injector nozzles and is calibrated in either gallons per hour or in pounds per hour of fuel burned.
Figure 15-8. A typical airplane using an RSA fuel injection system uses electric fuel pumps to deliver fuel to the engine(s). This system does not return fuel to the tanks as in the Teledyne-Continental fuel injection system.
75-74 Aircraft Fuel Systems
LARGE RECIPROCATING-ENGINE
AIRCRAFT FUEL SYSTEMS
Transport-category aircraft powered by reciprocat-ing engines are rapidly disappearreciprocat-ing from the active fleet. One exception seems to be the venerable Douglas DC-3. This aircraft has seen a working life of more than 60 years and is still being used for pas-senger and cargo applications. The fuel system installed on the DC-3 is typical for aircraft using large radial-type engines. [Figure 15-9]
Two main tanks and two auxiliary tanks mounted in the center wing section of the airplane supply the fuel. The capacity of each main tank is 202 gallons, and the auxiliary tanks hold 200 gallons each. Provisions are made for the installation of 2 to 8 long-range tanks, each holding 100 gallons. This makes it possible to carry a fuel load as large as 1604 gallons in 12 tanks.
Fuel quantity is measured by a liquidometer system, which consists of a float assembly and a liquidome-ter unit in each tank. These are connected electri-cally to the fuel gauge on the right instrument panel in the pilots' compartment. There are two tank selector valves, operated by dial and handle con-trols in the pilots' compartment. Ordinarily the left-hand engine draws fuel from the left tanks, and the right engine draws fuel from the right tanks, but by using the selector valves, any tank may supply fuel to either engine.
Two hand-operated wobble pumps are used to raise the fuel pressure when starting the engines, or before the engine-driven pumps are in operation. Fuel flows from the wobble pumps through lines to the strainers located in each nacelle, through the engine-driven pumps, and from there, under pressure, into the car-buretors. A cross-feed line is connected on the pres-sure side of each engine-driven pump, and the two cross-feed valves in this line are operated by a single control in the pilots' compartment. The cross-feed system enables both engines to receive fuel from one engine-driven pump in case either pump fails. On later model airplanes, two electric booster pumps replace the wobble pumps. A fuel strainer is in the center wing near each selector valve. The fuel, therefore, flows from the selector valves, through the strainers, through the booster pumps, through the engine-driven fuel pumps into the carburetor. There is no crossfeed system on airplanes equipped with electric booster pumps. The booster pumps will fur-nish ample pressure and supply for operation of the airplane in case either engine-driven pump fails.
A vapor overflow line connects from the top cham-ber of the carburetor to the main tanks, and a fuel line from the back of each carburetor operates the fuel pressure gauge in the pilots' compartment. This pressure gauge normally shows from 14 to 16 pounds of pressure. On some airplanes, a pressure-warning switch is installed in the fuel pressure gauge line. When the fuel pressure drops below 12 pounds, the switch illuminates a warning light on the instrument panel.
A restricted fitting on the fuel-pressure gauge line connects to the oil-dilution solenoid. This unit releases fuel into the engine oil system and the pro-peller feathering oil to aid in cold weather starting. Another solenoid valve in the fuel-pressure gauge line releases fuel into the eight upper cylinders of the engines for priming.
Vent lines from each tank vent overboard, and a vapor line connects each main tank with its corre-sponding auxiliary tank.
JET TRANSPORT AIRCRAFT
FUEL SYSTEMS
A large jet transport aircraft such as the Boeing 727 has a relatively simple fuel system that supplies its three engines from three fuel tanks. Tanks No. 1 and No. 3 are integral tanks, that is, part of the wing is sealed off and fuel is carried in the wing structure itself. Each of these tanks holds about 12,000 pounds of fuel. A fuselage tank, consisting of either two or three bladder-type fuel cells, holds another 24,000 pounds of fuel. [Figure 15-10]
Each of the wing tanks has two 115-volt AC electric boost pumps, and the fuselage tank has four of such pumps. Each of the three engines may be fed directly from one of the three fuel tanks, or all of the tanks and engines may be opened from the cross-feed manifold.
Perform pressure fueling by connecting the fuel supply to a single-point-fueling receptacle located under the leading edge of the right wing. Fuel flows from this receptacle, through the fueling and dump manifold, and into all three tanks through the appropriate fueling valves. When the tanks are completely filled, pressure shutoff valves sense the amount of fuel and shut off the fueling valve, which prevents the tank from being overfilled or damaged. If a partial fuel load is required, the person fueling the aircraft can mon-itor a set of fuel quantity gauges at the fueling
Figure 15-9. Older aircraft, such as the DC-3 shown here, had a complex, manual-fuel-management system.
station and shut off the fuel flow to any tank when the desired level is reached.
Defuel the airplane tanks by connecting the fuel receiving truck to the manual defueling valve,
close the engine shutoff valves, and open the cross-feed valve from the tank to be emptied. Either pump out the fuel from the tank with the boost pumps, or pull it from the tank by suction from the receiving truck. If it is pulled out by
Aircraft Fuel Systems 15-17
suction, it leaves the tank through the boost pump bypass valve.
Fuel may be dumped in-flight by opening the specific fuel dump valve and then opening the fuel dump noz-zle valve in the wing tip through which the fuel is to leave the airplane. Fuel can be dumped from individ-ual wing tips or from both tips at the same time. In-flight fuel jettison systems are divided into two sepa-rate systems, one for each wing. Dumping fuel from individual wings allows the pilot to control fuel bal-ance. Fuel is dumped from locations that allow it to remain clear of any part of the aircraft.
There is a fuel dump limit valve in each of the three systems that will shut off the flow if the pressure
drops below what is needed to supply the engine with adequate fuel. It will also shut off the dump valve when the level in the tank gets down to the preset dump shutoff level. This system is capable of dumping about 2,300 pounds of fuel per minute when all of the dump valves are open and all of the boost pumps are operating.
HELICOPTER FUEL SYSTEMS
A typical light turbine-powered helicopter system incorporates a single, bladder-type fuel cell, located below and aft of the rear passenger seat. Installed in the fuel cell are two submersible, centrifugal-type boost pumps, upper and lower fuel-quantity indi-cating probes, and a solenoid-operated sump drain. [Figure 15-11]
Figure 15-11. The fuel system for a light turbine-powered helicopter is simple since it has only one fuel tank that is usually located on the center of gravity of the helicopter.
The boost pumps are connected so that their outlet ports join to form a single line to the engine. Either pump is capable of supplying sufficient fuel to operate the engine. Check valves are installed at the outlet of each pump, and a pressure switch located in the outlet port of each pump will illuminate the FUEL BOOST CAUTION LIGHT in case of a pump failure. An electrically operated shut-off valve is installed in the fuel line running from the tank to the engine. A fuel selector valve is not necessary because only one tank is used in this system. Fuel is filtered twice before entering the engine, and each filter is equipped with a warning light to indi-cate clogging. Additional provisions are made in the system for a fuel pressure gauge, vent system, and a fuel quantity indication.
AIRCRAFT FUEL SYSTEM
COMPONENTS
The basic components of a fuel system include tanks, lines, valves, pumps, filtering units, gauges, warning systems, and for piston-engine aircraft, a primer. Some systems will include central refueling provisions, fuel dump valves, and a means for trans-ferring fuel. In order to clarify the operating princi-ples of complex aircraft fuel systems, the various units are discussed in the following paragraphs. TANKS
The location, size, shape, and construction of fuel tanks vary with the type and intended use of the aircraft.
Fuel tanks are manufactured from materials that will not react chemically with any aviation fuel and have a number of common features. Usually a sump and drain are provided at the lowest point in the tank, and the top of each tank is vented to the atmosphere. All except the smallest of tanks are fit-ted with baffles to resist fuel surging caused by changes in the attitude of the aircraft. Many fuel tanks incorporate flapper valves to prevent fuel from flowing away from the boost pump or tank out-let when the aircraft is in a high "G" maneuver. In this capacity, the flapper valves serve as check valves. An expansion space is provided in fuel tanks to allow for an increase in fuel volume due to increases in its temperature.
Some fuel tanks are equipped with dump valves that make it possible to jettison fuel during flight in order to reduce the weight of the aircraft to its spec-ified landing weight. In aircraft equipped with dump valves, the operating control is located within reach of the pilot, copilot, or flight engineer. Dump
valves are designed and installed to afford safe, rapid discharge of fuel.
WELDED OR RIVETED FUEL TANKS
Most older aircraft use welded or riveted gasoline tanks to hold their fuel, but because of the limita-tions of weight and space, these tanks have been replaced almost totally by either integral or bladder-type tanks.
The smaller fuel tanks are made of thin sheet steel coated with an alloy of lead and tin. This material is called terneplate. Terneplate sheets are formed into the shapes needed to construct the tank, and all of the seams are folded in the best tradition of commercial sheet metal practice. Solder is sweated into the seams. This provides a good leak-proof joint, and the tanks are relatively low cost. The weight of a terne-plate tank is more than that of an aluminum alloy tank, but for the type of airplane in which it is installed, the low cost advantage overcomes its weight disadvantage. Most terneplate tanks are of such small capacity that they seldom require baffles. The larger fuel tanks of older aircraft are generally made of either 3003 or 5052 aluminum alloy. Both of these metals are relatively lightweight and easily welded. The parts of the tank are stamped out of sheet metal and formed to the required shape, and the tank is often riveted together with soft alu-minum rivets to hold its parts in position. All of the seams are torch-welded to provide a fuel-tight seal. Many tanks are large enough to require baffles to prevent the fuel from sloshing around in flight and either damaging the tank or causing balance problems.
All rigid fuel tanks must be supported in the aircraft structure with hold-down straps that will prevent the tank from shifting during any maneuver. All of the tank mounts must be padded with some type of material, usually felt, to prevent the tank from chaf-ing against the structure.
Some modern small airplanes use fuel tanks that actually form part of the leading edge of the wing. Sealant is usually applied before these tanks are assembled. Others are welded together by electric resistance welding. Either of these tanks sometimes leak, and the manufacturer has approved a sloshing procedure to seal them. A sloshing compound is a liquid sealant that is poured into the tank, which flows over the entire inside surface and into the seams and crevices. The bulk of the compound is then poured out, and the sealant that remains inside the tank is allowed to cure. [Figure 15-12]
Aircraft Fuel Systems 15-19
Figure 15-12. In this built-up fuel tank that forms the leading edge of a wing, sealant is placed in all of the seams before the tank is assembled.
INTEGRAL FUEL TANKS
Rigid tanks require a large open space in the air-craft structure for their installation, and very few aircraft have this amount of space that is not crossed with structural members. However, most wings have large empty spaces, and with the availability of new, space-age sealants, it has become standard practice for many aircraft man-ufacturers to seal off a portion of the wing to form a fuel tank. This type of tank has the advantage of using a maximum amount of space for the fuel and having a minimum amount of weight. A typical light-aircraft integral fuel tank occupies the leading edge portion of the wing from the front spar forward, and it is sealed at both ends and all along the spar with a two-part sealant. All of the rivets and nutplates are sealed, as well as around all of the inspection openings. The sealant is spread along each seam individually rather than sloshing the entire tank. [Figure 15-13]
Figure 15-13. This drawing of an integral fuel tank illustrates a tank that makes up the bulk of the leading edge structure of a wing. All seams are sealed during assembly.
Some airplanes have the leading edge of the wing made of formed honeycomb, with facings of sheet aluminum or fiberglass on both the inside and out-side. This makes an excellent fuel tank with mini-mal sealing. [Figure 15-14]
Figure 15-14. Leading edges constructed of aluminum hon-eycomb can be easily sealed to form a fuel tank.
BLADDER TANKS
The bladder tank is an excellent substitute for a welded fuel tank. Bladder tanks have been success-fully used for both small and large aircraft. Prepare the fuel bay by covering all sharp edges of the metal structure with a chafe-resisting tape and install a bladder made of thin fabric, which is impregnated with neoprene or some similar material that is impervious to fuel. [Figure 15-15]
Figure 15-15. Bladder tanks are made of neoprene impreg-nated cloth that is snapped or laced into the fuel cell cavity in the wing of the airplane.
Put the bladder into the cavity prepared for it by folding and inserting it through an inspection open-ing. Snap or clip it in place, or in some instances,
lace it to the structure. Secure an opening in the bladder to the inspection opening and cover it with an inspection plate.
There are a few considerations that must be observed with aircraft bladder tanks. On each inspection, be sure that the bladder has not pulled away from any of its attachment points. If it has pulled away, the amount of fuel the tank can hold will be decreased and the fuel quantity gauge will be inaccurate. Inspect bladder tanks for wrinkles that can trap water and prevent it from reaching the sump for removal. Also, never allow these tanks to stand empty for any extended period. If it is ever necessary to leave the tank empty for an extended period, wipe the inside of the bladder carefully with an oily rag, leaving a film of engine oil on its inside surface.
FUEL TANK FILLER CAPS
The filler cap on a fuel tank is perhaps one of the least noticed, but most important components on an aircraft. Take care when installing a fuel tank cap, and carefully examine it on each routine mainte-nance inspection.
Almost all fuel tank caps are located on the upper surface of the wing, and it is possible for fuel to be siphoned from the tank if the cap is leaking or improperly installed. Some fuel tank caps are vented, and it is important that the vent hole be clear. Some caps have a gooseneck tube on the vent that sticks up above the tank cap, and it is extremely important that these tubes point forward to provide a slight positive pressure inside the tank while in flight.
There are numerous types of fuel tank caps used on modern aircraft, and only the tank cap approved for that particular aircraft should be used. The cap is actually part of the fuel tank filler adapter assembly, and the replacement of one type of adapter with another usually constitutes a major alteration, requiring approval of the aircraft manufacturer and/or the FAA. Complete the appropriate paper-work after making the alteration. [Figure 15-16] Locking fuel tank caps are popular on aircraft since vandalism has become so rampant. Foreign material put into a fuel tank can cause an expensive servic-ing problem, easily prevented with lockservic-ing fuel tank caps.
Lightning-safe fuel tank caps are often installed on aircraft that fly in all types of weather. These caps have no metal exposed on the inside of the tank and
Aircraft Fuel Systems 15-21
Figure 15-16. A typical fuel filler cap is part of a unit. If any part is damaged or lost, replace it with the proper part for that assembly.
will not conduct the lightning charge to the fuel. Even the lanyard that prevents the tank cap from being misplaced is made of a strong, nonconductive plastic material.
Non-siphoning fuel tank cap adapters have a small spring-loaded flapper inside the adapter that is pushed open by the fuel nozzle. The flapper closes when the nozzle is withdrawn from the tank. No fuel can siphon out of the tank even if the cap is left off the adapter.
FUEL LINES AND FITTINGS
The plumbing in aircraft fuel systems must be con-structed of the highest quality material, and all of the workmanship must conform to approved air-craft practices. The metal tubing is usually made of aluminum alloy, and the flexible hose is made of
synthetic rubber or Teflon. The engine's fuel flow requirements govern the diameter of the tubing. Most of the rigid fuel lines used in an aircraft are made of 5052 aluminum alloy, but in some aircraft, the lines that pass through the wheel wells and some of the lines in the engine compartments are made of stainless steel as insurance against damage from either abrasion or heat. The fittings used on the lines may be of either the AN or MS flare type or a flareless type, depending upon the system installed by the manufacturer.
Replacement of a fuel line is normally done by installing a new line furnished by the aircraft man-ufacturer. If it is ever necessary to fabricate a line in the shop, use only the correct material for the line, and do not use substitute fittings without specific approval of either the manufacturer or the FAA. Both the flare-type and the flareless fittings provide a good leak-proof connection if they are properly installed, and they will not usually develop a leak unless subjected to abuse or mistreatment. If a leaky fitting is found, remove the pressure from the sys-tem and re-tighten the fitting to proper torque spec-ifications. If it is sufficiently tight, check the sealing surface for any indication of damage. If there are scratches or damage, replace the fitting. Make no attempt to stop a leak by overtightening a fitting. This is especially true of the flareless fittings, as they are highly susceptible to damage caused by excess torque. Tighten a flareless fitting finger-tight and then turn the fitting with a wrench only one-sixth to, at the most, one-third of a turn.
When installing flexible hoses in a fuel system, be sure that they do not twist when tightening the fit-ting. The lay line (often a line of printing) that runs the length of the hose should be straight and show no indication of spiraling. [Figure 15-17]
MIL-H8794:SIZE-6-2/68-MFG SYMBOL
Figure 15-17. The lay line printed along a flexible hose pro-vides a means of identifying the hose material and shows whether it has been twisted during installation.
Many of the fuel lines in an engine compartment are encased in a fire sleeve. If the aircraft requires this protection, install the proper type of fire sleeve in the manner specified by the aircraft manufacturer.
Route the fuel lines in accordance with the manu-facturer's recommendations. In case of an alteration requiring new or re-routed fuel lines, there are cer-tain basic requirements for routing fuel lines in an aircraft:
1. If it is impossible to physically separate fuel lines from electrical wire bundles, locate the fuel lines below the wiring and clamp the wire bundle securely to the airframe structure. It is never per missible to clamp a wire bundle to a fuel line. 2. Support all fuel lines so there will be no strain on
the fittings, and never pull a line into place by the fitting.
3. There must always be at least one bend in rigid tubing between fittings. This allows for slight
misalignment of the ends, for vibration, and for expansion/contraction caused by
temperature changes.
4. Electrically bond all metal fuel lines at each
point that they are attached to the structure. Do
this by using bonded cushion clamps to hold the tubing. [Figure 15-18]
Figure 15-18. Support the fuel lines in an aircraft with bonded-type cushion clamps. Protect the edges of any hole that the tube passes through with a rubber or nylon grommet.
5. Protect all fuel lines from being used as a hand hold.
6. To protect fuel lines from being stepped on or damaged by baggage or cargo, route them along the sides or top of compartments where this type of damage could occur.
FUEL 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 installation. Valves may be hand-oper-ated, motor-operhand-oper-ated, or solenoid-operated. Valves must accommodate the full flow capacity of the fuel line, must not leak, and must operate freely. A man-ually operated valve must have a definite "feel" or "click" when it is in the correct position.
HAND OPERATED VALVES
Hand operated valves may be found on small and medium-sized aircraft, and ■will likely be either cone-type or poppet-type selector valves.
All fuel systems provide for shutting off the flow of fuel from the tanks to the engine, and most of the smaller aircraft use hand operated valves. The simplest valve is the cone-type, in which a cone, usually made of brass, fits into a conical recess in the valve body. The cone is drilled so it will allow flow from the inlet of the valve to any one of the outlets that is selected. A detent plate is installed on the shaft that is used to turn the cone, and a spring-loaded pin slips into the detent when the hole in the cone is accurately aligned with the holes in the valve body. This allows the pilot to tell by feel when the valve is in any given posi-tion. [Figure 15-19]
Figure 15-19. A cone-type fuel selector valve is used on
One problem with cone-type valves is that they can become difficult to turn. This can prevent the detent from providing a positive feel when the valve is properly centered in a selected position. In several instances, this condition has led to engine damage by preventing adequate fuel flow. The poppet-type valve overcomes this problem by using a camshaft, operated by the selector valve handle, to open the correct poppets and control the flow of fuel through the valve. The positive shutoff of fuel is provided by the spring on the valve, and it is easy for the pilot to tell by feel when the valve is centered in a selected position. [Figure 15-20]
MOTOR-OPERATED VALVES
Larger aircraft must use remotely operated valves in the fuel system. There are two basic types of remotely operated valves in popular use today: those driven by an electric motor, and those oper-ated by a solenoid.
There are two types of motor-operated valves. In one of them, the motor drives a drum that has holes cut in it. This is so fuel can flow through the drum when it is in one position, and is shut off when it is rotated ninety degrees.
Figure 15-20. Poppet-type valves use a cam to open the poppet valves. This type of valve is much less likely to become difficult to turn than the cone-type valve.
The other valve uses a motor-driven sliding gate. The gate is drawn to let fuel through and shut to stop it. [Figure 15-21]
75-24 Aircraft Fuel Systems
SOLENOID-OPERATED VALVES
A solenoid valve has the advantage over a motor-driven valve of being much quicker to open or close. When electrical current momentarily flows through the opening solenoid coil, it exerts a magnetic pull on the valve stem to open the valve. When the stem rises high enough, the spring-loaded locking plunger of the closing solenoid is forced into the notch in the valve stem. This holds the valve locked open until current is momentarily directed into the closing solenoid coil. The magnetic pull of this coil pulls the locking plunger out of the notch in the valve stem, and the spring closes the valve and shuts off the flow of fuel. [Figure 15-22]
Figure 15-22. A solenoid operated poppet-type valve. A plunger holds open the valve until a closing solenoid pulls the plunger, allowing the valve to close.
FUEL PUMPS
The purpose of an engine-driven fuel pump is to deliver a properly pressured, continuous supply of fuel during engine operation. Auxiliary fuel pumps may be installed in the system to aid in engine start-ing and to assure a positive pressure to the inlet of the engine-driven fuel pump.
HAND-OPERATED PUMPS
Hand-operated fuel pumps are often called "wobble pumps." The name comes from the method of oper-ation of one of the early types of hand fuel pumps. These pumps are used for backing up an engine-dri-ven pump and for transferring fuel from one tank into another. [Figure 15-23]
When the handle is moved up and down, the vane inside the pump rocks back and forth. When the handle is pulled down, the left side of the vane moves up, pulling fuel into chambers A and D
Figure 15-23. Larger aircraft that have the capability of transferring fuel from one part of the system to another are often equipped with a manual fuel-transfer pump called a "wobble pump."
through the flapper-type check valve and the drilled passage between the chambers. Fuel in chamber B is forced into chamber C through the passage drilled through the center of the vane, and out the pump discharge line through the check valve. When the handle is moved up, the vane moves in the opposite direction, pulling fuel into chambers B and C. The fuel in chamber A is forced out of the pump through chamber D. This is a double-acting pump since it moves fuel on each stroke of the handle.
CENTRIFUGAL BOOST PUMP
By far the most popular type of auxiliary fuel pump in use in modern aircraft is the centrifugal boost pump. These pumps are installed on either the inside or the outside of the fuel tank. [Figure 15-24] An electric motor drives a centrifugal pump, and it uses a small impeller to sling fuel out into the dis-charge line. These pumps are not of the constant displacement type, so restricting their outlet does not affect them. Many are two-speed types that use an electrical resistor in series with the motor to vary its speed.
Some centrifugal boost pumps have a small agitator on the pump shaft that stirs up the fuel being drawn into the impeller. Any of the tiny vapor bubbles that form in the fuel are forced to coalesce into larger bubbles and rise to the top of the tank rather than enter the fuel line.
This boost pump is used in its low-speed position for starting the engine and for minor vapor purging.
Figure 15-24. Centrifugal boost pumps are often submerged inside a fuel tank.
In its high-speed position, it is used as a backup for the engine-driven pump during takeoff and high power engine operation. It is also used in its high-speed position for major purging of fuel vapors. Some installations are quite critical with regard to the pressure delivered by the boost pump to the engine, and these systems have resistors in the boost pump circuit that are controlled by a precision switch on the throttle. When the throttle is opened and the boost pump switch is on, the pump operates at its high speed, but when the throttle is retarded, the pump speed will automatically decrease. This lowers the output pressure enough so that the boost pump will not flood the engine.
FUEL EJECTORS
To assure that there will always be an adequate sup-ply of fuel available, boost pumps are sometimes located in a fuel collector can. This is an area of the fuel tank that has been partitioned off and equipped with a flapper-type valve to allow fuel to flow into the collector from the tank. A fuel ejector system uses the venturi principle to supply additional fuel to the collector can, regardless of aircraft attitude. The submerged motor-driven boost pumps supply fuel from each tank to their respective engines. During operation of the boost pumps, a portion of their output is routed to the fuel ejectors. The flow of
