JET A JET A Civiliun (Kerosene—4Q°F) (Kerosene-58° F) JET A-1 JET A-1 Military JET B JET B Military (Naptha-Kerosene-76° F) FIGURE 4-3. Identification of jet fuels.
ruel Identification Markings The most positive method of identifying the type and grade of fuel includes the following:
1. Marking of Hose. A color band not less than one foot wide painted adjacent to the fitting on each end of hose used to dispense fuel. The bands completely encircle the hose, and the name and grade of the product is stenciled longitudinally in one-inch letters of a contrasting color over the color band.
2. Marking of Fuel Carriers, Pits and Fill Stands. Tags identifying the name and grade of the product permanently affixed to each discharge meter and fill pipe. Porcelain tags (4"X6") carrying the same information permanently bolted to the outside of the rear compartment of fuel servicing equipment. The delivery pipes of truck fill stands are banded with colors corresponding to that used on the dispensing hose.
TURBINE ENGINE FUELS The aircraft gas turbine is designed to operate on a distillate fuel, commonly called jet fuel. Jet fuels are also composed of hydrocarbons with a little more carbon and usually a higher sulphur content than gasoline. Inhibitors may be added to reduce corrosion and oxidation. Anti-icing additives are also being blended to prevent fuel icing.
Two types of jet fuel in common use today are:
(1) Kerosene grade turbine fuel, now named Jet A;
and (2) a blend of gasoline and kerosene fractions, designated Jet B. There is a third type, called Jet A-1, which is made for operation at extremely low temperatures. See figure 4-3.
There is very little physical difference between Jet A (JP-5) fuel and commercial kerosene. Jet A was developed as a heavy kerosene having a higher flash point and lower freezing point than most kerosenes. It has a very low vapor pressure, so there is little loss of fuel from evaporation or boiloff at higher altitudes. It contains more heat energy per gallon than does Jet B (JP-4).
Jet B is similar to Jet A. It is a blend of gasoline and kerosene fractions. Most commercial turbine engines will operate on either Jet A or Jet B fuel.
However, the difference in the specific gravity of the fuels may require fuel control adjustments.
Therefore, the fuels cannot always be considered interchangeable.
Both Jet A and Jet B fuels are blends of heavy distillates and tend to absorb water. The specific gravity of jet fuels, especially kerosene, is closer to water than is aviation gasoline; thus, any water introduced into the fuel, either through refueling or condensation, will take an appreciable time to settle out. At high altitudes, where low temperatures are encountered, water droplets combine with the fuel to form a frozen substance referred to as "gel." The mass of "gel" or "icing" that may be generated from moisture held in suspension in jet fuel can be much greater than in gasoline.
Volatility One of the most important characteristics of a jet fuel is its volatility. It must, of necessity, be a compromise between several opposing factors.
A highly volatile fuel is desirable to aid in starting in cold weather and to make aerial restarts easier and surer. Low volatility is desirable to reduce 77
20 Percent of gas by volume Avgas Jet B Too rich Flammable range OF 50F 100F 150F 200F Temperature FIGURE 4-4. Vaporization of aviation fuels at atmospheric pressure.
the possibility of vapor lock and to reduce fuel losses by evaporation.
At normal temperatures, gasoline in a closed container or tank can give off so much vapor that the 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 flammable or explosive fuel/air mixtures. Figure 4-4 shows the vaporization of aviation fuels at atmospheric pressure.
Identification Because jet fuels are not dyed, there is no onsight identification for them. They range in color from a colorless liquid to a straw-colored (amber) liquid, depending on age or the crude petroleum source.
Jet fuel numbers are type numbers and have no relation to the fuel's performance in the aircraft engine.
FUEL SYSTEM CONTAMINATION There are several forms of contamination in aviation fuel. The higher the viscosity of the fuel, the greater is its ability to hold contaminants in suspension. For this reason, jet fuels having a high viscosity are more susceptible to contamination than aviation gasoline. The principal contaminants that reduce the quality of both gasoline and turbine fuels are other petroleum products, water, rust or scale, and dirt.
Water Water can be present in the fuel in two forms:
(1) Dissolved in the fuel or (2) entrained or suspended in the fuel. Entrained water can be detected with the naked eye. The finely divided droplets reflect light and in high concentrations give the fuel a dull, hazy, or cloudy appearance.
Particles of entrained water may unite to form droplets of free water.
Fuel can be cloudy for a number of reasons.
If the fuel is cloudy and the cloud disappears at the bottom, air is present. If the cloud disappears at the top, water is present. A cloud usually indicates a water-in-fuel suspension. Free water can cause icing of the aircraft fuel system, usually in the aircraft boost-pump screens and low-pressure filters. Fuel gage readings may become erratic because the water short-circuits the aircrafts electrical fuel cell quantity probe. Large amounts of water can cause engine stoppage. If the free water is saline, it can cause corrosion of the fuel system components.
Foreign Particles Most foreign particles are found as sediment in the fuel. They are composed of almost any material with which the fuel comes into contact.
The most common types are rust, sand, aluminum and magnesium compounds, brass shavings, and rubber.
Rust is found in two forms: (1) Red rust, which is nonmagnetic and (2) black rust, which is magnetic. They appear in the fuel as red or black powder (which may resemble a dye), rouge, or grains. Sand or dust appears in the fuel in a crystalline, granular, or glasslike form.
Aluminum or magnesium compounds appear in the fuel as a form of white or gray powder or paste. This powder or paste becomes very sticky or gelatinous when water is present. Brass is found in the fuel as bright gold-colored chips or dust. Rubber appears in the fuel as fairly large irregular bits. All of these forms of contamination can cause sticking or malfunctions of fuel metering devices, flow dividers, pumps, and nozzles.
Contamination with Other Types or Grades of Fuel The unintentional mixing of petroleum products can result in fuels that give unacceptable per- 78
formance in the aircraft. An aircraft engine is designed to operate most efficiently on fuel of definite specifications. The use of fuels that differ from these specifications reduces operating efficiency and can lead to complete engine failure.
Operators of turbine-powered aircraft are sometimes forced by circumstances to mix fuels. Such mixing, however, has very definite disadvantages.
When aviation gasoline is mixed with jet fuel, the TEL in the gasoline forms deposits on the turbine blades and vanes. Continuous use of mixed fuels may cause a loss in engine efficiency. However, on a limited usage basis, they will have no detrimental effects on the engine.
Aviation gasoline containing by volume more than 0.5 percent of jet fuel may be reduced below the allowable limits in knock rating. Gasoline contaminated with turbine fuel is unsafe for use in reciprocating engines.
Microbial Growth Microbial growth is produced by various forms of micro-organisms that live and multiply in the water interfaces of jet fuels. These organisms may form a slime similar in appearance to the deposits found in stagnent water. The color of this slime growth may be red, brown, gray, or black. If not properly controlled by frequent removal of free water, the growth of these organisms can become extensive. The organisms feed on the hydrocarbons that are found in fuels, but they need free water in order to multiply.
Micro-organisms have a tendency to mat, generally appearing as a brown blanket which acts as a blotter to absorb more moisture. This mixture or mat accelerates the growth of micro-organisms.
The buildup of micro-organisms not only can interfere with fuel flow and quantity indication, but, more important, it can start electrolytic corrosive action.
Sediment Sediment appears as dust, powder, fibrous material, grains, flakes, or stain. Specks or granules of sediment indicate particles in the visible size range, i.e., approximately 40 microns or larger in size. (See figure 4-5.) The presence of any appreciable number of such particles indicates either a malfunction of the filter/separators or a source of contamination downstream of the filter/ separator, or else an improperly cleaned sample container. Even with the most efficient filter/ separators and careful fuel handling, an occasional TIME. I HOUR _ CLEAN FUEL CLEAN FUEL 5-MICRON S-MICRON
PARTICLES PARTICLES AVGAS JP-4 JP-5 FIGURE 4-5. Comparison of particle's rate of settling in three types of fuel.
visible particle will be encountered. These strays are usually due to particle migration through the filter media and may represent no particular problem to the engine or fuel control. The sediment ordinarily encountered is an extremely fine powder, rouge, or silt. The two principle components of this fine sediment are normally sand and rust.
Sediment includes both organic and inorganic matter. The presence of appreciable quantities of fibrous materials (close to naked eye visibility) is usually indicative of filter element breakdown, either because of a ruptured element or mechanical disintegration of a component in the system.
Usually, high metal content of relatively large particles suggest a mechanical failure somewhere in the system which is necessarily not limited to a metallic filter failure.
In a clean sample of fuel, sediment should not be visible except upon the most meticulous inspection. Persistent presence of sediment is suspect and requires that appropriate surveillance tests and corrective measures be applied to the fuel handling system.
Sediment or solid contamination can be separated into two categories: (1) coarse sediment and (2) fine sediment.
Course Sediment Sediment that can be seen and that easily settles out of fuel or can be removed by adequate filteration is coarse sediment. Ordinarily, particles 10 microns in size and larger are regarded as coarse sediment. (See figure 4-6.)
"HUMAN HAIR IS ABOUT I00 MICRONS
DIAMETER 40- MICRON IO-MICRON 5- MICRON
PARTICLE PARTICLE PARTICLE 25,400 MICRONS • | INCH FIGURE 4-6. Enlargement of small particles and comparison to human hair.
Coarse particles clog orifices and wedge in sliding valve clearances and shoulders, causing malfunctions and excessive wear of fuel controls and metering equipment. They are also effective in clogging nozzle screens and other fine screens throughout the aircraft fuel system.
Fine Sediment Particles smaller than 10 microns may be defined as fine sediment. (See figure 4-6.) Ninety eight percent of the fine sediment in fuel can be removed by proper settling, filtration, and centrifuging. Particles in this range accumulate throughout fuel controls, appearing as a dark shellac-like surface on sliding valves, and may also be centrifuged out in rotating chambers as sludge-like matter, causing sluggish operation of fuel metering equipment. Fine particles are not visible to the naked eye as distinct or separate particles; they will, however, scatter light and may appear as point flashes of light or a slight haze in fuel.
Maximum possible settling time should be allowed in fuel tanks after filling to allow reasonable settlement of water and sediment.
Contamination Detection Coarse contamination can be detected visually.
The major criterion for contamination detection is that the fuel be clean, bright, and contain no perceptible free water. Clean means the absence of any readily visible sediment or entrained water.
Bright refers to the shiny appearance of clean, dry fuels. Free water is indicated by a cloud, haze, or a water slug. A cloud may or may not be present when the fuel is saturated with water.
Perfectly clear fuel can contain as much as three times the volume of water considered to tolerable.
Several field methods for checking water content have been devised. One is the adding of a food color that is soluble in water, but not in fuel.
Colorless fuel samples acquire a definite tint if water is present. Another method uses a gray chemical powder that changes color to pink through purple, if 30 or more p.p.m. (parts per million) of water are present in a fuel sample.
In a third method a hypodermic needle is used to draw a fuel sample through a chemically treated filter. If the sample changes the color of the filter from yellow to blue, the fuel contains at least 30 p.p.m. of water.
Since fuel drained from tank sumps may have been cold-soaked, it should be realized that no method of water detection can be accurate while the fuel entrained water is frozen into ice crystals.
There is a good chance that water will not be drained or detected if the sumps are drained while the fuel is below 32° F. after being cooled in flight. The reason for this is that the sump drains may not be at the lowest point in the fuel tank while the airplane is in a flight attitude, and water may accumulate and freeze on other areas of the tank where it will remain undetected until it thaws.
Draining will be more effective if it is done after the fuel has been undisturbed for a period of time during which the free water can precipitate and settle to the drain point. The benefits of a settling period will be lost, however, unless the accumulated water is removed from the drains before the fuel is disturbed by internal pumps.
Contamination Control The aircraft fuel system can be considered as being divided into three parts when discussing clean fuel. The manufacturer produces clean fuel.
Contamination can occur at any time after the fuel is produced. The first part of the fuel system is the delivery and storage system between the refinery and the airport fuel service truck. Although this system is not physically a part of the aircraft, it is of equal importance in controlling contamination.
Anytime fuel is transferred it is susceptible to contamination. Therefore, all aviation maintenance personnel should be familiar with the following means of contamination control.
Fundamental in the control of contamination of turbine fuels are the methods followed by the industry in receiving and storing any bulk shipment of a petroleum product. These methods have long been established as sound, and they are too well known to need repetition here. The refueling facilities used by operators of turbine powered aircraft should incorporate the following features:
1. Fuel being pumped into airport storage should pass through a filter-separator.
should meet the requirements of U.S. Government Specification MIL-F-8508A.
2. Turbine fuels should be allowed to settle for a period of one hour per foot of depth of the fuel before being withdrawn for use. This means that ordinarily more than one storage tank must be primard provided for each grade of product.
3. Storage tanks should be checked with litmus paper after each new load of fuel is received and the fuel has settled. The litmus paper should remain submerged for a minimum of 15 seconds.
During periods of heavy rain underground tanks should be checked with litmus paper more frequently.
tank. Floating suction is the only logical way to take full advantage of gravity in removing water and particulate matter contamination. Its importance must not be minimized.
5. Fuel being withdrawn from storage should be passed through a filter-separator meeting the specification MIL-F-8508A.
6. Great care should be exercised in loading mobile fuelers to exclude airborne dust and dirt, rain or other foreign material.
7. To lessen the likelihood of rust and scale the tanks of mobile fuelers should be constructed of either stainless steel, nonferrous material or steel coated with a reliable, inert material.
8. As turbine fuel is being dispensed into the aircraft from truck or hydrant it should be filtered to a degree of 5 microns for solid particles and contain no more than 0.0015 per cent of free and entrained water. Bypass valves around the filter should not be permitted.
employed.
in use."
FUEL SYSTEM
4. Suction lines should be a minimum of 6 inches from the bottom of the tank. Kerosene storage tanks should be equipped with floating type suction lines. Floating suction does not remove the bottom product, which may not have settled sufficiently. It also prevents reintroduction into the fuel of any contamination at the bottom of the functioning.
To priming connection Hight tank Left tank Carburetor
• Primer Strainer Drain Selector valve FIGURE 4-7. Gravity feed fuel system.
The simplest type of fuel system is the gravity feed, which is still in use on many low-powered airplanes. A gravity feed system is shown in figure 4-7. The fuel tanks are mounted above the carburetor, with gravity causing the fuel to flow from the tanks to the carburetor. A selector valve is provided to stop the fuel fow or to select a particular tank in the system from which to draw fuel. A strainer filters the fuel before it reaches the carburetor. A drain is provided for removing water and sediment trapped at the strainer.
primer furnishes the additional fuel required for engine starting.
Airplanes equipped with a high-output engine require a fuel system that supplies fuel to the carburetor at a positive pressure. The basic source for this pressure is an engine-driven fuel pump, but auxiliary fuel pumps or booster pumps are required in every pressure feed system to: (1) supply fuel pressure for starting the engine; (2) supply fuel to the primer system; and (3) to serve as an emergency pump in case the engine-driven pump fails.
FUEL SYSTEM COMPONENTS The basic components of a fuel system include tanks, lines, valves, pumps, filtering units, gages, warning signal, and primer. Some systems will include central refueling provisions, fuel dump valves, and a means for transferring fuel. In order to clarify the operating principles of complex aircraft fuel systems, the various units are discussed in the following paragraphs.
Fuel Tanks The location, size, shape, and construction of fuel tanks vary with the type and intended use of the aircraft. In some aircraft, the fuel tanks are integral with the wing or other structural portions of the aircraft.
Fuel tanks are made of materials that will not react chemically with any aviation fuel. Aluminum alloy is widely used, and synthetic rubber bladdertype fuel cells are used in some installations.
Usually a sump and a drain are provided at the lowest point in the tank as shown in figure 4-8.
When a sump or low point is provided in the tank, the main fuel supply is not drawn from the bottom of the sump, but from a higher point in the tank.
The top of each tank is vented to the outside air in order to maintain atmospheric pressure within the tank. Air vents are designed to minimize the possibility of their stoppage by dirt and ice formation. In order to permit rapid changes in internal air pressure, the size of the vent is proportional to the size of the tank, thus preventing the collapse of the tank in a steep dive or glide. All except the very smallest of tanks are fitted with internal baffles to resist fuel surging caused by changes in the attitude of the aircraft. Usually an expansion space is provided in fuel tanks to allow for an increase in fuel volume due to expansion.
The filler neck and cap are usually located in a recessed well, equipped with a scupper and 82
Filler cap Vents Scupper Overboard drain line
- Baffle plates Sump - Finger screen Corrosion inhibitor Drain valve Standpipe FIÇURE 4-8. A typical metal fuel tank.
drain. The scupper is designed to prevent overflowing fuel from entering the wing or fuselage structure. Fuel caps have provisions for locking devices to prevent accidental loss during flight.
Filler openings are clearly marked with the word "FUEL", the tank capacity, and the type of fuel to be used. Information concerning the capacity of each tank is usually posted near the fuel selector valves, as well as on the tank filler caps.
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 specified maximum 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.
Fuel Cells Present day aircraft may be equipped with one or more of the following types of fuel cells: the bladder-type fuel cell and the integral fuel cell.
Bladder-Type Fuel Cells The bladder-type fuel cell is a nonself-sealing cell that is used to reduce weight. It depends entirely upon the structure of the cavity in which it sits to support the weight of the fuel within it.
For this reason, the cell is made slightly larger than the cavity. The bladder cells in use are made either of rubber or of nylon.
Integral Fuel Cells Since integral fuel cells are usually built into the wings of the aircraft structure, they are not removable. An integral cell is a part of the aircraft structure, which has been so built that after the seams, structural fasteners, and access doors have been properly sealed, the cell will hold fuel without leaking. This type of construction is usually referred to as a "wet wing."
Fuel Lines and Fittings In an aircraft fuel system, the various tanks and other components are usually joined together by fuel lines made of metal tubing connected, where flexibility is necessary, by lengths of flexible hose. The metal tubing usually is made of aluminum alloy, and the flexible hose is made of synthetic rubber or Teflon. The diameter of the tubing is governed by the fuel flow requirements of the engine.
Each fuel line is identified by a color-coded band near each end. Except for short lines between flexible connections, tubing should be properly supported by clamping to structural members of the aircraft.
A special heat-resistant hose is used where the 83
Fuel outlet Primer line connection Fuel inlet Fuel outlet Cast metal cover.
- Screen Clamp Thumb nut Transparent bowl Fuel inlet FIGURE 4-9. Main fuel strainer for light aircraft.
flexible lines will be subjected to intense heat.
For all flexible fuel lines located forward of the firewall, fire-resistant hose is used.
In many installations, the fuel lines are designed to be located within the tanks. Therefore, minor leaks occurring within the tank are classified as internal leaks and will not cause fire hazards.
Fuel Strainers Strainers are installed in the tank outlets and frequently in the tank filler necks. These are of fairly coarse mesh and prevent only the larger particles from entering the fuel system. Other, fine-mesh, strainers are provided in the carburetor fuel inlets and in the fuel lines.
The function of the main strainer is important:
it not only prevents foreign matter from entering the carburetor, but also, because of its location at the low point of the fuel system, traps any small amount of water that may be present in the system. In multiengine aircraft, one main strainer is usually installed in each engine nacelle.
A main fuel strainer for a light airplane is shown in figure 4-9. It consists of a cast metal top, a screen, and a glass bowl. The bowl is attached to the cover by a clamp and thumb nut.
Fuel enters the unit through the inlet port, filters through the screen, and exits through the outlet port. At regular intervals the glass bowl is drained, and the screen is removed for inspection and cleaning.
The main fuel strainer shown in figure 4-10 is so Out - Drain cock Fuel flow through strainer element FIGURE 4-10. Main fuel strainer.
installed that the fuel flows through it before reaching the engine-driven pump. It is located at the lowest point in the fuel system. The shape and construction of the fine-mesh screen provides 84
HI OFF LOW FIGURE 4-11. Centrifugal fuel booster pump.
a large screening surface encased in a compact housing. Reinforcing the screen is a coarse, heavywire mesh.
Auxiliary Fuel Pumps The electrically driven centrifugal booster pump, shown in figure 4-I1, supplies fuel under pressure to the inlet of the engine-driven fuel pump. This type of pump is an essential part of the fuel system, particularly at high altitudes, to keep the pressure on the suction side of the engine-driven pump from becoming low enough to permit the fuel to boil. This booster pump is also used to transfer fuel from one tank to another, to supply fuel under pressure for priming when starting the engine, and, as an emergency unit, to supply fuel to the carburetor in case the enginedriven pump fails. To increase the capacity of the pump under emergency conditions, many pumps are equipped with a two-speed or variable-speed control so that the recommended fuel inlet pressure to the carburetor can be maintained. As a precautionary measure, the booster pump is always turned on during takeoffs and landings to ensure a positive supply of fuel.
The booster pump is mounted at the tank outlet within a detachable sump or is submerged in fuel at the bottom of the fuel tank. The seals between the impeller and the power section of the pump prevent leakage of fuel or fumes into the motor.
If any liquid or vapor should leak past the seal, it is vented overboard through a drain. As an added precaution in nonsubmerged-type pumps, air is allowed to circulate around the motor to remove dangerous fuel vapor.
As fuel enters the pump from the tank, a highspeed impeller throws the fuel outward in all directions at high velocity. The high rotational speed swirls the fuel and produces a centrifuge action that separates air and vapor from the fuel before it enters the fuel line to the carburetor.
This results in practically vapor-free fuel delivery to the carburetor and permits the separated vapors to rise through the fuel tank and escape through the tank vents. Since a centrifugal-type pump is not a positive-displacement pump, no relief valve is necessary.
Although the centrifugal type is the most common type of booster pump, there are still a few sliding-vane-type booster pumps in service.
This type, too, is driven by an electric motor.
Unlike the centrifugal type, it does not have the advantage of the centrifuge action to separate the vapor from the fuel. Since it is a positivedisplacement-type pump, it must have a reliet valve to prevent excessive pressure. Its construction and operation are identical to the enginedriven pump.
Hand Pump The hand, or wobble, pump is frequently used on light aircraft. It is generally located near other fuel system components and operated from the cockpit by suitable controls. A diagram of a wobble pump is shown in figure 4-12. When the handle attached to the central blade is operated, the low pressure created on the chamber below the upward moving blade, permits the incoming fuel pressure to lift the lower flapper and allows fuel to flow into this chamber. At the same time fuel flows through a drilled passageway to fill the chamber above the downward moving blade. As the blade moves downward, the lower flapper closes, preventing fuel from escaping back into the inlet line. The fuel below the downward moving blade flows through a passageway into another chamber and is discharged through an outlet flapper valve to the carburetor. The cycle is re- 85
Fuel outlet Flapper Blade Inlet fuel Discharge fuel Fuel inlet Booster pump pressure A. Balance line C. Pump inlet FICURE 4-12. Schematic diagram of a wobble B. Pump outlet D. Bypass valve pump.
FIGURE 4-14. Engine-driven fuel pump peated each time the handle is moved in either (bypass flow).
direction.
Engine-Driven Fuel Pump The purpose of the engine-driven fuel pump is to deliver a continuous supply of fuel at the proper pressure at all times during engine operation. The pump widely used at the present time is the positive-displacement, rotary-vane-type A schematic diagram of a typical engine-driven pump (vane-type) is shown in figure 4-13. Regardless of variations in design, the operating principle of all vane-type fuel pumps is the same.
= Booster pump pressure Eta Engine-driven pump pressure A. Balance line B. Pump outlet C. Pump inlet FIGURE 4-13. Engine-driven fuel pump (pressure delivery).
The engine-driven pump is usually mounted on the accessory section of the engine. The rotor, with its sliding vanes, is driven by the crankshaft through the accessory gearing. Note how the vanes carry fuel from the inlet to the outlet as the rotor turns in the direction indicated. A seal prevents leakage at the point where the drive shaft enters the pump body, and a drain carries away any fuel that leaks past the seal. Since the fuel provides enough lubrication for the pump, no special lubrication is necessary.
Since the engine-driven fuel pump normally discharges more fuel than the engine requires, there must be some way of relieving excess fuel to prevent excessive fuel pressures at the fuel inlet of the carburetor. This is accomplished through the use of a spring-loaded relief valve that can be adjusted to deliver fuel at the recommended pressure for a particular carburetor. Figure 4-13, shows the pressure relief valve in operation, bypassing excess fuel back to the inlet side of the pump. Adjustment is made by increasing or decreasing the tension of the spring The relief valve of the engine-driven pump is designed to open at the set pressure regardless of the pressure of the fuel entering the pump. To maintain the proper relation between fuel pressure and carburetor inlet air pressure, the chamber above the fuel pump relief valve is vented either to the atmosphere or through a balance line to carburetor air inlet pressure. The combined pressures of spring tension and either atmospheric or 86
carburetor inlet air pressure determine the absolute pressure at which the relief valve opens. This balanced-type relief valve has certain objectionable features that must be investigated when encountering fuel system troubles. A sylphon or diaphragm failure will allow air to enter the fuel on the inlet side of the pump if the pump inlet pressure is less than atmospheric. Conversely, if the pump inlet pressure is above atmospheric pressure, fuel will be discharged from the vent.
For proper altitude compensation the vent must be open. If it should become clogged by ice or foreign matter while at altitude, the fuel pressure will decrease during descent. If the vent becomes clogged during ascent, the fuel pressure will increase as the altitude is increased.
In addition to the relief valve, the fuel pump has a bypass valve that permits fuel to flow around the pump rotor whenever the pump is inoperative. This valve, shown in figure 4-14, consists of a disk that is lightly spring-loaded against a series of ports in the relief valve head. When fuel is needed for starting the engine, or in the event of engine-driven pump failure, fuel at booster-pump pressure is delivered to the fuel pump inlet. When the pressure is great enough to move the bypass disk from its seat, fuel is allowed to enter the carburetor for priming or metering.
When the engine-driven pump is in operation, the pressure built up on the outlet side of the pump, together with the pressure of the bypass spring, holds the disk on its seat and prevents fuel flow through the ports.
Valves Selector valves are installed in the fuel system to provide a means for shutting off the fuel flow, for tank and engine selection, for crossfeed, and for fuel transfer. The size and number of ports (openings) vary with the type of installation, For example, a single-engine aircraft with two fuel tanks and a reserve fuel supply requires a valve with four ports--three inlets from the tanks and a common outlet. The valve must accommodate the full flow capacity of the fuel line, must not leak, and must operate freely with a definite "feel" or "click" when it is in the correct position.
Selector valves may be operated either manually or electrically. A tube, rod, or cable is attached to a manually operated valve so that it can be operated from the cockpit. Electrically operated valves have an actuator, or motor. The three main types of selector valves are the poppet, cone, and disk.
The poppet-type selector valve has an individual poppet valve at each inlet port. A cam and yoke on the same shaft act to open the selected poppet valve as the yoke is turned. Figure 4-15 shows how the cam lifts the upper poppet valve from its seat when the control handle is set to the "number 2" tank. This opens the passage from the "number 2" tank to the engine. At the same time, a raised portion of the index plate drops into a notch in the side of the cam. (See the detail of the index mechanism.) This produces the "feel" that indicates the valve is in the wide open position. The control handle should always be set by "feel" rather than by the marking on the indicator dial. The index mechanism also keeps the valve in the desired position and prevents creeping caused by vibration. Some valves have more than one raised portion on the cam to allow two or more ports to be opened at the same time.
The cone-type selector valve has either an allmetal or a cork-faced aluminum housing. The cone, which fits into the housing, is rotated by means of a cockpit control. To supply fuel from the desired tank, the cockpit control is turned until the passages in the cone align with the correct ports in the housing. An indexing mechanism aids in obtaining the desired setting and also holds the cone in the selected position. Some cone-type valves have a friction release mechanism that reduces the amount of turning torque required to make a tank selection and that can be adjusted to prevent leakage.
The rotor of the disk-type selector valve fits into a cylindrical hole in the valve body. A disktype valve is shown in figure 4-16. Note that the rotor has one open port and several sealing disksone for each port in the housing. To select a tank, the rotor is turned until the open port aligns with the port from which fuel flow is desired. At this time, all other ports are closed by the sealing disks. In this position, fuel will flow from the desired tank to the selector valve and out through the engine-feed port at the bottom of the valve.
To ensure positive port alignment for full fuel flow, the indexing mechanism (shown in the center of figure 1-16 forces a spring-loaded ball into a ratchet ring. When the selector valve is placed in the closed position, the open port in the rotor is opposite a blank in the valve body, while each sealing disk covers a tank port.
Reserve tank From tank 1 Jam Index plate 'Tank No. 2 From reserve To main strainer tank From tank 2 To main strainer The index mechanism Tank No. 1 FIGURE 4-15. Poppet-type selector valve.
Fuel tank shutoff valves have two positions, open and closed. They are installed in the system to prevent fuel loss when a fuel system component is being removed or when a part of the system is damaged. In some installations they are used to control the fuel flow during fuel transfer. They are operated either manually or electrically. An electrically operated fuel shutoff valve includes a reversible electric motor linked to a sliding-valve assembly. The motor moves the valve gate in and out of the passage through which the fuel flows, thus, shutting off or turning on the fuel flow.
FUEL SYSTEM INDICATORS Fuel Quantity Gages Fuel quantity gages are necessary so that the operator may know the quantity of fuel remaining in the tanks during operation of the aircraft. The four general types of fuel gages are: (1) Sight glass, (2) mechanical, (3) electrical, and (4) electronic. The type of fuel gage installation depends on the size of the aircraft and the number and location of the fuel tanks. Since the sight glass and mechanical fuel gages are not suitable for aircraft where tanks are located an appreciable distance from the cockpit, larger aircraft use either electrical or electronic fuel quantity gages. On some aircraft, one fuel gage, called a totalizer, indicates the total amount of fuel remaining in all the fuel tanks.
The sight glass is the simplest form of fuel quantity gage. The indicator is a glass or plastic tube placed on the same level as the tank. It operates on the principle that a liquid seeks its Valve body mechanism Rotor Sealing disk Index mechanism FIGURE 4-16. Disk-type selector valve.