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 Turbine fuels should be allowed to settle for a period of one hour per foot of depth of the fuel before being withdrawn for use.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 Fuel being withdrawn from storage should be passed through a filter-separator meeting the specification MIL-F-8508A.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.
Float FIGURE 4-17. Float-and-lever type fuel level gage.
own level. The tube is calibrated in gallons or has a metal scale near it. The sight glass may have a shutoff valve so that the fuel can be shut off for cleaning and for preventing loss of fuel if the tube is broken.
The mechanical-type fuel quantity gage is usually located in the tank and is known as a direct reading gage. It has an indicator connected to a float resting on the surface of the fuel. As the fuel level changes, the float mechanically operates the indicator, thus showing the level of fuel in the tank. One type of mechanical fuel gage is illustrated in figure 4-17.
The electrical-type quantity gage consists of an indicator in the cockpit and a float-operated transmitter installed in the tank. As the fuel level changes, the transmitter sends an electric signal to the indicator, which shows the changing fuel level. Two important advantages of this fuel quantity gage (and the electronic type discussed in the next paragraph) are that the indicator can be located any distance from the tank and the fuel levels of several tanks can be read on one indicator.
The electronic-type (capacitance) fuel quantity gage differs from the other types in that it has no movable devices in the fuel tank. Instead of floats and their attendant mechanical units, the dielectric qualities of fuel and air furnish a measurement of fuel quantity. Essentially, the tank transmitter is a simple electric condenser. The dielectric (or nonconducting material) of the condenser is fuel and air (vapor) above the fuel.
The capacitance of the tank unit at any one time will depend on the existing proportion of fuel and vapors in the tank. The capacitance of the transmitter is compared to a reference capacitor in a rebalance-type bridge circuit. The unbalanced signal is amplified by the voltage amplifiers that drive a phase discriminating power stage. The output stage supplies power to one phase of a two-phase a.c. motor that mechanically drives a rebalancing potentiometer and indicator pointer.
The electronic type system of measuring fuel quantity is more accurate in measuring fuel level, as it measures the fuel by weight instead of in gallons. Fuel volume will vary with temperature (a gallon of gasoline weighs more when it is cold than when it is hot); thus, if it is measured in pounds instead of gallons, the measurement will be more accurate.
In addition to the cockpit fuel quantity indicating system, some aircraft are provided with a means to determine the fuel quantity in each tank when the aircraft is on the ground. This is accomplished in several different ways. Some manufacturers use float-operated, direct-reading fuel gages mounted in the lower surface of the wing. Another means is to use under-wing bayonet gages. There are two types in use, the drip gage and the sight gage.
When using the drip gage it is necessary to proceed slowly, using the trial-and-error method to find the exact fuel level. In large area tanks a proportionately large amount of fuel is represented by a fraction of an inch variation in fuel level.
The long, hollow drip tubes require some time to drain once they are filled with fuel, and a substantial error in reading will be made if the diminishing drainage drip is mistaken for the steady drip that signifies that the tube is properly po- When the cap and hollow drip tube are drawn out from the lower wing surface, the fuel enters the open top of the tube when it reaches the level of the fuel. As stated previously steady drip from a drip hole signifies that the tube is properly positioned with a tiny head of fuel above the opening. The drip gage tube may be calibrated in pounds or inches. When calibrated in inches, the reading is compared with a special chart to give a reading of fuel quantity in gallons.
The sight gage is somewhat simpler in construction than the drip gage, and offers unmistakable visual evidence when it is properly positioned for reading. As shown in figure 4-18, the sight gage is basically a long lucite rod, protected by a calibrated tube, which terminates at the top in an exposed quartz tip. When the tip is above the fuel it acts as a reflector. Light rays traveling up the lucite rod are deflected at right angles by 89
Fuel Flowmeter Quartz tip (May be either conical The fuel flowmeter is normally used only in or chisel point) multiengine aircraft. The system consists of a transmitter and an indicator. The transmitter is Lucite rod installed in the fuel inlet line to the engine, where Permanently it measures the rate of fuel flow. The transmitter mounted is electrically connected to the indicator located Protective tube housing in the cockpit. This gage shows the rate of fuel (Has embossed consumption in pounds per hour.
calibration marks) The transmitter signal may be developed by a Removable guide movable vane mounted in the fuel flow path.
housing The impact of fuel causes the vane to swing and move against the restraining force of a calibrated spring. The final position assumed by the vane represents a measure of the rate at which fuel is Latch spring passing through the flowmeter and the corresponding signal to be sent to the indicator. A vane-type fuel flowmeter system is illustrated in figure 4-19.
The transmitter used with turbine engines is the mass-flow type having a range of 500 to 2,500 Read tank quantity at this point Latching lug and pin, Lower surface of wing typical three places (Retains bayonet Sight in stowed window position) FIGURE 1-18. Under-wing sight gage.
the 45° surface at one side of the tip and deflected 90° again by the 45° surface at the opposite side and returned down the lucite rod.
Any portion of the tip submerged in fuel will not act as a reflector. Consequently, when the fuel level is part way up the taper, a light pattern is created that is visible at the lower end of the lucite rod and that has the dimension and shape described by the intersection of the tip and the fuel. When the reflected light is reduced to the smallest perceptible point in the case of conetipped gages, or hairline in the case of chisel-tipped gages, the rod is properly positioned. The fuel tank quantity can be read on the tube where it emerges from the recessed guide housing. Drip gage readings are taken at this location also.
pounds per hour. It consists of two cylinders placed in the fuel stream so that the direction of fuel flow is parallel to the axes of the cylinders.
(See figure 4-20.) The cylinders have small vanes in the outer periphery. The upstream cylinder, called the impeller, is driven at a constant angular velocity by the power supply. This velocity imparts an angular momentum to the fuel. The fuel then transmits this angular velocity to the turbine (the downstream cylinder), causing the turbine to rotate until a restraining spring force balances the force due to the angular momentum of the fuel. The deflection of the turbine positions a magnet in the second harmonic transmitter to a position corresponding to the fuel flow. The turbine position is transmitted to the flight station indicator by means of a selsyn system.
Fuel Pressure Gage The fuel pressure gage indicates the pressure of the fuel entering the carburetor. This gage may be included with the oil pressure gage and the oil temperature gage in one casing, called the engine gage unit. Most aircraft today have separate gages for these functions. An engine gage unit is shown in figure 4-21.
The fuel pressure gage is a differential pressure indicator with two connections on the back of the indicator housing. The air connection (see figure 4-22) is vented to the carburetor air inlet, and the fuel connection is attached to the fuel 90
26 V 400 - AC Transmitter Fuel flow — Restraining spring FIGURE 4-19. Vane-type fuel flowmeter system.
inlet chamber of the carburetor. In this way the gage indicates the difference between the fuel pressure entering the carburetor and the air pressure at the carburetor air inlet. In some installations, the air fitting on the gage is left open to the air pressure of the cockpit, which is generally the same as the pressure of the atmosphere. When this venting arrangement is used, the relief valve of the engine-driven fuel pump is also vented to the atmosphere, and the gage indicates the fuel pressure resulting from the adjusted spring pressure only. In order to dampen pressure pulsations that cause pointer fluctuation, a restrictor fitting (A) is installed at the carburetor end of the fuel gage line. (See the Y connection shown in figure
4-22.) The second restrictor (B) meters fuel to the oil system during oil dilution. The arrangement of these restrictors provides an indicated drop in fuel pressure when the oil dilution system is used. The oil-dilution system will be discussed Decoupling disk Fuel flow Second harmonic transmitter Impeller motor Calibrated restraining Fluid passage Fluid passage springs | 115VAC Timing contacts B 00 28VDC Indicator Power supply FIGURE 4-20. Schematic of a turbine engine fuel flow indicating system.
200 200 20 FUEL / 11 FIGURE 4-21. Engine gage unit.
thoroughly in the Powerplant Handbook, and is mentioned at this time only because the fuel pressure indicator provides a means for a check on the operation of other fuel system units.
In small aircraft the fuel pressure gage may be actuated by a Bourdon tube (an instrument that converts changes in pressure to mechanical motion), or an aneroid and bellows type, installed with a pressure line leading directly from the carburetor to the indicator. On larger aircraft, where the fuel pressure gage is located some distance from the carburetor, a transmitter is usually installed. The pressure transmitter may be a simple cast metal cell that is divided into two chambers by a flexible diaphragm. Pressure applied by the fuel source to the transmitter inlet pushes against the diaphragm and builds up an equal pressure to a thin fluid (highly refined kerosene), which transfers the pressure to the indicator mechanism. Some installations, however, use electrical transmitters to register fuel pressure on the gage. In this electrical arrangement, the pressure-indicating unit is contained in the transmitter. Fuel pressure, acting upon the aneroid and bellows portion of the unit, causes motion of one part of an electrical unit (the synchro transmitter). As the unit turns, it causes a similar movement of a corresponding unit (the synchro motor). This receiving unit actuates the indicator on the instrument panel. These pressure and electrical arrangements make it unnecessary for combustible fuel to enter the cockpit or flight deck, thereby reducing fire risk.
A fuel pressure gage often used with fuel injection systems on light aircraft engines is illustrated in figure 4-23. A gage of this type registers metered fuel pressure at the fuel injection unit distributor valve and is a direct indication of engine power output when installed in a fuel injection system for light aircraft engines. The dial of the gage is marked to indicate percent of power. The gage does not indicate either the engine-driven pump or the boost pump pressure.
Pressure Warning Signal In an aircraft with several tanks, there is always the possible danger of allowing the fuel supply in one tank to become exhausted before the selector valve is switched to another. To prevent this, pressure-warning signals are installed in some aircraft. The complete installation, shown in figure 4-22, consists of a pressure-sensitive mechanism and a warning light. The warning mechanism has both a fuel and an air connection.
The connection marked "fuel is connected to the fuel pressure line of the carburetor. The air connection is vented to either atmospheric or carburetor air inlet pressure. This arrangement prevents the warning mechanism from acting in response to changes in the absolute pressure of the fuel. If, for example, the absolute pressure of the fuel decreases because of a change in atmospheric or carburetor air inlet pressure, the change is also reflected at the warning mechanism, which then cancels the effects of the change. Normal fuel pressure against the power surface of the diaphragm holds the electrical contacts apart. When the fuel pressure drops below specifed limits, the contacts close and the warning light is turned on.
This alerts the operator to take whatever action is necessary to boost the fuel pressure.
Valve-In-Transit Indicator Lights On large multiengine aircraft, each of the fuel crossfeed and line valves may be provided with a valve-in-transit indicator light. This light is on only during the time the valve is in motion and is off when movement is complete.
Fuel Temperature Indicator A means for checking the temperature of the fuel in the tanks and at the engine is provided on some turbine-powered aircraft. During extreme cold, especially at altitude, the gage can 92