The basic components of a fuel system include tanks, lines, valves,
pumps, filtering units, gauges, 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
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 bladder-type 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
The filler neck and cap are usually located in a recessed well, equipped with a scupper and 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.
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 nonselfsealing 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 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.
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 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 a large
screening surface encased in a compact housing. Reinforcing the screen
is a coarse, heavy wire mesh.
|Auxiliary Fuel Pumps
The electrically driven centrifugal booster pump, shown in figure 4-11, 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 engine driven 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 high speed 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 positive displacement type pump, it must have a relief valve to prevent excessive pressure. Its construction and operation are identical to the engine driven 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 repeated each time the handle is moved in either 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 pump.
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.
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 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 syphon 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.
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 all metal 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 disk-type valve is shown in figure 4-16. Note that the rotor has one open port and several sealing disks - one 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 4-16 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 scaling disk covers a tank port.
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.