Fuel Management and Indicating

Tank E F Tank resistor N Float S A B + DC power Dial Housing Pinion Cover glass Crank Gasket Float Pointer magnet Float primary sensing element, but a mechanical linkage is connected to move a pointer across the dial face of an instrument. This can be done with a crank and pinion arrangement that drives the pointer with gears, or with a magnetic coupling, to the pointer. [Figure 14-68] Electric fuel quantity indicators are more common than mechanical indicators in modern aircraft. Most of these units operate with direct current (DC) and use variable resistance in a circuit to drive a ratiometer-type indicator. The movement of a float in the tank moves a connecting arm to the wiper on a variable resistor in the tank unit. This resistor is wired in series with one of the coils of the ratiometer-type fuel gauge in the instrument panel. Changes to the current flowing through the tank unit resistor change the current flowing through one of the coils in the indicator. This alters the magnetic field in which the indicating pointer pivots. The calibrated dial indicates the corresponding fuel quantity. [Figure 14-69] Digital indicators are available that work with the same variable resistance signal from the tank unit. They convert the variable resistance into a digital display in the flight deck instrument head. [Figure 14-70] Fully digital instrumentation systems, such as those found in a glass flight deck aircraft, 14-37 LEFT RIGHT FUEL L R 200 FUEL QTY GAL ENGINE MAP A B C indication of a Garmin G-1000 flat screen display is shown in C.

convert the variable resistance into a digital signal to be processed in a computer and displayed on a flat screen panel. Large and high-performance aircraft typically utilize electronic fuel quantity systems. This is also known as a capacitance-type fuel quantity indicating system. These more costly systems have the advantage of having no moving parts in the tank sending units. Variable capacitance transmitters are installed in the fuel tanks extending from the top to the bottom of each tank in the usable fuel. Several of these tank units, or fuel probes as they are sometimes called, may be installed in a large tank. [Figure 14-71] They are wired in parallel. As the level of the fuel changes, the capacitance of each unit changes. The capacitance transmitted by all of the probes in a tank is totaled and compared in a bridge circuit by a microchip computer in the tank’s digital fuel quantity indicator in the flight deck. As the aircraft maneuvers, some probes are in more fuel than others due to the attitude of the aircraft. The indication remains steady, because the total capacitance transmitted by all of the probes remains the same.

A trimmer is used to match the capacitance output with the precalibrated quantity indicator. A capacitor is a device that stores electricity. The amount it can store depends on three factors: the area of its plates, the distance between the plates, and the dielectric constant of the material separating the plates. A fuel tank unit contains two concentric plates that are a fixed distance apart. Therefore, the capacitance of a unit can change if the dielectric constant of the material separating the plates varies. The units are open at the top and bottom, so they can assume the same level of fuel as is in the tanks. Therefore, the material between the plates is either fuel (if the tank is full), air (if the tank is empty), or some ratio of fuel and air depending on how much fuel remains in the tank. Figure 14-72 shows a simplified illustration of this construction.

The bridge circuit that measures the capacitance of the tank units uses a reference capacitor for comparison. When voltage is induced into the bridge, the capacitive reactance of the tank probes and the reference capacitor can be equal or different. The magnitude of the difference is translated into an indication of the fuel quantity in the tank calibrated in pounds. Some European aircraft display this in kilograms 14-38 Outer plate Inner plate capacitance-type fuel tank indicator system as the space between the inner and outer plates is filled with varying quantities of fuel and air depending on the amount of fuel in the tank.

(kg). Figure 14-73 represents the nature of this comparison bridge circuit. The use of tank unit capacitors, a reference capacitor, and a microchip bridge circuit in the fuel quantity indicators is complicated by the fact that temperature affects the dielectric constant of the fuel. A compensator unit (mounted low in the tank so it is always covered with fuel) is wired into the bridge circuit. It modifies current flow to reflect temperature variations of the fuel, which affect fuel density and thus capacitance of the tank units. [Figure 14-74] An amplifier is also needed in older systems. The amplitude of the electric signals must be increased to move the servo motor in the analog indicator. Additionally, the dielectric constant of different turbine-engine fuels approved for a particular aircraft may also vary. Calibration is required to overcome this.

Fuel tank probe Reference capacitor 115 V 400 Hz AC Indicator Fuel system. FWD Compensator unit Electrical terminal Tank unit Upper wing skin Lower wing skin installed inside a wing tank. A fuel totalizer is part of the capacitance-type fuel quantity indication system. It is used to add the tank quantities from all indicators. This total aircraft fuel quantity can be used by the crew and by flight management computers for calculating optimum airspeed and engine performance limits for climb, cruise, descent, etc. Capacitance-type fuel quantity system test units are available for troubleshooting and ensuring proper functioning and calibration of the indicating system components.

Many aircraft with capacitance-type fuel indicating systems also use a mechanical indication system to cross-check fuel quantity indications and to ascertain the amount of fuel onboard the aircraft when electrical power is not available. A handful of fuel measuring sticks, or drip sticks, are mounted throughout each tank. When pushed and rotated, the drip stick can be lowered until fuel begins to exit the hole on the bottom of each stick. This is the point at which the top of the stick is equal to the height of the fuel. The sticks have a calibrated scale on them. By adding the indications of all of the drip sticks and converting to pounds or gallons via a chart supplied by the manufacturer, the quantity of the fuel in the tank can be ascertained. [Figure 14-75] Fuel Flow Meters A fuel flow meter indicates an engine’s fuel use in real time. This can be useful to the pilot for ascertaining engine performance and for flight planning calculations. The types of fuel flow meter used on an aircraft depends primarily on the powerplant being used and the associated fuel system.

14-39 11 12Read fuel quantity here Drip hole Locking pin Fuel tank access panel until fuel drips out the hole at the bottom. By reading the calibrated scale and adding readings from all tank drip sticks, a chart can be consulted to arrive at the total fuel quantity on the aircraft by weight or by volume. Measuring fuel flow accurately is complicated by the fact that the fuel mass changes with temperature or with the type of fuel used in turbine engines. In light aircraft with reciprocating engines, systems have been devised to measure fuel volume. The actual mass of fuel flowing to the engine is based on an assumption of the average weight of the fuel per unit volume.

The simplest fuel flow sensing device is used in conjunction with fuel injection systems installed on horizontally opposed reciprocating engines. A pressure gauge is used but it is calibrated in gallons per hour or pounds per hour. The amount of fuel that is flowing through the fuel injectors has a direct relationship to the pressure drop across the fuel injector orifices. Therefore, monitoring fuel pressure at the injector(s) closely approximates fuel flow and provides useful flow information for mixture control and flight planning. There is a major limitation to the use of fuel pressure as a flow indicator. Should an injector become clogged, fuel flow is reduced. However, the pressure gauge indicates a higher fuel pressure (and greater fuel flow) due to the restriction.

Operators must be aware of this potential condition and check the flow meter against EGT to determine the nature of the elevated indication. [Figure 14-76] Large reciprocating engine fuel systems may use a vane- type fuel flow meter that measures the volume of the fuel consumed by the engine. The fuel flow unit is typically located between the engine-driven fuel pump and the carburetor. The entire volume of fuel delivered to the engine is made to pass through the flow meter. Inside, the fuel pushes against the vane, which counters the force of the fuel flow with a calibrated spring. The vane shaft rotates varying degrees matching the fuel flow rate through the unit. An autosyn transmitter deflects the pointer on the flight deck fuel flow gauge the same amount as the vane deflects. The dial face of the indicator is calibrated in gallons per hour or pounds per hour based on an average weight of fuel.

Since fuel fed to the engine must pass through the flow meter unit, a relief valve is incorporated to bypass the fuel around the vane should it malfunction and restrict normal fuel flow. The vane chamber is eccentric. As more fuel pushes against the vane, it rotates further around in the chamber. The volume of the chamber gradually increases to 0 5 10 15 20 25 30 35 40 Nozzle pressure gauge calibrated in pounds per hour of fuel flow Fuel injection system flow divider From fuel injector regulator Fuel nozzle 14-40 20 40 10 30 Fuel outletFuel inlet Fuel flow Measuring vane Rotor Measuring vane Stator Damper vane Hairspring RotorStator transmitter replicates the vane shaft rotation on the flight deck indicator that is calibrated in gallons or pounds of fuel flow per hour.

permit the greater flow of fuel without restriction or pressure flight deck. The mass flow indicator takes advantage of the buildup. [Figure 14-77] direct relationship between fuel mass and viscosity. Fuel is swirled by a cylindrical impeller that rotates at a fixed speed. Turbine-engine aircraft experience the greatest range of fuel The outflow deflects a turbine just downstream of the impeller. density from temperature variation and fuel composition. The turbine is held with calibrated springs. Since the impeller An elaborate fuel flow device is used on these aircraft. It motor swirls, the fuel at a fixed rate, any variation of the measures fuel mass for accurate fuel flow indication in the turbine deflection is caused by the volume and viscosity of P MS N PM S N E A B C D E A B C D A B C Indicator Transmitter Fluid passage Impeller motor Decoupling discCalibrated restraining springs Impeller Turbine Fuel flow Fluid passage 115-volt AC To three-phase power supply mass to display fuel flow in pounds per hour.

14-41 the fuel. The viscosity component represents the mass of the fuel. [Figure 14-78] An alternating current (AC) synchro system is part of the mass fuel flow meter. It is used to position a pointer against the flight deck indicator scale calibrated in pounds per hour. With accurate fuel flow knowledge, numerous calculations can be performed to aid the pilot’s situational awareness and flight planning. Most high-performance aircraft have a fuel totalizer that electronically calculates and displays information, such as total fuel used, total fuel remaining onboard the aircraft, total range and flight time remaining at the present airspeed, rate of fuel consumption, etc. On light aircraft, it is common to replace the original analog fuel indicators with electronic gauges containing similar capabilities and built-in logic. Some of these fuel computers, as they are called, integrate global positioning satellite (GPS) location information. [Figure 14-79] Aircraft with fully digital flight decks process fuel flow data in computers and display a wide array of fuel flow related information on demand.

Relatively new types of fuel flow sensors/transmitters are available in new aircraft and for retrofit to older aircraft. One type of device found in home-built and experimental aircraft uses a turbine that rotates in the fuel flow. The higher the flow rate is, the faster the turbine rotates. A Hall effect transducer is used to convert the speed of the turbine to an electrical signal to be used by an advanced fuel gauge similar GAL. GAL. REM. L. FUEL FLOW R. FUEL FLOW USED G P H ADD FULL TEST ENTER GAL TO DESTINATION GAL RESERVE ENDURANCE NM/GAL microprocessor to display fuel flow and numerous other fuel consumption related calculations.

to a fuel computer to produce a variety of calculated readouts and warnings. The turbine in this unit is in line with the fuel flow but is fail-safe to allow adequate fuel flow without interruption should the unit malfunction. [Figure 14-80] Another fuel flow sensor used primarily on light aircraft also detects the spinning velocity of a turbine in the fuel path. It too has a fail-safe design should the turbine malfunction. In this unit, notches in the rotor interrupt an infrared light beam between an LED and phototransistor that creates a signal proportional to the amount fuel flow. [Figure 14-81] This type of sensor may be coupled with an electronic indicator.

Power Fuel FLOW T OTAL This side slightly lower This side slightly higher (allows bubbles to exit) Flow direction is menu driven with numerous display options. 14-42 produces a current pulse signal from an opto-electronic pickup with a preamplifier. Increasing use of microprocessors and computers on aircraft enable the integration of fuel temperature and other compensating factors to produce highly accurate fuel flow information. Fuel flow sensing with digital output facilitates this with a high degree of reliability. Thermal dispersion technology provides flow sensing with no moving parts and digital output signals. The sensor consists of two resistance temperature detectors (RTDs). One is a reference RTD that measures the temperature of the fuel. The other is the active RTD. It is heated by an adjacent element to a temperature higher than the fuel. As the fuel flows, the active element cools proportionally to the fuel flow. The temperature difference between the two RTDs is highest at no flow.

The RTDs are connected to an electronic assembly that supplies power to the heater and uses sensing circuitry and a microprocessor to control a constant temperature difference between the heated and unheated RTDs. The electrical current to the heater is proportional to the mass flow of the fuel. As mentioned, the reference RTD is used as a temperature sensor to provide a temperature output and allow for temperature compensation of the flow measurement. [Figure 14-82] Fuel Temperature Gauges As previously mentioned, monitoring fuel temperature can inform the pilot when fuel temperature approaches that which could cause ice to form in the fuel system, especially at the fuel filter. Many large and high-performance turbine aircraft use a resistance type electric fuel temperature sender in a main fuel tank for this purpose. It can display on a traditional ratiometer gauge [Figure 14-65] or can be input into a computer for processing and digital display.

A low fuel temperature can be corrected with the use of a fuel heater if the aircraft is so equipped. Also as mentioned, fuel temperature can be integrated into fuel flow processing calculations. Viscosity differences at varying fuel technology have no moving parts and output digital signals. temperatures that affect fuel flow sensing accuracy can be corrected via microprocessors and computers. Fuel Pressure Gauges Monitoring fuel pressure can give the pilot early warning of a fuel system related malfunction. Verification that the fuel system is delivering fuel to the fuel metering device can be critical. Simple light reciprocating-engine aircraft typically utilize a direct reading Bourdon tube pressure gauge. It is connected into the fuel inlet of the fuel metering device with a line extending to the back of the gauge in the flight deck instrument panel. A more complex aircraft may have a sensor with a transducer located at the fuel inlet to the metering device that sends electrical signals to a flight deck gauge.

[Figure 14-83] In aircraft equipped with an auxiliary pump for starting and to back up the engine-driven pump, the fuel pressure gauge indicates the auxiliary pump pressure until the engine is started. When the auxiliary pump is switched off, the gauge indicates the pressure developed by the engine- driven pump. More complex and larger reciprocating engine aircraft may use a differential fuel pressure gauge. It compares fuel inlet pressure to the air inlet pressure at the fuel metering device. A 0 5 10 15 20 25 30 Fuel Press. Psi transducer to display fuel inlet pressure at the metering device.

14-43 0 1 2 3 4 5 6 7 Airbox Diff. PSI FUEL and high-performance reciprocating-engine aircraft compares the fuel inlet pressure to the air inlet pressure at the fuel metering device. bellows type pressure gauge is normally used. [Figure 14- 84] Modern aircraft may use a variety of sensors including solid state types and those with digital output signals or signals that are converted to digital output. These can be processed in the instrument gauge microprocessor, if so equipped, or in a computer and sent to the display unit. [Figure 14-85] Pressure Warning Signal On aircraft of any size, visual and audible warning devices are used in conjunction with gauge indications to draw the pilot’s attention to certain conditions. Fuel pressure is an important parameter that merits the use of a warning signal when it falls outside of the normal operating range. Low fuel pressure warning lights can be illuminated through the use of simple pressure sensing switches. [Figure 14-86] The contacts of the switch will close when fuel pressure against the diaphragm is insufficient to hold them open. This allows current to flow to the annunciator or warning light in the flight deck.

Electrical connections to warning light Contacts Adjustment screw Diaphragm Air Fuel switch that closes when fuel pressure is low. Most turbine-powered aircraft utilize a low-pressure warning switch at the outlet of each fuel boost pump. The annunciator for each is typically positioned adjacent to the boost pump ON/ OFF switch on the fuel panel in the flight deck. [Figure 14-87] Valve-In-Transit Indicator Lights Aircraft with multiple fuel tanks use valves and pumps to move fuel and to have it flow to desired locations, such as the engines, a certain tank, or overboard during fuel jettison. The functioning of the valves in the fuel system is critical.

20+ LEFT TNK3 N/A 20+ RGHT TNK4 N/A FUEL COMPUTER 51.7 GALS REM 10.3 GALS USED 4:17 TIME REM 7.0 NM/GAL 0.0 GALS AT WPT 382 RANGE NM 11.5 GPH 9.8 PSI FlightDEK-D180 fuel pressure. pressure warning lights for each fuel boost pump. 14-44 Some aircraft indicate to the crew when the valve is opening or closing with the use of valve-in-transit lights. Contacts in the valve control the lights that go out when the valve is fully open or when it is fully closed. Alternately, annunciator lights that show the valve position as OPEN or CLOSED are also used. Valve-in-transit and valve position indicators, or lights, are located on the fuel panel in the flight deck adjacent to the valve ON/OFF switches. [Figure 14-88] Sometimes the switch mechanism has the annunciator light built into it. Digital display systems graphically depict valve positions on screen.

Fuel System Repair The integrity of an aircraft fuel system is critical and should not be compromised. Any evidence of malfunction or leak should be addressed before the aircraft is released for flight. The danger of fire, explosion, or fuel starvation in flight makes it imperative that fuel system irregularities be given top priority. Each manufacturer’s maintenance and operation instructions must be used to guide the technician in maintaining the fuel system in airworthy condition. Follow the manufacturer’s instructions at all times. Component CLOSE FUEL QTY TEST ON OFF ON OFF LOW PRESS VALVE IN TRANSIT SHUT OFF VALVE IN TRANSIT CROSS FEED OPEN ENG 1 FUEL QUANTITY 20 0 I0 25 15 5 30 20 0 I0 25 15 5 300 2 4 6 NO 1 TANK FWD AFT BOOST PUMP transport category aircraft fuel panel. Low boost pump pressure lights that look the same are also on the panel.

manufacturers and STC holder instructions should be used when applicable. Some general instructions for fuel system maintenance and repair are given in the following sections. Troubleshooting the Fuel System Knowledge of the fuel system and how it operates is essential when troubleshooting. Manufacturers produce diagrams and descriptions in their maintenance manuals to aid the technician. Study these for insight. Many manuals have troubleshooting charts or flow diagrams that can be followed. As with all troubleshooting, a logical sequence of steps to narrow the problem to a specific component or location should be followed. Defects within the system can often be located by tracing the fuel flow from the tank through the system to the engine. Each component must be functioning as designed and the cause of the defect symptom must be ruled out sequentially.

Location of Leaks & Defects Close visual inspection is required whenever a leak or defect is suspected in a fuel system. Leaks can often be traced to the connection point of two fuel lines or a fuel line and a component. Occasionally, the component itself may have an internal leak. Fuel leaks also occur in fuel tanks and are discussed below. Leaking fuel produces a mark where it travels. It can also cause a stronger than normal odor. Gasoline may collect enough of its dye for it to be visible or an area clean of dirt may form. Jet fuel is difficult to detect at first, but it has a slow evaporation rate. Dirt and dust eventually settle into it, which makes it more visible.

When fuel leaks into an area where the vapors can collect, the leak must be repaired before flight due to the potential for fire or explosion. Repair could be deferred for external leaks that are not in danger of being ignited. However, the source of the leak should be determined and monitored to ensure it does not become worse. Follow the aircraft manufacturer’s instructions on the repair of fuel leaks and the requirements that need to be met for airworthiness. Detailed visual inspection can often reveal a defect. Fuel Leak Classification Four basic classifications are used to describe aircraft fuel leaks: stain, seep, heavy seep, and running leak.

[Figure 14-89] In 30 minutes, the surface area of the collected fuel from a leak is a certain size. This is used as the classification standard. When the area is less than ¾ inch in diameter, the leak is said to be a stain. From ¾ to 1½ inches in diameter, the leak is classified as a seep. Heavy seeps form an area from 1½ inches to 4 inches in diameter. Running leaks pool and actually drip from the aircraft. They may follow the contour of the aircraft for a long distance. 14-45 Fuel will usually flow in this area along skin contour after it is wiped dry Fuel usually drips at this point Size will vary with location and intensity Stain Seep Heavy seep Running leak 3/4" Max.

3/4" to 11/2" 11/2" to 4" Replacement of Gaskets, Seals, & Packings A leak can often be repaired by replacing a gasket or seal. When this occurs, or a component is replaced or reassembled after a maintenance operation, a new gasket, seal, or packing must be installed. Do not use the old one(s). Always be sure to use the correct replacement as identified by part number. Also, most gaskets, seals, and packings have a limited shelf life. They should be used only if they are within the service life stamped on the package. Remove the entire old gasket completely and clean all mating surfaces. Clean surfaces and grooves allow a tight seal. Inspect new gaskets and seals for any flaws. Follow the manufacturer’s instructions for replacement, including cleaning procedures and any sealing compound that you may need to apply during replacement. Torque assembly bolts evenly so as to provide even pressure and prevent pinching.

Fuel Tank Repair Whether rigid removable, bladder-type, or integral, all fuel tanks have the potential to develop leaks. Repair a tank according to the manufacturer’s instructions. Some general notes for repair of each tank type follow. At the time a tank is repaired, a thorough inspection should be made. Corrosion, such as that caused by water and microbes, should be identified and treated at this time, even if it is not the cause of the leak. Rigid removable fuel tanks can be riveted, welded, or soldered together. A leak can develop at any of these types of seams or can be elsewhere on the tank. Generally, the repair must match the construction in technique.

Some metal fuel tanks experiencing minor seepage can be repaired with a sloshing procedure. An approved sloshing compound is poured into the tank, and the tank is moved so that the compound coats the entire inner surface area of the tank. Any excess compound is then poured out and the compound in the tank is allowed to cure for a specified amount of time. Minor gaps in the seams of the tank and repairs are filled in this manner. The compound is fuel resistant once dry. Check with the aircraft manufacturer to ensure that sloshing is an airworthy repair for the aircraft fuel tank in question. Welded Tanks Welded tank repairs are usually done by welding. These tanks can be constructed from steel or weldable aluminum, such as 3003S or 5052SO. The tank is removed from the aircraft for the repair. It must be treated to remove any fuel vapors that remain in the tank before it is welded. This is critical to avoid serious injury from explosion should the fuel vapor ignite. The manufacturer usually gives a procedure for doing this. Some common methods for purging the tank include steam cleaning, hot water purging, and inert gas purging.

Most procedures involve running the steam, water, or gas through the tank for a stated period of time. Adapters may need to be fashioned or purchased for the fill port to enable proper cleaning. Follow the manufacturer’s procedure for the proper time to keep the cleaning medium in the tank and for prepping the tank for welding in general. 14-46