4 Quantity Measurement

4.1 Mechanical indicators

4.1.1 Describe the construction and principles of operation of a typical mechanical fuel quantity indication system with particular regard to the following:

a. Dial indicator b. Magnetic coupling c. Float d. Unit mounting e. Damping (2)

a. Dial indicator: Some fuel tanks have a wire arm that rides on top of the fuel. The wire arm moves a bevel gear which drives a pinion. A pointer is attached to the pinion which rides over a dial to indicate the level of fuel in the tank. One type is shown in the figure below.

Figure 1: A direct reading fuel quantity gauge indicates the level of fuel in the tank by converting movement of the float arm into rotation of the pointer

b. Magnetic coupling: In this type of mechanical fuel indicating system, the mechanical fuel indicator operates with nearly a direct connection between the fuel float and the indicator. A magnet on the pointer is magnetically coupled to a magnet inside the tank that is moved by the float arm in the mechanism. Movement of the float arm rotates the pointer and as the parts are connected only by magnetic forces, there is no possibility of leaking through the indicator.

Jeppesen describes the above in this way: Direct reading fuel quantity indicators move a pointer across the dial by magnetic coupling. A float rides on top of the fuel which, through a bevel gear, rotates a horseshoe permanent magnet inside the indicator housing. A pointer attached to a small permanent magnet and mounted on a pivot is separated from the horseshoe magnet by an aluminium alloy diaphragm. The permanent magnet is coupled to the to the horseshoe magnet by the magnetic fields. The pointer will move around the dial to indicate the level of fuel in the tank (see diagram overleaf).

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Figure 2: The mechanical fuel indicator operates with nearly a direct connection between the fuel float and the indicator. The magnetic coupling dampens the sloshing of the fuel and reduces wear on the gauge.

c. Float: As explained above, the float rides on top of the fuel and as it is connected indirectly by an arm to an indicator.

d. Unit mounting: There are various methods of mounting indicator units. All the figures above are direct reading indicators. A sight glass, for example, is a transparent tube connected between the top and bottom of the tank. It can be slanted and the fuel quantity is indicated against a calibrated scale to make it easier to read – especially for shallow tanks. Figure 1 mounts the indicator outside the tank while float mechanism is inside, directly connected by some mechanical means. The figure 2 above has the entire mechanism inside the tank, while the indicator is outside – the only connection being magnetic coupling.

e. Damping: Damping is the resistance to movement of the system components, due to factors such as air resistance, friction and fluid viscosity. Fuel in an aircraft’s tanks slosh around. As the fuel moves it causes the arm to move up and down which in turn causes the instrument to fluctuate. A magnetic type coupling dampens the sloshing effects of the fuel (as do baffle plates and flapper valves in fuel tanks) and helps to reduce wear on the gauge.

4.2 Direct Current Indicators

4.2.1 Describe the construction and principles of operation of a typical DC fuel contents indication system with particular regard to the following:

a. Conversion of float movement to electrical current b. Wiper arm operation c. Resistance material d. Ratiometer-type gauges (2)

a. Conversion of float movement to electrical current:

In most cases, a direct connection between the float and the indicator is not possible. A DC electrical indicator solves this problem. It converts mechanical motion of the float into varying direct current. This current then drives a mechanical indicator or is converted to a digital readout. The components of a float-type system are shown schematically in Fig 3, together with the methods of transmitting electrical signals. The float is attached to an arm pivoted to permit angular movement which is transmitted to an electrical element consisting of either a wiper arm and potentiometer, or a Desyn.

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b. Wiper arm operation:

For many years the most widely used fuel quantity measuring system has been the electrical resistance-type system. These systems use a sender, or transmitter, that consists of a variable resistor mounted on the outside of the fuel tank and operated by an arm connected to float that rides on the surface of the fuel tank. Movement of the arm is transmitted through a bellows-type seal to operate the wiper of the resistor.

Figure 3: A simple float type of fuel quantity indicator. The float rides on the fuel and drives the wiper across the resistance element.

This type of system uses current-measuring instruments which are calibrated in fuel quantity. When the tank is empty, the float is at the bottom of the tank and the resistance is at maximum. This drives the pointer on the gauge to the EMPTY mark. When the tank is full, the wiper resistance is at its minimum and the gauge reads FULL.

c. Resistance material:

Instead of using a resistor, some units use a segment of composition resistance material. A wiper arm driven by the float moves across the segment of composition resistance material, changing the circuit resistance.

d. Ratiometer-type gauges:

There are two types of indicators. Both use current-measuring instruments calibrated in fuel quantity. In the diagram below, the “empty” coil would be the coil on the left and the “full” coil the one on the right. Some of these units signal a full tank with maximum resistance while others do so with minimum resistance. They are common in modern small airplanes and cars.

Figure 4: Ratiometer indicator, when the tank is full, the tank unit resistance is at a minimum and current through the full coil inside the gauge is at a maximum. A permanent magnet attached to the pointer pulls the pointer into alignment with the full coil

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The majority of small general aviation (GA) aircraft use a float gauge system similar to that used in motor vehicles as shown in Fig. 4; this is based on a float connected to a variable resistor adjacent to the tank. The variable resistor is connected into a DC ratiometer circuit where two opposing magnetic fields are created in each of the coils. The pointer is formed with a permanent magnet and is aligned with the resulting magnetic field created by the coils; the pointer moves in accordance with the ratio of currents in the coil.

4.3 Maintenance of Mechanical and DC Systems

4.3.1 Describe the following maintenance practices associated with DC and mechanical fluid contents indicating systems:

a. Installation of system components b. Fuel quantity calibration c. Determining fuel level/resistance relationship d. Wiper deterioration e. Setting of low fuel level warning indicators f. The effects of defects associated with float and float mechanism, wiper, variable

resistance, wiring and quantity gauge (2)

a. Installation of system components:

An aircraft maintenance technician is not allowed to repair aircraft instruments. He is allowed to perform a number of functions:

Installation: He can install instruments into instrument panels, connect power supplies. Simple adjustments: Such as those outlines in a maintenance manual, for example adjusting

the float lever arm so that the arm does not come into contact with the bottom of the tank so the lever arm has a full range of movement.

Place range markings on the faces of the instruments.

b. Fuel quantity calibration:

Each fuel quantity indicator must be calibrated to read zero during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply. Look for dents or other distortions, such as a partially-collapsed tank caused by an obstructed fuel tank vent; these can adversely affect fuel quantity gauge accuracy and tank capacity. Compare the indications of the gauges when dipping the tanks so accuracy can be checked. Remember that during some manoeuvres, slipping skidding and in turbulence, fuel tank indications can and will vary accordingly.

If the aircraft is equipped with a fuel flow indicator its indications should be checked against the level of fuel in the tank after the flight and the expected calculated fuel consumption. In some countries it is common to dip the tanks to verify the amount of fuel. It is the most accurate way. But as each aircraft (and tank in that aircraft) is somewhat different it is important to use the correct dipstick for your aircraft. Each dipstick is calibrated to the fuel tanks of that aircraft. It should have the aircraft registration number marked on it too.

c. Determining fuel level/resistance relationship:

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The gauge senses the resistance sent by the transmitter and thereby indicates the level of fuel. By checking the resistance of the transmitter and gradually increasing the amount of fuel in a tank, a relationship between resistance and quantity can be determined.

d. Wiper deterioration:

Because the wiper moves across the variable resistor, it will inevitably wear out and dead spots can occur on the resistor, where contact is lost and therefore the gauge reading will be inaccurate.

e. Setting of low fuel level warning indicators:

Depends on the aircraft, but when fuel in a tank gets to a certain level (in a Jetstream 4100 this is 279 lbs), a sensor in the tank causes a low fuel level warning indicator to come on. The warning indication is usually only accurate in level flight. Some tank sensors will have a timer delay built in to allow for sloshing in the tank.

f. The effects of defects associated with float and float mechanism, wiper, variable resistance, wiring and quantity gauge:

Check with the appropriate AMM. If the float arm touches the bottom of the tank and does not have the full range of movement, then the arm can be ‘bent’. Check the float to see that it is not punctured or come loose in any way from the arm. Check that the arm can pivot properly. Use an ohmmeter to check the resistance of the variable resistor and to see if there are any dead spots. For dead spots the unit has to be replaced – usually the whole unit. For a unit that is working properly, connect the ohmmeter to the transmitter unit and move the float arm slowly up and down. The ohmmeter indicator needle must move steadily up and down scale without fluctuation.

4.4 Capacitance Fuel Quantity Systems

4.4.1 Outline the construction and principles of operation of a capacitance-type fuel contents indication system with particular regard to the following:

a. Probe construction b. Capacitance determination c. Correction for fuel permittivity d. Dielectrics and dielectric constants e. Fuel density factoring f. Fuel mass determination g. Units of contents measurement h. Volume (gallon or litre) compensation i. Aircraft attitude and fuel sloshing compensation j. Integration of multiple fuel tanks k. Installation of probes l. Indicator circuits m. Wiring type and electrical connections (2)

a. Probe construction:

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Fuel tank units are formed by concentric aluminium tubes; the inner and outer tubes are the capacitor plates, see Fig. 6.

Figure 6: Capacitance type fuel indicator system

b. Capacitance determination:

Figure 7: Capacitance fuel quantity system - principles

The primary advantages of capacitance-type technology are no moving parts and fuel quantity is measured in mass rather than volume. (The mass of fuel determines the amount of energy available.) The dielectric is either fuel or air depending on the quantity of fuel in tank. Air has a dielectric of 1.0006 (practically unity); fuel has a dielectric of approximately two; this provides a good relationship for the measurement of a variable quantity which is the level of fuel in the tank. From basic fundamentals, we know that capacitance is proportional to:

Plate area

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Air gap between the plates Dielectric strength.

In the capacitive tank unit, the first two parameters are fixed; the capacitance varies in accordance with the dielectric, i.e. the amount of fuel in the tank as illustrated in figure 7 overleaf. With a high quantity of fuel in the tank, the capacitance is high; capacitance varies in direct proportion to the amount of fuel in the tank.

c. Correction for fuel permittivity (dielectric constant): The capacitance type probes take into account the density of the fuel. They provide an accurate indication of the mass of the fuel in the tanks, which more accurately reflects the available energy in the fuel than volume. This is negligible in smaller aircraft powered by Avgas, but in larger aircraft powered by kerosene, it can be significant. Fuel density can change due to variations in temperature or due to substitution of a different grade of fuel. The capacitive probes sense this because the denser fuel has a higher dielectric constant, which increases the measured fuel quantity at a given volume.

d. Dielectrics and Dielectric constants: As explained above, the dielectric constant is a value given to the gap between the two plates in the capacitor. The dielectric for air is unity or approximately one, while that of fuel is nearly two. When the tank is full the dielectric is fuel and the dielectric constant is two, while if the tank is empty, the dielectric is air and the dielectric constant is one. In any condition between full and empty, part of the dielectric is air and part is fuel, and so the capacitance of the probe varies according to level of the tank.

e. Fuel density factoring: The capacitance type probes take into account the density of the fuel (see point c above). Cold fuel is denser than warm fuel, hence there are more pounds of cold fuel in a gallon than warm fuel, which is more important in large aircraft because the power of the engine is determined by the pounds of fuel burned not the gallons.

f. Fuel mass determination: Fuel density can change due to variations in temperature or due to substitution of a different grade of fuel. The capacitive probes sense this because the denser fuel has a higher dielectric constant, which increases the measured fuel quantity at a given volume.

g. Units of contents measurement: The capacitive fuel sensor measures the mass of the fuel, usually given in pounds.

h. Volume (gallon or litre compensation): Since the capacitive fuel sensor measures the mass of the fuel, some compensation is required if the desired indication is to be in gallons or litres. To accomplish this, a compensator is built into the bottom of one of the tank units. It is electrically in parallel with the probes which cancels the changes in dielectric constant caused by the changes in fuel temperature. To provide accurate indications, the compensator should be calibrated for the grade of fuel normally used in the aircraft.

i. Aircraft attitude and fuel sloshing compensation: To compensate for change in aircraft attitude the capacitive system may have many capacitive probes in the tank connected in parallel to ‘average; the measurement of the fuel in the tank. This enables the system to give an accurate indication irrespective of the aircraft’s attitude (Figure 8 below). Baffles are fitted within the tank to minimise the large inertial forces generated when the fuel surges during aircraft manoeuvres, acceleration, deceleration or sideslip for example. Some large aircraft may be fitted with baffle check valves which allow fuel to flow inboard but not outboard during wingtip manoeuvres.

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Figure 8: Attitude and fuel sloshing compensation

j. Integration of multiple fuel tanks: Capacitive fuel systems can be tailored for tanks of all sizes and shapes. Another advantage is all the probes can be connected so the system that integrates their output to show the total amount of fuel onboard. On a typical twin-engine passenger aircraft the fuel tanks are contained within the aircraft structure. Fuel tank sensors are located throughout each tank and are monitored by an electronic control system. Accurate readings can be obtained by capacitive fuel quantity systems in large and irregular-shaped tanks.

Aircraft instrument panel space is always limited, and one advantage of the capacitance-type fuel quantity indicating system is its ability to measure the fuel in several tanks and give the pilot an indication of the total number of pounds of fuel remaining on one indicator, called a totalizer.

k. Installation of probes: Multiple tank units are often employed in larger aircraft as illustrated in Fig. 9. On a typical medium sized passenger aircraft there are twelve capacitive tank units in each main fuel tank, and nine in the centre fuel tank. The construction of all the tank units is the same, except for their length; this depends on the depth of the tank at that particular location. Each tank unit consists of two concentric aluminium tubes and a terminal block. The aluminium tubes are anodized and polyurethane coated to protect against corrosion. The air gap between the tubes is relatively wide to avoid electrical short-circuiting, caused by contamination in the fuel or fungus coating on the tubes.

Several probes can be installed in a fuel tank to measure the quantity of fuel in odd-shaped tanks. These capacitors are connected in parallel and their total capacitance is the sum of all the individual capacitances (CT = C1 + C2 + C3 …). The probes are connected into a bridge circuit and the indicator is servo-driven to make the bridge self-balancing.

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Figure 9: Capacitance fuel quantity system – multiple tank units. Note the co-axial cable joining the amplifier to the tank.

Figure 10 represents the contour of a tank located in an aircraft wing, and as will be noted the levels of fuel from points A, B and C are not the same. When standard tank units are positioned at these points the total capacitance will be the sum of three different values due to fuel, and as the units produce the same change of capacitance for each inch of wetted length, the indicator scales will be non-linear corresponding to the non-linear characteristic of the tank contour.

Figure 10: Non-linear fuel tank

The non-linear variations in fuel level are unavoidable, but the effects on the graduation spacing of the indicator scale can be overcome by designing tank units which measure capacitance changes proportional to tank contour. The non-linear tank units so designed are called characterized tank units, the required effect being achieved either by altering the diameter of the centre electrode or by varying the area of its conducting surface at various points over its length, to suit the tank contour.

l. Indicator circuits: Figure 11 is a simplified diagram of a basic capacitance bridge circuit. The bridge is excited with 400 Hz AC through the centre-tapped secondary of a transformer. One half of the secondary windings is in series with the tank unit capacitors while the other half of the secondary windings is in series with a reference capacitor. The two halves of the centre-tapped windings are 180 degrees out of phase with each other. If the capacitance of the tank units and the reference capacitor are exactly the same, their capacitive reactances will be the same, and the current through the top half of the bridge will exactly cancel the current through the bottom half, therefore no current flows through the indicator.

The self-balancing bridge in figure 12 works in the same way as the capacitance bridge circuit. As the fuel quantity in the fuel tank changes, the capacitance of the probes change and therefore, there is a shift in phase of the current in the top half of the bridge – the bridge is unbalanced. The out of phase current causes a signal to be sent to an amplifier, which amplifies the out of phase current through one set of windings in a two phase motor within an

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indicator. The other winding in the motor is fed with a reference AC, thus with the bridge unbalanced the motor turns and drives a rebalancing potentiometer until the bridge balances. Connected to the potentiometer is a shaft that drives an indicator which indicates the number of pounds of fuel remaining in an aircraft.

Figure 11: Capacitance bridge diagrams with the simplified diagram of a capacitance bridge on the left, while the right shows how the current in each half of a bridge circuit cancel out

So, when the fuel is consumed during a flight, and the fuel level decreases, the capacitance decreases and the reactance increases; less current flows in the tank unit (compared with the reference capacitor). This unbalance causes a potential difference proportional to fuel quantity; this signal is amplified and used to drive an indicator. Capacitance trimmers are used for calibration of the fuel indicators (or gauges).

Figure 12: A simplified self-balancing bridge circuit used in a capacitance-type fuel quantity indicating system

m. Wiring type and electrical connections: A tank unit wiring harness attaches to the terminal block of the tank unit. The terminals on the tank wiring harness and the terminal posts have different dimensions to prevent cross-connection during installation. Tank unit end caps are

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insulated to ensure that the unit does not short to the ground; they also control stray capacitance that forms between the tank unit and ground plane (capacitance fringing). Two brackets made of nonferrous glass-filled nylon attach each tank unit to the fuel tank structure. Intrinsic safety is a technique used for safe operation of electrical and electronic equipment in explosive atmospheres. It is essential that the available electrical and thermal energy in the fuel quantity system is always low enough that ignition of the hazardous atmosphere cannot occur.

4.5 Maintenance of Capacitance-Type Contents Indicating Systems

4.5.1 Describe the following maintenance practices associated with capacitance type systems: a. Capacitance testing b. Contents calibration c. Cockpit test facility d. System defects and rectification (2)

a. Troubleshooting a capacitive-type fuel-quantity indicating system is quite simple. Following the appropriate instructions, connect a test unit into the system. The tester substitutes known capacitance values for the probes and includes an accurate amplifier and indicator into the system. The testing unit may also provide a technician with the capability to calibrate the system following troubleshooting or repairs. AN alternative is to connect a known operating system into the circuit – such as connecting the right wing fuel tanks to the left fuel gauge.

b. The most important setting on a fuel tank is the empty or zero reading on the fuel gauge – you don’t want to be in the air thinking you have fuel and then run out because the gauge is not calibrated correctly. The maintenance manual will give instructions for calibrating the probes. In the Diamond it involves first draining the tanks of all fuel with the aircraft on jacks as if in level flight. A quantity of fuel is always unusable to the aircraft’s engine. This amount is specified in the AMM and should be put in the tank being tested. The gauge should now read zero or empty – if not following instructions reset the probes resistance following the AMM. The gauge can be checked for accuracy by filling the tank in set amounts. In the Diamond DA40 this means resetting the computer to zero (with 1.895 litres unusable fuel in the tank) and then checking the accuracy of the gauge in 5 gallon increments.

c. Most capacitance-type systems have a test feature. Actuating the test switch causes the gauges to drive towards zero or empty. When the switch is released, the pointer(s) should promptly return to the original quantity indication. In computer controlled systems, the test feature is done automatically when the system is initially powered.

d. Graphite dust and particles are conductors and can cause shorts in electrical motors, avionics circuitry as well as affect the aircraft’s fluid systems. In the fuel system, these particles can be introduced during wet wing repairs, causing clogged filters and erroneous readings in capacitance fuel quantity probes. The abrasiveness of these dust particles can cause failures to fuel controls and other close tolerance fuel valves. To correct this problem, drain the tank and flush the fuel probe. If the problem persists change the probe. Wiring could be defective in which case a check of the wiring is done using wiring diagrams. Or, in the case of the DA40, the engine integrated indicating system is defective and again the AMM will have to be consulted.

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