LEFT RIGHT FUEL FUEL L R 20 0 FUEL QTY GAL ENGINE MAP A B C Figure 14-70.  Digital fuel quantity gauges that work off of variable resistance from the tank unit are shown in A and B. The fuel quantity  indication of a Garmin G-1000 flat screen display is shown in C. Large and high-performance aircraft typically utilize  electronic fuel quantity systems. 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 cockpit. 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. 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.  Figure 14-73  represents the nature of this comparison  bridge circuit. 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  The use of tank unit capacitors, a reference capacitor, and  a microchip bridge circuit in the fuel quantity indicators is  Figure 14-71.  A fuel tank transmitter for a capacitance-type fuel quantity indicating system.  14-39