Outer plate Inner plate FWD Compensator unit Electrical terminal Tank unit Upper wing skin Lower wing skin Figure 14-72.  The capacitance of tank probes varies in a  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. Figure 14-74.  A fuel quantity tank unit and compensator unit  installed inside a wing tank.  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.  A fuel summation unit 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]  A fuel flowmeter 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. 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  Figure 14-73.  A simplified capacitance bridge for a fuel quantity  system. Fuel tank probe Reference capacitor 115 V 400 Hz AC Indicator Fuel Fuel Flowmeters 14-40