A probe or a series of probes is used in what kind of fuel quantity indicating system?

In fuel quantity indicating systems, a capacitor-based system operates by using probes that are placed in the fuel tank. These probes measure the capacitance of the fuel, which varies with the amount of fuel present. As the fuel level changes, so does the dielectric constant of the fuel-air mixture, affecting the capacitance readings. The proportional change in capacitance is then converted into an electrical signal that can be interpreted as fuel quantity, usually displayed on an indicator.

This method is prevalent because it offers reliable measurements, is relatively simple to implement, and requires less maintenance than some other systems. Capacitive fuel quantity systems are known for their accuracy and effectiveness in various aircraft, making them a common choice in modern aviation.

The other choices relate to different principles of operation. Synchro and Selsyn are electromechanical systems that do not specifically utilize probes for measuring fuel quantity, while electromechanical systems generally involve mechanical components to provide readings, which differ from the capacitive technology.