Definition
A fuel quantity measuring system that uses one or more tank-mounted probes acting as electrical capacitors to measure fuel by mass rather than by volume. Each probe consists of two concentric metal tubes separated by the fuel and any air space between them. Because fuel and air have different dielectric constants, the probe's electrical capacitance changes as the ratio of fuel to air around it changes. The system's electronics convert this capacitance into a cockpit reading of fuel quantity, typically displayed in pounds.
Plain English
A fuel gauge system that measures fuel by sensing how it affects an electrical signal in tube-shaped sensors inside the tanks, rather than by floats. Because it responds to the actual mass of fuel, it gives a more accurate reading than older float-and-pointer gauges, especially in larger aircraft.
Context Anchor
Seen in aircraft systems, fuel gauge discussions, maintenance troubleshooting, and preflight checks of fuel quantity indications.
Derivation
Capacitance is the ability of two conductors separated by an insulator to store an electric charge. The probes in the tanks act as capacitors, and the fuel between their walls is the insulator. Because the amount of fuel changes how much charge the probe can hold, the system reads fuel quantity by measuring capacitance — hence the name.
Why Pilots Care
Provides reliable fuel quantity readings that remain accurate regardless of aircraft attitude or fuel sloshing.
Analogy
Think of the tank sensor like a ruler that does not move. Instead of a float rising and falling, the system notices how the fuel changes the sensor’s electrical behavior and turns that into a fuel reading.
Intuition Check
Do not assume this system uses a simple float arm like some basic fuel gauges. In a capacitance-type system, the fuel quantity indication comes from an electrical change sensed inside the tank.
Example Sentence 1
The aircraft's capacitance-type fuel quantity indicating system showed 4,200 pounds remaining, matching the fueler's slip closely enough for dispatch.
Example Sentence 2
During the preflight, the mechanic verified that the capacitance-type fuel quantity indicating system probes were clean and properly bonded.