FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsMedium
A capacitance-type fuel quantity indicating system uses probes mounted inside the fuel tank. As the fuel level changes, the system determines quantity by measuring a change in which electrical property?
- ACapacitance between the probe plates, caused by the change in dielectric as fuel replaces air
- BResistance of the fuel between the probe plates
- CInductance of a coil wound around the probe
- DFrequency of oscillation in a synchro transmitter
Show answer & explanationAnswer & explanation
Correct answer: A. Capacitance between the probe plates, caused by the change in dielectric as fuel replaces air
Capacitance fuel probes act as capacitors with fuel and air serving as the dielectric between two concentric plates. As fuel level rises, more of the dielectric becomes fuel instead of air, changing the capacitance, which the indicating circuit converts into a quantity reading.
Why the other options are wrong
- B. Fuel resistance is not the measured property in capacitance systems.
- C. Inductance is not used in capacitance-type fuel probes.
- D. Synchro transmitters are used in other remote-indicating systems, not capacitance fuel probes.
Capacitance Fuel Quantity System
A fuel quantity indicating system that measures capacitance between tank probes, which changes as fuel (a different dielectric than air) rises or falls between the plates.
- Probes act as a capacitor with fuel/air as dielectric
- Capacitance increases as fuel level rises
- Indicating circuit converts capacitance change to a quantity reading
Memory trick: Fuel fills the gap like a bigger dielectric raising capacitance