FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationMedium
A technician is troubleshooting a capacitance-type fuel quantity indicating system on a transport-category aircraft. Which statement best describes the principle used by this system to measure fuel quantity?
- AA float mechanically moves a variable resistor to change indicator voltage
- BFuel acts as the dielectric between tank probe plates, changing capacitance in proportion to fuel mass
- CA pressure transducer at the tank bottom measures hydrostatic head to infer volume
- DAn ultrasonic transducer measures the time for a sound pulse to reflect off the fuel surface
Show answer & explanationAnswer & explanation
Correct answer: B. Fuel acts as the dielectric between tank probe plates, changing capacitance in proportion to fuel mass
Capacitance-type systems use tubular probes in the tank as capacitor plates; fuel and air have different dielectric constants, so as fuel level changes, probe capacitance changes proportionally, and the system is calibrated to indicate fuel quantity by weight, which compensates for density variation with temperature.
Why the other options are wrong
- A. That describes a float-and-resistance (rheostat) type system, not capacitance type.
- C. Pressure-based sensing is used in some simple systems but is not the capacitance principle.
- D. Ultrasonic sensing is a different, non-capacitance technology.
Capacitance Fuel Quantity System
A fuel quantity indicating system that measures fuel level via changes in electrical capacitance between tank probes, using fuel as the dielectric.
- Probes act as capacitor plates in the tank
- Fuel's dielectric constant differs from air, changing capacitance with level
- Calibrated to read fuel quantity by weight, compensating for density/temperature changes
Memory trick: Fuel is the 'in-between stuff' that changes the capacitor's story.