FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationMedium
A mechanic is troubleshooting a capacitance-type fuel quantity indicating system on a turbine aircraft. What is the primary principle of operation for this type of system?
- AIt relies on ultrasonic waves to determine the fuel depth.
- BIt measures the pressure exerted by the fuel in the tank.
- CIt uses floats and rheostats to detect fuel level.
- DIt measures the dielectric constant of the fuel and air in the tank.
Show answer & explanationAnswer & explanation
Correct answer: D. It measures the dielectric constant of the fuel and air in the tank.
Capacitance-type fuel quantity systems utilize probes (capacitors) within the fuel tank. The dielectric constant of fuel is different from that of air. As the fuel level changes, the amount of fuel and air between the capacitor plates changes, altering the capacitance, which is then converted to a fuel quantity reading.
Why the other options are wrong
- A. Ultrasonic systems exist but are not the principle of a capacitance system.
- B. Pressure measurement is used in some specialized systems but not the primary principle of capacitance systems.
- C. Floats and rheostats describe a resistance-type fuel quantity system, not capacitance.
Capacitance Fuel Quantity System
An aircraft fuel quantity system that measures the change in electrical capacitance between probes in the fuel tank, which varies with the amount of fuel (dielectric medium) present.
- More accurate than float-type systems.
- Measures fuel by weight, not volume (due to specific gravity variations).
- Probes act as capacitors; fuel and air act as dielectric.
Memory trick: Capacitance is like a 'dielectric detective', sensing fuel's electrical presence.