FAA Aviation Mechanic Airframe (AMA)Fuel, Cabin and Ice Control SystemsMedium
A technician troubleshoots a capacitance-type fuel quantity indicating system that reads incorrectly after the aircraft was refueled with a batch of fuel of unusually different density. What is the underlying operating principle that explains this sensitivity to fuel density?
- AThe system measures fuel weight directly using a float and lever arm
- BThe system uses a pressure transducer calibrated only for a single fuel type
- CThe system relies on fuel flow rate through a turbine sensor to calculate quantity
- DThe system senses the change in capacitance between tank probes, which depends on the dielectric constant of the fuel
Show answer & explanationAnswer & explanation
Correct answer: D. The system senses the change in capacitance between tank probes, which depends on the dielectric constant of the fuel
Capacitance fuel probes act as capacitors with fuel and air as the dielectric; since fuel and air have different dielectric constants, the capacitance changes as the fuel level changes, and it can be affected by the dielectric constant (density) of the fuel itself.
Why the other options are wrong
- A. Float-and-lever systems are mechanical, not capacitance-type.
- B. Capacitance systems do not use pressure transducers.
- C. Flow-rate turbine sensors measure fuel used, not quantity in the tank.
Capacitance Fuel Quantity System
A fuel quantity indicating system that uses tank probes acting as capacitors, where capacitance varies with the ratio of fuel to air (differing dielectric constants) surrounding the probe.
- Probes are coaxial tubes forming a capacitor
- Fuel and air have different dielectric constants
- System compensated for temperature and fuel type variations
Memory trick: Capacitance probes 'feel' fuel through electric field changes.