FAA Aviation Mechanic Airframe (AMA)Fuel, Cabin and Ice Control SystemsHard
A technician troubleshoots a capacitance-type fuel quantity indicating system that reads inaccurately across varying fuel temperatures. Inspection reveals a small tube-shaped sensing probe mounted separately from the main tank probes. What is the function of this component?
- AIt is a backup float switch that triggers a low fuel warning light
- BIt is a vent line pressure sensor used to detect tank overpressure
- CIt is a compensator probe that measures the dielectric constant of the fuel to correct for density variations with temperature
- DIt is a fuel temperature probe used solely for engine fuel control input
Show answer & explanationAnswer & explanation
Correct answer: C. It is a compensator probe that measures the dielectric constant of the fuel to correct for density variations with temperature
The compensator (reference) tube in a capacitance fuel quantity system is fully immersed in fuel and measures the fuel's dielectric constant, which changes with temperature and density, allowing the system to correct tank probe readings for accurate mass/volume indication.
Why the other options are wrong
- A. Float switches are mechanical low-level warnings, unrelated to capacitance compensation.
- B. Vent pressure sensing is unrelated to capacitance fuel quantity compensation.
- D. While related to fuel temperature effects, its specific function is dielectric compensation, not engine fuel control input.
Fuel Quantity Compensator Tube
A fully-submerged reference probe in a capacitance fuel quantity system that measures fuel dielectric constant to correct readings for temperature/density variations.
- Always fully immersed regardless of fuel level
- Compensates for changes in fuel dielectric constant with temperature
- Improves accuracy of capacitance-type fuel quantity indication
Memory trick: Compensator tube is the system's built-in thermometer for fuel density.