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Compensator unit in a capacitance quantity system

Aviation Mechanic Airframe (AMA) practice question 10 of 12 on Aircraft Fuel Systems. ACS element AM.II.J.K8 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONIn a capacitance-type fuel quantity indicating system, what is the function of the compensator unit mounted low in the tank?

  1. ATo detect free water lying in the sump
  2. BTo correct for fuel temperature changes
  3. CTo total the readings of all tank probes
Show the answer and explanation

Correct answer: B. To correct for fuel temperature changes

  • A — incorrect Water detection is done by sumping and field test kits, not by the compensator unit in the indicating bridge.
  • B — correct Temperature affects the dielectric constant of the fuel, and the compensator modifies current flow in the bridge circuit to reflect fuel temperature variations that change density and capacitance.
  • C — incorrect The tank probes are wired in parallel and their capacitance is totaled and compared in the bridge circuit by the indicator's microchip computer.

BACKGROUND

Large and high-performance aircraft typically use capacitance-type, or electronic, fuel quantity systems, which have no moving parts in the tank sending units. Variable capacitance tank units, also called probes, run from the top to the bottom of the usable fuel and are wired in parallel, and their total capacitance is compared in a bridge circuit against a reference capacitor. Because the dielectric constant of fuel varies with temperature, a compensator unit is mounted low in the tank so it stays covered with fuel and is wired into the bridge to modify current flow accordingly. The magnitude of the bridge difference is translated into a quantity indication calibrated in pounds. A fuel totalizer adds the tank quantities from all indicators for use by the crew and the flight management computers.

STUDY TIP

Keep the three capacitance-system parts straight: tank probes sense, the reference capacitor compares, and the compensator corrects for temperature.

KEY POINTS

  • Capacitance systems have no moving parts in the tank units.
  • The compensator corrects the bridge for fuel temperature and density effects.
  • Capacitance-type quantity is calibrated in pounds, and a totalizer sums the tanks.

SOURCES

  • sources/ama-airframe/ch14.txt, "Fuel Quantity Indicating Systems"
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MORE ON AIRCRAFT FUEL SYSTEMS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.