Incorrect Fuel Quantity Indication
An Airbus A320 reports that the fuel quantity indication for the left wing tank is erratic and sometimes shows 500 kg less than actual. The aircraft uses a capacitance-type fuel quantity indicating system. The AME suspects a problem with the tank unit (probe) or the wiring. The fuel quantity indicating system has two channels: primary and secondary. The primary channel uses probes in the tank, while the secondary uses a different set of probes. The fault is present on both channels. The AME must troubleshoot.
1. What is the most likely cause of the erratic fuel quantity indication and what troubleshooting steps should be taken?
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Since both channels are affected, the fault is likely common to both, such as a faulty fuel quantity indicator, a wiring issue common to both channels, or a problem with the fuel density or dielectric constant. Steps: 1) Check fuel quantity indicator for fault codes. 2) Perform a capacitance test on the probes using a bonding meter or capacitance tester. 3) Inspect wiring and connectors for corrosion or damage. 4) Verify fuel type and density settings. 5) If no fault found, replace the fuel quantity indicator.
2. Describe how a capacitance fuel quantity system measures fuel quantity and the effect of fuel dielectric constant changes.
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The system measures capacitance between probes in the tank, which changes with fuel level (fuel has a different dielectric constant than air). The indicator converts capacitance to volume, then to mass using fuel density. Changes in fuel dielectric constant (due to fuel type or temperature) can affect accuracy. Some systems compensate using a densitometer or reference capacitor.