Fuel Quantity Gauge Discrepancy After Refuelling
After refuelling a PA-28-181 Archer at a NZ airfield, the pilot reports the left fuel gauge reads 14 US gal and the right gauge reads 24 US gal. Dipstick checks show left 22 US gal and right 24 US gal. The left gauge was replaced six months ago; the right sender was replaced last annual. The aircraft has two resistance-type tank units and a fuel totalizer. With master on and engine off, the left gauge reads 14; tapping the panel changes it to 17 and then back to 14. The wiring connector at the left sender is corroded. The AME must decide whether to sign off, calibrate, or replace the sender. The aircraft is scheduled for a 2-hour VFR training flight with 45 minutes reserve. POH unusable fuel is 2 US gal per tank; total usable fuel is 48 US gal.
1. Determine the likely cause of the left gauge discrepancy and explain why tapping the panel matters.
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The left gauge is underreading by about 8 US gal compared with the dipstick, while the right gauge agrees. The change from 14 to 17 when the panel is tapped indicates an intermittent electrical connection, not a stable calibration error. The corroded connector at the left sender is the first suspect: resistance in the sender circuit will bias the gauge low, and vibration or tapping can temporarily change that resistance. The sender arm could also be sticking, but the connector corrosion and the tap response make the circuit the priority. The gauge itself could be faulty, but it was replaced recently and the right gauge is stable. The AME should not treat the dipstick as a substitute for repairing an intermittent fuel quantity indication; the dipstick verifies actual fuel, but it does not make the gauge system airworthy.
2. Describe the troubleshooting and return-to-service requirements.
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Disconnect the left sender connector, inspect and clean the corrosion, and repair or replace the terminal as required. Measure the sender resistance at a known fuel quantity and compare it with the AMM resistance chart; check the gauge by substituting a known resistance if the manual permits. Move the sender arm through its range and check for smooth resistance change and no dead spots. Check the ground return and the wiring from the sender to the gauge. If cleaning the connector restores a stable indication, verify the gauge at empty, full, and at least one intermediate quantity against the approved calibration procedure. If the sender or gauge is out of tolerance, replace it and recalibrate. Return to service only when the indication is stable, the calibration check is within the AMM limits, and the fuel totalizer is correctly set. Record the defect and the corrective action.
3. The aircraft has 46 US gal total and needs 2 hours plus 45 minutes reserve. Can the AME release it with the gauge still intermittent if the dipstick shows enough fuel?
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The fuel quantity is sufficient on paper: about 44 US gal usable, a training burn near 10 US gal/h for 2 hours, plus about 7.5 US gal reserve, gives roughly 27.5 US gal required. But fuel quantity is not the limiting issue. An intermittent fuel gauge is an airworthiness defect, and the dipstick does not cure it. The pilot would have unreliable fuel indication in flight, and the totalizer depends on correct initial fuel data. The AME should not release the aircraft with a known intermittent gauge merely because the dipstick shows enough fuel. Repair the connector or sender, verify the calibration, and then release. If the repair cannot be completed, the aircraft should remain unserviceable for the planned flight, or the operator must use an approved alternative that does not rely on the faulty gauge.