Fuel quantity mismatch and free water after refuelling
A turboprop commuter aircraft has two 450 L wing tanks and capacitance fuel quantity probes. After a 300 L refuel, the cockpit gauges indicate 260 L total. The left tank sump drain yields 500 mL: the first 100 mL is cloudy with free water, and the rest is clear. The right tank drain is clear. The aircraft was parked outside for five days in heavy rain, and both fuel caps have worn seals. The fuel receipt is Jet A-1. The engines start and run normally. The operator wants to depart using the gauges as the only quantity reference because the totalizer is placarded inoperative. Outside air temperature is +2 degrees C and the freezing level is 6 000 ft. The AME must decide whether the aircraft can be released and how to resolve the quantity discrepancy.
1. Interpret the sump sample and decide whether the aircraft can be released. State the reasoning and any further checks.
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Free water in the left sump sample means the fuel system is contaminated, even though the engines run normally. At +2 degrees C, water can freeze in filters, lines or the fuel control and cause a power loss, so warm engine operation on the ground is not reassurance. I would not release the aircraft. The immediate action is to drain every sump and drain point on both tanks until the sample is clear and free of cloudiness, then repeat after a settling period. I would also inspect the fuel caps, filler necks, vents and upper wing skin for water paths, because worn cap seals match the five-day rain exposure. The right tank being clear does not prove it is dry; water can sit elsewhere in the tank. If water returns after draining, the aircraft should be defuelled and the source corrected before further flight.
2. How would you troubleshoot the 40 L quantity mismatch?
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I would not adjust the gauge to match the refuel figure until I know which indication is wrong. First compare the gauge with dipsticks or magnetic level indicators, the refuel receipt and the tank geometry. Then check the capacitance probe connections, bonding, wiring and the gauge calibration. Water in the tank can affect capacitance and quantity indication, so the contamination finding must be resolved first. Swapping or substituting a known-good probe or gauge, where the manual permits, helps separate an indication fault from a wiring fault. A totalizer placarded inoperative cannot be used as the sole reference. If the gauge remains 40 L low after the tank is known dry and correctly filled, the probe or gauge must be calibrated or replaced in accordance with the maintenance manual, and the correction recorded.
3. If water is found in both tanks, what additional actions and records are required?
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If both tanks contain water, I would treat the whole fuel system as contaminated. That means defuelling, flushing and re-draining the tanks, replacing or cleaning fuel filters, checking the fuel control and injector or carburettor screens, and verifying that no water remains at any drain point. I would also correct the cap seals and any vent or filler defect that allowed ingress. The maintenance record should state the water quantity, the drains affected, the flushing and filter actions, the cap and seal replacement, and the final clear samples. The aircraft should not be released until the fuel system passes the manufacturer's contamination checks and the engine runs are satisfactory. If the contamination is significant, the operator's fuel quality and storage procedures should be reviewed as well.