Uncommanded Fuel Transfer in Cruise
A medium twin-turboprop is cruising at FL310 with 4,000 kg total fuel. At the start of the cruise the left tank indicates 2,100 kg and the right 1,900 kg, which is at the 200 kg maximum permitted lateral imbalance. Total fuel flow is 900 kg/h, split equally between engines. The crossfeed selector is CLOSED. After one hour, the left tank has fallen to 1,500 kg and the right tank to 1,600 kg. Both engine fuel-flow indications remain equal at 450 kg/h. There is no fuel smell, no visible leak, and no cabin fume warning. The fuel quantity indication system has no fault flag. Total fuel is now 3,100 kg, giving about 3.4 hours at the current flow. The destination is 1.5 hours away; a suitable airport is 30 minutes away.
1. Reconstruct the fuel movement over the hour. Is this a leak or a transfer? Show the arithmetic and state the rate.
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Each engine burns 450 kg/h, so with crossfeed closed the left tank should have fallen from 2,100 to 1,650 kg and the right from 1,900 to 1,450 kg. Actual quantities are left 1,500 kg and right 1,600 kg. The left tank is 150 kg below its expected value; the right tank is 150 kg above its expected value. Total fuel is 1,500 + 1,600 = 3,100 kg, which exactly matches 4,000 − 900 kg burned. There is no missing fuel, so this is not an external leak. Fuel is moving from the left tank to the right tank at about 150 kg/h, probably through a failed transfer/refuel valve or an internal transfer path, even though the crossfeed selector is closed. The initial 200 kg left-heavy imbalance has reversed to about 100 kg right-heavy in one hour.
2. State the immediate actions and give a timeline. Explain why simply opening the crossfeed is not automatically the correct response.
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Treat the uncommanded transfer as a fuel-system malfunction. Attempt to stop it by switching off the transfer pumps or closing the affected transfer valve, and keep the crossfeed closed unless the checklist directs otherwise. Monitor both quantities and total fuel every few minutes. The current imbalance is 100 kg right-heavy and the transfer rate is 150 kg/h, so the 200 kg lateral limit will be reached in about (200 − 100)/150 = 0.67 h, or 40 minutes, if the transfer continues. The destination is 1.5 h away, so it cannot be reached without exceeding the limit; the 30-minute alternate is the correct plan. Opening the crossfeed is not an automatic fix because it connects the tanks but does not stop the transfer. It may let both engines feed from the right tank and temporarily reduce the imbalance, but it can also mask the fault, complicate fuel accounting, and leave the crew managing an uncommanded fuel path rather than a known leak. Use the checklist, but plan to land before the limit is exceeded.
3. If total fuel had fallen by 1,000 kg instead of 900 kg, what would that indicate and how would it change your decision?
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Total fuel would then be 3,000 kg rather than 3,100 kg, so 100 kg/h more fuel is leaving the system than the engines are burning. That points to an external leak or a leak into another structure, not a simple transfer. The transfer arithmetic would still show 150 kg/h left-to-right movement, but the total loss rate would be 1,000 kg/h, giving only 3.0 hours of fuel at the current indicated total burn instead of 3.4 hours. More importantly, an unknown leak is a fire and fuel-exhaustion risk. The decision would change from 'monitor and divert within 40 minutes' to 'land as soon as possible at the nearest suitable airport', with no crossfeed opening that could feed the leak, and with consideration of engine shutdown only if the leak is associated with one side and the checklist supports it.