Fuel Imbalance and Transfer in a Twin Turboprop

The imbalance could be due to a fuel leak in the left main tank or its plumbing, a faulty fuel quantity indicating system, or an imbalance in fuel flow due to engine performance differences. The left engine fuel flow is only 20 lb/h higher than the right, which does not explain a 20 gal/h imbalance (20 gal of Jet A is about 134 lb, so 20 gal/h is 134 lb/h, far more than the 20 lb/h difference). Therefore, a leak or a transfer/indication issue is more likely.

Immediate actions: (1) Cross-check the fuel quantity indicators with the totaliser and dipsticks if available. (2) Visually inspect the left wing for fuel venting or leakage (if visible from the cockpit). (3) Turn off the left auxiliary tank transfer if it is on, to see if the imbalance changes. (4) Consider crossfeeding: open the crossfeed valve and turn off the left engine-driven pump (if safe) to feed both engines from the right main tank, which will reduce the left tank quantity further? Actually, crossfeed from right to left engine would use right tank fuel for the left engine, balancing the draw. But if there is a leak in the left tank, you want to isolate it.

You should first determine if the imbalance is real. If the left main tank quantity is decreasing at 20 gal/h more than expected, and the totaliser is also decreasing at a rate consistent with total flow plus 20 gal/h, then it is a leak. If the totaliser shows normal total flow, then the quantity indicator is faulty.

Given the maximum imbalance of 100 gal, you are currently at 100 gal (450-350=100), at the limit. You must act to prevent exceeding it. You could transfer fuel from the right main to the left main if a transfer system exists, but typically transfer is from auxiliary to main. You could crossfeed to draw from the right tank to the left engine, which would reduce the right tank and increase the imbalance? No, crossfeed from right to left engine means the left engine consumes right tank fuel, so right tank decreases faster, making imbalance worse. To correct a left tank that is too low, you want to draw from the right tank to the left engine? That would decrease right tank, increasing imbalance. Actually, if left tank is low, you want to add fuel to it or reduce its consumption. You could turn off the left engine and fly on the right engine, but that is drastic.

A better action: if the aircraft has a fuel transfer from main to main (some do), use it. If not, you can crossfeed from the left tank to both engines, which would draw from the left tank and increase imbalance? No, crossfeed from left tank to both engines means both engines feed from left tank, which is already low, making it worse.

Wait, crossfeed typically allows either engine to feed from either tank. If you select crossfeed and turn off the right tank feed, both engines feed from the left tank, which is low, so that's bad. If you select crossfeed and turn off the left tank feed, both engines feed from the right tank, which is high, so the right tank decreases and the left tank stays the same, reducing the imbalance. That is the correct action: feed both engines from the right main tank, which will lower the right tank quantity and bring the imbalance within limits. However, you must ensure the right tank has enough fuel and that the crossfeed system can supply both engines. Also, if there is a leak in the left tank, feeding from the right tank avoids using the leaking tank.

So immediate actions: (1) Confirm imbalance is real. (2) If real and not a leak, open crossfeed and feed both engines from the right main tank, monitoring left tank quantity. (3) If a leak is suspected, avoid using the left tank and consider landing as soon as possible.

You should also consider the auxiliary tanks: you can transfer from auxiliary to main to replenish the left main if the transfer system allows selective transfer. If you can transfer from left auxiliary to left main, that would add fuel to the left main and help balance. But if there is a leak, transferring would just leak more. So first determine leak vs indication.

Given the time to destination (1 hour) and the increasing imbalance, you should plan to land with a safe fuel state. If the imbalance reaches 100 gal, you are at the limit; exceeding it may affect lateral control or fuel system operation. You should not delay.

If a leak is suspected, you should isolate the left main tank as much as possible. Turn off the left engine-driven fuel pump and any transfer pumps that could feed the left main tank. If the crossfeed system allows, feed the left engine from the right main tank by selecting crossfeed and turning off the left tank feed. This will prevent further loss of fuel from the left tank and use the right tank for both engines. However, you must monitor the right tank quantity and ensure you have enough fuel to reach destination.

You should also consider shutting down the left engine if the leak is severe or if there is a risk of fire. A fuel leak in the wing could be dangerous, especially if it contacts hot exhaust or electrical components. If you shut down the left engine, you can fly on the right engine, but performance will be reduced. You should then plan to land as soon as possible, perhaps at a closer airport.

You should declare an emergency and advise ATC of the fuel leak. Request priority handling and a long runway. Prepare for a possible single-engine approach and landing.

If the leak is in the left main tank, transferring fuel from the left auxiliary to the left main would likely worsen the leak, so do not transfer. If you need fuel, transfer from the right auxiliary to the right main, if possible.

You should also consider the maximum imbalance: if you stop using the left tank, the right tank will decrease and the left tank will remain at 350 gal, so the imbalance will decrease (since right tank goes down). That is good. But you must ensure the right tank has enough fuel: currently 450 gal, and you need about 2 hours 30 minutes of fuel at 780 lb/h total, which is about 1,950 lb, or about 290 gal (Jet A at 6.7 lb/gal). So 450 gal is plenty. However, if you shut down the left engine, fuel flow will be about 400 lb/h, so you need less.

After landing, you should have the leak inspected. The key judgement is to prioritise safety over schedule and to isolate the leak while maintaining sufficient fuel to land.