2022-10 · NASA ASRS report 1946175
Air Carrier Pilot Crew reported a continuing fuel imbalance during climb out. The pilots stated eventually the imbalance triggered fuel warning indications prompting a return to the origin station. There was an MEL regarding the fuel system on this aircraft.
During preflight I noticed that Aircraft X had a MELed item from Date 0 referencing a fuel seep on the fuel panel of the NO. 2 Tank. During the flight everything proceeded uneventfully until climb out through 19;000 ft. when we noticed a Amber Fuel Alert Indication on the fuel indicators stating an imbalance (IMBAL) on the NO. 2 Tank. We didn't notice any fuel transferring to any other tank and the NO. 2 Tank was indicating that there was about an 1;800 lbs deficit compared to the NO. 1 Tank. While looking at the quantity indicators and observing a fuel Leak on the NO. 2 tank; plus the MEL leakage history on that fuel tank; I decided that the safest and more conservative course of action would be to [advise ATC] and turn back to ZZZ to make sure we were not leaking an amount of fuel that could create a fire hazard or a fuel starvation situation. We landed on ZZZ uneventfully. While on the runway CFR (Crash Fire Rescue) personnel inspected the aircraft and we noticed both Center Pumps were indicating LOW PRESSURE with 2;300 lbs still left on the center tank. We taxied to the gate uneventfully.Suggestion: Better fleet fuel system maintenance.
When we reviewed the AML we noticed a MELed item referencing a fuel seep/leak from Date 0. I took an extra close look at all fuel related items during the preflight; but didn't see any leaks. We departed and at about 19000 ft. noticed a significant fuel imbalance of 1800 lbs; increasing at a significant rate. The right tank quantity was lower than the left tank. Due to the MEL item and continuing loss of fuel in that tank; we were certain that we had a fuel leak. We [advised ATC] and turned around to land at ZZZ. I started the checklist; communicated with ZZZ ATC; while the Captain retained his Pilot Flying duties and communication with the cabin. Due to the existing MEL item; reports from the cabin and a slowing rate of leaking; we did not assume that the fuel leak was coming from the engine and therefore stopped the checklist prior to the point where the engine had to be shut down. As we started descending; the change in fuel quantity had slowed significantly. The airplane had a tendency to bank to the left due to the imbalance. As we were on final approach the fuel quantity in the right tank stayed constant. We stopped on the runway and received the confirmation from the fire trucks that there was no obvious leak. At that point we noticed that the center fuel tank 'low pressure' lights were on; even though the pumps were on and the center tank quantity at 2300 lbs. We checked the 'fuel used' indication and saw an equal amount of fuel used by both engines. I am not sure what caused this imbalance. I don't have enough systems insight to understand why we could have a decreasing fuel quantity on the right tank only; with 'low pressure' and fuel remaining in the center tank. Maintenance did several tests and when we flew this plane the next day; everything worked normally.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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