Flight Crew reported multiple Level 1 and Level 2 alerts concerning the fuel system including Lateral Fuel Unbalanced; Fuel Qty Fault; Fuel Qty/Used Check; tank quantity gauge failure; fuel totalizer quantity failure; and Select Fuel Manual. The flight crew elected to perform an air turn back and precautionary landing at the departure airport.
Synopsis
Flight Crew reported multiple Level 1 and Level 2 alerts concerning the fuel system including Lateral Fuel Unbalanced; Fuel Qty Fault; Fuel Qty/Used Check; tank quantity gauge failure; fuel totalizer quantity failure; and Select Fuel Manual. The flight crew elected to perform an air turn back and precautionary landing at the departure airport.
Narrative
Air Turn Back for multiple Level 1 and Level 2 alerts concerning the fuel system including Lateral Fuel Unbalanced; Fuel Qty Fault; Fuel Qty/Used Check; Tank Quantity Gauge Failure; Fuel Totalizer Quantity Failure; and Select Fuel Manual. All these indications were the result of intermittent; erroneous and sometimes accurate fuel quantity indications in tank 1. This; in turn; caused the Fuel System Controller (FSC) to constantly 'chase' proper fuel balance based on erroneous and inaccurate input from the fuel quantity gauge in Tank 1. Ultimately; the Fuel System Controller took itself offline and directed us to operate the system in manual mode. Given the large quantity fluctuations in Tank 1 and multiple instance of the FSC both adding and removing fuel from tank 1 in its attempts to balance; it became impossible to determine the true quantity in Tank 1. We had quantity indications both high and low outside of aircraft fuel balance limitations. Manual computations to estimate our fuel state showed as much 3;700 pounds of fuel unaccounted for; so we could not positively rule out a fuel leak. Given the dynamic fuel quantity situation with its cascade of resultant effects to the fuel system and the possibility of a fuel leak; we decided to [request priority handling] and return to ZZZ1. An unsuccessful attempt to contact Operations/Maintenance via SATCOM was made but not re-attempted to the exigencies of the situation. A Diversion ACARS message to Operations was successful; however. Clearance was coordinated with ZZZ Center and an uneventful arrival; approach and landing was made. We cleared the runway and asked ARFF to inspect for fuel leak. Uneventful taxi back to the gate. As an aside; ZZZ ATC was superb in their support!Faulty components in the fuel system. Rigorous troubleshooting of fuel system anomalies as there was a previous write-up with similar issues.
Second reporter narrative
Approximately 15 minutes after departure; we received a LAT FUEL UNBAL due to erratic indications on Tank #1. Fuel quantity indications fluctuated +/- 2;500 pounds; which resulted in multiple level 1 & 2 alerts associated with the fuel system. Erroneous fluctuating indications lasted for the duration of the flight; with intermittent indications including amber numerical quantity indications and amber X's. These fluctuations resulted in the FSC (Fuel System Controller) Auto 'hunting' the quantity as the #1Tank quantity was regularly indicating out of tolerances; FSC filling the tank when it indicated low and emptying the tank when it indicated high. We manually calculated our fuel quantity multiple times. Each time there was around 3700 pounds of unaccounted fuel; so we could not definitively rule out a fuel leak. We attempted to contact Operations/Maintenance but the SATCOM failed. During our previous flight; we had a similar SATCOM problem; failing 6 times in a row before we were able to positively contact anyone. Due to the compounding nature of the situation and the inability to determine the quantity in the #1 tank; the decision to return to ZZZ1 was made. We [requested priority handling]; proceeded direct ZZZ1 for the ILS XXC; and landed uneventfully. We were inspected by ARFF after taxiing clear of Runway XXC and confirmed there was no fuel leak. We taxied to Gate X uneventfully. The summary of our indications received includes: FUEL QTY FAULT; FUEL QTY/USED CHK; #1 tank quantity failure resulting in an amber X; fuel totalizer failure; and an eventual SEL FUEL SYS MAN; as the system automatically transitioned to the manual mode.Faulty components in the fuel system. Continue to ensure awareness of fuel system indicator problems are circulated through maintenance and aircrew personnel. Ensure aircrew write-ups and maintenance corrective actions are documented in a detailed manner. This is an important factor; as there was a previous write up on this aircraft for a similar problem that the Captain and I reviewed during preflight. I believe this helped us expedite our decision making and maintain situational awareness while we were troubleshooting real-time.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.