WASHINGTON—The FAA plans to require affected Boeing 777 operators to modify fuel systems to ensure they are accurately tracking the amount of fuel going into center wing tanks, correcting a design flaw that has led to over-reporting of fuel amounts—and some diversions as a result.
A draft airworthiness directive (AD) published March 21 calls for mandating Boeing-recommended changes to fuel quantity processor units (FQPUs)—either updated software or replacing existing units with new ones containing the new software. The AD refers to Boeing’s March 30, 2021, service bulletin for proposed compliance deadlines and specific aircraft applicability.
The change corrects a 777 fuel quantity indication system (FQIS) design flaw that can cause the system to mis-state the amount of fuel in the center wing tank. That can lead to too little fuel being added, and the aircraft being dispatched without enough fuel for its mission.
“At least 25 in-service events have been reported by operators,” the draft AD’s preamble said. In 11 cases, the flight “had to land at a diversion airport,” it added.
Boeing found the discrepancy was caused by FQIS ultrasonic tank units calibrating incorrect velocity of sound (VOS) in the fuel during center tank fueling, the FAA explained. The problem “leads to an improper fuel density calculation, and results in the FQIS showing a different fuel amount from the actual fuel quantity in the CWT,” the FAA said. The current FQPU software “is subjected to this stale VOS value under certain conditions due to insufficient VOS reset functions,” the agency added.
The FAA in 2020 mandated a temporary procedure that verifies the amount of fuel in the CWT during refueling. Modifying the FQPU eliminates the need to perform the procedure on affected aircraft.
Boeing argued in comments on the 2020 rule that the procedure addressed the issue, and operators should have the option of modifying the FQPU or retaining the procedure as part of its maintenance program. It also noted that confirmed in-service events are limited to 777-300ERs.
It is not clear whether the issue has been detected on other 777 variants. Boeing declined to comment on the incidents.
United Airlines (UAL), in comments on the 2020 directive, welcomed the prospect of a permanent modification. “While UAL has implemented robust procedures to comply with the AD, human factors remain a concern for 100% adherence,” the airline told the FAA.
The FAA AD would affect 257 U.S.-registered aircraft, the FAA said. Adoption by other regulators is likely. Proposed compliance times for the FQPU changes follow Boeing’s recommendations and range from 6-30 months, depending on the aircraft configuration.
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