FAA Rule Targets Inaccurate Boeing 777 Fuel-Quantity Systems

Credit: Boeing

WASHINGTON—The U.S. FAA will order Boeing 777 operators to validate the accuracy of fuel-quantity check systems following reports that inaccurate tank status data caused aircraft to depart with too little fuel for their planned missions, leading to at least 10 diversions.  

The issue affects center wing tank (CWT) fuel quality indication systems (FQIS) data, the FAA explained in an airworthiness directive set for publication June 3. An FQIS “design flaw” causes the FQIS display to mis-state the amount of fuel in the CWT. The FQIS reports the amount of fuel, via weight, in fuel tanks, and fueling systems rely on this figure when adding fuel. If the FQIS overstates the amount of fuel onboard when fueling starts, too little fuel can be loaded. 

“Comparing the fuel volume upload with the final fuel load mass, which also accounts for the remaining fuel in the tanks from previous flight, is not an easy calculation and is prone to significant inaccuracies,” the FAA said, adding that it has “received reports that verification tasks are either not accomplished or done incorrectly.”

Onboard fuel management systems report fuel-quantity anomalies within the first several hours of a flight. Discrepancies trigger advisory messages that prompt crews to monitor the aircraft’s fuel system. 

In some cases, a misstated fuel load combined with a crew’s decision to continue flying can lead to diversions. The FAA said that it knows of “at least” 25 in-service events linked to fuel-quantity inaccuracies. Sixteen of the flights opted to continue, but 10 of those were forced to divert before reaching their destinations because of low-fuel conditions.

Boeing is developing a permanent fix for the FQIS problem. In the interim, the FAA will require operators to perform a new FQIS validation process, the “refueling station door cycling procedure,” within 30 days and following any CWT refueling. The issue does not affect other tanks.

Documentation of each check must be provided to flight crews, so they are aware that the validation has been done. The FAA also is requiring operators to document their general FQIS validation procedures and submit them to the agency.

The directive applies to 777s with CWTs having a capacity of at least 26,100 U.S. gallons. The agency said 255 U.S.-registered 777s will be affected by the rule.

Sean Broderick

Senior Air Transport & Safety Editor Sean Broderick covers aviation safety, MRO, and the airline business from Aviation Week Network's Washington, D.C. office.


Experienced similar problem on a brand new 773 on its first commercial (13 hour) flight two years ago, (for the first time) carrying fuel in the CWT: difference between fuel indicated and (fuel remaining on last flight + fuel tanked) was 4.5 tons. It appeared fuel density indication of the CWT (MAINTENANCE pages) was 0.9 instead of actual 0.8. Took extra fuel to compensate. Later on maintenance informed me that the default density indication of the system in case of a fault is 0.9 (inherently dangerous). Also, refueling these days is based on fuel indicated (auto shutoff) in stead of amount refueled. Boeing no longer provides any information on maximum difference between fuel indicated and (fuel remaining on last flight + fuel tanked) so it can no longer be checked by the pilots. Pilots need to apply common sense these days iso following FAA approved Boeing manuals.
It seems ROBBEDOES has it right about common sense. Why shouldn't the pilot be expected to check "fuel added" by ground crew against the cockpit indication? The pilot should be informed of gallons/liters/weight of fuel loaded. It should be simple to detect improper indications and plan accordingly.