A B737-300 developed a faulty fuel quantity indicating system in flight and subsequently a 1;400 LB fuel imbalance developed as the crew attempted to balance inaccurately presented quantities.
Synopsis
A B737-300 developed a faulty fuel quantity indicating system in flight and subsequently a 1;400 LB fuel imbalance developed as the crew attempted to balance inaccurately presented quantities.
Narrative
During preflight I accomplished the required fuel audit and verified the correct fuel onboard for Dispatch. Approximately one hour into the flight; I corrected an imbalance of 500 LBS between the main tanks with the number two tank indicating higher. The center tank was empty. Later in the flight while on descent during the arrival; the number two tank was again indicating an imbalance of approximately 300 LBS; again higher than number one. I attempted to balance again and noticed the number two tank continued to increase in quantity along with the totalizer. The number one tank quantity remained steady. Crossfeed operations were discontinued after the number two indicator continued to increase with the boost pumps on. The number two fuel quantity indication continued to increase until it indicated a full tank and the fuel totalizer continued to increase as well. At this point; both the number two fuel quantity indicator and the totalizer were considered unreliable and unusable. We were now on the arrival; ten minutes from touchdown. I attempted to compute the actual quantity in the number two main tank. I used the starting fuel; subtracting the fuel used and then subtracting the fuel from tank number one. This was accomplished on approach and resulted in a condition where we thought the tanks were 3;600 in tank one and 3;700 in tank one. We landed without incident and I noticed no unusual controllability problems on approach and landing. After landing a logbook entry was made regarding the number two fuel tank indicator. Dispatch and Maintenance Control were notified and Contract Maintenance was called out. After consulting with the Fueler and dripping the number two tank; it was determined we landed with 2.1 in the number two tank and 3.5 in the number one tank. This resulted in a landing imbalance of 1;400 LBS. Looking back on the situation; I should have been more suspect of the initial 500 LB split. I also should have done a quick check of the number two tank quantity by checking the fuel used on the fuel flow gauge prior to opening the crossfeed valve; as once I opened the crossfeed; the results of the above procedure were invalid. This would have verified that a split had indeed developed or that the fuel gauge was inaccurate.
Second reporter narrative
The proper MEL procedure was done by Contract Maintenance and we continued our day without incident.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.