2011-12 · NASA ASRS report 986796
A B737-400 flight crew diverted after the cause of multiple large fuel imbalances could not be determined and destination weather would prevent continuous crew monitoring of the fuel system.
During climbout; at about 15;000 FT; the Captain noticed that there was an 800 LBS imbalance between the left and right main tanks (left heavy). He called it out; and announced that he would wait to balance until the aircraft was in cruise; straight and level (aircraft was in a right turn; climbing). Within a few minutes; he announced that the imbalance had increased to 1;100 LBS; and that he was going to crossfeed to correct. Cross feeding was used to return the two tanks to within about 100 LBS of each other. This series of events was repeated twice more (once as we neared cruise altitude; and once in cruise). As the third event was occurring; we began attempting to contact Maintenance Control; but were in a dead zone and couldn't reach them for several minutes. At that time; the main tanks had been balanced and seemed to be holding even; however; we were concerned by the rapidity with which the imbalances were occurring (500 plus LBS within 10 minutes) and by the weather at destination (less than 1/8 SM with fog). Specifically; what if a significant imbalance occurred with lesser amounts of fuel in the mains; during a go-around or a divert if the weather went below minimums? After contact was made with Maintenance Control; the Captain informed Maintenance Control and Dispatch of his intent to divert to sort the problem out on the ground; with which they concurred. We received an amended release to the divert airport; informed ATC; received a clearance and proceeded to the airport where we conducted an overweight landing; with a sink rate of 250 FPM. Aircraft main tank fuel was not being consumed in an appropriate manner. Right wing tank appeared to be being consumed faster than left tank; although total consumption was normal. Cause unknown; but suspected to be a fuel system check valve.
During flight had multiple large fuel imbalance discrepancies. Accomplished three fuel balancing procedures. Despite Maintenance Control assistance unable to determine exact cause of fuel problem. With possibility of continuing large rapid fuel imbalance problems of unknown cause and with destination weather forecast to require approach to cat I minimums with dispatch concurrence diverted to an enroute airport and accomplished overweight landing. Fuel imbalance or possible fuel transfer malfunction.
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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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