Narrative:

First officer's leg. During initial cruise; he mentioned aircraft flew crooked. Fuel difference minimal. I considered and started balancing fuel. During vectoring for approach into ZZZ; yoke was quite offset and first officer noticed left tank approximately 4500 pounds; right tank approximately 1000 pounds. Center tank empty; but xfeed open. Main tank pumps on. I attempted to xfeed to the right tank by turning the respective right pumps off. But fuel was still feeding some out of the right side and we decided to declare an emergency. Aircraft control was no problem. We landed and taxied normal to the gate. Supplemental information from acn 765940: shortly before starting our descent into ZZZ; I noticed the control yoke was unusually displaced. I had initially thought this was just the particular airplane characteristic; as shortly after takeoff I made the comment to the captain that this plane was 'bent' as some airplanes do not fly perfectly straight. The yoke seemed too far 'turned' to the right though and I looked at the fuel gauges. This is when I noticed we had a fuel balance problem with approximately 1000 pounds in the right tank and 4600 pounds in the left. I then looked at the fuel pump switches and noticed the xfeed valve was open. I had no idea when this switch was moved to 'open' as we had accomplished the before takeoff checklist that calls for it 'closed.' the center tank did not have fuel in it and its switches were 'off;' and all the main tank switches were 'on' as they should have been. During the flight; I disconnected the autoplt momentarily just to confirm that aircraft controllability was not a problem which it wasn't. We talked about entering a hold to discuss checklists; etc; and at that time noticed that even with the fuel switches now properly configured for re-balancing; the right gauge appeared to be creeping down in spite of being configured to feed fuel only from the left tank. At that point we both concluded that declaring an emergency; starting the APU for backup; and landing as soon as possible was the only correct choice; as we didn't know if this slow reduction of the amount of fuel in the right tank was a real concern or just normal quantity fluctuations. We landed uneventfully in ZZZ. I can only think that we were both 'led down the wrong path' as a result of my comment of the plane flying 'bent.' I think this deflected the amount of attention to the fuel balance condition as in some airplanes it is 'normal' for the yoke to be somewhat displaced from 'wings level.' this may have caused us to not monitor the fuel during this 1 hour flight as frequently as we should have. I also feel that not being informed by the captain that a switch was moved (the fuel xfeed switch) was a major contributor to this problem.

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Original NASA ASRS Text

Title: B737-300 DISCOVERS FUEL IMBALANCE AND LOW FUEL QUANTITY IN RIGHT TANK DURING DESCENT FOR LNDG. DECLARE EMERGENCY AND LAND WITHOUT INCIDENT.

Narrative: FO'S LEG. DURING INITIAL CRUISE; HE MENTIONED ACFT FLEW CROOKED. FUEL DIFFERENCE MINIMAL. I CONSIDERED AND STARTED BALANCING FUEL. DURING VECTORING FOR APCH INTO ZZZ; YOKE WAS QUITE OFFSET AND FO NOTICED L TANK APPROX 4500 LBS; R TANK APPROX 1000 LBS. CTR TANK EMPTY; BUT XFEED OPEN. MAIN TANK PUMPS ON. I ATTEMPTED TO XFEED TO THE R TANK BY TURNING THE RESPECTIVE RIGHT PUMPS OFF. BUT FUEL WAS STILL FEEDING SOME OUT OF THE R SIDE AND WE DECIDED TO DECLARE AN EMER. ACFT CTL WAS NO PROB. WE LANDED AND TAXIED NORMAL TO THE GATE. SUPPLEMENTAL INFO FROM ACN 765940: SHORTLY BEFORE STARTING OUR DSCNT INTO ZZZ; I NOTICED THE CTL YOKE WAS UNUSUALLY DISPLACED. I HAD INITIALLY THOUGHT THIS WAS JUST THE PARTICULAR AIRPLANE CHARACTERISTIC; AS SHORTLY AFTER TKOF I MADE THE COMMENT TO THE CAPT THAT THIS PLANE WAS 'BENT' AS SOME AIRPLANES DO NOT FLY PERFECTLY STRAIGHT. THE YOKE SEEMED TOO FAR 'TURNED' TO THE R THOUGH AND I LOOKED AT THE FUEL GAUGES. THIS IS WHEN I NOTICED WE HAD A FUEL BAL PROB WITH APPROX 1000 LBS IN THE R TANK AND 4600 LBS IN THE L. I THEN LOOKED AT THE FUEL PUMP SWITCHES AND NOTICED THE XFEED VALVE WAS OPEN. I HAD NO IDEA WHEN THIS SWITCH WAS MOVED TO 'OPEN' AS WE HAD ACCOMPLISHED THE BEFORE TKOF CHKLIST THAT CALLS FOR IT 'CLOSED.' THE CTR TANK DID NOT HAVE FUEL IN IT AND ITS SWITCHES WERE 'OFF;' AND ALL THE MAIN TANK SWITCHES WERE 'ON' AS THEY SHOULD HAVE BEEN. DURING THE FLT; I DISCONNECTED THE AUTOPLT MOMENTARILY JUST TO CONFIRM THAT ACFT CONTROLLABILITY WAS NOT A PROB WHICH IT WASN'T. WE TALKED ABOUT ENTERING A HOLD TO DISCUSS CHKLISTS; ETC; AND AT THAT TIME NOTICED THAT EVEN WITH THE FUEL SWITCHES NOW PROPERLY CONFIGURED FOR RE-BALANCING; THE R GAUGE APPEARED TO BE CREEPING DOWN IN SPITE OF BEING CONFIGURED TO FEED FUEL ONLY FROM THE L TANK. AT THAT POINT WE BOTH CONCLUDED THAT DECLARING AN EMER; STARTING THE APU FOR BACKUP; AND LNDG ASAP WAS THE ONLY CORRECT CHOICE; AS WE DIDN'T KNOW IF THIS SLOW REDUCTION OF THE AMOUNT OF FUEL IN THE R TANK WAS A REAL CONCERN OR JUST NORMAL QUANTITY FLUCTUATIONS. WE LANDED UNEVENTFULLY IN ZZZ. I CAN ONLY THINK THAT WE WERE BOTH 'LED DOWN THE WRONG PATH' AS A RESULT OF MY COMMENT OF THE PLANE FLYING 'BENT.' I THINK THIS DEFLECTED THE AMOUNT OF ATTN TO THE FUEL BAL CONDITION AS IN SOME AIRPLANES IT IS 'NORMAL' FOR THE YOKE TO BE SOMEWHAT DISPLACED FROM 'WINGS LEVEL.' THIS MAY HAVE CAUSED US TO NOT MONITOR THE FUEL DURING THIS 1 HR FLT AS FREQUENTLY AS WE SHOULD HAVE. I ALSO FEEL THAT NOT BEING INFORMED BY THE CAPT THAT A SWITCH WAS MOVED (THE FUEL XFEED SWITCH) WAS A MAJOR CONTRIBUTOR TO THIS PROB.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.