Narrative:

We were cruising at FL410 and our main tanks were full along with fuel in our center tank. We depleted the center tanks per our 700 fuel specs. After the center tanks were empty; I put the crossfeed closed and went back to a normal fuel operation. About that time I noticed we had a about 400-500 pound difference on #1 and #2 tank; so I went ahead and corrected it. A little while later we got a #1 tank fuel imbalance indication; with #1 low and a 1000 pound difference. I opened the crossfeed and turned off the #1 fuel pumps to correct our fuel imbalance. As we were monitoring it; we noticed that the #1 tank was still going down faster with the #2 tank staying relatively the same amount; now a 1300 pound difference. We pulled our QRH checklist out for the crossfeed valve. The QRH called for using differential thrust which we were using; but with very little effect. The circuit breaker was checked to be in too. All engine parameters (N1's; N2's; egt's; and fuel flows were all normal within each other's indications). Now our fuel imbalance was 1600 pound. At this time; we were about 260 NM from ZZZ. I contacted dispatch; and had a patch with maintenance control to consult about our situation. Now we were speculating a fuel leak; because fuel out of the tank was depleting faster than normal (greater than 500 pounds/30 minutes). At this point in time I asked dispatch for planning purposes; our plan of diversion was going to be ZZZ (now 200 NM out). After further talking with maintenance; our speculation was fuel leak; fuel imbalance now reaching 2000 pounds difference. There was also a deflection in the control wheel showing the fuel imbalance. Dispatch and I made the necessary changes to flight plan for the divert and an emergency was declared. The first officer advised ATC of our situation and got direct routing. I advised the flight attendants and the passengers of our situation and divert plan; including shutting down the engine. Also; I let ATC know we wanted the fire trucks standing by for us as a precaution and most likely will just taxi to the gate after landing. With the possible fuel leak; the QRH has you shut down that engine to isolate the condition. We waited until we descended into the high 20's to get a good start on APU and not as critical flying with 1 engine. We shut down the #1 engine and planned on a flap 15 landing. I took control of the aircraft and made the landing. Once again; after I turned off the autopilot; we had a deflection in the control wheel showing we had more fuel in the #2 tank. Our fuel imbalance now was about 1000 pound difference. After landing; the fire trucks inspected the runway for fuel and none was found. We taxied to the gate with 1 fire truck following us. Maintenance met us at the gate. I gave them the overview of the situation. I called our dispatcher to advise him we were at the gate and to help write up his report. I made a logbook entry about it; including that all engine indications were normal even at engine shut down. The passengers were transferred to another aircraft and we continued uneventfully.; after the center tank was depleted; the captain noticed a 400 pound imbalance with the left side low. He properly balanced the fuel and returned the fuel panel to a normal configuration. Not a great deal of time later; he said; 'we have an imbalance.' the left fuel gauge had turned amber as the imbalance had exceeded 1000 pounds. He opened the crossfeed valve and turned off the left pumps. We noticed that the imbalance was not improving; but getting worse. He then pointed out that the crossfeed open light was not indicating properly. (Don't know when he first noticed this). The light would not cycle bright/dim; but would go from off to dim; dim to off. I looked at the QRH for a stuck crossfeed valve. It refers to the light staying bright to indicate a stuck valve. No reference to what we were seeing. We were not certain if the valve was working properly or not. Though; had it been stuck closed; it would certainly have flamed out #1 at FL410 with the left boost pumps off (or so I assume). In the imbalance checklist; it mentions using differential thrust to improve the imbalance. I then set the right engine to about 2600 pph fuel flow and the left at about 1600 pph. The imbalance was about 1600 pounds at this point. We continued to lose ground on the imbalance and it reached 2000 pounds. I looked at the engine fuel leak checklist to see if anything applied. None of the list of indications in the notes section applied except the last; which was an 'uncontrolled fuel imbalance.'I suggested to the captain that we may need to consider the possibility of a leak. It was a very dark; moonless night; so we didn't try to visually check for trailing fuel. The captain was talking with maintenance and dispatch during the time I was trying the asymmetrical thrust. I don't know what all was discussed. I was quickly becoming convinced that we needed to get the airplane on the ground before we got to a point where the imbalance gave us serious control problems. To this point; I hadn't noticed any real out of trim problems. While the captain finished up with maintenance/dispatch; I coordinated the divert with ATC.once we were on our way to ZZZ and in the decent; as required by the engine fuel leak QRH; we shutdown the left engine using the engine shutdown checklist. As a precaution; we arranged for fire/rescue to be available after landing. The rest of the flight was uneventful. The captain took the airplane about 3000 ft and greased her on. We taxied to the gate while the fire trucks followed. I believe we had about a 1200 pounds imbalance at landing.

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Original NASA ASRS Text

Title: B737-700 flight crew experienced uncontrolled fuel imbalance at FL410. As the imbalance reached 2000 pounds the crew elected to divert to nearest suitable airport. Left engine was shut down per QRH for suspected fuel leak.

Narrative: We were cruising at FL410 and our main tanks were full along with fuel in our center tank. We depleted the center tanks per our 700 fuel specs. After the center tanks were empty; I put the crossfeed closed and went back to a normal fuel operation. About that time I noticed we had a about 400-500 LB difference on #1 and #2 tank; so I went ahead and corrected it. A little while later we got a #1 tank fuel imbalance indication; with #1 low and a 1000 LB difference. I opened the crossfeed and turned off the #1 fuel pumps to correct our fuel imbalance. As we were monitoring it; we noticed that the #1 tank was still going down faster with the #2 tank staying relatively the same amount; now a 1300 LB difference. We pulled our QRH checklist out for the crossfeed valve. The QRH called for using differential thrust which we were using; but with very little effect. The circuit breaker was checked to be in too. All engine parameters (N1's; N2's; EGT's; and fuel flows were all normal within each other's indications). Now our fuel imbalance was 1600 LB. At this time; we were about 260 NM from ZZZ. I contacted Dispatch; and had a patch with Maintenance Control to consult about our situation. Now we were speculating a fuel leak; because fuel out of the tank was depleting faster than normal (greater than 500 LBS/30 minutes). At this point in time I asked Dispatch for planning purposes; our plan of diversion was going to be ZZZ (now 200 NM out). After further talking with Maintenance; our speculation was fuel leak; fuel imbalance now reaching 2000 LBS difference. There was also a deflection in the control wheel showing the fuel imbalance. Dispatch and I made the necessary changes to flight plan for the divert and an emergency was declared. The First Officer advised ATC of our situation and got direct routing. I advised the flight attendants and the passengers of our situation and divert plan; including shutting down the engine. Also; I let ATC know we wanted the fire trucks standing by for us as a precaution and most likely will just taxi to the gate after landing. With the possible fuel leak; the QRH has you shut down that engine to isolate the condition. We waited until we descended into the high 20's to get a good start on APU and not as critical flying with 1 engine. We shut down the #1 engine and planned on a Flap 15 landing. I took control of the aircraft and made the landing. Once again; after I turned off the autopilot; we had a deflection in the control wheel showing we had more fuel in the #2 tank. Our fuel imbalance now was about 1000 LB difference. After landing; the fire trucks inspected the runway for fuel and none was found. We taxied to the gate with 1 fire truck following us. Maintenance met us at the gate. I gave them the overview of the situation. I called our Dispatcher to advise him we were at the gate and to help write up his report. I made a logbook entry about it; including that all engine indications were normal even at engine shut down. The Passengers were transferred to another aircraft and we continued uneventfully.; After the center tank was depleted; the Captain noticed a 400 LB imbalance with the left side low. He properly balanced the fuel and returned the fuel panel to a normal configuration. Not a great deal of time later; he said; 'We have an imbalance.' The left fuel gauge had turned amber as the imbalance had exceeded 1000 LBS. He opened the crossfeed valve and turned off the left pumps. We noticed that the imbalance was not improving; but getting worse. He then pointed out that the crossfeed open light was not indicating properly. (Don't know when he first noticed this). The light would not cycle bright/dim; but would go from off to dim; dim to off. I looked at the QRH for a stuck crossfeed valve. It refers to the light staying bright to indicate a stuck valve. No reference to what we were seeing. We were not certain if the valve was working properly or not. Though; had it been stuck closed; it would certainly have flamed out #1 at FL410 with the left boost pumps off (or so I assume). In the Imbalance checklist; it mentions using differential thrust to improve the imbalance. I then set the right engine to about 2600 PPH fuel flow and the left at about 1600 PPH. The imbalance was about 1600 LBS at this point. We continued to lose ground on the imbalance and it reached 2000 LBS. I looked at the Engine Fuel Leak checklist to see if anything applied. None of the list of indications in the notes section applied except the last; which was an 'uncontrolled fuel imbalance.'I suggested to the Captain that we may need to consider the possibility of a leak. It was a very dark; moonless night; so we didn't try to visually check for trailing fuel. The Captain was talking with Maintenance and Dispatch during the time I was trying the asymmetrical thrust. I don't know what all was discussed. I was quickly becoming convinced that we needed to get the airplane on the ground before we got to a point where the imbalance gave us serious control problems. To this point; I hadn't noticed any real out of trim problems. While the Captain finished up with Maintenance/Dispatch; I coordinated the divert with ATC.Once we were on our way to ZZZ and in the decent; as required by the Engine Fuel Leak QRH; we shutdown the left engine using the Engine Shutdown checklist. As a precaution; we arranged for fire/rescue to be available after landing. The rest of the flight was uneventful. The Captain took the airplane about 3000 FT and greased her on. We taxied to the gate while the fire trucks followed. I believe we had about a 1200 LBS imbalance at landing.

Data retrieved from NASA's ASRS site as of July 2010 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.