Narrative:

We received the aircraft and the only crew to crew communication came from the inbound captain to my first officer (first officer). We were told that on their first leg in the aircraft they experienced a fuel imbalance on the number two main of 1150 pounds. Inbound captain relayed that he didn't think he had done anything to cause the imbalance; (though it was possible) and they did speak with a mechanic but nothing was documented in the book. They flew without incident the second leg.I saw in the logbook that on (date) the aircraft was written up for a fuel leak and had been cleared by maintenance (mx) with a note that the scavenge tank had no leaks; ok to continue (as best I can remember the verbiage).we were planned out with 22.6 and actually got fueled to 22.8. At push we had noted the right main had 300 hundred pounds more than the left main. All fuel tank pumps were on prior to before start originating checklist and the APU was operated with all pumps running for pushback. During the checklist we made note of the difference in number one and number two mains.our flight was scheduled with 4.7k of dispatch add due to our flight through [tropical weather].takeoff and climb were uneventful. We reached cruise altitude FL370 and started pulling weather for destination. We noticed that the main tanks were only slightly different with the number two main being slightly higher than the number one; though it started about 300 pounds higher. We continued to deal with the eventual weather at destination; and at next glance at the fuel gauges; our number two main had less than the number one. This; despite the fact that the centers were running; we were stable at cruise; and noted fuel flow on the gauges was 2440 and 2460 respectively (with minimal fluctuations; but within 10-20 pounds of each other). I called to the B flight attendant (flight attendant); and asked him to see if he saw anything coming from the wing or engine. He asked a deadheading captain (who until that point; I wasn't aware was on; traffic delayed our arrival at the airport; and my sign-in process at tsa took longer than usual; so I got down as boarding had started) who reported that they could see nothing unusual.we opted to pull the QRH and look at our options for dealing with fuel issues since it was becoming apparent that the number two main was losing fuel somewhere.we started with the fuel leak checklist as I know it has the procedure for determining tolerances. My first goal was to start the process of recording our fuel information. We recognized the first step (possible diversion) and knew that if we did have a fuel issue; we would not be continuing to [destination] due to the weather. We complied with steps two thru four and got to step five where I recorded main one of 8500 and main two of 8440. We still had 3000 plus; in the center but those pumps were turned off during the checklist.for step six; we could not confirm spray but within 14 minutes we had an imbalance of 380 pounds with our left main at 8100 and our right main at 7720. During the timing and recording process; I sent an ACARS to dispatch letting them know we were running that checklist. Dispatch then sent quite a few messages; that I quite honestly don't recall much of their content as we also decided to review the imbal QRH procedure and determine if there were any other checklists that may pertain since we knew that the fuel leak checklist leads to an engine shutdown. Running the imbal checklist; we ruled out step one immediately. We got slowed on step two simply because we didn't have an actual imbal alert but moved to step three where we noted the first item: the total fuel remaining is less than the planned fuel remaining. We climbed to cruise quickly and had already received a shortcut; plus we departed with 300 extra pounds; so at this point we didn't see a discrepancy. The next suspected item says: an engine has excessive fuel flow. During our process we both interpreted that to be excessive flow from the tank and in reviewing the QRH as we discussed our event; we determined that it most likely referred to the actual fuel flow on the gauges; which again were equal. At this particular point in the flight; my concern was there is excessive fuel flowing from that tank and I didn't know where it was going - especially since the center tanks had been running when we noticed the discrepancy. In our minds at that point; we had excessive fuel flowing out of the tank. This led us to step four and back to the fuel leak engine checklist. Because I wanted another set of eyes to review the QRH prior to shutting down the motor; I took control of the aircraft and asked my first officer to review the fuel section in the QRH and make sure we were not missing anything. He reviewed a couple of the checklists as well and we both felt that the safest course of action was to divert to ZZZ; which was 100-110 NM away and was certainly the nearest suitable. It was also just north of the line of tcus that we could see. Our concern was not knowing where our fuel was actually going and perhaps running the risk of running a main dry in the event of a divert due to weather later in the flight. The next question after the divert to ZZZ decision; was to complete the checklist and shut down the motor or continue on both. Since the checklist was clear; we continued the checklist to secure the engine; notified dispatch; briefed the approach; briefed the fas; briefed the passengers; advised ATC and proceeded to ZZZ. From that point; the flight was uneventful and we proceeded to the gate after landing and spoke with the mechanics; who met the flight.learning items:1. The old checklists had notes that directed you to read very pertinent information. In this checklist; the condition and objective are right at the beginning; but the additional information section is at the back. I missed that note when I read the statement perhaps because I thought it was going to be part of the checklist. That would have been good information to have at the beginning of the checklist to ensure it was read because by the time we got to step 23; the next line directs us to the single-engine inoperative checklist. So our second chance to review that very good information was missed. I mention this; because the only item (in hindsight review) that applied for our thinking was the engine has excessive fuel flow which we again read to mean excessive fuel flow from the tank. We may have taken another look at securing the engine versus just diverting with two operating; given that none of the other items applied.2. Dispatch blew up our ACARS with messages which was distracting; but more importantly I think also tunneled our vision just a touch. As we were working through the checklist; we received his message to divert to ZZZ with all of the weather and release information. While this was good information; we wanted to make sure we were on the same page in the cockpit before diverting. Obviously the final say was mine; but from a timing standpoint; it interrupted calculations and checklists we were doing. At the end of the day; my responsibility is to manage the distractions; and that was something I could have done better.3. A discreet frequency from ATC would have been helpful. There was a lot of traffic inbound; lots of radio congestion; and once we informed ATC; it was distracting to listen to so much radio traffic while running checklists; briefing approaches; passengers; etc. 4. Hindsight can be eye opening; and in retrospect; I don't think fuel was leaving the aircraft; but was definitely leaving the tank. I estimated 1600 pounds an hour were leaving that tank (400 in 15 minutes) and my overriding concern was not knowing where it was. If it was moving to another tank; fine; but that could create a greater imbalance issue or may suggest a possible cross feed valve problem or a boost pump issue. I wanted all of my fuel for dealing with the weather and without setting up a hold on top of ZZZ to continue the troubleshooting process; I knew the best course of action would be to divert. Again; the question remains; run the engine or shut it down. I think the imbal checklist was probably best suited to our situation especially if we follow the excessive fuel flow logic to be the gauge and not the tank. 5. Preconceived notions and communications also may have altered our perception. From the previous write up; to the outgoing crew having an issue; and dispatch early divert message all factored into our thought process; and not in a positive way; certainly a mitigating factor.6. In the end; this may have been nothing more than a leaking valve or problematic pump that required fuel management as opposed to an emergency. Had weather not been so impending; we certainly would have taken more time with the situation to really look at the numbers; other checklists in more detail (imbal in particular); as well as had time to discuss excessive fuel flow and even contact mx control to ensure we used all resources. I don't want to imply that we rushed; as I think we were very diligent in our process. We know that main two should not have been losing fuel when the center tanks still had 3000 plus pounds and the pumps were still operating (and we could see the fuel burning down from that tank) 7. Finally; we had a deadheading captain in the back. After he checked for the leak; I should have asked him to come to the jump seat to observe and assist. Another set of eyes and another 20 plus years of experience could have made our jobs easier allowing us to offload functions like landing calculations; weather (getting ATIS on COMM2); reviewing the checklists; etc.

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Original NASA ASRS Text

Title: B737NG flight crew reported noting fuel disappearing from the right main fuel tank at FL370 even though it should have been stable as the center tank was supplying fuel to both engines. The crew eventually diverted to a suitable airport after deciding that a fuel leak existed. After the event they believed that fuel may have been migrating to the center tank and not actually leaking.

Narrative: We received the aircraft and the only Crew to Crew communication came from the inbound Captain to my First Officer (FO). We were told that on their first leg in the aircraft they experienced a fuel imbalance on the number two main of 1150 pounds. Inbound Captain relayed that he didn't think he had done anything to cause the imbalance; (though it was possible) and they did speak with a Mechanic but nothing was documented in the book. They flew without incident the second leg.I saw in the logbook that on (date) the aircraft was written up for a fuel leak and had been cleared by Maintenance (MX) with a note that the scavenge tank had no leaks; ok to continue (as best I can remember the verbiage).We were planned out with 22.6 and actually got fueled to 22.8. At push we had noted the right main had 300 hundred pounds more than the left main. All fuel tank pumps were on prior to Before Start Originating Checklist and the APU was operated with all pumps running for pushback. During the checklist we made note of the difference in number one and number two mains.Our flight was scheduled with 4.7k of Dispatch add due to our flight through [tropical weather].Takeoff and climb were uneventful. We reached cruise altitude FL370 and started pulling weather for destination. We noticed that the main tanks were only slightly different with the number two main being slightly higher than the number one; though it started about 300 pounds higher. We continued to deal with the eventual weather at destination; and at next glance at the fuel gauges; our number two main had less than the number one. This; despite the fact that the centers were running; we were stable at cruise; and noted fuel flow on the gauges was 2440 and 2460 respectively (with minimal fluctuations; but within 10-20 pounds of each other). I called to the B Flight Attendant (FA); and asked him to see if he saw anything coming from the wing or engine. He asked a deadheading Captain (who until that point; I wasn't aware was on; traffic delayed our arrival at the airport; and my sign-in process at TSA took longer than usual; so I got down as boarding had started) who reported that they could see nothing unusual.We opted to pull the QRH and look at our options for dealing with fuel issues since it was becoming apparent that the number two main was losing fuel somewhere.We started with the fuel leak checklist as I know it has the procedure for determining tolerances. My first goal was to start the process of recording our fuel information. We recognized the first step (possible diversion) and knew that if we did have a fuel issue; we would not be continuing to [destination] due to the weather. We complied with steps two thru four and got to step five where I recorded Main One of 8500 and Main Two of 8440. We still had 3000 plus; in the center but those pumps were turned off during the checklist.For step six; we could not confirm spray but within 14 minutes we had an imbalance of 380 pounds with our left main at 8100 and our right main at 7720. During the timing and recording process; I sent an ACARS to Dispatch letting them know we were running that checklist. Dispatch then sent quite a few messages; that I quite honestly don't recall much of their content as we also decided to review the IMBAL QRH procedure and determine if there were any other checklists that may pertain since we knew that the Fuel Leak Checklist leads to an engine shutdown. Running the IMBAL Checklist; we ruled out step one immediately. We got slowed on step two simply because we didn't have an actual IMBAL alert but moved to step three where we noted the first item: The total fuel remaining is less than the planned fuel remaining. We climbed to cruise quickly and had already received a shortcut; plus we departed with 300 extra pounds; so at this point we didn't see a discrepancy. The next suspected item says: an engine has excessive fuel flow. During our process we both interpreted that to be excessive flow from the tank and in reviewing the QRH as we discussed our event; we determined that it most likely referred to the actual fuel flow on the gauges; which again were equal. At this particular point in the flight; my concern was there is excessive fuel flowing from that tank and I didn't know where it was going - especially since the center tanks had been running when we noticed the discrepancy. In our minds at that point; we had excessive fuel flowing out of the tank. This led us to step four and back to the Fuel Leak Engine Checklist. Because I wanted another set of eyes to review the QRH prior to shutting down the motor; I took control of the aircraft and asked my FO to review the fuel section in the QRH and make sure we were not missing anything. He reviewed a couple of the checklists as well and we both felt that the safest course of action was to divert to ZZZ; which was 100-110 NM away and was certainly the nearest suitable. It was also just north of the line of TCUs that we could see. Our concern was not knowing where our fuel was actually going and perhaps running the risk of running a main dry in the event of a divert due to weather later in the flight. The next question after the divert to ZZZ decision; was to complete the checklist and shut down the motor or continue on both. Since the checklist was clear; we continued the checklist to secure the engine; notified Dispatch; briefed the approach; briefed the FAs; briefed the Passengers; advised ATC and proceeded to ZZZ. From that point; the flight was uneventful and we proceeded to the gate after landing and spoke with the Mechanics; who met the flight.Learning Items:1. The old checklists had notes that directed you to read very pertinent information. In this checklist; the condition and objective are right at the beginning; but the Additional Information section is at the back. I missed that note when I read the statement perhaps because I thought it was going to be part of the checklist. That would have been good information to have at the beginning of the checklist to ensure it was read because by the time we got to step 23; the next line directs us to the Single-Engine Inoperative Checklist. So our second chance to review that very good information was missed. I mention this; because the only item (in hindsight review) that applied for our thinking was the engine has excessive fuel flow which we again read to mean excessive fuel flow from the tank. We may have taken another look at securing the engine versus just diverting with two operating; given that none of the other items applied.2. Dispatch blew up our ACARS with messages which was distracting; but more importantly I think also tunneled our vision just a touch. As we were working through the checklist; we received his message to divert to ZZZ with all of the weather and Release information. While this was good information; we wanted to make sure we were on the same page in the cockpit before diverting. Obviously the final say was mine; but from a timing standpoint; it interrupted calculations and checklists we were doing. At the end of the day; my responsibility is to manage the distractions; and that was something I could have done better.3. A discreet frequency from ATC would have been helpful. There was a lot of traffic inbound; lots of radio congestion; and once we informed ATC; it was distracting to listen to so much radio traffic while running checklists; briefing approaches; Passengers; etc. 4. Hindsight can be eye opening; and in retrospect; I don't think fuel was leaving the aircraft; but was definitely leaving the tank. I estimated 1600 pounds an hour were leaving that tank (400 in 15 minutes) and my overriding concern was not knowing where it was. If it was moving to another tank; fine; but that could create a greater imbalance issue or may suggest a possible cross feed valve problem or a boost pump issue. I wanted all of my fuel for dealing with the weather and without setting up a hold on top of ZZZ to continue the troubleshooting process; I knew the best course of action would be to divert. Again; the question remains; run the engine or shut it down. I think the IMBAL Checklist was probably best suited to our situation especially if we follow the excessive fuel flow logic to be the gauge and not the tank. 5. Preconceived notions and communications also may have altered our perception. From the previous write up; to the outgoing Crew having an issue; and Dispatch early divert message all factored into our thought process; and not in a positive way; certainly a mitigating factor.6. In the end; this may have been nothing more than a leaking valve or problematic pump that required fuel management as opposed to an emergency. Had weather not been so impending; we certainly would have taken more time with the situation to really look at the numbers; other checklists in more detail (IMBAL in particular); as well as had time to discuss excessive fuel flow and even contact MX Control to ensure we used all resources. I don't want to imply that we rushed; as I think we were very diligent in our process. We know that Main Two should not have been losing fuel when the center tanks still had 3000 plus pounds and the pumps were still operating (and we could see the fuel burning down from that tank) 7. Finally; we had a deadheading captain in the back. After he checked for the leak; I should have asked him to come to the jump seat to observe and assist. Another set of eyes and another 20 plus years of experience could have made our jobs easier allowing us to offload functions like landing calculations; weather (getting ATIS on COMM2); reviewing the checklists; etc.

Data retrieved from NASA's ASRS site 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.