B737-800
Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 14000
Time Pressure; Distraction; Confusion; Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Weight And Balance; Inflight Event / Encounter Fuel Issue
Flight Crew Overcame Equipment Problem; General Maintenance Action
Aircraft; MEL
This report describes a fuel imbalance while flying to ZZZ.Preflight dealt with several issues.  First; the aft pump in the number 1 main tank had been placarded inoperative; so we had to carry 5;000 lbs extra fuel to comply with the MEL.  I reviewed the MEL requirements with my First Officer (FO) a few minutes before pushback.  Second; we had to carry extra fuel because of weather in the ZZZ area.  ZZZ1 and ZZZ2 were our alternates.  Third; we were close to our maximum takeoff weight.  Our planned takeoff weight was 160.0 with a maximum takeoff weight of '160.9L' (landing weight restriction).  Fourth; this was the first time that the FO and I had flown together; so we talked briefly about crew coordination.  Fifth; I had great difficulty finding a computer terminal that had CCI (Crew Check In) and which also had a working printer.  This consumed time that contributed to a somewhat rushed preflight.After engine start; we encountered two 'slot time' delays apparently because of weather near ZZZ.  The first time was XB:10 (35 minutes after our scheduled push time of XA:35).  The second time was XB:20.  These delays consumed more fuel and brought us close to our minimum takeoff fuel of 29.0.  I looked at our total fuel onboard several times and was preparing to contact dispatch to negotiate a new minimum takeoff fuel when tower finally cleared us for takeoff.  We pulled on to the runway with 29.2 fuel onboard.Takeoff and climb were normal.  I was the pilot flying.  After climbing through 10;000 feet; I noticed that the right center tank fuel pump 'LOW PRESSURE' light was on and the 'IMBAL' alert had annunciated on the Upper Display Unit.  We ran the 'FUEL PUMP LOW PRESSURE' abnormal procedure and stopped the imbalance at 1.6.  I took the following fuel readings about 8 or 9 minutes after takeoff:  8.6; 12.0; 7.0 (number 1 main; center; number 2 main).We were now in a situation where we had exceeded two limitations.  First; number 1 and number 2 main tank quantities must remain within 1;000 lbs.  We had exceeded this by 600 lbs.  Second; number 1 and number 2 main tanks must be kept full if the center tank has more than 1;000 lbs.  Our number 1 tank was full; but our number 2 tank was not full.We took the following actions to bring the aircraft back within these two limitations.  We continued to run the 'FUEL PUMP LOW PRESSURE' abnormal procedure until the center tank was empty.  Then; we ran the 'IMBAL' abnormal procedure to balance the fuel between the number 1 and number 2 main tanks.  These procedures were completed before we started down from cruise altitude.  The total fuel onboard closely matched the planned fuel onboard during the flight; so we did not think that we had a fuel leak.Arrival into ZZZ was uneventful.  I wrote up both the fuel imbalance and the failed center tank pump in the maintenance logbook.When did the center tank pump fail?  About 8 or 9 minutes after takeoff; we had 27.6 fuel onboard (8.6 plus 12.0 plus 7.0).  Our planned fuel onboard at the gate was 30.1.  We may have actually had 30.2.  Subtracting 27.6 from 30.2 gives 2.6; so each engine had burned about 1.3 since engine start.  (We kept both engines running after pushback.)  The number 2 main tank had 1.6 less than the number 1 main tank (7.0 versus 8.6).  While the exact point of failure is unknown; these data suggest that the right center tank pump probably failed before takeoff.If true; how did we miss the failed pump?  There appear to have been three factors.  First; we had a rushed preflight; so this 'rush-to-comply' mindset may have contributed to missing the failed pump.  Second; the aft pump in the number 1 main tank had already been placarded inoperative; and its LOW PRESSURE light was illuminated.  We expected to see a light in our peripheral vision up on the overhead fuel panel.  This may have been a case of 'Expectation Bias' where we saw what we expected to see.  Third; several tight constraints (i.e. MEL; weather near ZZZ; takeoff weight; and minimum takeoff fuel) probably directed our attention away from indications of a failed center tank pump.The common theme running through these three factors is attention.  Paying close attention to one item allows you to examine it in great detail.  However; this close attention comes at a price:  reduced attention to other items in your environment.  The challenge here is to deploy your attention and your FOs attention in a manner best suited to the situation.  As Captain; I could have and should have slowed down the pace of events so that we could have had more attention available to pick up on unusual items; items like a failed pump.
The Captain of a 737 reported experiencing a fuel imbalance issue due to a failed center tank fuel pump. The troubleshooting of this event was complicated by an already deferred main tank fuel pump. However; the crew correctly identified the problem; accomplished the appropriate checklist; rebalanced the fuel to within limits; and landed uneventfully.