Date: 201703
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: Fuel Distribution System
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Troubleshooting
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Maintenance Action
Contributing factors: Aircraft; Procedure; Human Factors
Primary problem: Ambiguous

Narrative: We were getting ready for our redeye and started the APU.  After power transfer we got a left center tank boost pump low pressure light that came on.  We contacted maintenance and they began to troubleshoot the problem.  We were done boarding and waiting for the repair.  It was determined they would MEL the left center boost pump.  The mechanic went to finish up and we were waiting for a new release.  The gate agent was waiting to get word to close up and we were briefing.  The Captain read the MEL aloud as we looked at our Ipad.  I thought we got finished; but may have got distracted before getting to the second page. (I do not recall).  I know we read the part about maintain fuel crossfeed in closed position prior to takeoff.  I also know we saw the part about 'flight planning restrictions' that said Dispatch must notify flight planning about the center tank fuel being in the ZFW calculations.  The captain called Dispatch to make sure they were working on this and that they were aware of the MEL being added since it may affect our load.  As he was on the phone we got a new release and I heard him discussing that we may need to remove people.  A flight attendant came up and informed us of a sick passenger vomiting in the bathroom and a couple concerned about our slight delay and connections in ZZZ1. I pulled up the MEL again to review one more time; but got sidetracked when we were informed the couple plus the sick passenger and their companion were getting off.  After that the captain called OPS and informed them of the 4 passengers getting off and was told weights should be good. Our release started printing and we were ready to close up.  We made sure the release had the MEL on it that we just looked at.I was the flying pilot and we were climbing out and given higher when I heard the Captain state that we had a fuel imbalance.  We got cleared higher and he grabbed the QRH.  I noticed no fuel being used in the right tank and the left and center were being used.  We were again disrupted with a climb higher to FL250 and the CA informed them we were going to stop there to figure out a fuel issue.  I looked over and saw the crossfeed open and fuel being used in the left and center tanks with over 1000 LB imbalance.  I was not sure if the crossfeed was opened when he ran the QRH for imbalance or if it was opened earlier.  He was able to stabilize the tank and only burn out of the center; but there was still the imbalance.  He briefly tried to crossfeed; but it appeared to go back to burning center and left again; so he repositioned to try not making the situation worse.  We figured it was safer to get on the ground and figure the problem out and not continue and get a bigger problem if the right tank would not burn fuel. We turned back. We ran our checklists and looked over performance and the approach.  We landed and returned to the gate.




Synopsis: B737 First Officer reported being dispatched with the left center tank fuel pump inoperative. Once airborne a fuel imbalance quickly developed and the crew elected to return to the departure airport.