Date: 201510
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Climb
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Fuel Crossfeed
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting; Situational Awareness
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Maintenance Action
Contributing factors: Human Factors; MEL
Primary problem: Human Factors

Narrative: Day two of a three day trip; conducting operating experience for the first officer returning from furlough. During our preflight duties; the first officer and I noticed the left center boost pump was on MEL. We also noticed that the aircraft was over fueled by more than 14;000 pounds. I called dispatch and load planning to remedy this issue. After speaking with both dispatch and load planning I reviewed the MEL and then had the first officer review the MEL as well. Things were getting very busy and we were trying to get the flight out as close to on time as possible. The first officer was very busy because we had a new flight plan based on the increased fuel load. This completely changed our planned gate fuel; cruise altitude; briefing new takeoff data; etc; a lot of new information to go over. We pushed back and made our way to the runway. Passing 14;000 feet; I noticed we were a couple hundred pounds from a fuel imbalance. ATC leveled us off at an intermediate altitude. We ran the QRH and as we watched the quantity indicators; the left main tank was decreasing at an abnormal rate. That instantly led us to think we had a fuel leak. We decided the safest course of action was an air return (about 65 NM). We landed uneventfully.


Second narrative: On climb out; the Captain noticed a significant fuel imbalance and called for the QRH. We determined that we had a fuel leak and returned to ZZZ. Upon landing; emergency crews responded that they could not see any leakage. We taxied to the gate uneventfully.During preflight the fueler handed me the fuel slip and I noticed we were tankering fuel. Our flight plan did not reflect the extra fuel. The Captain called dispatch for clarification. We requested a new release and lower filed altitude to accommodate the extra weight. The aircraft had a write-up for an inoperative left center tank boost pump. The Captain reviewed the MEL and showed it to me as well. Both of us missed the step that said 'once airborne; open the crossfeed valve' as we were focused on the extra fuel situation. Subsequently; on climb out; the number one engine was running on the left [wing tank] fuel pumps while the number two was feeding from the right center tank boost pump. An imbalance quickly occurred. While running the fuel leak QRH for the suspected leak; step three directs the center tank pumps to be turned off (if on). In retrospect; once this was done; the two engines burned at the same rate; (but we determined this once on the ground.) We elected to keep the engines running given the proximity to the airfield and the desire to land on two engines instead of one. The landing; taxi in and shutdown were uneventful. Upon discussion with the maintenance representative; we determined the cause of the imbalance was not opening the crossfeed valve once airborne.


Synopsis: B737 flight crew reported being dispatched with a left center tank fuel pump on MEL. Once airborne the MEL calls for the fuel cross feed to be opened; but this was not accomplished and a fuel imbalance developed. Suspecting a fuel leak the crew returned to the departure airport.