Date: 201401
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Fuel System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer
Human factors: Troubleshooting; Confusion
Second reporter function: Captain
Anomaly: Aircraft Equipment Problem Critical
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Inflight Shutdown; General Declared Emergency; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: During climbout; passing through about FL250; we received a Fuel Master Caution with associated fuel imbalance pictured on the main fuel tank gauge display. As I recall; when we first noticed the imbalance after receiving the caution; we had about 7;300 LBS in the left main and 6;100 LBS in the right with the right going down quickly. We started with the Fuel Imbalance Alert Checklist in the QRH. As we worked through the possible causes listed in the checklist; we enlisted the help of a Captain that was jumpseating and riding in the back to help us look for visible signs of a fuel leak. As he was looking for that out the passenger windows; we tried the crossfeed valve and it appeared to work correctly; meanwhile; the right tank was going down at a rate of double or a little faster than double of the left tank. The jumpseating Captain then joined us on the flight deck as we decided an extra opinion and help would be beneficial. It was determined that it would be necessary to continue to the Engine Fuel Leak Checklist. By this time we had climbed to and leveled off at FL350. As we looked ahead at the checklist and saw that it mandated an engine shutdown; we declared an emergency with Center and coordinated our return to the departure airport. We were now heading west back when I ran the Engine Fuel Leak Checklist. That checklist then refers to the Engine Failure/Shutdown Checklist. I read through every item and every note in all the checklists and conferred with both these Captains to ensure that we weren't missing something or misdiagnosing the problem before we shut down that #2 Engine. All three of us agreed that the fuel was going down on the right side at a rapid rate and it warranted continuing with the Shutdown Checklist. At this time; as I recall; we had about 6;900 in the left tank and about 5;400 in the right. The checklist was run and the right engine was shutdown. We made a long slow controlled descent back into the runway with no further incident. We were greeted by the fire trucks; informed Tower that we needed no further assistance; and proceeded to the gate where we were met by several Maintenance personnel; a Chief Pilot and several other people. After our long descent with the right motor shut down; upon arrival at the gate; the fuel gauges were nearly equal again. We had time on our next deadhead flight to review what we'd done. The question comes to mind that what if it was an issue with the fuel crossfeed valve that we somehow overlooked? We feel that we checked it adequately during the event enough to rule it out; but the possibility exists. We feel that if anything we erred to the side of safety in making an air return. We did have questions about why the engine shutdown was mandated through the series of checklists; but can only assume it is to reduce the chance of fire from not knowing where the fuel leak could be. I felt good about our CRM; teamwork; and the added benefit of having an additional Captain helping back up our decisions.

Callback: The reporter stated that no mechanical faults were found and in the final analysis Maintenance assumed that for an unknown reason the fuel crossfeed valve did not close completely which allowed the right tank to feed both engines. The reporter stated however that the crew operated the crossfeed valve several times during troubleshooting and it appeared to operate normally with the light either OFF; BRIGHT OR DIM when in the commanded position.

Second narrative: During step climb to cruise; Master Caution illuminated and a fuel imbalance alert was identified: left 7;300 LBS; right 6;100 LBS. I asked deadheading Captain to assist with a visual inspection of the #2 Engine and wing area to determine if there was an external fuel leak. None found. Captain then joined crew in cockpit. First Officer referred to QRH for Fuel Imbalance Alert Checklist; as directed; and we began to address the fuel problem. After going through that checklist; the fuel imbalance increased to: left 6;900 LBS; right 5;400 LBS. A fuel audit showed approximately the correct fuel for our position; however; the bulk of the fuel was coming from the right main tank. The cross feed and fuel pump operations all indicated they were functioning normally.We proceeded with the Engine Fuel Leak Checklist because of the increasing imbalance. We requested return to the departure station; declared an emergency; shutdown #2 Engine; per the QRH; and landed.


Synopsis: A B737-700 FUEL MASTER CAUTION alerted when the right tank fuel quantity decreased more quickly than the left tank and after completing the Fuel Leak Checklist the crew shutdown the right engine; declared an emergency and returned to the departure airport. A faulty crossfeed valve was suspected.