Date: 201408
Local time of day: 0601-1200
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: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Fuel Crossfeed
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Landed in Emergency Condition; General Maintenance Action
Contributing factors: Procedure; Aircraft
Primary problem: Aircraft

Narrative: At 35 minutes into flight at cruise; fuel IMBAL light on right main tank illuminated. Verified cross feed valve was closed and main fuel pumps were on. Center tank fuel was 0.00. Started fuel leak- engine checklist in QRH. Suspected fuel leak and requested diversion to ZZZ which was nearest suitable airport. [Advised ATC] minimum fuel. Landed and contacted Maintenance Control. Maintenance determined cross feed valve malfunctioning. Aircraft was grounded.As an addition Contract Maintenance was instructed to stick the tanks which confirmed that there was a fuel imbalance but there were no other tests done as far as finding a leak or checking the fuel system. Maintenance stated to me that there was nothing else to test and they thought everything was fine and to take the plane to destination. I insisted on more investigation which revealed that the cross feed valve was allowing fuel to be transferred from the right tank to the left tank with the cross feed valve in the closed position. During this lengthy process the gate agents were pressing me to board the aircraft for departure. I stated that we were not finished with the testing and to hold off until I advise her. We came to find out that our Maintenance personnel were making comments to the Contract Mechanic that 'the pilots don't want to fly today' and 'they wanted to come here'. Comments like that are very unprofessional and are not conducive to a safe operation. Contract Maintenance confirmed that the cross feed valve was not functioning correctly and informed Maintenance Control.


Second narrative: During cruise at flight level 380 (filed for 340; but climbed for turbulence avoidance); approximately 35-40 minutes into the flight; the right fuel quantity indicator amber alert imbalance annunciated. We immediately checked that the crossfeed and center tank pumps were off; the main tank pumps were on and looked back at the gauges. We noticed that the right fuel quantity was decreasing by the tens by the second and that the left indicator seemed normal. The FF for both sides appeared normal as did the fuel burn/usage and totalizer quantity per the flight plan. We determined that we should pull out the QRH suspecting a possible Fuel Leak. Coincidentally; we got a vector to the north of course; assessed our position via STA and ARPT on the ND and saw that we were 60+ miles south of ZZZ.We entered the Suspected Fuel Leak Checklist at 120.3. It starts with a condition paragraph and points to the additional information on 120.5. We went to bullet items and list on 120.5 to help ascertain the suspicion of a fuel leak or not. One of the items is an amber alert annunciated for an imbalance. We went back to step one and knowing that it was an abnormal indication and proceeded to discuss options and procedures. The Captain informed the flight attendants of the situation and asked for a visual to check for spray or leak at the engine or off/under the wing on the right side. The flight attendants informed us that they and an off duty pilot looked and everything appeared normal. Step one stated that a diversion might be needed. We were vectored back to the west and direct on our filed course as we continued with the checklist agreeing that if we decided to divert; we would ask for vectors back to the north for ZZZ as our first option.We reviewed Steps 2; 3 and 4 and confirmed that they had already been done in the initial assessment upon first noticing the imbalance alert light. At step 5; we took a time hack at 43 minutes into the flight; and the Captain wrote down the quantities of the left and right indicators. We monitored and noticed that the right indicator was still decreasing by the tens by the seconds. The left indicator appeared normal as did the fuel flows. In fact; the right FF and the right fuel usage were both slightly less than the left. Three minutes later the imbalance had changed by an additional 100 pounds and at the next check it had an increased fuel balance of 200 pounds since our initial time hack 5 minutes earlier. So now it was at 1200+ pounds of imbalance; outside of the limitations and changing rapidly. We could not determine where the fuel was going and everything indicated a fuel leak and we continued the checklist. The checklist stated that if there was an imbalance of 500 pounds or more in 30 minutes or less; then a fuel leak is confirmed.We continued the checklist while also requesting a divert and minimum fuel with ATC for priority handling and descent into ZZZ. We got vectors for a descent and to the north. The confirmation step flows us to Step 11 which is to continue the steps to shutdown the engine. After discussion of the options and the continuing imbalance; we [advised] ATC. With the increased descent rate and the thrust at idle; we continued to monitor the indicators as we paused to assess whether we really wanted to shutdown the engine or not. We still had plenty of fuel for a landing and we just wanted to get the plane safely on the deck ASAP. While monitoring the indicators; the imbalance seemed to stabilize and not continue to decrease. We determined to hold off on the remaining steps and not shut down the engine and focus on the normal checklists in preparation for landing.The Captain continued communication with the flight attendants and updated them on our divert and that we were electing not to shutdown the engine and continuing for a normal landing. He then made a PA announcement to the passengers and we continued normal checklists for approach and landing. I ran the checklists; changed destination in the FMC; changed the pressurization panel appropriate for ZZZ and confirmed that men and emergency equipment were still standing by at the airport for our approach and landing.The Captain had begun to type a message to dispatch via ACARS and decided that timing would not allow it and we would just continue to landing. I kept the QRH at the ready; ensured normal checklists and procedures were complete and that we had the ILS in the box with an extended centerline as a back up to the vectors to the visual approach.We landed; taxied off the active runway and had the emergency response crew look at our #2 engine and wing to ascertain if they could see a leak. They reported all normal and we then shutdown #2 and taxied to the gate single engine. Once parked and checklists complete; deplaning of the passengers had begun and airport emergency ground personnel met us on the jetway and wanted to confirm that we had [advised] ATC and confirm our reported SOB and FOB.The Captain then began the communication phase using his phone to call the Duty Manager; Dispatch; and Maintenance Control. He filled out the logbook and then we began the long wait while Contract Maintenance and Maintenance Control determined the best course of action.I am filing this not because I think we committed any errors; but because the scenario is interesting and can be learned from. The Captain did an awesome job taking command and control and using his emergency authority. He was directive and receptive and really fostered a team atmosphere while keeping the safety of the passengers and crew number 1 and wanting to get the aircraft on the deck ASAP after we ascertained a definite emergency and suspected fuel leak as dictated by the checklists and manuals.


Synopsis: B737 flight crew reports diverting for a possible fuel leak and a fuel imbalance.