Narrative:

At a cruise altitude of 39000 ft on flight to ZZZ1 approximately 100 NM west of ZZZ; I noticed a blue light in full bright on the overhead fuel control panel. The light was a bright blue starboard spar-valve-closed light. I quickly glanced at the engine instruments but everything appeared to be normal: N1; N2; fuel flow; etc; were all reading the same for both engines. At first glance; the first officer and I discussed the significance of the indication and surmised that the bright blue indication meant that either the valve was in transit or that it indicated some sort of disagreement between the actual valve position and the detected valve position. I decided to maintain control of the aircraft since it was still my leg; and directed the first officer to pull out the QRH and look for any popped circuit breakers. We could not find anything in the QRH to help with the spar-valve indication; so I instructed the first officer to send an ACARS message to dispatch to inform them of our situation and see if they had anything to offer. After a couple of minutes went by; I noticed that we had developed a 300 pound split between the left and right fuel tanks. This was a bit disconcerting and we were both amazed at how quickly the split had developed. The right (#2) fuel tank showed (6.1) while the left (#1) fuel tank had decreased to (5.8). The fuel cross-feed valve was still closed and the number 2 engine was still performing normally but the number 2 fuel tank showed no indication of fuel consumption. Now we were faced with a situation involving either multiple failures or more likely a single failure that somehow has affected these two different systems: right fuel quantity indication system and right wing spar valve position light. Not knowing the actual condition of the aircraft; I had to assume that we were at risk with possible multiple failures. Assuming that the #2 spar valve was only partially closed; I decided to open the fuel cross-feed valve to ensure that the number 2 engine was getting positive fuel pressure from either tank. My main concern was to provide positive fuel pressure for the number 2 engine. As the split continued to develop; I directed the first officer to send an ACARS message to dispatch and to have them contact us on the number 2 radio so that we could talk to them of our fuel split situation. At the same time; I contacted center to inform them that we were declaring an emergency with a fuel problem and requested an immediate descent for divert to ZZZ 80 NM ahead. I could not risk over flight of a perfectly good airfield based on an assumption of a series of false indications. The safest thing was to land and investigate the problem. As the split continued in the descent; we still could not understand why the number 2 fuel tank would not show an indication of fuel consumption while all four fuel boost pumps appeared to be operating normally. There was no indication of a low fuel pressure or fuel filter bypass light from any of the four main fuel boost pumps. The first officer and I were somewhat hesitant to attempt the fuel balance by switching the fwd and aft fuel boost pumps in the number 1 tank to the off position. We thought it safer not to risk removing positive fuel pressure from the 'good' number 1 tank since both engines appeared to be running normally in this configuration (with the fuel cross-feed valve open). We also figured that if the right (#2) fuel quantity indicator was inoperative or reading erroneously; then any attempt to balance would produce a self-induced fuel split anyway. There were too many unknowns in this situation to begin down a path of investigation in flight with passengers on board. Therefore; we stuck to our original plan to divert; land; and investigate before something unforeseen or catastrophic occurred. In the descent; I coordinated with our flight attendants to explain the situation and told them that as a precaution; we were declaring a fuel emergency with center due to a possible low fuel situation. I emphasized that everything was ok and we would be safely on the ground shortly; but that we were extremely busy and would get back with them in a minute. The a flight attendant asked if I was going to make an announcement to the passengers or if he should inform them. I responded that I would make a P.a. In a little bit but am very busy right now; so I authorized him to go ahead and let them know why we were descending and to inform them that I would make my P.a. Shortly. When approach offered emergency vehicles to be put on standby; we weren't going to refuse because it was the safest thing to do in this unknown situation. I then asked the first officer to explain to the passengers over the P.a. That we would have emergency vehicles standing by as a routine procedure so as to alleviate any concerns when the passengers saw the vehicles approaching the aircraft. Note: it was a very busy 10 minutes as we descended from FL390 approximately 80 NM from ZZZ for a visual approach to runway. The issues mentioned above were all being handled in conjunction with the chaotic duties of pulling out the appropriate charts; setting the pressure controller to the correct altitude; getting ATIS; setting up the navigational aids for the approach; briefing the approach; running checklists; and generating the opc data as well as programming the FMC. During the entire idle descent with the cross feed valve open; the split in the fuel tanks continued to diverge and we were coming up on a 1000 pound split with the imbal warning indication as we approached final. Therefore; I decided to make a wider than normal approach in order to have wings level time to trim the aircraft before intercepting the glide path. We fully configured the aircraft for landing; intercepted the glide path; and at approximately 1000 ft AGL the bright blue starboard spar-valve-closed light extinguished. At the very least; a crew brief message needs to be sent to all crewmembers advising them of the possibility of this happening in flight and some guidance issued since none of it is covered in the QRH. Maintenance seems to think that there is some problem with the hmu giving erroneous spar valve lights. Maybe the hmu is affecting the fuel quantity indicator as well. I don't know; but hopefully boeing will have an answer. In the meantime let's put out a crew brief message to discuss the risks and procedures to follow.supplemental information from acn 794983: at this point; we were on about a 10 mile right base to runway and we had the airport in sight. I glanced over at the fuel quantity indicators and noticed about a 950-1000 pound split. The captain turned about a 9 mile final when we received the fuel imbalance message on the left quantity indicator. We discussed this earlier so we expected it prior to landing. The captain disengaged the autopilot prior to turning final. I asked him if it felt out of trim and he stated that it did. At this point; we both knew that this was not an indication problem if the aircraft reacted to the imbalance. The landing was normal and uneventful except that we landed with a 1500 pound fuel imbalance split.callback conversation with reporter 794983 revealed the following information: the reporter stated that the B737-ng aircraft engine spar valve opening and closing operation is automatic during crossfeed operation. The flight crew controls the center tank valves and pumps but the spar valve function is controlled by the system. The spar valve blue light is only an agree/disagree indicator. In this event the spar light illuminated bright blue indicating a disagreement with the fuel system's desired position. The air carrier's maintenance personnel were unable to the tell the reporter what had occurred and could not explain why the engine did not suction feed from the tank. The reporter could only guess that in fact the valve did not open and so the opposite wing tank fed fuel to both engines.

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Original NASA ASRS Text

Title: DUE TO A WING SPAR VALVE MALFUNCTION; A FUEL IMBALANCE DEVELOPED WHEN THE B737-700'S LT FUEL TANK BEGAN SUPPLYING BOTH ENGINES. AN EMER WAS DECLARED FOLLOWED BY A LNDG AT A NEARBY ARPT.

Narrative: AT A CRUISE ALTITUDE OF 39000 FT ON FLIGHT TO ZZZ1 APPROXIMATELY 100 NM WEST OF ZZZ; I NOTICED A BLUE LIGHT IN FULL BRIGHT ON THE OVERHEAD FUEL CONTROL PANEL. THE LIGHT WAS A BRIGHT BLUE STARBOARD SPAR-VALVE-CLOSED LIGHT. I QUICKLY GLANCED AT THE ENGINE INSTRUMENTS BUT EVERYTHING APPEARED TO BE NORMAL: N1; N2; FUEL FLOW; ETC; WERE ALL READING THE SAME FOR BOTH ENGINES. AT FIRST GLANCE; THE FIRST OFFICER AND I DISCUSSED THE SIGNIFICANCE OF THE INDICATION AND SURMISED THAT THE BRIGHT BLUE INDICATION MEANT THAT EITHER THE VALVE WAS IN TRANSIT OR THAT IT INDICATED SOME SORT OF DISAGREEMENT BETWEEN THE ACTUAL VALVE POSITION AND THE DETECTED VALVE POSITION. I DECIDED TO MAINTAIN CONTROL OF THE AIRCRAFT SINCE IT WAS STILL MY LEG; AND DIRECTED THE FIRST OFFICER TO PULL OUT THE QRH AND LOOK FOR ANY POPPED CIRCUIT BREAKERS. WE COULD NOT FIND ANYTHING IN THE QRH TO HELP WITH THE SPAR-VALVE INDICATION; SO I INSTRUCTED THE FIRST OFFICER TO SEND AN ACARS MESSAGE TO DISPATCH TO INFORM THEM OF OUR SITUATION AND SEE IF THEY HAD ANYTHING TO OFFER. AFTER A COUPLE OF MINUTES WENT BY; I NOTICED THAT WE HAD DEVELOPED A 300 LB SPLIT BETWEEN THE LEFT AND RIGHT FUEL TANKS. THIS WAS A BIT DISCONCERTING AND WE WERE BOTH AMAZED AT HOW QUICKLY THE SPLIT HAD DEVELOPED. THE RIGHT (#2) FUEL TANK SHOWED (6.1) WHILE THE LEFT (#1) FUEL TANK HAD DECREASED TO (5.8). THE FUEL CROSS-FEED VALVE WAS STILL CLOSED AND THE NUMBER 2 ENGINE WAS STILL PERFORMING NORMALLY BUT THE NUMBER 2 FUEL TANK SHOWED NO INDICATION OF FUEL CONSUMPTION. NOW WE WERE FACED WITH A SITUATION INVOLVING EITHER MULTIPLE FAILURES OR MORE LIKELY A SINGLE FAILURE THAT SOMEHOW HAS AFFECTED THESE TWO DIFFERENT SYSTEMS: RIGHT FUEL QUANTITY INDICATION SYSTEM AND RIGHT WING SPAR VALVE POSITION LIGHT. NOT KNOWING THE ACTUAL CONDITION OF THE AIRCRAFT; I HAD TO ASSUME THAT WE WERE AT RISK WITH POSSIBLE MULTIPLE FAILURES. ASSUMING THAT THE #2 SPAR VALVE WAS ONLY PARTIALLY CLOSED; I DECIDED TO OPEN THE FUEL CROSS-FEED VALVE TO ENSURE THAT THE NUMBER 2 ENGINE WAS GETTING POSITIVE FUEL PRESSURE FROM EITHER TANK. MY MAIN CONCERN WAS TO PROVIDE POSITIVE FUEL PRESSURE FOR THE NUMBER 2 ENGINE. AS THE SPLIT CONTINUED TO DEVELOP; I DIRECTED THE FIRST OFFICER TO SEND AN ACARS MESSAGE TO DISPATCH AND TO HAVE THEM CONTACT US ON THE NUMBER 2 RADIO SO THAT WE COULD TALK TO THEM OF OUR FUEL SPLIT SITUATION. AT THE SAME TIME; I CONTACTED CENTER TO INFORM THEM THAT WE WERE DECLARING AN EMERGENCY WITH A FUEL PROBLEM AND REQUESTED AN IMMEDIATE DESCENT FOR DIVERT TO ZZZ 80 NM AHEAD. I COULD NOT RISK OVER FLIGHT OF A PERFECTLY GOOD AIRFIELD BASED ON AN ASSUMPTION OF A SERIES OF FALSE INDICATIONS. THE SAFEST THING WAS TO LAND AND INVESTIGATE THE PROBLEM. AS THE SPLIT CONTINUED IN THE DESCENT; WE STILL COULD NOT UNDERSTAND WHY THE NUMBER 2 FUEL TANK WOULD NOT SHOW AN INDICATION OF FUEL CONSUMPTION WHILE ALL FOUR FUEL BOOST PUMPS APPEARED TO BE OPERATING NORMALLY. THERE WAS NO INDICATION OF A LOW FUEL PRESSURE OR FUEL FILTER BYPASS LIGHT FROM ANY OF THE FOUR MAIN FUEL BOOST PUMPS. THE FIRST OFFICER AND I WERE SOMEWHAT HESITANT TO ATTEMPT THE FUEL BALANCE BY SWITCHING THE FWD AND AFT FUEL BOOST PUMPS IN THE NUMBER 1 TANK TO THE OFF POSITION. WE THOUGHT IT SAFER NOT TO RISK REMOVING POSITIVE FUEL PRESSURE FROM THE 'GOOD' NUMBER 1 TANK SINCE BOTH ENGINES APPEARED TO BE RUNNING NORMALLY IN THIS CONFIGURATION (WITH THE FUEL CROSS-FEED VALVE OPEN). WE ALSO FIGURED THAT IF THE RIGHT (#2) FUEL QUANTITY INDICATOR WAS INOPERATIVE OR READING ERRONEOUSLY; THEN ANY ATTEMPT TO BALANCE WOULD PRODUCE A SELF-INDUCED FUEL SPLIT ANYWAY. THERE WERE TOO MANY UNKNOWNS IN THIS SITUATION TO BEGIN DOWN A PATH OF INVESTIGATION IN FLIGHT WITH PASSENGERS ON BOARD. THEREFORE; WE STUCK TO OUR ORIGINAL PLAN TO DIVERT; LAND; AND INVESTIGATE BEFORE SOMETHING UNFORESEEN OR CATASTROPHIC OCCURRED. IN THE DESCENT; I COORDINATED WITH OUR FLIGHT ATTENDANTS TO EXPLAIN THE SITUATION AND TOLD THEM THAT AS A PRECAUTION; WE WERE DECLARING A FUEL EMERGENCY WITH CENTER DUE TO A POSSIBLE LOW FUEL SITUATION. I EMPHASIZED THAT EVERYTHING WAS OK AND WE WOULD BE SAFELY ON THE GROUND SHORTLY; BUT THAT WE WERE EXTREMELY BUSY AND WOULD GET BACK WITH THEM IN A MINUTE. THE A FLIGHT ATTENDANT ASKED IF I WAS GOING TO MAKE AN ANNOUNCEMENT TO THE PASSENGERS OR IF HE SHOULD INFORM THEM. I RESPONDED THAT I WOULD MAKE A P.A. IN A LITTLE BIT BUT AM VERY BUSY RIGHT NOW; SO I AUTHORIZED HIM TO GO AHEAD AND LET THEM KNOW WHY WE WERE DESCENDING AND TO INFORM THEM THAT I WOULD MAKE MY P.A. SHORTLY. WHEN APPROACH OFFERED EMERGENCY VEHICLES TO BE PUT ON STANDBY; WE WEREN'T GOING TO REFUSE BECAUSE IT WAS THE SAFEST THING TO DO IN THIS UNKNOWN SITUATION. I THEN ASKED THE FIRST OFFICER TO EXPLAIN TO THE PASSENGERS OVER THE P.A. THAT WE WOULD HAVE EMERGENCY VEHICLES STANDING BY AS A ROUTINE PROCEDURE SO AS TO ALLEVIATE ANY CONCERNS WHEN THE PASSENGERS SAW THE VEHICLES APPROACHING THE AIRCRAFT. NOTE: IT WAS A VERY BUSY 10 MINUTES AS WE DESCENDED FROM FL390 APPROXIMATELY 80 NM FROM ZZZ FOR A VISUAL APCH TO RWY. THE ISSUES MENTIONED ABOVE WERE ALL BEING HANDLED IN CONJUNCTION WITH THE CHAOTIC DUTIES OF PULLING OUT THE APPROPRIATE CHARTS; SETTING THE PRESSURE CONTROLLER TO THE CORRECT ALTITUDE; GETTING ATIS; SETTING UP THE NAVIGATIONAL AIDS FOR THE APPROACH; BRIEFING THE APPROACH; RUNNING CHECKLISTS; AND GENERATING THE OPC DATA AS WELL AS PROGRAMMING THE FMC. DURING THE ENTIRE IDLE DESCENT WITH THE CROSS FEED VALVE OPEN; THE SPLIT IN THE FUEL TANKS CONTINUED TO DIVERGE AND WE WERE COMING UP ON A 1000 POUND SPLIT WITH THE IMBAL WARNING INDICATION AS WE APPROACHED FINAL. THEREFORE; I DECIDED TO MAKE A WIDER THAN NORMAL APPROACH IN ORDER TO HAVE WINGS LEVEL TIME TO TRIM THE AIRCRAFT BEFORE INTERCEPTING THE GLIDE PATH. WE FULLY CONFIGURED THE AIRCRAFT FOR LANDING; INTERCEPTED THE GLIDE PATH; AND AT APPROXIMATELY 1000 FT AGL THE BRIGHT BLUE STARBOARD SPAR-VALVE-CLOSED LIGHT EXTINGUISHED. AT THE VERY LEAST; A CREW BRIEF MESSAGE NEEDS TO BE SENT TO ALL CREWMEMBERS ADVISING THEM OF THE POSSIBILITY OF THIS HAPPENING IN FLIGHT AND SOME GUIDANCE ISSUED SINCE NONE OF IT IS COVERED IN THE QRH. MAINTENANCE SEEMS TO THINK THAT THERE IS SOME PROBLEM WITH THE HMU GIVING ERRONEOUS SPAR VALVE LIGHTS. MAYBE THE HMU IS AFFECTING THE FUEL QUANTITY INDICATOR AS WELL. I DON'T KNOW; BUT HOPEFULLY BOEING WILL HAVE AN ANSWER. IN THE MEANTIME LET'S PUT OUT A CREW BRIEF MESSAGE TO DISCUSS THE RISKS AND PROCEDURES TO FOLLOW.SUPPLEMENTAL INFO FROM ACN 794983: AT THIS POINT; WE WERE ON ABOUT A 10 MILE RIGHT BASE TO RUNWAY AND WE HAD THE AIRPORT IN SIGHT. I GLANCED OVER AT THE FUEL QUANTITY INDICATORS AND NOTICED ABOUT A 950-1000 LB SPLIT. THE CAPTAIN TURNED ABOUT A 9 MILE FINAL WHEN WE RECEIVED THE FUEL IMBALANCE MESSAGE ON THE LEFT QUANTITY INDICATOR. WE DISCUSSED THIS EARLIER SO WE EXPECTED IT PRIOR TO LANDING. THE CAPTAIN DISENGAGED THE AUTOPILOT PRIOR TO TURNING FINAL. I ASKED HIM IF IT FELT OUT OF TRIM AND HE STATED THAT IT DID. AT THIS POINT; WE BOTH KNEW THAT THIS WAS NOT AN INDICATION PROBLEM IF THE AIRCRAFT REACTED TO THE IMBALANCE. THE LANDING WAS NORMAL AND UNEVENTFUL EXCEPT THAT WE LANDED WITH A 1500 LB FUEL IMBALANCE SPLIT.CALLBACK CONVERSATION WITH RPTR 794983 REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT THE B737-NG ACFT ENG SPAR VALVE OPENING AND CLOSING OPERATION IS AUTOMATIC DURING CROSSFEED OPERATION. THE FLT CREW CONTROLS THE CENTER TANK VALVES AND PUMPS BUT THE SPAR VALVE FUNCTION IS CONTROLLED BY THE SYSTEM. THE SPAR VALVE BLUE LIGHT IS ONLY AN AGREE/DISAGREE INDICATOR. IN THIS EVENT THE SPAR LIGHT ILLUMINATED BRIGHT BLUE INDICATING A DISAGREEMENT WITH THE FUEL SYSTEM'S DESIRED POSITION. THE ACR'S MAINTENANCE PERSONNEL WERE UNABLE TO THE TELL THE REPORTER WHAT HAD OCCURRED AND COULD NOT EXPLAIN WHY THE ENG DID NOT SUCTION FEED FROM THE TANK. THE REPORTER COULD ONLY GUESS THAT IN FACT THE VALVE DID NOT OPEN AND SO THE OPPOSITE WING TANK FED FUEL TO BOTH ENGINES.

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.