Narrative:

Fuel on board was 20200#, with approximately 9400 pounds in each tank 1 and 2, and approximately 1400 pounds in the center tank. The minimum release fuel was 19900#. Because we had less than 2000# of fuel in the center tank, the center tank fuel boost pumps were left turned off for takeoff and then turned on later in the climb, as per our manual. The fuel burned out of the center tank normally, and as fuel from the main tanks began to feed, I used the xfeed to balance them, as they became somewhat unbalanced during the transition from center tank fuel to main tank fuel (the imbalance at this point was a normal and not uncommon occurrence). #1 and #2 fuel flows were even, during climb and cruise. Sometime later, maybe 15 or 20 mins later, I noticed a fuel imbalance of approximately 500 pounds between tanks 1 and 2. I xfed them back to even. A while later, maybe 15 or 20 mins or so, the fuel was again unbalanced by approximately 500 pounds and again, I xfeed them back to even. About this time, I noticed #2 fuel gauge fluctuating, a not uncommon occurrence). Sometime shortly before or after the above, we xed hlc and our fuel checked to be approximately 500 pounds less then scheduled on the computer flight plan for that chkpoint. This was felt to be not all that unusual, especially at heavier gross weights and when ATC levels one off for traffic at lower altitudes for extended periods during the initial climb out. All or most of the deficit is usually made up during the en route phase, especially on the longer flts. Later, we noticed that #2 fuel gauge had stopped at about 7000 pounds while #1 gauge appeared to be operating normally. Both gauges tested ok using the fuel quantity test button, the circuit breaker's were checked to be in, and we recycled them. It was assumed that the #2 fuel gauge had become inoperative. Nearing a point approximately southeast of ALS, with the #1 fuel gauge reading approximately 3000# and the #2 gauge still at about 7000 pounds, assuming that #1 was correct, we would be going deep into our reserve fuel by the time we reached phx. The different possibilities were discussed, including one gauge being correct and the other in error, both being correct, both being in error, as well as the possibility of a fuel leak. We then decided the safest course of action would be to divert to abq, about 100 to 125 NM from our present position. I opened the xfeed with all 4 main fuel boost pumps operating. I asked the first csr to look out at the wings to see if he could see any fuel leaking, and while he was doing that, I called abq operations to advise them of our problem, our ETA, to notify dispatch, an have a mechanic standing by. Csr reported back, that he wasn't sure, but he thought there might be fuel coming from the left wing. Several times during the flight, as well as for landing, we disconnected the autoplt aileron channel to check for any latitude imbalance. None was apparent. We landed uneventfully at abq. The mechanic reported that no fuel leak was apparent as we taxied in. We had him drip the tanks and he reported 4 1/4' in #1 tank and 20' in #2 tank. According to the center tank fuel gauge, the quantity was less than 200#. While the mechanic was dripping the tanks, I spoke with dispatch and maintenance control to discuss the problem. Maintenance control stated that the drip stick readings correlated to about 1600# in #1 tank and 6600 pounds in #2 tank, which agreed with the fuel gauges in the cockpit. Phx maintenance spoke with the mechanic a number of times and the mechanic checked quite a few items pertaining to the aircraft fuel system. He reported to me that everything he checked appeared to be operating normally, but the possibility of his cycling valves, and switches, etc, could have loosened a stuck or intermittent valve, etc. Phx maintenance felt the aircraft could safely be flown to phx, and we coordinated with dispatch, refueled the aircraft to 15700#, which included 6000 pounds of extra fuel, and flew to phx uneventfully. The main tanks 1 and 2 remained in balance the entire 55 min flight. When we arrived at gate in phx, I discussed the problem with maintenance and they took the aircraft OTS. I don't believe there is anything I could have or should have done, except it may have been a good idea to declare an emergency and have emergency equipment standing by at abq. I did not discuss our decision to divert to abq (with dispatch) because as soon as the decision to divert was made, we needed to start our descent and prepare for landing. We were too busy to converse with anyone. Callback conversation with reporter revealed the following: advised he has not discussed with maintenance as to the possible cause. On the advice of his attorney, reporter refused to give any additional information until he has received his identify strip. Advised report would be processed and that we could not contact him because we do not keep records of reporters. Did volunteer the FAA is investigating the incident.

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Original NASA ASRS Text

Title: UNABLE TO KEEP ACFT FUEL BURN IN BALANCE AND ELECTED TO MAKE PRECAUTIONARY LNDG SHORT OF DESTINATION.

Narrative: FUEL ON BOARD WAS 20200#, WITH APPROX 9400 LBS IN EACH TANK 1 AND 2, AND APPROX 1400 LBS IN THE CENTER TANK. THE MINIMUM RELEASE FUEL WAS 19900#. BECAUSE WE HAD LESS THAN 2000# OF FUEL IN THE CENTER TANK, THE CENTER TANK FUEL BOOST PUMPS WERE LEFT TURNED OFF FOR TKOF AND THEN TURNED ON LATER IN THE CLB, AS PER OUR MANUAL. THE FUEL BURNED OUT OF THE CENTER TANK NORMALLY, AND AS FUEL FROM THE MAIN TANKS BEGAN TO FEED, I USED THE XFEED TO BALANCE THEM, AS THEY BECAME SOMEWHAT UNBALANCED DURING THE TRANSITION FROM CENTER TANK FUEL TO MAIN TANK FUEL (THE IMBALANCE AT THIS POINT WAS A NORMAL AND NOT UNCOMMON OCCURRENCE). #1 AND #2 FUEL FLOWS WERE EVEN, DURING CLB AND CRUISE. SOMETIME LATER, MAYBE 15 OR 20 MINS LATER, I NOTICED A FUEL IMBALANCE OF APPROX 500 LBS BTWN TANKS 1 AND 2. I XFED THEM BACK TO EVEN. A WHILE LATER, MAYBE 15 OR 20 MINS OR SO, THE FUEL WAS AGAIN UNBALANCED BY APPROX 500 LBS AND AGAIN, I XFEED THEM BACK TO EVEN. ABOUT THIS TIME, I NOTICED #2 FUEL GAUGE FLUCTUATING, A NOT UNCOMMON OCCURRENCE). SOMETIME SHORTLY BEFORE OR AFTER THE ABOVE, WE XED HLC AND OUR FUEL CHKED TO BE APPROX 500 LBS LESS THEN SCHEDULED ON THE COMPUTER FLT PLAN FOR THAT CHKPOINT. THIS WAS FELT TO BE NOT ALL THAT UNUSUAL, ESPECIALLY AT HEAVIER GROSS WTS AND WHEN ATC LEVELS ONE OFF FOR TFC AT LOWER ALTS FOR EXTENDED PERIODS DURING THE INITIAL CLBOUT. ALL OR MOST OF THE DEFICIT IS USUALLY MADE UP DURING THE ENRTE PHASE, ESPECIALLY ON THE LONGER FLTS. LATER, WE NOTICED THAT #2 FUEL GAUGE HAD STOPPED AT ABOUT 7000 LBS WHILE #1 GAUGE APPEARED TO BE OPERATING NORMALLY. BOTH GAUGES TESTED OK USING THE FUEL QUANTITY TEST BUTTON, THE CB'S WERE CHKED TO BE IN, AND WE RECYCLED THEM. IT WAS ASSUMED THAT THE #2 FUEL GAUGE HAD BECOME INOP. NEARING A POINT APPROX SE OF ALS, WITH THE #1 FUEL GAUGE READING APPROX 3000# AND THE #2 GAUGE STILL AT ABOUT 7000 LBS, ASSUMING THAT #1 WAS CORRECT, WE WOULD BE GOING DEEP INTO OUR RESERVE FUEL BY THE TIME WE REACHED PHX. THE DIFFERENT POSSIBILITIES WERE DISCUSSED, INCLUDING ONE GAUGE BEING CORRECT AND THE OTHER IN ERROR, BOTH BEING CORRECT, BOTH BEING IN ERROR, AS WELL AS THE POSSIBILITY OF A FUEL LEAK. WE THEN DECIDED THE SAFEST COURSE OF ACTION WOULD BE TO DIVERT TO ABQ, ABOUT 100 TO 125 NM FROM OUR PRESENT POS. I OPENED THE XFEED WITH ALL 4 MAIN FUEL BOOST PUMPS OPERATING. I ASKED THE FIRST CSR TO LOOK OUT AT THE WINGS TO SEE IF HE COULD SEE ANY FUEL LEAKING, AND WHILE HE WAS DOING THAT, I CALLED ABQ OPS TO ADVISE THEM OF OUR PROB, OUR ETA, TO NOTIFY DISPATCH, AN HAVE A MECH STANDING BY. CSR RPTED BACK, THAT HE WASN'T SURE, BUT HE THOUGHT THERE MIGHT BE FUEL COMING FROM THE LEFT WING. SEVERAL TIMES DURING THE FLT, AS WELL AS FOR LNDG, WE DISCONNECTED THE AUTOPLT AILERON CHANNEL TO CHK FOR ANY LAT IMBALANCE. NONE WAS APPARENT. WE LANDED UNEVENTFULLY AT ABQ. THE MECH RPTED THAT NO FUEL LEAK WAS APPARENT AS WE TAXIED IN. WE HAD HIM DRIP THE TANKS AND HE RPTED 4 1/4' IN #1 TANK AND 20' IN #2 TANK. ACCORDING TO THE CENTER TANK FUEL GAUGE, THE QUANTITY WAS LESS THAN 200#. WHILE THE MECH WAS DRIPPING THE TANKS, I SPOKE WITH DISPATCH AND MAINT CTL TO DISCUSS THE PROB. MAINT CTL STATED THAT THE DRIP STICK READINGS CORRELATED TO ABOUT 1600# IN #1 TANK AND 6600 LBS IN #2 TANK, WHICH AGREED WITH THE FUEL GAUGES IN THE COCKPIT. PHX MAINT SPOKE WITH THE MECHANIC A NUMBER OF TIMES AND THE MECH CHKED QUITE A FEW ITEMS PERTAINING TO THE ACFT FUEL SYS. HE RPTED TO ME THAT EVERYTHING HE CHKED APPEARED TO BE OPERATING NORMALLY, BUT THE POSSIBILITY OF HIS CYCLING VALVES, AND SWITCHES, ETC, COULD HAVE LOOSENED A STUCK OR INTERMITTENT VALVE, ETC. PHX MAINT FELT THE ACFT COULD SAFELY BE FLOWN TO PHX, AND WE COORDINATED WITH DISPATCH, REFUELED THE ACFT TO 15700#, WHICH INCLUDED 6000 LBS OF EXTRA FUEL, AND FLEW TO PHX UNEVENTFULLY. THE MAIN TANKS 1 AND 2 REMAINED IN BALANCE THE ENTIRE 55 MIN FLT. WHEN WE ARRIVED AT GATE IN PHX, I DISCUSSED THE PROB WITH MAINT AND THEY TOOK THE ACFT OTS. I DON'T BELIEVE THERE IS ANYTHING I COULD HAVE OR SHOULD HAVE DONE, EXCEPT IT MAY HAVE BEEN A GOOD IDEA TO DECLARE AN EMER AND HAVE EMER EQUIP STANDING BY AT ABQ. I DID NOT DISCUSS OUR DECISION TO DIVERT TO ABQ (WITH DISPATCH) BECAUSE AS SOON AS THE DECISION TO DIVERT WAS MADE, WE NEEDED TO START OUR DSCNT AND PREPARE FOR LNDG. WE WERE TOO BUSY TO CONVERSE WITH ANYONE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: ADVISED HE HAS NOT DISCUSSED WITH MAINT AS TO THE POSSIBLE CAUSE. ON THE ADVICE OF HIS ATTORNEY, RPTR REFUSED TO GIVE ANY ADDITIONAL INFO UNTIL HE HAS RECEIVED HIS IDENT STRIP. ADVISED RPT WOULD BE PROCESSED AND THAT WE COULD NOT CONTACT HIM BECAUSE WE DO NOT KEEP RECORDS OF REPORTERS. DID VOLUNTEER THE FAA IS INVESTIGATING THE INCIDENT.

Data retrieved from NASA's ASRS site as of August 2007 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.