A DC9-30 IN CRUISE AT FL310 DIVERTED DUE TO INDICATION OF UNWANTED FUEL XFER WITH #2 TANK FUEL DECREASING WITH FUEL STILL IN THE CTR TANK.

Narrative

DURING CRUISE FLT WE NOTICED A SLIGHT DECREASE OF FUEL IN THE R MAIN TANK WITH FUEL STILL IN THE CTR TANK (ALL PUMPS ON AND FUEL XFEED OFF). OUR L MAIN FUEL TANK GAUGE WAS MEL'ED INOP AND WAS NOT WORKING. AFTER THE CTR TANK HAD APPEARED TO BE EMPTY (THE GAUGE HOWEVER INDICATED 200-300 LBS FOR REMAINDER OF FLT) THE R MAIN TANK APPEARED TO BE DECREASING AT A FASTER THAN NORMAL RATE. OUR FLT PLAN FUEL XCHK S OF AU SABLE INDICATED THAT WE WERE DOWN APPROX 1500 LBS OF FUEL; AS FAR AS WE COULD CALCULATE AT THAT TIME; PREVIOUS XCHKS WERE NORMAL. WITH A POSSIBLE FUEL IMBALANCE OCCURRING A DIVERSION AND PRECAUTIONARY LNDG WAS MADE IN MBS. AN AUTOPLT OFF; AILERON TRIM CHK WAS MADE PRIOR TO LNDG AND WAS NORMAL. LNDG WAS UNEVENTFUL AND THE ACFT WAS TAXIED TO THE GATE FOR MAINT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS A DC9-30 AND HAD A #1 MAIN TANK FUEL QTY INDICATOR DEFERRED AS INOP. THE RPTR SAID DURING THE FLT IT WAS NOTED A DECREASE OF #2 MAIN TANK FUEL WITH FUEL STILL IN THE CTR TANK NOT A NORMAL SIT. THE RPTR SAID THE ACFT DISPLAYED NO ABNORMAL TRIM TO INDICATE AN UNWANTED XFER OF FUEL. THE RPTR SAID THE FLT DIVERTED TO MBS AND MECHS WERE SENT IN TO WORK THE MALFUNCTION. THE RPTR STATED THE CREW REMAINED OVERNIGHT IN MBS TO FERRY THE ACFT WHEN MAINT WAS COMPLETED. THE RPTR SAID MAINT RPTED THAT ALL TANKS WERE CALIBRATED INCLUDING #1 MAIN TANK AND SOME UNDEFINED VALVE WORK WAS ACCOMPLISHED. THE RPTR SAID THE RPTR FLEW THE ACFT IN THE MORNING AND ALL SYS AND INDICATIONS WERE NORMAL.