B737-400 DEPARTS WITH MEL ITEM REGARDING AUX TANK FUEL GAUGE READINGS AND 0 FUEL IN AUX TANK. AUX TANK ACTUALLY HAD FUEL; AS FLC DISCOVERED INFLT.

Narrative

B737 WAS PARKED AT THE DEP JETWAY WHEN THE FLC ARRIVED FOR A SCHEDULED 3:00 HR FLT. THE ACFT WAS ALREADY FUELED AND THE ACFT LOGBOOK INDICATED THAT THE ACFT WAS BEING RETURNED TO SVC AFTER EXTENSIVE MAINT. ALL LOGBOOK ENTRIES WERE COMPLETE AND THE ACFT WAS CERTIFIED AIRWORTHY WITH 1 MEL ITEM OPEN CONCERNING AN INOP AUX FUEL QUANTITY GAUGE. THE LOGBOOK ENTRY BY MAINT STATED THAT THE 'AUX FUEL TANK INDICATION SHOWS 3400 LBS WITH TANK VERIFIED AS EMPTY. AUX TANK NOT TO BE FILLED/PLACED ON MEL' ETC. ACTUAL TKOF WT WAS WELL UNDER THE MAX TKOF WT FOR THE EXISTING CONDITIONS AND ALL ASPECTS OF THE DEP WERE NORMAL. DURING THE CRUISE PORTION OF THE FLT; THE FLCS CHKED THE AUX FUEL TRANSFER SYS. THE AUX FUEL SYS APPEARED TO OPERATE PROPERLY AND AFTER 18 MINS THE AUX TANK QUANTITY INDICATOR INDICATED '0.' THE CTR TANK FUEL INCREASED; INDICATING THAT A SUBSTANTIAL AMOUNT OF FUEL HAD TRANSFERRED FROM THE AUX TANK. THE ADDITIONAL WT WAS ACCOUNTED FOR DURING LNDG PERFORMANCE CALCULATIONS AND THE PROPER RPTS WERE COMPLETED ON ARR AT DEST. THE AUX FUEL SYS INVOLVED IN THIS INCIDENT IS AN AFTER FACTORY MODIFICATION AND IS COMPLEX AND DIFFICULT TO MAINTAIN. HOWEVER; IF PROPER MAINT PROCS WERE FOLLOWED; THIS INCIDENT WOULD NOT HAVE OCCURRED.