B757-200
Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 23000; Flight Crew Type 5000
Troubleshooting; Time Pressure; Situational Awareness; Communication Breakdown
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural MEL / CDL
General Maintenance Action; General Flight Cancelled / Delayed
Logbook Entry; Aircraft; Company Policy; Procedure; Human Factors
Flight departed ZZZ July 2012 with Potable Water Quantity Gage on MEL. Water tank confirmed full prior to departure in accordance with MEL procedure.  On approach to ZZZZ the following morning; a split flap condition was encountered. Suspect that the leak from the water tank was directly related to the flap anomaly.  MEL was incorrectly applied. The problem was in fact a ruptured/leaking potable water tank; not an inoperative quantity gage. The tank was confirmed full prior to departure from ZZZ.  Within one hour after departure there was no water available in the galleys and lavatories. No water uptake was available from the water tank.  Pressure was OK.  Advised Maintenance Control and Dispatch with HF Phone Patch. Dispatch advised to continue to destination. Flight crew was concerned that water had leaked into unknown areas of the airframe and would freeze at altitude.  Captain complied with Dispatch recommendation to continue; but configured the aircraft for landing early on approach; in anticipation of potential landing gear and/or flight control problems.  When Flaps were extended beyond Flaps 20; a split flap condition was encountered. A landing was made with Flaps 20 and increased speed additives were applied. The longest available runway was used and no control issues were noted. A previous flight; on the same B757-200 aircraft earlier in the day had departed ZZZZ1 with this same problem. The Logbook entry indicated that the quantity gage read one-quarter (1/4) 'Full' prior to dispatch. Ground personnel in ZZZZ1 filled the tank prior to departure.  During the flight back to ZZZ a Logbook entry was made that all the water in the tank was depleted.  Prior to our later dispatch on ZZZ-ZZZZ; later that day; the Potable Water Quantity Gage was placed on MEL.  Due to the nature of the Logbook write-up; it would have been prudent to further troubleshoot the Potable Water System before applying the MEL for the quantity gage.  The safety of the passengers and crew were compromised by this situation. Luckily; the split flap situation was well controlled but the possibility of other complications from the frozen water could have been much worse.  On-time performance and dispatch reliability should never take precedence over thorough maintenance trouble shooting and corrective action. Complacency.
Reporter stated that Maintenance troubleshooting had determined the split flap condition was due to a faulty flap position indicator; not related to the ruptured Potable Water Tank located aft of the Aft Cargo Compartment. He has 23;000 flight hours and 5;000 plus hours in B757-200 aircraft.
Pilot reports an MEL deferral of a Potable Water Quantity Gage was incorrectly applied on a B757-200 aircraft.The Potable Water tank was later found ruptured and leaking after repeated refills. Lack of adequate Maintenance troubleshooting; prior to deferring the Quantity Gage; was also noted.