B737-300 CABIN ATTENDANT INADVERTENTLY OPENED THE LAVATORY DRAIN VALVE WHEN ATTEMPTING TO DIRECT HOT WATER TO THE LAVATORY RESULTING IN A CHUNK OF ICE FALLING ON A PUBLIC RESIDENCE SOON AFTER TKOF.

Narrative

ENRTE FROM BNA-ONT IN THE VICINITY OF MEM; A FLT ATTENDANT INFORMED ME WE HAD NO POTABLE WATER; AND THAT WE HAD LEFT BNA WITH A FULL TANK. I HAD SOME CONCERN ABOUT ICE BUILDUP; BUT NOTICED NO CHANGE IN FLT CHARACTERISTICS. I COULD ONLY SPECULATE THAT WE HAD LEFT WITH ALMOST NO WATER AND A FAULTY GAUGE; OR WE HAD AN OPEN OR FAULTY VALVE. I CONSIDERED CALLING DISPATCH ENRTE; BUT ELECTED NOT TO RPT THE PROB UNTIL I WAS IN RANGE; AS I BELIEVED THEY WOULD HAVE ME TAKE NO FURTHER COURSE OF ACTION THAN TO CONTINUE TO ONT. IF WE HAD AN OPEN VALVE; I HOPED THAT THE WATER DRAINED ON TAXI AND CLBOUT. CALLING IN-RANGE TO ONT; I ASKED IF THEY COULD CHK FOR AN OPEN VALVE. AFTER LNDG; I WENT OUT AND OBSERVED A SMALL ACCUMULATION OF ICE ON THE TAILCONE. THE VALVES WERE IN THE CLOSED POS. AFTER FILLING THE TANK; WATER STARTED TO DRAIN. I WAS TALKING TO MAINT WHEN ONE OF THE RAMPERS TOLD ME THE PURGE VALVE IN AFT LAVATORY HAD BEEN LEFT OPEN. I AM ASHAMED TO ADMIT THAT I HAD FORGOTTEN ABOUT THAT VALVE. I CAN ONLY SPECULATE IT WAS LEFT OPEN IN BDL; WHERE THE ACFT ORIGINATED. IT IS POSSIBLE A FLT ATTENDANT OPENED IT AFTER I ASKED IF WE HAD HOT WATER IN THE LAVATORIES; WE THE WASH PRIOR; THE WATER HEATERS HAD BEEN LEFT OFF ON THE SAME FLT. WE DID HAVE HOT WATER; AND WHEN HE INFORMED ME ABOUT THE LOSS OF WATER; I ASKED HIM IF WE DID ANYTHING BACK THERE; AND HE SAID NO. ABOUT 15 MINS AFTER ARR; THE OPS AGENT TOLD ME THE TWR HAD CALLED BECAUSE SOMEONE HAD RPTED A CHUNK OF ICE HAD HIT THEIR ROOF. TWR BELIEVED IT COULD HAVE BEEN FROM MY ACFT OR ANOTHER ONE. I BELIEVE I PROBABLY SHOULD HAVE CALLED DISPATCH ENRTE TO GIVE THEM A HEADS UP TO A POTENTIAL PROB. I SUPPOSE THAT ONE WAY TO PREVENT THIS FROM HAPPENING AGAIN WOULD BE TO HAVE EITHER THE PLTS OR FLT ATTENDANTS INCLUDE CHKING THE VALVES IN THE LAVATORY CLOSED ON AN ORIGINATOR.