C172 ENGINE QUIT ON APPROACH. PLT EXECUTED AN EMERGENCY LANDING WITHOUT HARM TO PERSONS OR THE AIRCRAFT.

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ENG QUIT WHILE EXECUTING THE RNAV APCH AT THE ZZZ ARPT. A FORCED LNDG WAS MADE INTO A FARMER'S FIELD WITHOUT ANY INJURY OR DAMAGE TO THE ACFT OR PROPERTY. THE EVENT OCCURRED AT THE LEVELOFF AT THE MINIMUM DSCNT ALT ON THE APCH. THE REASON THE ENG QUIT WAS DUE TO FUEL LINE ICING WHICH CUT OFF THE FUEL FLOW FROM THE R TANK. AFTER 3.5 HRS OF FLT TIME THE L TANK WENT DRY EVEN THOUGH THE TANK GAUGE WAS AT 1/4 TANK WHICH IS A WHOLE OTHER ISSUE TO BE ADDRESSED. THE R TANK SHOWED 1/3 OF A TANK AND THE FUEL SELECTOR WAS TURNED TO R TANK POS DURING THE FORCED LNDG WITHOUT ANY SIGN OF THE ENG RESTARTING. THE EVENT OCCURRED RATHER FAST SINCE THE ACFT WAS ONLY 500 FT AGL AT THE TIME OF THE OCCURRENCE. INVESTIGATING THE SITUATION ON THE GND; THE R TANK HAD 10 USABLE GALLONS AND THE L TANK WAS DRY. WHEN I CHKED THE DRAIN SUMP ON THE R TANK; I NOTICED ICE CRYSTALS IN THE SUMP CUP; THEREFORE; CONCLUDED THE R TANK MUST HAVE FROZE UP AT THE TANK SCREEN INSIDE THE TANK. THE REASON THE TANK HAD WATER IN IT WAS DUE TO A TANK REPAIR THAT WAS DONE THE DAY BEFORE THE FLT. THE TANK HAD BEEN WELDED ON PRIOR TO THIS FLT TO FIX A SMALL CRACK IN THE TANK. THE WELDING PROCESS REQUIRES FILLING THE TANK WITH WATER IN ORDER TO KEEP ANY FUMES IN THE TANK FROM IGNITING. THE MAINT STAFF THOUGHT THEY RID THE TANK OF ANY WATER BY SLOSHING FUEL THROUGH IT SEVERAL TIMES. AFTER MAINT; THE PLANE WAS BROUGHT BACK TO ITS COLD STORAGE HANGAR. AWARE OF A POSSIBLE WATER ISSUE; THE PLANE WAS BROUGHT BACK TO BE HEATED AT THE MAINT HANGAR 45 MINS PRIOR TO THE FLT. THE R TANK HAD BEEN CHKED TWICE FOR WATER CONTAMINATION WITH NO VISUAL SIGN OF CONTAMINATION. THE TEMP THE NIGHT BEFORE WAS IN THE TEENS AND IT COULD THEREFORE BE CONCLUDED THAT THERE MUST HAVE BEEN FROZEN ICE PARTICLES IN THE FUEL AND THE 45 MINS PRIOR TO THE FLT WAS NOT LONG ENOUGH FOR THE WATER TO COME OUT OF SUSPENSION IN THE FUEL. TO CONCLUDE; IT WAS UNFORTUNATE THAT WE WERE NOT ABLE TO REMOVE THE WATER VAPOR FROM THE FUEL PRIOR TO THE FLT. IT WAS ALSO UNFORTUNATE THAT WE WERE 3 MI SHY OF THE ARPT WHERE WE PLANNED TO STOP AND REFUEL AT.