A C-172 CFI RPTED THAT; DUE TO FUEL CONTAMINATION; THE ENG QUIT 100 FT OVER 3G3; CAUSING AN OFF ARPT LNDG.

Narrative

AFTER BOARDING THE ACFT; A CESSNA 172; I ASKED MY STUDENT IF THE ACFT WAS 'AIRWORTHY' AND THE RESPONSE WAS 'YES.' THE ONLY ANOMALY POINTED OUT TO ME DURING THE PREFLT INSPECTION WAS THE R FUEL TANK CAP BEING LOOSE AND HAVING REQUIRED ADDITIONAL SECURING DURING THE FUEL DEPTH TEST. NO WATER WAS MENTIONED DURING THE SUMPING PROCESS. THE LAST TIME THE ACFT WAS FLOWN HAD BEEN THE DAY PRIOR IN WHICH THE TIME IN BTWN FLTS THERE HAD BEEN HVY RAIN. I NEVER PUT 2 AND 2 TOGETHER TO THINK ABOUT THE POSSIBILITY OF WATER IN THE TANKS BECAUSE I HAD FORGOTTEN ABOUT THE WX THAT HAD MOVED THROUGH OVERNIGHT; AND THAT THE ACFT HAD NOT FLOWN SINCE. AFTER A SUCCESSFUL RUN-UP; A 'GO' DECISION WAS MADE TO DEPART EBOUND TO K3G3 FOR TOUCH-AND-GO PRACTICE. AFTER THE FIRST TOUCH-AND-GO; PWR WAS APPLIED AND FLAPS WERE RETRACTED; INITIATING A SUCCESSFUL CLBOUT. THEN; ABRUPTLY AT ABOUT 100 FT AGL; THE ENG CAME TO A SUDDEN STOP. THERE WAS A SHORT PAUSE OF DISBELIEF; AND THEN I TOOK THE CTLS FROM MY STUDENT AND MADE SURE SEATBELTS WERE FASTENED; AND LOOKED FOR A PLACE TO SET DOWN IN FRONT OF US. THE ONLY SAFE CHOICE TO LAND WAS A CORN FIELD TO MY L; WITH A NARROW OPENING BTWN A BARN AND A ROW OF TREES TO SQUEEZE THROUGH. AT THE END OF THE SMALL OPENING WAS A COUNTY ROAD THAT I COULD NOT CLR; SO I HAD TO LAND BEFORE IT AND STOP QUICKLY TO AVOID A COLLISION WITH A VEHICLE. I SHUT THE FUEL SELECTOR VALVE OFF; CUT THE MIXTURE TO IDLE-CUTOFF POS; AND TURNED THE IGNITION OFF AND PULLED THE KEY OUT; LEAVING THE MASTER SWITCH ON TO BE ABLE TO EXTEND FLAPS TO SLOW DOWN BEFORE THE ROAD. I ALSO USED A MOMENTARY FORWARD SLIP TO GET DOWN FASTER AND SLOWER. WE SET DOWN RELATIVELY SMOOTHLY AND PLOWED THROUGH SOME OF THE CORN. WE CAME TO A STOP NEXT TO A BARN. WE EVACED AND DETERMINED THAT NO INJURIES OCCURRED; AND THE ACFT WAS VISUALLY UNDAMAGED. MINIMAL DAMAGE TO PROPERTY WAS DONE. THE CAUSE OF THE FAILURE WAS DETERMINED BY MAINT TECHNICIANS TO BE A LARGE QUANTITY OF WATER FOUND IN THE R FUEL TANK. THE ONLY REASON THAT I CAN IMAGINE THE WATER WAS NOT FOUND DURING THE SUMPING PROCESS; WAS THAT MY RELATIVELY LOW TIME STUDENT PLT WAS UNABLE TO OBSERVE THE DISCOLORATION OF WATER AT THE BOTTOM AT THE FUEL TESTER; DUE TO POOR BACKGND LIGHTING; MAKING IT HARD TO DIFFERENTIATE. THE LESSON LEARNED WAS THAT INSTRUCTORS MUST SUMP FUEL THEMSELVES ON EVERY FLT!