C172 INSTRUCTIONAL FLT WITH PIC AND STUDENT OUT OF MRB; VA. ROUGH RUNNING ENG. PICKED PRIMARY OFF ARPT LNDG SITE. DIVERTED TO ALTERNATE OFF ARPT SITE BECAUSE OF TREES ON PRIMARY SITE. SUCCESSFUL LNDG WITHOUT DAMAGE OR INJURY.

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I WAS FLT INSTRUCTING WITH A STUDENT WITH WHOM I HAD NEVER FLOWN BEFORE AND AN AIRPLANE I HAD NEVER FLOWN BEFORE. THE STUDENT AND I WERE ABOUT 10 MINS INTO THE FLT AND THEN EXPERIENCED A LITTLE ROUGHNESS IN THE ENG. AT THAT TIME I HEADED BACK TO THE ARPT AND ABOUT 1 MIN LATER EXPERIENCED A LOSS OF ENG PWR. AT THAT POINT I PICKED THE BEST SUITABLE OFF-ARPT LNDG SITE AND HEADED TOWARDS IT. WHILE HEADED TOWARDS IT; I TROUBLESHOT THE PROB INCLUDING USING THE CARB HEAT AND NO CHANGE IN ENG PERFORMANCE. AT THAT POINT I WAS STILL HEADED TOWARDS MY FIELD. AS I GOT TO FINAL; I REALIZED THAT WE HAD SOME TREES IN THE WAY SO I WENT TO AN ALTERNATE FIELD AND LANDED THERE WITHOUT INCIDENT OR ANY INJURY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT; AS OF THE DATE OF THE CALLBACK; NO CAUSE FOR THE ENG FAILURE HAD BEEN DETERMINED. THE FUEL HAD BEEN CHKED DURING PREFLT FOR QUANTITY AND CONTAMINATION. THERE WERE NO OBVIOUS SIGNS OF ENG DAMAGE SUBSEQUENT TO THE FORCED LNDG. THE ACFT WINGS WERE REMOVED AND ACFT TRANSPORTED TO MAINT FACILITY.