AN AERONCA 65 ON A TRAINING FLT HAS ITS CARB ICE UP WHEN TRAINEE PLACES THROTTLE TO LOW POS FOR DSCNT. INSTRUCTOR PLT TAKES OVER FOR A PRECAUTIONARY LNDG IN A GRASS FIELD.

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I WAS WITH A TAILWHEEL STUDENT; ENTERING ON A 45 DEG TO DOWNWIND; RWY 23 AT FDK. I CALLED '4 MI 45' AND HEARD 2 OTHERS CALL 45; NO DISTANCE. STUDENT AND I COULD NOT SEE THEM; BUT AS I WAS CONCERNED AND AT LEAST 4 ACFT WERE ALREADY IN PATTERN; I TOOK OVER; CLRED MY R AND TURNED OUT; AND BEGAN TO CLB. I TURNED THE CTLS OVER TO MY PVT PLT STUDENT FOR RE-ENTRY AND HE CONTINUED TO TURN R FOR RE-ENTRY. I SAW CONFLICTING TFC AND POINTED IT OUT TO STUDENT. HE REDUCED THE THROTTLE AND MADE A DSNDING R TURN TO AVOID. TFC WAS APPROX 1 1/2 MI TO OUR 10 O'CLOCK AND SLIGHTLY ABOVE; COMING AT US. NEITHER HE NOR I APPLIED CARB HEAT. MY ATTN WAS ON TFC AS THE 45 DEG ENTRY NOW HAD AT LEAST 3 ACFT AND ALL OF THE OTHERS WERE FASTER; ABOVE; BEHIND AND PRESUMABLY DSNDING TO TPA. WE WERE THEN 1000 FT AGL. I TOLD HIM TO CONTINUE THE DSCNT AND TURN OUT AGAIN AND WE WOULD WAIT FOR A BETTER TIME TO ENTER. WHEN HE ADVANCED THE THROTTLE TO LEVEL OUT AT APPROX 900 FT AGL AND CLB OUT; ENG DID NOT RESPOND AND WE CONTINUED TO DSND. I TOOK OVER THE CTLS; APPLIED CARB HEAT; TOLD HIM TO CHK THE FUEL SWITCH AND BEGAN LOOKING FOR A PLACE TO PARK. ENG BEGAN RUNNING ROUGH WITH NO INCREASE IN RPM; SO AS I WAS ROUGHLY 500 FT AGL DOWNWIND TO A 1 MI LONG GRASS FIELD; A ROUGH IDLING ENG AND DSNDING; I DECIDED TO MAKE A LNDG IN THE FIELD. I CLOSED THE THROTTLE; SLIPPED ROUGHLY 300 FT AND MADE AN UNEVENTFUL LNDG. STUDENT AND I WALKED TO THE NEAREST HOUSE APPROX 1/2 MI AWAY; CALLED THE FBO. ABOUT 1 HR LATER; WE RESTARTED THE ENG; CHKED EVERYTHING AND FLEW BACK TO THE ARPT. I SHOULD HAVE CHKED THE CARB HEAT SOONER. AS I KNOW THE A65 ENGS PROCLIVITY FOR ICING; BUT I WAS MORE CONCERNED WITH TFC. BY THE TIME I APPLIED IT; THE ENG WAS ALREADY AFFECTED; AND I DECIDED TO LAND AS I HAD AN UNKNOWN SIT.