ENG FAILS WHILE IN TFC PATTERN RESULTING IN AN OFF ARPT LNDG.

Narrative

ACFT STARTED VIBRATING ON DOWNWIND. STUDENT PLT CONTINUED TO FLY PATTERN AS I TROUBLESHOT CAUSE; WITH MIXED SUCCESS (VIBRATION ABATED A BIT). CONTINUED PATTERN AND PWR OK. ON FINAL AT 800 FT; 1/2 MI; PUT IN FULL FLAPS AND ALL OK. THEN PROP STARTED TO JERK (SAME LOOK AS A STROBE LIGHT ON PROP) AND GLIDE PATH ALTERED GREATLY. THE ENG FAILED. AT THAT TIME I TOOK CTL OF ACFT. WITH SHRED OF DOUBT ABOUT GLIDING OVER WIRES AND TREES; I TURNED ACFT AND LANDED ON FIELD SAFELY. NO DAMAGE TO ACFT; OCCUPANTS; FIELD (FROZEN). CAUSE: CARB ICE. LESSONS: #1 -- THE ENG WILL QUIT. PLAN FOR THIS. PLAN FOR WORST CASE BUT REALIZE IDLING PROP HAS DIFFERENT GLIDE THAN STOPPED PROP! #2 -- DON'T WORRY ABOUT 'FIXING' ENG IN AIR. PUT IT DOWN THEN FIX IT. #3 -- AS CFI (OR PIC) DECIDE WHEN YOU WILL TAKE OVER ACFT; UNDER WHAT CONDITIONS; ETC; SO THAT DECISION NEED NOT BE MADE UNDER OR DURING EMER. THIS WILL DIFFER FROM PERSON TO PERSON AND WITH EXPERIENCE OF CFI/STUDENT. I'M WRITING THIS RPT FOR ANYONE WHO HAS NOT THOUGHT HYPOTHETICALLY ABOUT AN INCIDENT SUCH AS ABOVE. I NEVER HAD; AND I LOST SUCH VALUABLE TIME AS I MUDDLED THROUGH #1-3.