A C172 PVT PLT ABORTS HIS TKOF AT 30 FT AGL WHEN HE ENCOUNTERS TURB AND PERCEIVES A ROUGH ENG. UPON LNDG HE ROLLS OFF THE END OF THE RWY; SOFTLY; INTO THE GRASS.

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OCCURRENCE WAS ON RETURN TRIP TO HOME BASE AFTER A STOP OF ABOUT 1/2 HR AT OVO ARPT. AFTER TKOF FROM RWY 5; ACFT ROLLED ABOUT 15 DEGS R DUE TO TURB AT ABOUT 20-30 FT; RECOVERED IMMEDIATELY TO LEVEL FLT. JUST THEN; ENG SOUNDS AND VIBRATION SUGGESTED A ROUGH RUNNING ENG. PLT DETERMINED THERE WAS SUFFICIENT RWY LEFT TO LAND SAFELY AND DECISION WAS MADE IMMEDIATELY TO ABORT TKOF. PWR WAS PULLED TO IDLE AND ACFT SIDE SLIPPED TO ABOVE RWY; ACFT CTRED AND TOUCHDOWN MADE ON RWY. ACFT WAS SLOWED CONSIDERABLY ON RWY SURFACE AND CAME TO A SAFE STOP IN SOFT WET GND AND GRASSY AREA AT THE END OF THE RWY. THERE WAS NO DAMAGE TO ACFT AND NO PERSONAL INJURY. ACFT EXAMINED BY A&P MECH AT FIELD AND ALSO BY OWNER'S (RENTAL) A&P WHO FLEW IN AT PLT'S REQUEST. MAGNETO CHK; FULL PWR RUN-UP; FUEL SUMPS; VISUAL STRUCTURE; ETC; SHOWED OK TO A&P'S. AFTER EXAMINATION ON THE GND; A&P FLT TESTED ACFT AND FLEW IT BACK TO BASE. ACFT LOG SHOWED 100 HR HAD BEEN COMPLETED IN LAST 4 HRS. NO FUEL WAS PURCHASED DURING TRIP. PLT DURING CHKOUT HAD DRAINED SUMPS -- NO WATER. MAGNETO CHK AT RUN-UP BEFORE TKOF OK; ALSO HAD RUN CARB HEAT FOR ABOUT 5-10 SECONDS AT 1700 RPM AND IDLE DURING RUN-UP WITH NO ROUGHNESS. MIXTURE FULL RICH FOR TKOF. FIELD ELEVATION 758 FT. WIND 090 DEGS; 120 KTS; GUSTING AT 8-12 KTS. TEMP ABOUT 10 DEGS C; DEWPOINT ABOUT 3-5 DEGS. CARB ICE UNLIKELY -- FULL PWR. THEN SUDDEN; NOT GRADUAL; ROUGHNESS. EXACT CAUSE UNDETERMINED. POSSIBILITY -- SUDDEN WINDSHEAR OR TURB FROM XWIND OVER ADJACENT TREES; PRODUCED PROP CAVITATION VIBRATION OR OTHER RAPID VIBRATION OR CHOP THAT WAS PERCEIVED BY PLT AS ENG ROUGHNESS. POSSIBILITY OF WATER IN ENG SUMP BEING INGESTED AFTER SUDDEN ROLL; BUT SUMPS DRAINED -- VERY SLIGHT CHANCE. CONTINUE PLT EDUCATION AND DUAL TRAINING AND EXPERIENCE -- WINGS II; INST INSTRUCTION.