ACTUAL MIDAIR COLLISION BTWN A PIPER PA28-161 AND A C152 ON FINAL APCH TO AN UNCTLED ARPT. THE PIPER COLLIDED ON TOP OF THE CESSNA WITHOUT KNOWING WHAT HAPPENED; BUT HEARD A LOUD BANG. THE RPTR IN THE PIPER THEN ADDED FULL PWR IN AN ATTEMPT TO MAKE THE RWY. HE LANDED OFF TO THE SIDE OF THE RWY IN A GRASSY AREA. THERE WERE NO INJURIES AND BOTH ACFT SUSTAINED ONLY MINOR DAMAGE.

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I ANNOUNCED TURNING BASE TO FINAL FOR RWY 10 FOR TOUCH-AND-GO; FOLLOWED BY A SBOUND DEP. HEARING NO TFC ON OR ABOUT TO TURN FINAL; I CONTINUED MY APCH AT THE ACFT'S RECOMMENDED SPD OF 63 KIAS. WITH FULL FLAPS AND ALMOST IDLE PWR; I HEARD A LOUD 'BANG; BANG' AND IMMEDIATELY BEGAN DSNDING. I PUSHED THE THROTTLE TO FULL AND ATTEMPTED TO CLB OR AT LEAST HOLD ALT; WITH NO SUCCESS. TO AVOID A STALL; I KEPT THE THROTTLE IN FULL AND NOSED TO A SLIGHTLY NOSE-LOW ATTITUDE AND HEADED FOR THE ARPT (TRYING TO AVOID THE TREES BEFORE THE BOUNDARY). BEING PULLED TO THE L; I KNEW I COULDN'T MAKE THE RWY BUT THERE WAS GRASS TO THE L OF IT. KEEPING THE THROTTLE IN ALL THE WAY TO THE GND (UNTIL FLARE); I LANDED ON THE GRASS SAFELY. AT THAT POINT; I SHUT DOWN THE ENG; ELECTRICAL SYS OFF; MAGNETOS OFF/KEYS OUT; AND I; KNOWING THEN THAT I HAD COLLIDED WITH ANOTHER PLANE (AND CONTINUED DOWN; EVENTUALLY LNDG ON IT) JUMPED OUT AND CHKED FOR THE CESSNA'S PAX SAFETY. NO ONE WAS HURT AND WE WERE ALL AMAZED AT THE SIGHT. I HAD NOT HEARD THE CESSNA CALL FINAL OR THE APCH; NOR DID I HEAR THE CALL ANYWHERE ELSE (LIKE DOWNWIND; BASE; ETC). WHEN I ANNOUNCED FINAL; THERE WAS NO XMISSION OF CONFLICT BY THE OTHER PLT(S). NEXT TIME I WILL DEFINITELY ANNOUNCE SHORT FINAL AND RECHK MY APCH CORRIDOR AT THAT TIME.