AN MD80 ON APCH AT 500 FT AGL EXPERIENCED A RUNAWAY STABILIZER TRIM IN SLOW MODE. AUTOPLT WAS DISCONNECTED PRIOR TO APCH.

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WE EXPERIENCED A RUNAWAY PRIMARY STAB TRIM ON VERY SHORT FINAL; APPROX 500 FT AGL. WE ACCOMPLISHED THE IMMEDIATE ACTION (RED BOX) ITEM WHICH CORRECTED THE PROB. THE ACFT WAS COMPLETELY CONFIGURED FOR LNDG (STAB TRIM WAS ABOUT 5 DEGS NOSE UP) AND I WAS USING ONLY SLIGHT NOSE PRESSURE ON THE YOKE TO KEEP THE ACFT UNDER CONTROL; SO I CONTINUED THE LNDG. I ELECTED TO DO SO BECAUSE IN MY BEST JUDGMENT AT THE TIME; GIVEN THE CIRCUMSTANCES; IT WAS THE SAFEST COURSE OF ACTION. WE EXPERIENCED A NORMAL TOUCHDOWN IN THE TDZ. OBVIOUSLY BECAUSE OF THE SUDDENNESS OF THE EVENT AT SUCH LATE POINT IN THE FLT; THERE WASN'T TIME TO DECLARE AN EMER WITH ATC EVEN THOUGH THIS EVENT DURING ANY OTHER PHASE OF FLT CERTAINLY WOULD HAVE REQUIRED I EXERCISE THAT AUTHORITY. AFTER GATE ARR; WE NOTIFIED MAINT AND MADE AN ENTRY IN THE ACFT LOGBOOK. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THE STABILIZER TRIM WAS TRIMMING NOSE DOWN SLOWLY AND THE AUTOPLT HAD BEEN DISCONNECTED PRIOR TO THE START OF THE APCH. THEY DID NOT HAVE TIME TO DECLARE AN EMER BEING CLOSE TO THE GND AND THE LNDG WAS CONTINUED. MAINT RPTED REPLACING THE PRIMARY TRIM MOTOR ACTUATOR AND THE ALTERNATE TRIM MOTOR ACTUATOR.