A B727-200 FLC EXPERIENCES AN ALT LOSS OF 200 FT AND A POTENTIAL CONFLICT DURING A REQUESTED SPD REDUCTION FROM ATC 10 MI NW OF ATL; GA.

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THE FO WAS FLYING THE ARR TO ATL WHEN APCH CTL REQUESTED A RAPID SPD REDUCTION FROM 300 KIAS TO 210 KIAS. AUTOPLT WAS ENGAGED IN LEVEL FLT MODE AND WAS MAINTAINING ALT. FO DEPLOYED 20 DEGS SPD BRAKES AND SET IDLE PWR TO EXPEDITE SPD REDUCTION. INITIALLY; ALT DECREASED ABOUT 50 FT; WHICH IS NOT UNUSUAL DURING SPD CHANGES WITH THE BLOCK V AUTOPLT. THEN; ALT LOSS BEGAN TO INCREASE. WHEN 100 FT LOW; I DIRECTED FO TO DISCONNECT THE AUTOPLT; STOW SPD BRAKE; AND RETURN TO 11000 FT. WHEN THE AUTOPLT WAS DISCONNECTED; THE FO ENCOUNTERED LARGE DOWN ELEVATOR FORCE AND LOST ANOTHER 100 FT WHILE ADJUSTING TO THE STICK FORCES. HE WAS ALREADY MAKING A NICE RECOVERY TO 11000 FT WHEN ATL APCH ADVISED WE HAD TFC LESS THAN 1 MI AT 10000 FT. APCH ASKED US TO CONFIRM OUR 11000 FT ASSIGNMENT; WHICH WE DID. WROTE UP AUTOPLT FOR REPAIR. SUPPLEMENTAL INFO FROM ACN 493492: ON THE ARR TO ATL WE WERE CLRED TO DSND TO 10000 FT AND THE ACFT WAS ON AUTOPLT AT 300 KIAS. JUST AFTER INITIATING THE DSCNT; WE WERE ASKED BY APCH CTL TO ALSO SLOW DURING THE DSCNT TO 210 KIAS. THE ACFT WAS ALREADY AT IDLE PWR; SO I EXTENDED THE SPOILERS TO MAINTAIN OUR DSCNT AND ALSO SLOW DOWN. THE ALT CAPTURE FEATURE OF THE AUTOPLT WAS ENGAGED TO AUTOMATICALLY LEVEL OFF AT THE DESIRED ALT. AS THE ACFT APCHED THE LEVELOFF ALT; THE AUTOPLT BEGAN TO RAISE THE PITCH ATTITUDE AND THE AUTOPLT STATUS LIGHT TURNED TO GREEN INDICATING THAT THE AUTOPLT WAS LEVELING OFF. DUE TO THE OUT OF TRIM CONDITION; THE ACFT DSNDED TO APPROX 300 FT BELOW ASSIGNED ALT. AFTER THE FLT; THE ACFT AUTOPLT WAS WRITTEN UP IN THE LOGBOOK FOR NOT TRIMMING THE ACFT CORRECTLY AND NOT CAPTURING THE ALT AS IT SHOULD HAVE.