AN MD88 FLT DSNDS 300 FT BELOW ITS ASSIGNED ALT DURING A LEVELOFF; A SPD REDUCTION AND A 30 DEG BANK WHILE EXTENDING FLAPS/SLATS 10 MI S OF PHL; PA.

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ACFT WAS ON AUTOPLT. AT LEVELOFF at 6000 ft; ACFT WAS ALSO IN A 25 DEG BANK; ACFT WAS SLOWING FROM 250 KTS TO 210 KTS AND SLATS WERE BEING EXTENDED. AUTOPLT WAS UNABLE TO HANDLE ALL OF THESE INPUTS AT ONCE AND AS LEVELOFF COMMENCED; ACFT DSNDED 250 FT BELOW 6000 FT. ALT CAPTURE WAS ILLUMINATED ON FLT MGMNT ADVISORY PANEL. AS THE ACFT DSNDED BELOW 6000 FT; ALT HOLD WAS SELECTED AS AN ATTEMPT TO KEEP ACFT FROM GOING ANY LOWER. UNFORTUNATELY; ACFT WAS STILL DSNDING SO ALT HOLD WAS SELECTED AGAIN AND AUTOPLT WAS DISCONNECTED TO BEGIN A MANUAL CLB UP TO 6000 FT. I BELIEVE THAT THE ACFT ONLY WENT 250-300 FT BELOW 6000 FT AND CORRECTION WAS MADE IMMEDIATELY. WE CHKED TCASII TO SEE IF WE MAY HAVE INFRINGED ON ANOTHER ACFT'S AIRSPACE AND SAW NO CONFLICT ON THE TCASII SCREEN. ATC CTLR ASKED US TO CONFIRM THAT WE WERE LEVELING AT 6000 FT. SUPPLEMENTAL INFO FROM ACN 493485: ON A VECTOR OF 360 DEGS; PHL APCH GAVE US A VECTOR TO 030 DEGS AND ADVISED US TO SLOW FROM 250 KIAS TO 210 KIAS. AS THE ACFT APCHED 6000 FT AND TURN ROLLOUT; WE NOTICED THE AUTOPLT WASN'T SLOWING ITS RATE OF DSCNT FOR ALT HOLD. I ADVISED THE CAPT. HE ACKNOWLEDGED HE WAS AWARE OF IT. LESSON LEARNED: MD88 AUTOPLT MAY NOT HANDLE TURNING; SLOWING; AND LEVELOFF SIMULTANEOUSLY AS WELL AS IT SHOULD.