A B757-200F FLC HAD THE AUTOPLT MALFUNCTION AND PITCH UP AND DOWN AND THE FLC COULD NOT DISCONNECT THE AUTOPLT EXCEPT BY OVERPWRING IT. THE OVERSPD WARNING ALSO SOUNDED DURING THE SECOND EPISODE AND THE AUTOTHROTTLES ADVANCED WHEN THE AIRSPD WAS MORE THAN ADEQUATE. MAINT FOUND THAT THE CTR AUTOPLT WAS FAULTY.

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WE WERE WITH GSO DEP AND LEVEL AT 10000 FT; AUTOPLT ON; WHEN WE EXPERIENCED AUTOPLT PROBS. THE AUTOPLT TOOK US 300 FT ABOVE AND 300 FT BELOW THE ASSIGNED 10000 FT. WE ADVISED GSO OF OUR SIT AND THEY SAID IT WAS NOT A PROB. WHEN WE CHANGED OVER TO ROA APCH WE ENCOUNTERED MORE AUTOPLT; AUTOTHROTTLE; AND FLT WARNING PROBS. THE AUTOPLT SYS WOULD NOT DISENGAGE AND IT MADE IT VERY DIFFICULT TO MAINTAIN ALT AND AIRSPD. WHEN WE FINALLY GOT THE PLANE UNDER MANUAL CTL THE ROA APCH CTLR ADVISED US THAT WE WERE INDICATING 300 KTS ON THE DSCNT THROUGH 8000 FT. WE ADVISED HIM OF OUR SIT AND READ BACK OUR INDICATED AIRSPD AT THAT TIME OF 250 KTS. HE THEN ASKED US IF WE WERE GOING TO REQUIRE ASSISTANCE AND WE REPLIED NO; SINCE EVERYTHING WAS UNDER CTL AT THAT TIME. WE LANDED WITHOUT FURTHER PROBS AND MADE SEVERAL MAINT LOGBOOK ENTRIES. BOTH CTLRS WERE ADVISED OF OUR DIFFICULTIES AND PROBS WITH OUR FLT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS FLYING A B757-200 FREIGHTER WHEN THE AUTOPLT AND THE OVERSPD WARNING SYS STARTED TO MALFUNCTION. THE FO WAS FLYING THE ACFT ON AUTOPLT AT 10000 FT AND THE CAPT WAS READING AN APCH CHART WHEN THE ACFT PITCHED UP THEN DOWN. THEY CLBED 300 FT ABOVE THEIR ASSIGNED ALT AND THEN DSNDED 300 FT BELOW IT. THE FO ATTEMPTED TO DISCONNECT THE AUTOPLT USING THE CTL WHEEL SWITCH AND FAILING THAT HE PULLED THE PADDLES ON THE GLARE SHIELD PANEL TO OFF; BUT THAT DID NOT WORK EITHER. FINALLY HE SIMPLY KEPT OVERPWRING THE AUTOPLT UNTIL IT DISCONNECTED BECAUSE OF THE EXCESSIVE FORCE; ACCORDING TO THE CAPT. A LITTLE LATER; THE FLC RE-ENGAGED THE AUTOPLT AS THEY WERE APCHING ROA. THE OVERSPD WARNING STARTED TO SOUND EVEN THOUGH THEY WERE SLOWING. THEN THE AUTOPLT STARTED TO PITCH UP AND DOWN AND THE AUTOTHROTTLES ADVANCED AS THE ACFT STARTED TO DSND. AGAIN THE FLC WAS UNABLE TO DISCONNECT THE AUTOPLT UNTIL THEY HAD OVERPWRED IT SEVERAL TIMES. THEY WERE AT 8000 FT AND 300 KIAS AT THIS TIME. THE OVERSPD WARNING HAD STOPPED WHILE THE FLC WAS STRUGGLING WITH THE AUTOPLT AND THE AUTOTHROTTLES DISCONNECTED NORMALLY. THE FLC WAS ABLE TO MAKE A NORMAL; HAND FLOWN; APCH AND LNDG. ROA MAINT CHKED THE ACFT AND FOUND THAT THE CTR AUTOPLT WAS MALFUNCTIONING. THEY MEL THE CTR AUTOPLT AND CLASSIFIED THE ACFT FOR CAT I APCHS ONLY AND THE FLC FLEW THE NEXT LEG WITH NO PROBS. CALLBACK CONVERSATION WITH RPTR ACN #366638 REVEALED THE FOLLOWING INFO: THE FO OF THIS B757- 200 SAID THAT THE AUTOPLT WOULD NOT DISENGAGED USING THE NORMAL DISCONNECT SWITCH ON THE CTL COLUMN OR THE MAIN SWITCHES ON THE GLARE SHIELD SO HE SIMPLY OVERPWRED IT. HE ESTIMATED THAT THE STICK FORCES WERE 40-50 LBS; BUT THE AUTOPLT DISCONNECTED DUE TO HIS EFFORTS. HE SAID THAT HE DOES NOT KNOW WHY THE SYS WOULD NOT DISCONNECT; BUT THE TECHNICIAN TOLD HIM THAT THE CTR AUTOPLT HAD SEVERAL FAULTS IN IT WHEN SHE DID A BYTE CHK. THE FO SAID THAT WHEN THE FLC FLEW THE ACFT THAT EVENING WITH THE CTR AUTOPLT MEL'ED OFF THE AUTOPLT WORKED NORMALLY.