AN ACR MLG OVERSHOT ITS ALT IN DSCNT AND THEN BALLOONED BACK UP OVER ITS ASSIGNED ALT. THE AUTOPLT WAS USED IN BOTH ACTIONS.

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WE WERE CLRED TO CROSS RIDGY AT FL240 ON THE ROBINSVILLE 1 ARR. WE WERE CRUISING AT FL310 MSL WHEN WE RECEIVED OUR CLRNC TO CROSS RIDGY AT FL240. WE SET AND ARMED THE ALT OF FL240. WE BOTH VERIFIED OUR ALT AFTER IT WAS SET IN ALT BOX. AS ACFT DSNDED; EVERYTHING APPEARED NORMAL. WHEN IT CAME TIME FOR LEVELOFF; THE AUTOPLT DID NOT CAPTURE. WHEN IT WAS OBVIOUS ACFT WAS NOT GOING TO LEVEL OFF AT FL240; CAPT DISCONNECTED AUTOPLT AND MANUALLY LEVELED ACFT HIMSELF. IN THE PROCESS; WE SANK 280 FT BELOW ASSIGNED; THEN BALLOONED TO 500 FT ABOVE FL240. AFTER ACFT SETTLED DOWN; WE TURNED AUTOPLT BACK ON. AUTOPLT FUNCTIONED NORMALLY REMAINDER OF FLT. SUPPLEMENTAL INFO FROM ACN 415618: I SET FL240 IN ALT WINDOW; ARMED IT; AND STATED '240 SET AND ARMED' AS CALLED FOR IN OUR PROCS. AT FL250; FO CALLED '1000 FT TO GO' AND I RECHKED THE ALT ARM. AFTER RECONNECTING THE AUTOPLT AND HITTING ALT HOLD AT FL240; ACFT CLBED RAPIDLY TO FL245; SO I DISCONNECTED THE AUTOPLT AGAIN AND FLEW ACFT BACK DOWN TO FL240. AT THIS TIME; CTR ADVISED US TO DSND TO FL190.