THE FO (PF) OF AN MD80 OVERSHOT THE ASSIGNED ALT DURING THE DSCNT WHILE USING THE AUTOPLT. WHEN IT WAS RECOGNIZED THAT THE ACFT FAILED TO LEVEL; THE AUTOPLT WAS DISCONNECTED AND THE ACFT MANUALLY LEVELED OFF. THE FLC NOTIFIED THE ARTCC RADAR CTLR WHO ALSO ASSIGNED A LOWER ALT.

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WE WERE AT FL290 WITH A CLRNC AT PLT'S DISCRETION TO DSND TO FL240 AND WERE FLYING THE MEINZ 7 ARR INTO MSP. I CALCULATED A DSCNT POINT BASED ON THE USUAL XING RESTR OF 20 DME S OF FARMINGTON; MN; VOR AT 10000 FT. FL240 WAS SET IN THE ALT SELECT WINDOW ON THE FLT GUIDANCE CTL PANEL. I THOUGHT I HAD INITIALLY ARMED THE 'ALT CAPTURE ARM' MODE OF THE AUTOPLT AFTER SELECTING FL240 WHEN WE WERE GIVEN THE DSCNT CLRNC. AFTER STARTING THE DSCNT I WAS DISTRACTED BY THE FLT ATTENDANT ENTERING THE COCKPIT DISCUSSING WHEEL CHAIR REQUIREMENTS; ETC; AND OTHER SMALL TALK FROM THE CAPT. NOTICED THE ALT AS IT PASSED THROUGH FL240; DISCONNECTED THE AUTOPLT AND CLBED THE ACFT BACK TO FL240. THE LOWEST ALT NOTED WAS FL236. THERE WERE NO ACFT NOTED ON THE TCASII AND THUS THERE WERE NO CONFLICTS. AS WE LEVELED OFF AT FL240 ZMP CLRED US TO DSND TO 15000 FT. I HAVE JUST STARTED FLYING THE MD80 THIS MONTH HAVING PREVIOUSLY FLOWN THE DC9. THE ALT ALERT SYS ON THE DC9 HAS AN AURAL CHIME THAT GOES OFF IN ADDITION TO AN AMBER LIGHT AS YOU APCH THE SELECTED ALT. THE MD80 ONLY HAS AN AMBER LIGHT. THE INCIDENT WAS CAUSED BY AN INADVERTENT DESELECTING THE 'ALT CAPTURE ARM' MODE OF THE AUTOPLT AND THE DISTR IN THE COCKPIT. AN AURAL CHIME WARNING AS YOU APCH THE SELECTED ALT (SIMILAR TO THE DC9 ALT ALERT SYS) WOULD HAVE OVERCOME BOTH THE ABOVE CAUSES AND PREVENTED THE INCIDENT. SUPPLEMENTAL INFO FROM ACN 334196: CALLED CTR AND DSCNT CLRNC TO 15000 FT ISSUED. NEW FO AND LAST LEG OF 3 DAY TRIP WITH A 7 HR FLY DAY. ALSO MISSED THE ALT ARM.