AN LTT DSNDING INTO MSP; MN; OVERSHOT ALT LEVELOFF DUE TO AUTOPLT BEING ACCIDENTALLY TRIPPED OFF.

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WE WERE INBOUND ON THE EAU 3 ARR INTO MSP ARPT BTWN THE INTXNS OF WOLVS AND ZASKE. WE WERE AT 11000 FT MSL AND WERE CLRED TO DSND TO 9000 FT. THE INBOUND TFC WAS VERY LIGHT AND THE WX WAS DAYLIGHT VFR WITH CLR SKIES. WHILE IN THE DSCNT; THE PNF BEGAN TO READ THE 10000 FT CHKLIST AT 10000 FT. AT APPROX 9500 FT I INADVERTENTLY BUMPED THE AUTOPLT PITCH CTL WHEEL THEREBY DISENGAGING THE ALT CAPTURE FEATURE. WHEN THE AIRPLANE REACHED 9000 FT THE AUTOPLT SHOULD HAVE CAPTURED. SOMETIMES THE AUTOPLT LETS THE AIRPLANE GO A LITTLE BELOW THE PRESELECTED ALT BUT NOT MORE THAN 300 FT EVER. WHEN THE AIRPLANE WAS AT 8700 FT AND STILL DSNDING; I REALIZED THAT THE AUTOPLT HAD NOT CAPTURED. I IMMEDIATELY TOOK OVER AND BEGAN TO REVERSE THE TREND. THE LOWEST ALT THAT THE PLANE REACHED WAS APPROX 8400 FT. THE TOTAL TIME THAT THE AIRPLANE HAD DEVIATED FROM THE 9000 FT ASSIGNED ALT WAS APPROX 10-15 SECONDS. THE CTLR ASKED US WHAT OUR ALT WAS AND I TOLD HIM WE WERE AT 8500 FT AND EXPEDITING OUR CLB BACK TO 9000 FT AND THAT OUR ALT DIDN'T CAPTURE. THE CTLR THEN GAVE US A PHONE NUMBER TO CALL WHEN ON THE GND. ON THE GND; I CALLED THE NUMBER AND TALKED TO ABC; A SUPVR AT MSP APCH. I COOPERATED FULLY AND TOLD HIM EVERYTHING WRITTEN HERE. IN THE FUTURE; TO AVOID SUCH A PROB ARISING AGAIN; IT IS IMPERATIVE THAT THE PF NOT LET THE ALT EVER GET BELOW THE ASSIGNED ALT; AUTOPLT OR NOT. I BELIEVE THAT I REACTED VERY QUICKLY TO THE SIT AND TOOK IMMEDIATE CTL TO GET TO THE ASSIGNED ALT. SUPPLEMENTAL INFO FROM ACN 379896: WE WERE LEVEL AT 11000 FT MSL AND WERE INSTRUCTED TO DSND AND MAINTAIN 9000 FT MSL. AT THAT POINT; I SET 9000 FT IN THE ALT ALERT PRESELECT AND THE CAPT BEGAN THE DSCNT AND CALLED FOR THE 10000 FT CHKLIST. I BEGAN THE CHKLIST. A FEW MINS LATER THE CAPT RECONFIRMED THE ALT OF 9000 FT. THE ACFT WAS AT 8400 FT. THE CAPT BEGAN A RAPID CLB BACK TO 9000 FT. EVIDENTLY THE AUTOPLT WENT OFF LINE AND DID NOT CAPTURE THE ALT.