Narrative:

We were inbound on the eau 3 arrival into msp airport between the intxns of wolvs and zaske. We were at 11000 ft MSL and were cleared to descend to 9000 ft. The inbound traffic was very light and the WX was daylight VFR with clear skies. While in the descent, the PNF began to read the 10000 ft checklist at 10000 ft. At approximately 9500 ft I inadvertently bumped the autoplt pitch control wheel thereby disengaging the altitude 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 altitude but not more than 300 ft ever. When the airplane was at 8700 ft and still descending, I realized that the autoplt had not captured. I immediately took over and began to reverse the trend. The lowest altitude that the plane reached was approximately 8400 ft. The total time that the airplane had deviated from the 9000 ft assigned altitude was approximately 10-15 seconds. The controller asked us what our altitude was and I told him we were at 8500 ft and expediting our climb back to 9000 ft and that our altitude didn't capture. The controller then gave us a phone number to call when on the ground. On the ground, I called the number and talked to abc, a supervisor at msp approach. I cooperated fully and told him everything written here. In the future, to avoid such a problem arising again, it is imperative that the PF not let the altitude ever get below the assigned altitude, autoplt or not. I believe that I reacted very quickly to the situation and took immediate control to get to the assigned altitude. Supplemental information from acn 379896: we were level at 11000 ft MSL and were instructed to descend and maintain 9000 ft MSL. At that point, I set 9000 ft in the altitude alert preselect and the captain began the descent and called for the 10000 ft checklist. I began the checklist. A few mins later the captain reconfirmed the altitude of 9000 ft. The aircraft was at 8400 ft. The captain began a rapid climb back to 9000 ft. Evidently the autoplt went off line and did not capture the altitude.

Related reports:

Google
 

Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.