Narrative:

During our initial descent into msp we were cleared to descend from an intermediate altitude to FL240. During this intermediate period our altitude control bug on the mode control panel (MCP) failed to operate at about FL246 and stuck in this position for the rest of the flight. The first officer was flying the aircraft and I (the captain) instructed him to fly the aircraft manually, since all leveloff logic was based on the MCP altitude window. We were kept high on the arrival because of traffic and then were cleared to cross gollf intersection at 11000 ft. The first officer descended manually, deploying the speed brakes and increasing airspeed to 340 KTS in order to make the restr. I then started to troubleshoot the problem with hopes of restoring the MCP and returning to automatic flight while the first officer continued to fly the aircraft. As we approached 11000 ft, the overspd warning sounded on the aircraft and distracted us just long enough to dip below the 11000 ft restr. We leveled at 10400 ft and started an immediate climb back to 11000 ft. As we started our climb back to 11000 ft we were almost immediately given the clearance to descend to 10000 ft. We descended to 10000 ft and flew the rest of the flight manually with a normal landing at msp. The altitude bust was the result of 4 factors working in concert: 1) mechanical failure of the MCP altitude bug. 2) the sudden requirement to fly the aircraft manually (very rare situation on the B757) with no flight guidance. 3) the aggressive descent rate required to make the STAR altitude requirement. 4) the last min distraction of the aircraft overspd warning while approaching leveloff altitude.

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Original NASA ASRS Text

Title: FLC OF A B757 OVERSHOT ASSIGNED ALT DURING A STAR ARR DUE TO THE ACFT'S OVERSPD WARNING DISTR AND IMMEDIATELY STARTED RETURN WHEN ATC CLRED THEM TO A LOWER ALT. THE FO WAS MANUALLY FLYING ACFT DUE TO THE ALT BUG OF THE MODE CTL PANEL BECOMING INOP.

Narrative: DURING OUR INITIAL DSCNT INTO MSP WE WERE CLRED TO DSND FROM AN INTERMEDIATE ALT TO FL240. DURING THIS INTERMEDIATE PERIOD OUR ALT CTL BUG ON THE MODE CTL PANEL (MCP) FAILED TO OPERATE AT ABOUT FL246 AND STUCK IN THIS POS FOR THE REST OF THE FLT. THE FO WAS FLYING THE ACFT AND I (THE CAPT) INSTRUCTED HIM TO FLY THE ACFT MANUALLY, SINCE ALL LEVELOFF LOGIC WAS BASED ON THE MCP ALT WINDOW. WE WERE KEPT HIGH ON THE ARR BECAUSE OF TFC AND THEN WERE CLRED TO CROSS GOLLF INTXN AT 11000 FT. THE FO DSNDED MANUALLY, DEPLOYING THE SPD BRAKES AND INCREASING AIRSPD TO 340 KTS IN ORDER TO MAKE THE RESTR. I THEN STARTED TO TROUBLESHOOT THE PROB WITH HOPES OF RESTORING THE MCP AND RETURNING TO AUTOMATIC FLT WHILE THE FO CONTINUED TO FLY THE ACFT. AS WE APCHED 11000 FT, THE OVERSPD WARNING SOUNDED ON THE ACFT AND DISTRACTED US JUST LONG ENOUGH TO DIP BELOW THE 11000 FT RESTR. WE LEVELED AT 10400 FT AND STARTED AN IMMEDIATE CLB BACK TO 11000 FT. AS WE STARTED OUR CLB BACK TO 11000 FT WE WERE ALMOST IMMEDIATELY GIVEN THE CLRNC TO DSND TO 10000 FT. WE DSNDED TO 10000 FT AND FLEW THE REST OF THE FLT MANUALLY WITH A NORMAL LNDG AT MSP. THE ALT BUST WAS THE RESULT OF 4 FACTORS WORKING IN CONCERT: 1) MECHANICAL FAILURE OF THE MCP ALT BUG. 2) THE SUDDEN REQUIREMENT TO FLY THE ACFT MANUALLY (VERY RARE SIT ON THE B757) WITH NO FLT GUIDANCE. 3) THE AGGRESSIVE DSCNT RATE REQUIRED TO MAKE THE STAR ALT REQUIREMENT. 4) THE LAST MIN DISTR OF THE ACFT OVERSPD WARNING WHILE APCHING LEVELOFF 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.