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.