CAPT INADVERTENTLY MOVED THE ALT ALERTER WHILE SILENCING THE GEAR HORN IN A DC9-30. HE RESET IT TO THE WRONG ALT AND FAILED TO LEVEL OFF AT THE ASSIGNED ALT.

Narrative

ACFT WAS ASSIGNED 11000 FT MSL FOR DSCNT INTO MINNEAPOLIS. THE ALT ALERT ALT WAS SET AT 11000 FT AND THE CAPT INITIATED THE DSCNT. WHILE SILENCING THE GEAR WARNING ALERT AS THROTTLES WERE RETARDED TO IDLE; THE ALT KNOB FOR THE ALT ALERT WAS MOVED INDICATING 10400 FT. THE CAPT (PF) SET THE ALERT KNOB TO 10000 FT. PASSING 10500 FT THE CAPT ASKED WHAT ALT WE WERE CLRED TO. I IMMEDIATELY CALLED ZMP AND ADVISED THAT WE WERE DSNDING THROUGH 10500 FT AND ASKED WHAT WAS OUR ASSIGNED ALT; 10000 FT OR 11000 FT? ZMP ADVISED US TO CONTINUE OUR DSCNT TO 10000 FT. THIS WAS A VERY BUSY TIME IN THE COCKPIT AND AS THE FO (PNF) I DID NOT SEE THE CAPT RESET THE ALT ALERT ALT. I WAS ACCOMPLISHING MY CHKLIST DURING THE UNINTENTIONAL DSCNT BELOW THE ASSIGNED ALT OF 11000 FT. I BELIEVE IT IS IMPORTANT TO MONITOR THE LEVELOFF WHILE ACCOMPLISHING OTHER COCKPIT DUTIES. FORTUNATELY; NO OTHER ACFT WERE IN OUR VICINITY AT THE TIME OF THIS INCIDENT. SUPPLEMENTAL INFO FROM ACN 396410: IF I HAD CONFIRMED THE 10000 FT WITH THE FO; AND/OR ATC; WE COULD HAVE AVOIDED THIS. THE GEAR HORN IS ALSO VERY CLOSE TO THE ALT ALERTER IN SOME DC9'S. ON OTHER ACFT THE GEAR HORN BUTTON IS FARTHER DOWN THE DASH AND AWAY FROM THE ALERTER.