ALTDEV ALT OVERSHOT IN DSCNT.

Narrative

WHILE ON A FLT FROM ORD TO BWI; WE WERE CLRED TO CROSS MUMSY INTXN (14 NM WEST OF MARTINSBURG VOR) AT 15000 FT. DSNDING THROUGH 16000 FT; WE ENCOUNTERED LIGHT CHOP. I TURNED TO THE SO TO ASK HIM TO TURN ON THE SEATBELT SIGN AND TO MAKE THE LNDG ANNOUNCEMENT TO THE PAX. NOTICING HE WAS STILL BUSY OBTAINING THE ATIS INFO AND COMPLETING THE LNDG DATA CARD; I THEN TURNED ON THE SEATBELT SIGN MYSELF AND DIRECTED THE FO TO MAKE THE ANNOUNCEMENT. TURNING MY ATTN BACK TO FLYING THE ACFT; I NOTICED THE ALT ALERT LIGHT AND THE ALT WARNING HORN SOUND PASSING 14700 FT. SUDDENLY REALIZING THE CLRNC WAS TO 15000 FT; I DISCONNECTED THE AUTOPLT; ADVANCED THE THROTTLES AND IMMEDIATELY CLBED BACK TO 15000 FT. THE LOWEST ALTIMETER READING WAS 14400 FT. THERE WERE NO OTHER ACFT OBSERVED IN THE AREA (EITHER VISUALLY OR ON TCASII) AND ATC DID NOT MENTION THE DEV. CONTRIBUTING FACTORS: ATIS AT BWI IS WEAK AND UNREADABLE UNTIL FAIRLY CLOSE IN; PREVENTING THE SO FROM COMPLETING THE LNDG DATA CARD IN A MORE TIMELY MANNER. WITH THE SO PRE-OCCUPIED; I DIRECTED THE FO TO PERFORM A DUTY NOT NORMALLY ASSIGNED TO A FO AT A BUSY TIME. THIS DIRECTED THE FO'S ATTN AWAY FROM BACKING UP THE PF. A CLRNC TO 15000 FT AT 14 MI CAN BE MILDLY CONFUSING (WAS THAT 15000 FT AT 14 MI OR 14000 FT AT 15 MI?). PERHAPS CHANGING THE ARR PROC TO CROSS MUMSEY AT 14000 FT OR CHANGE MUMSY TO 15 MI WOULD PREVENT A FUTURE OCCURRENCE. THE CAPT RECENTLY FLEW BOEING 767 ACFT WHICH LEVELS OFF AT THE PRESELECTED ALT ON AUTOPLT. NONE OF THE CONTRIBUTING FACTORS IS MEANT TO IMPLY ANYTHING OTHER THAN THE PLT HAS COMPLETE CTL OF THE ACFT AND MUST INSURE IT IS FLOWN IN A SAFE MANNER. I HAVE LEARNED FROM THIS EXPERIENCE TO REFRAIN FROM ASSIGNING LESS IMPORTANT DUTIES OR TO BE PRE-OCCUPIED WITH LESS IMPORTANT DUTIES DURING CRITICAL TIMES OF FLT; SUCH AS LEVEL OFF; TURNING TO A HDG OR INTERCEPTING A COURSE.