ALTDEV ALT OVERSHOT. ACR; WITH A MALFUNCTIONING CENTRAL AIR DATA COMPUTER; DSNDED BELOW CLRNC ALT.

Narrative

THE FLT FROM PHI-ORD STARTED WITH A PROB WITH THE CAPT'S ALTIMETER (WOULD NOT STAY IN 'CORRECTED' MODE). DURING FLT; IT WAS NOTICED; ALONG WITH OTHER INDICATIONS; THAT AUTOPLT WOULD NOT HOLD ALT IN 'ALT HOLD' MODE EVEN THOUGH IT WAS SELECTED. BOTH PLTS TRIED TO ANALYZE THE PROB(S) AND DEDUCED THAT THE #1 AIR DATA COMPUTER WAS CAUSING THE PROB. WX WAS NOT VERY GOOD IN ORD. COPLT (PNF) WENT OFF FREQ TO GET WX IN ORD; LEAVING ME TO ATTEND TO ATC COMS; NAV IN MARGINAL VFR WX; AND FLYING ACFT. ORD APCH ISSUED CLRNC TO 12000 FT FROM 14000 FT AND I BEGAN A DSCNT PER SOP. I CALLED (TO MYSELF ALOUD) 'PASSING 13000 FT FOR 12000 FT.' AT THIS POINT I WAS DISTRACTED FOR SOME REASON; AND NEXT NOTICED OUR ALT AT 11300 FT AND DSNDING. I IMMEDIATELY REVERSED DSCNT; AT WHICH TIME APCH CLRED US FOR A HDG AND I RESPONDED AND ALSO INFORMED THEM THAT I HAD DSNDED BELOW ASSIGNED ALT OF 12000 FT AND WAS CLBING. APCH CTL THEN RECLRED US TO MAINTAIN 11000 FT. NO CONFLICT OF TFC WAS INVOLVED. AFTER THE INCIDENT; THE COPLT NOTED THAT THE ALT ALERT HAD NEVER GONE OFF; EITHER APCHING THE ALT OR WHERE I DSNDED PASSING IT. WE DEDUCED THAT THERE WAS PROBABLY SOME CONNECTION TO THE #1 AIR DATA COMPUTER PROB AS NEITHER OF US REMEMBERED THE ALT ALERT WORKING AT ALL DURING FLT. I FEEL THIS OCCURRED DUE TO 2 REASONS: 1) FAILURE OF AN IMPORTANT ACFT SYS DESIGNED TO PREVENT EXACTLY THIS SIT. 2) LACK OF ADEQUATE XCHK DUE TO EITHER DISTR OR OVERRELIANCE ON THE ALT ALERT. I KNOW OF NO CORRECTIVE ACTION OTHER THAN INCREASED DILIGENCE DURING EQUIP MALFUNCTION.