Narrative:

The flight from phi-ord started with a problem with the captain's altimeter (would not stay in 'corrected' mode). During flight, it was noticed, along with other indications, that autoplt would not hold altitude in 'altitude hold' mode even though it was selected. Both pilots tried to analyze the problem(south) and deduced that the #1 air data computer was causing the problem. WX was not very good in ord. Copilot (PNF) went off frequency to get WX in ord, leaving me to attend to ATC communications, navigation in marginal VFR WX, and flying aircraft. Ord approach issued clearance to 12000 ft from 14000 ft and I began a descent 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 altitude at 11300 ft and descending. I immediately reversed descent, at which time approach cleared us for a heading and I responded and also informed them that I had descended below assigned altitude of 12000 ft and was climbing. Approach control then reclred us to maintain 11000 ft. No conflict of traffic was involved. After the incident, the copilot noted that the altitude alert had never gone off, either approaching the altitude or where I descended passing it. We deduced that there was probably some connection to the #1 air data computer problem as neither of us remembered the altitude alert working at all during flight. I feel this occurred due to 2 reasons: 1) failure of an important aircraft system designed to prevent exactly this situation. 2) lack of adequate xchk due to either distraction or overreliance on the altitude alert. I know of no corrective action other than increased diligence during equipment malfunction.

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Original NASA ASRS Text

Title: 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.

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.