Narrative:

We were flying the last of 6 segments on day 3 of a 3 day schedule. This day started with a early am wake up call and ended with a landing on the sixth leg at XX24, a 14 hour and 59 min duty day. There had been long delays on earlier segments due to WX. We approached chicago on the heels of a cold front with associated IMC, thunderstorms, rain changing to snow, wind and turbulence. We were cleared to descend to 9000 ft. At about the same time, my attention was diverted to the ACARS panel where I was entering data for an arrival time estimate, a report 'highly desired' by the company. While entering the data, the altitude alert sounded, I looked up and observed that we were descending through 8700 ft. I pulled the yoke back and returned to 9000 ft. No apparent conflict occurred. A few mins later, we were cleared to 7000 ft. The first officer descended at 1000 FPM through 7000 ft, but I caught it at 6800 ft and returned to 7000 ft. He made a text book landing in a 90 degree, 26 KT crosswind. Prior to takeoff on this final segment, I mentioned to my first officer 'we're both tired, let's watch each other!' this is a classic example of fatigue induced error brought on by a 15 hour duty day flown in adverse WX on a 2 man cockpit in night IMC. It occurred during a high workload portion of the flight when other duties diverted my attention away from monitoring the first officer as he flew the approach. Errors will occur. I have listed several contributing factors leading to this one.

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Original NASA ASRS Text

Title: MLG FLC DSNDS BELOW ASSIGNED ALT DURING APCH.

Narrative: WE WERE FLYING THE LAST OF 6 SEGMENTS ON DAY 3 OF A 3 DAY SCHEDULE. THIS DAY STARTED WITH A EARLY AM WAKE UP CALL AND ENDED WITH A LNDG ON THE SIXTH LEG AT XX24, A 14 HR AND 59 MIN DUTY DAY. THERE HAD BEEN LONG DELAYS ON EARLIER SEGMENTS DUE TO WX. WE APCHED CHICAGO ON THE HEELS OF A COLD FRONT WITH ASSOCIATED IMC, TSTMS, RAIN CHANGING TO SNOW, WIND AND TURB. WE WERE CLRED TO DSND TO 9000 FT. AT ABOUT THE SAME TIME, MY ATTN WAS DIVERTED TO THE ACARS PANEL WHERE I WAS ENTERING DATA FOR AN ARR TIME ESTIMATE, A RPT 'HIGHLY DESIRED' BY THE COMPANY. WHILE ENTERING THE DATA, THE ALT ALERT SOUNDED, I LOOKED UP AND OBSERVED THAT WE WERE DSNDING THROUGH 8700 FT. I PULLED THE YOKE BACK AND RETURNED TO 9000 FT. NO APPARENT CONFLICT OCCURRED. A FEW MINS LATER, WE WERE CLRED TO 7000 FT. THE FO DSNDED AT 1000 FPM THROUGH 7000 FT, BUT I CAUGHT IT AT 6800 FT AND RETURNED TO 7000 FT. HE MADE A TEXT BOOK LNDG IN A 90 DEG, 26 KT XWIND. PRIOR TO TKOF ON THIS FINAL SEGMENT, I MENTIONED TO MY FO 'WE'RE BOTH TIRED, LET'S WATCH EACH OTHER!' THIS IS A CLASSIC EXAMPLE OF FATIGUE INDUCED ERROR BROUGHT ON BY A 15 HR DUTY DAY FLOWN IN ADVERSE WX ON A 2 MAN COCKPIT IN NIGHT IMC. IT OCCURRED DURING A HIGH WORKLOAD PORTION OF THE FLT WHEN OTHER DUTIES DIVERTED MY ATTN AWAY FROM MONITORING THE FO AS HE FLEW THE APCH. ERRORS WILL OCCUR. I HAVE LISTED SEVERAL CONTRIBUTING FACTORS LEADING TO THIS ONE.

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.