Narrative:

On descent into daytona beach we had been given clearance to 12000 ft. The first officer was flying the aircraft on autoplt. I was on interphone talking with the flight attendants about the numerous low altitude cumulus clouds and the possibility of a bumpy ride on approach. I looked up and noticed that the aircraft was descending through 12000 ft. My first thought was that we must have been given a clearance that I did not hear while I was on interphone. I then noticed that the altitude window of the autoplt control panel still displayed 12000 ft. The first officer appeared to be looking at his approach plate in preparation for our approach. I grabbed the yoke and asked the first officer if we had been given clearance below 12000 ft. He realized the situation and replied that we had not been given clearance below 12000 ft. He immediately took control of the aircraft, disconnected the autoplt and manually flew the aircraft up to 12000 ft. In our overshoot we descended to approximately 11300 ft. At that time ZJX transferred us to approach control. I informed center that we were switching to approach control and climbing back to 12000 ft. I do not believe we caused a conflict. Center did not mention a conflict and approach control immediately cleared us to descend below 12000 ft. I do not know why the autoplt failed to capture the selected altitude. Between analyzing the situation, taking manual control of the aircraft, looking for potential traffic hazards and switching radios from center to approach control, I did not notice if the altitude capture armed light was illuminated as we descended through 12000 ft. The first officer said that when he had last checked the armed light was illuminated. I believe our biggest mistake was in trusting the autoplt during the critical (leveloff) phase of flight. We had also been on duty for almost 8 hours and were flying the next to last leg of a 4 day trip. Fatigue could have been a factor. Supplemental information from acn 303236: somehow the altitude armed annunciator dropped out at the end. No clue how or why. I was further distracted with FMS programming. The real key to this attention failure was fatigue. The first day of the trip ended after 6 legs at the hotel at XA00. I ended up with about 6 hours of sleep. The following night's layover was scheduled for 9 hours of which I again got about 6 hours. The last night's layover also gave me about 6 hours of sleep due to the necessary wake-up call at XA15 am. The end result was the worst fatigue of my career. Consequently, my xchk was mostly focused on staying awake rather than the altimeter.

Related reports:

Google
 

Original NASA ASRS Text

Title: ALTDEV ON DSCNT BY 700 FT. FLC FATIGUE. RPTR ERROR ADMITTED.

Narrative: ON DSCNT INTO DAYTONA BEACH WE HAD BEEN GIVEN CLRNC TO 12000 FT. THE FO WAS FLYING THE ACFT ON AUTOPLT. I WAS ON INTERPHONE TALKING WITH THE FLT ATTENDANTS ABOUT THE NUMEROUS LOW ALT CUMULUS CLOUDS AND THE POSSIBILITY OF A BUMPY RIDE ON APCH. I LOOKED UP AND NOTICED THAT THE ACFT WAS DSNDING THROUGH 12000 FT. MY FIRST THOUGHT WAS THAT WE MUST HAVE BEEN GIVEN A CLRNC THAT I DID NOT HEAR WHILE I WAS ON INTERPHONE. I THEN NOTICED THAT THE ALT WINDOW OF THE AUTOPLT CTL PANEL STILL DISPLAYED 12000 FT. THE FO APPEARED TO BE LOOKING AT HIS APCH PLATE IN PREPARATION FOR OUR APCH. I GRABBED THE YOKE AND ASKED THE FO IF WE HAD BEEN GIVEN CLRNC BELOW 12000 FT. HE REALIZED THE SIT AND REPLIED THAT WE HAD NOT BEEN GIVEN CLRNC BELOW 12000 FT. HE IMMEDIATELY TOOK CTL OF THE ACFT, DISCONNECTED THE AUTOPLT AND MANUALLY FLEW THE ACFT UP TO 12000 FT. IN OUR OVERSHOOT WE DSNDED TO APPROX 11300 FT. AT THAT TIME ZJX TRANSFERRED US TO APCH CTL. I INFORMED CTR THAT WE WERE SWITCHING TO APCH CTL AND CLBING BACK TO 12000 FT. I DO NOT BELIEVE WE CAUSED A CONFLICT. CTR DID NOT MENTION A CONFLICT AND APCH CTL IMMEDIATELY CLRED US TO DSND BELOW 12000 FT. I DO NOT KNOW WHY THE AUTOPLT FAILED TO CAPTURE THE SELECTED ALT. BTWN ANALYZING THE SIT, TAKING MANUAL CTL OF THE ACFT, LOOKING FOR POTENTIAL TFC HAZARDS AND SWITCHING RADIOS FROM CTR TO APCH CTL, I DID NOT NOTICE IF THE ALT CAPTURE ARMED LIGHT WAS ILLUMINATED AS WE DSNDED THROUGH 12000 FT. THE FO SAID THAT WHEN HE HAD LAST CHKED THE ARMED LIGHT WAS ILLUMINATED. I BELIEVE OUR BIGGEST MISTAKE WAS IN TRUSTING THE AUTOPLT DURING THE CRITICAL (LEVELOFF) PHASE OF FLT. WE HAD ALSO BEEN ON DUTY FOR ALMOST 8 HRS AND WERE FLYING THE NEXT TO LAST LEG OF A 4 DAY TRIP. FATIGUE COULD HAVE BEEN A FACTOR. SUPPLEMENTAL INFO FROM ACN 303236: SOMEHOW THE ALT ARMED ANNUNCIATOR DROPPED OUT AT THE END. NO CLUE HOW OR WHY. I WAS FURTHER DISTRACTED WITH FMS PROGRAMMING. THE REAL KEY TO THIS ATTN FAILURE WAS FATIGUE. THE FIRST DAY OF THE TRIP ENDED AFTER 6 LEGS AT THE HOTEL AT XA00. I ENDED UP WITH ABOUT 6 HRS OF SLEEP. THE FOLLOWING NIGHT'S LAYOVER WAS SCHEDULED FOR 9 HRS OF WHICH I AGAIN GOT ABOUT 6 HRS. THE LAST NIGHT'S LAYOVER ALSO GAVE ME ABOUT 6 HRS OF SLEEP DUE TO THE NECESSARY WAKE-UP CALL AT XA15 AM. THE END RESULT WAS THE WORST FATIGUE OF MY CAREER. CONSEQUENTLY, MY XCHK WAS MOSTLY FOCUSED ON STAYING AWAKE RATHER THAN THE ALTIMETER.

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.