Narrative:

After leveling at 14000 ft, approach issued a descent to 12000 ft. Copilot was flying on autoplt, and I thought he set 12000 ft in MCP, and began a vertical speed descent. I read back out of 14000 ft for 12000 ft. He apparently heard 11000 ft and set that. I didn't catch the error, though I thought we both pointed at the altitude window. He then called for the approach check, which I accomplished. When it was complete, I noticed the aircraft leveling at 11000 ft. I said 'we're only cleared to 12000 ft,' and about that time approach control asked us our altitude. I responded with 11000 ft and he said to maintain it. TCASII showed traffic at 11 O'clock at 5 mi climbing, departing atl. The clearance out of atlanta is to 10000 ft, and we never lost separation. I suspect fatigue was a factor in this occurrence. We had already flown from dfw to lga, switched planes, and experienced a mechanical problem in lga after pushback. This involved a lengthy process of procedures, radios, MEL, passenger, etc, while taxiing around a very busy airport, and resulted in 1 hour away from the gate. This was followed by another aircraft swap, and many passenger concerns. We departed lga approximately 2 hours late, and were pretty tired on arrival in atl.

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Original NASA ASRS Text

Title: ALT ALERT SYS NOT SET TO CLRED ALT AND ALT OVERSHOOT OCCURS.

Narrative: AFTER LEVELING AT 14000 FT, APCH ISSUED A DSCNT TO 12000 FT. COPLT WAS FLYING ON AUTOPLT, AND I THOUGHT HE SET 12000 FT IN MCP, AND BEGAN A VERT SPD DSCNT. I READ BACK OUT OF 14000 FT FOR 12000 FT. HE APPARENTLY HEARD 11000 FT AND SET THAT. I DIDN'T CATCH THE ERROR, THOUGH I THOUGHT WE BOTH POINTED AT THE ALT WINDOW. HE THEN CALLED FOR THE APCH CHK, WHICH I ACCOMPLISHED. WHEN IT WAS COMPLETE, I NOTICED THE ACFT LEVELING AT 11000 FT. I SAID 'WE'RE ONLY CLRED TO 12000 FT,' AND ABOUT THAT TIME APCH CTL ASKED US OUR ALT. I RESPONDED WITH 11000 FT AND HE SAID TO MAINTAIN IT. TCASII SHOWED TFC AT 11 O'CLOCK AT 5 MI CLBING, DEPARTING ATL. THE CLRNC OUT OF ATLANTA IS TO 10000 FT, AND WE NEVER LOST SEPARATION. I SUSPECT FATIGUE WAS A FACTOR IN THIS OCCURRENCE. WE HAD ALREADY FLOWN FROM DFW TO LGA, SWITCHED PLANES, AND EXPERIENCED A MECHANICAL PROB IN LGA AFTER PUSHBACK. THIS INVOLVED A LENGTHY PROCESS OF PROCS, RADIOS, MEL, PAX, ETC, WHILE TAXIING AROUND A VERY BUSY ARPT, AND RESULTED IN 1 HR AWAY FROM THE GATE. THIS WAS FOLLOWED BY ANOTHER ACFT SWAP, AND MANY PAX CONCERNS. WE DEPARTED LGA APPROX 2 HRS LATE, AND WERE PRETTY TIRED ON ARR IN ATL.

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.