Narrative:

We were on approach into charlotte on the unarm arrival. We'd been given a descent to 9000 ft, and then were vectored off the arrival (355 degree heading) with a speed reduction to 210 KTS (from 250 KTS). As we started to roll out on the 355 degree heading, we got a TA indicating traffic at about 2 O'clock (on TCASII). I looked to the right and located the traffic, below and slightly ahead of us. I looked back inside the aircraft and noticed the aircraft had deviated below 9000 ft. I called this to the captain's attention, after which we got a TCASII RA prompting us to climb. At no time did we come close enough to each other to create a hazard and the other plane was in visual contact. What I believe caused the deviation was (as usual) a combination of circumstances. We had a lot of things happen at the same time, multiple traffic calls, airspeed change, altitude change/leveloff, heading change and locating traffic. As each of us became occupied with different duties, we weren't able to xchk each other, or the autoplt, and when the autoplt did not perform as expected we were late in catching it. What I think happened to the autoplt was that as we were beginning to level off (and therefore in the altitude capture mode) we were told to reduce speed to 210 KTS. The captain moved the thumbwheel to reduce the speed, but since the autoplt was in an altitude capture mode, that caused the autoplt to cancel the capture and go to a vertical speed mode. Since we were busy with other duties we did not notice it until we had deviated below 9000 ft. Supplemental information from acn 263692: I believe that the autoplt did not level off at 9000 ft because when I updated the airspeed as we leveled off, the autoplt computer did not level the aircraft but reduced the descent rate to slow to 210 KTS but did not level off at 9000 ft even though 9000 ft was set in the altitude window.

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Original NASA ASRS Text

Title: ALT BUST.

Narrative: WE WERE ON APCH INTO CHARLOTTE ON THE UNARM ARR. WE'D BEEN GIVEN A DSCNT TO 9000 FT, AND THEN WERE VECTORED OFF THE ARR (355 DEG HDG) WITH A SPD REDUCTION TO 210 KTS (FROM 250 KTS). AS WE STARTED TO ROLL OUT ON THE 355 DEG HDG, WE GOT A TA INDICATING TFC AT ABOUT 2 O'CLOCK (ON TCASII). I LOOKED TO THE R AND LOCATED THE TFC, BELOW AND SLIGHTLY AHEAD OF US. I LOOKED BACK INSIDE THE ACFT AND NOTICED THE ACFT HAD DEVIATED BELOW 9000 FT. I CALLED THIS TO THE CAPT'S ATTN, AFTER WHICH WE GOT A TCASII RA PROMPTING US TO CLB. AT NO TIME DID WE COME CLOSE ENOUGH TO EACH OTHER TO CREATE A HAZARD AND THE OTHER PLANE WAS IN VISUAL CONTACT. WHAT I BELIEVE CAUSED THE DEV WAS (AS USUAL) A COMBINATION OF CIRCUMSTANCES. WE HAD A LOT OF THINGS HAPPEN AT THE SAME TIME, MULTIPLE TFC CALLS, AIRSPD CHANGE, ALT CHANGE/LEVELOFF, HDG CHANGE AND LOCATING TFC. AS EACH OF US BECAME OCCUPIED WITH DIFFERENT DUTIES, WE WEREN'T ABLE TO XCHK EACH OTHER, OR THE AUTOPLT, AND WHEN THE AUTOPLT DID NOT PERFORM AS EXPECTED WE WERE LATE IN CATCHING IT. WHAT I THINK HAPPENED TO THE AUTOPLT WAS THAT AS WE WERE BEGINNING TO LEVEL OFF (AND THEREFORE IN THE ALT CAPTURE MODE) WE WERE TOLD TO REDUCE SPD TO 210 KTS. THE CAPT MOVED THE THUMBWHEEL TO REDUCE THE SPD, BUT SINCE THE AUTOPLT WAS IN AN ALT CAPTURE MODE, THAT CAUSED THE AUTOPLT TO CANCEL THE CAPTURE AND GO TO A VERT SPD MODE. SINCE WE WERE BUSY WITH OTHER DUTIES WE DID NOT NOTICE IT UNTIL WE HAD DEVIATED BELOW 9000 FT. SUPPLEMENTAL INFO FROM ACN 263692: I BELIEVE THAT THE AUTOPLT DID NOT LEVEL OFF AT 9000 FT BECAUSE WHEN I UPDATED THE AIRSPD AS WE LEVELED OFF, THE AUTOPLT COMPUTER DID NOT LEVEL THE ACFT BUT REDUCED THE DSCNT RATE TO SLOW TO 210 KTS BUT DID NOT LEVEL OFF AT 9000 FT EVEN THOUGH 9000 FT WAS SET IN THE ALT WINDOW.

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.