Narrative:

We had been cleared to descend to 16000 ft the autoplt was being used with IAS hold, heading select, and altitude select. Approaching 19000 ft, I called for the in range checklist. As that was being run, the captain suggested changing the autoplt elevator servo so we could run a check of the CAT ii ILS approach system. I disconnected the autoplt. He transferred the servo and I reengaged the autoplt. As I punched the altitude select button, I noticed it didn't arm but went to selected (green light). I checked the altitude. We were at a large rate of descent, approaching 16000 ft. Also at this time, the captain and also the controller called the altitude. I assisted the autoplt in the level off. The aircraft settled through 16000 ft by about 300 ft. Additionally, center had called traffic below us on a parallel course which was in sight. We were close enough apparently to give the other aircraft a TCASII alert (our aircraft is not TCASII equipped). This was a case of a lot of things occurring at once. Normally, anyone would not have been any problem. Factors: aircraft had a more complex autoplt than I had been using normally but with a feature (altitude select) that I expect to work. Also heads down set-up. A delay in descent so descent rate was high. ATC call of traffic coinciding with time for preparation for level off running checklist which entailed a need for a change in the autoplt setup. With the additional workload I missed the aural altitude alert and call. I didn't take into account the high rate of descent existing as I reconnected the autoplt. This was a situation which went from the most ordinary to complex without seeming to be because each event was so every day -- that they all combined in a period F flight attendant few mins is probably the stuff of most incidents/accidents. More situational awareness and a conscious effort to not allow events to overlap but either slowing down the sequence and/or prioritizing and accomplishing the tasks would help avoid this. Supplemental information from acn 223958: we reemphasized cockpit resource management with respect to division of duties. All of us were 'outside' looking for traffic. One of us should have hawked altitude and remained scanning gauges. Also, our faith in the autoplt to level off lulled us into complacency.

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Original NASA ASRS Text

Title: ALT BUST.

Narrative: WE HAD BEEN CLRED TO DSND TO 16000 FT THE AUTOPLT WAS BEING USED WITH IAS HOLD, HDG SELECT, AND ALT SELECT. APCHING 19000 FT, I CALLED FOR THE IN RANGE CHKLIST. AS THAT WAS BEING RUN, THE CAPT SUGGESTED CHANGING THE AUTOPLT ELEVATOR SERVO SO WE COULD RUN A CHK OF THE CAT II ILS APCH SYS. I DISCONNECTED THE AUTOPLT. HE TRANSFERRED THE SERVO AND I REENGAGED THE AUTOPLT. AS I PUNCHED THE ALT SELECT BUTTON, I NOTICED IT DIDN'T ARM BUT WENT TO SELECTED (GREEN LIGHT). I CHKED THE ALT. WE WERE AT A LARGE RATE OF DSCNT, APCHING 16000 FT. ALSO AT THIS TIME, THE CAPT AND ALSO THE CTLR CALLED THE ALT. I ASSISTED THE AUTOPLT IN THE LEVEL OFF. THE ACFT SETTLED THROUGH 16000 FT BY ABOUT 300 FT. ADDITIONALLY, CTR HAD CALLED TFC BELOW US ON A PARALLEL COURSE WHICH WAS IN SIGHT. WE WERE CLOSE ENOUGH APPARENTLY TO GIVE THE OTHER ACFT A TCASII ALERT (OUR ACFT IS NOT TCASII EQUIPPED). THIS WAS A CASE OF A LOT OF THINGS OCCURRING AT ONCE. NORMALLY, ANYONE WOULD NOT HAVE BEEN ANY PROBLEM. FACTORS: ACFT HAD A MORE COMPLEX AUTOPLT THAN I HAD BEEN USING NORMALLY BUT WITH A FEATURE (ALT SELECT) THAT I EXPECT TO WORK. ALSO HEADS DOWN SET-UP. A DELAY IN DSCNT SO DSCNT RATE WAS HIGH. ATC CALL OF TFC COINCIDING WITH TIME FOR PREPARATION FOR LEVEL OFF RUNNING CHKLIST WHICH ENTAILED A NEED FOR A CHANGE IN THE AUTOPLT SETUP. WITH THE ADDITIONAL WORKLOAD I MISSED THE AURAL ALT ALERT AND CALL. I DIDN'T TAKE INTO ACCOUNT THE HIGH RATE OF DSCNT EXISTING AS I RECONNECTED THE AUTOPLT. THIS WAS A SITUATION WHICH WENT FROM THE MOST ORDINARY TO COMPLEX WITHOUT SEEMING TO BE BECAUSE EACH EVENT WAS SO EVERY DAY -- THAT THEY ALL COMBINED IN A PERIOD F FA FEW MINS IS PROBABLY THE STUFF OF MOST INCIDENTS/ACCIDENTS. MORE SITUATIONAL AWARENESS AND A CONSCIOUS EFFORT TO NOT ALLOW EVENTS TO OVERLAP BUT EITHER SLOWING DOWN THE SEQUENCE AND/OR PRIORITIZING AND ACCOMPLISHING THE TASKS WOULD HELP AVOID THIS. SUPPLEMENTAL INFO FROM ACN 223958: WE REEMPHASIZED COCKPIT RESOURCE MGMNT WITH RESPECT TO DIVISION OF DUTIES. ALL OF US WERE 'OUTSIDE' LOOKING FOR TFC. ONE OF US SHOULD HAVE HAWKED ALT AND REMAINED SCANNING GAUGES. ALSO, OUR FAITH IN THE AUTOPLT TO LEVEL OFF LULLED US INTO COMPLACENCY.

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.