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.