Narrative:

On a scheduled flight from stl to las, we were cleared, pilot's discretion, to descend to FL240. At 90 DME from las, first officer began his descent and I reported leaving FL310. At about FL260, I saw a bogie on TCASII at 11 O'clock about 12 mi, 3000 ft low. Our altitudes were closing but it appeared that his trajectory would take him slightly left of our course, so I was not concerned. Shortly later, at about FL255, the TCASII gave us a TA. I checked the TCASII where the bogie was now at 10:30 O'clock and about 5 mi. I immediately looked out the window and began a rapid scan for traffic. I repeatedly checked TCASII display and out the window, but never saw the traffic. I kept this up until the bogie passed our 9 O'clock position. I was just resuming my normal instrument scan when ATC asked us what our altitude was. I checked, saw that it was FL232 and reported that to ATC. He then cleared us for further descent. This incident had several contributing factors: 1) the flight engineer was occupied copying ATIS information which had just become legible. 2) first officer had planned his descent point based on an optimum descent profile for landing, and was clearly anticipating further descent clearance. 3) at 1000 ft above our assigned altitude, where I should have made a callout, I was busy looking for traffic. 4) ATC never mentioned the traffic, although in retrospect, it's obvious that he was waiting for that traffic to pass before issuing further descent clearance. 5) TCASII, which is generally of great value to flcs, this time contributed to a loss of separation by distracting me enough to interfere with normal cockpit procedures.

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Original NASA ASRS Text

Title: B727 FLC DSNDS BELOW ASSIGNED ALT WHILE LOOKING FOR TFC INDICATED ON TCASII.

Narrative: ON A SCHEDULED FLT FROM STL TO LAS, WE WERE CLRED, PLT'S DISCRETION, TO DSND TO FL240. AT 90 DME FROM LAS, FO BEGAN HIS DSCNT AND I RPTED LEAVING FL310. AT ABOUT FL260, I SAW A BOGIE ON TCASII AT 11 O'CLOCK ABOUT 12 MI, 3000 FT LOW. OUR ALTS WERE CLOSING BUT IT APPEARED THAT HIS TRAJECTORY WOULD TAKE HIM SLIGHTLY L OF OUR COURSE, SO I WAS NOT CONCERNED. SHORTLY LATER, AT ABOUT FL255, THE TCASII GAVE US A TA. I CHKED THE TCASII WHERE THE BOGIE WAS NOW AT 10:30 O'CLOCK AND ABOUT 5 MI. I IMMEDIATELY LOOKED OUT THE WINDOW AND BEGAN A RAPID SCAN FOR TFC. I REPEATEDLY CHKED TCASII DISPLAY AND OUT THE WINDOW, BUT NEVER SAW THE TFC. I KEPT THIS UP UNTIL THE BOGIE PASSED OUR 9 O'CLOCK POS. I WAS JUST RESUMING MY NORMAL INST SCAN WHEN ATC ASKED US WHAT OUR ALT WAS. I CHKED, SAW THAT IT WAS FL232 AND RPTED THAT TO ATC. HE THEN CLRED US FOR FURTHER DSCNT. THIS INCIDENT HAD SEVERAL CONTRIBUTING FACTORS: 1) THE FE WAS OCCUPIED COPYING ATIS INFO WHICH HAD JUST BECOME LEGIBLE. 2) FO HAD PLANNED HIS DSCNT POINT BASED ON AN OPTIMUM DSCNT PROFILE FOR LNDG, AND WAS CLRLY ANTICIPATING FURTHER DSCNT CLRNC. 3) AT 1000 FT ABOVE OUR ASSIGNED ALT, WHERE I SHOULD HAVE MADE A CALLOUT, I WAS BUSY LOOKING FOR TFC. 4) ATC NEVER MENTIONED THE TFC, ALTHOUGH IN RETROSPECT, IT'S OBVIOUS THAT HE WAS WAITING FOR THAT TFC TO PASS BEFORE ISSUING FURTHER DSCNT CLRNC. 5) TCASII, WHICH IS GENERALLY OF GREAT VALUE TO FLCS, THIS TIME CONTRIBUTED TO A LOSS OF SEPARATION BY DISTRACTING ME ENOUGH TO INTERFERE WITH NORMAL COCKPIT PROCS.

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.