Narrative:

On descent from FL330 to FL290 I had called passing FL300 for FL290 and turned my attention to navigation duties. I looked back at the instrument panel shortly and saw us passing FL290. I called attention to this and grabbed the yoke to stop descent and climb back to FL290. We had probably gone down as far as FL287. Once back level at FL290 ATC advised us to maintain FL290 as we had opposing traffic. The traffic was another air carrier Y B747 and we did have a visual on them. Just after ATC reminded us to maintain FL290 I looked out and saw the B747 in a slight right turn. They ask ATC for our flight number and advised they were making a turn as their TCASII had alerted merging traffic with less than 1000 ft separation. I advised them we were level at FL290 and they acknowledged. ATC did not respond. This incident was caused a momentary distraction of attention to other duties in the cockpit on arrival, and abetted by failure of the altitude alert system to sound or light. Supplemental information from acn 344956: at this time the autoplt was disengaged and manually climbed back to FL290. The other aircraft reported that their TCASII gave them a warning of conflicting traffic of less than 1000 ft. We had the traffic in sight at all times even before going below FL290 by 200 ft. I believe had the altitude alert system be working it would have helped to avoid the situation in a very busy cockpit environment.

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Original NASA ASRS Text

Title: POTENTIAL CONFLICT DEVELOPS DURING ALTDEV ALT OVERSHOT IN DSCNT. ACR X FREIGHTER WITH OPPOSITE DIRECTION TFC OF A B747 700 FT BELOW. OCCURRED IN FOREIGN AIRSPACE.

Narrative: ON DSCNT FROM FL330 TO FL290 I HAD CALLED PASSING FL300 FOR FL290 AND TURNED MY ATTN TO NAV DUTIES. I LOOKED BACK AT THE INST PANEL SHORTLY AND SAW US PASSING FL290. I CALLED ATTN TO THIS AND GRABBED THE YOKE TO STOP DSCNT AND CLB BACK TO FL290. WE HAD PROBABLY GONE DOWN AS FAR AS FL287. ONCE BACK LEVEL AT FL290 ATC ADVISED US TO MAINTAIN FL290 AS WE HAD OPPOSING TFC. THE TFC WAS ANOTHER ACR Y B747 AND WE DID HAVE A VISUAL ON THEM. JUST AFTER ATC REMINDED US TO MAINTAIN FL290 I LOOKED OUT AND SAW THE B747 IN A SLIGHT R TURN. THEY ASK ATC FOR OUR FLT NUMBER AND ADVISED THEY WERE MAKING A TURN AS THEIR TCASII HAD ALERTED MERGING TFC WITH LESS THAN 1000 FT SEPARATION. I ADVISED THEM WE WERE LEVEL AT FL290 AND THEY ACKNOWLEDGED. ATC DID NOT RESPOND. THIS INCIDENT WAS CAUSED A MOMENTARY DISTR OF ATTN TO OTHER DUTIES IN THE COCKPIT ON ARR, AND ABETTED BY FAILURE OF THE ALT ALERT SYS TO SOUND OR LIGHT. SUPPLEMENTAL INFO FROM ACN 344956: AT THIS TIME THE AUTOPLT WAS DISENGAGED AND MANUALLY CLBED BACK TO FL290. THE OTHER ACFT RPTED THAT THEIR TCASII GAVE THEM A WARNING OF CONFLICTING TFC OF LESS THAN 1000 FT. WE HAD THE TFC IN SIGHT AT ALL TIMES EVEN BEFORE GOING BELOW FL290 BY 200 FT. I BELIEVE HAD THE ALT ALERT SYS BE WORKING IT WOULD HAVE HELPED TO AVOID THE SIT IN A VERY BUSY COCKPIT ENVIRONMENT.

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.