Narrative:

Climbed from filed cruise altitude of FL370 to FL390 per ZDV request. Traffic was given as reason for climb, checked current weight of aircraft and maneuver capability of aircraft before accepting climb. Upon reaching FL390 aircraft did not accelerate properly, appeared to be heavier than computer showed. Conflicting traffic was at FL370, 1 O'clock position, and visual. After less than 2 mins at FL390 the aircraft experienced a very slight low speed buffet, a request was made for a lower altitude. While the dialogue between my first officer and ZDV was taking place, the second light buffet was experienced. I told my first officer to notify center that I was leaving altitude and commencing a slow, shallow turn to left, to clear traffic even more. After turn, we were approximately 30 degrees left correction from original course, and received clearance to continue descent to FL330 with a direct obh VOR out of FL350. When established at FL330 I requested a discreet frequency to explain why I vacated FL390. A phone call was placed to ZDV upon arrival at destination. An inaccurate bag count, plus using an estimated weight versus actual weight of bags was determined to be the main problem. Since we loaded erroneous data into the FMC (weights), it correspondingly returned to us equally bad information.

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Original NASA ASRS Text

Title: POTENTIAL CONFLICT ENSUES WHEN A B757 IS UNABLE TO MAINTAIN ITS ASSIGNED ALT OF FL390. ALTDEV ALT EXCURSION. NON ADHERENCE TO ATC CLRNC. OPPOSITE DIRECTION TFC.

Narrative: CLBED FROM FILED CRUISE ALT OF FL370 TO FL390 PER ZDV REQUEST. TFC WAS GIVEN AS REASON FOR CLB, CHKED CURRENT WT OF ACFT AND MANEUVER CAPABILITY OF ACFT BEFORE ACCEPTING CLB. UPON REACHING FL390 ACFT DID NOT ACCELERATE PROPERLY, APPEARED TO BE HEAVIER THAN COMPUTER SHOWED. CONFLICTING TFC WAS AT FL370, 1 O'CLOCK POS, AND VISUAL. AFTER LESS THAN 2 MINS AT FL390 THE ACFT EXPERIENCED A VERY SLIGHT LOW SPD BUFFET, A REQUEST WAS MADE FOR A LOWER ALT. WHILE THE DIALOGUE BTWN MY FO AND ZDV WAS TAKING PLACE, THE SECOND LIGHT BUFFET WAS EXPERIENCED. I TOLD MY FO TO NOTIFY CTR THAT I WAS LEAVING ALT AND COMMENCING A SLOW, SHALLOW TURN TO L, TO CLR TFC EVEN MORE. AFTER TURN, WE WERE APPROX 30 DEGS L CORRECTION FROM ORIGINAL COURSE, AND RECEIVED CLRNC TO CONTINUE DSCNT TO FL330 WITH A DIRECT OBH VOR OUT OF FL350. WHEN ESTABLISHED AT FL330 I REQUESTED A DISCREET FREQ TO EXPLAIN WHY I VACATED FL390. A PHONE CALL WAS PLACED TO ZDV UPON ARR AT DEST. AN INACCURATE BAG COUNT, PLUS USING AN ESTIMATED WT VERSUS ACTUAL WT OF BAGS WAS DETERMINED TO BE THE MAIN PROB. SINCE WE LOADED ERRONEOUS DATA INTO THE FMC (WTS), IT CORRESPONDINGLY RETURNED TO US EQUALLY BAD INFO.

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.