Narrative:

We were cleared to cross braid intersection at 9000 ft on buj arrival to dfw. I was flying as first officer, and it was my leg. Passing approximately 13000 ft, I attempted to adjust the leveloff point using vertical speed mode. Flight mode panel had been in level change. The vertical speed window remained blank, and level change light remained on. We decided to ignore the anomaly and let the autoplt level the aircraft, as it was going to intercept 9000 ft prior to braid. Just prior to leveloff, we were given a heading change, and also encountered wake turbulence behind an S80 6 1/2 mi ahead. Distracted, the autoplt flew through 9000 ft without capturing. At approximately 8700 ft, I disconnected the autoplt and flew the aircraft back to 9000 ft. The lowest altitude observed was 8600 ft. ATC did not comment on the deviation. No further autoplt anomalies were observed. Callback conversation with reporter revealed the following information: callback in conjunction with wake turbulence study. Reporter first officer believes, though the wake encounter was a distraction, it wasn't a major event. He says it was more of a moderate bump. At all times he had complete control of the aircraft, only displacement being slight roll, less than 5 degrees. But, not being absolutely certain of the autoplt mode, it was a distraction just when the aircraft should have been leveling with the autoplt. Supplemental information from acn 336205: we had been having problems with the vertical speed mode of the flight mode panel. We were cleared to cross braid at 9000 ft. We set and confirmed using point and shoot as always. First officer was flying, I was entering flap setting. With autoplt on, the aircraft went through 9000 ft. First officer turned off autoplt at 8600 ft and climbed back to 9000 ft. Approach then cleared us to 5000 ft and said nothing. We could not figure out why this happened, but we did get into the wake of an S80 right at altitude capture, and encountered a rapid roll to the right. When we recovered, the FMC was in level change mode. As usual, it was a long day, last leg of trip, and I guess we could have been more vigilant. Callback conversation with reporter revealed the following information: reporter participated in the wake turbulence callback program. Reporter felt this was minor in occurrence. It was not severe nor long lasting. Reporter doesn't see any way to change this from happening as all aircraft in this area have a structured routing, therefore ground tracks are all the same. Aircraft had a high speed at time of encounter (250 KIAS) which gave good flight control response to reporter's aircraft. Reporter felt the most significant cause of the incident was ATC spacing.

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Original NASA ASRS Text

Title: WITH ACFT ON AUTOPLT, RPTR ATTEMPTED TO ADJUST LEVELOFF POINT USING VERT SPD MODE, BUT THE VERT SPD WINDOW REMAINED BLANK AND LEVEL CHANGE LIGHT REMAINED ON. THE FLC DECIDED TO IGNORE THE ANOMALY AND JUST AS THEY WERE REACHING LEVELOFF ALT, WAKE TURB WAS ENCOUNTERED WHICH WAS A DISTR AND THE AUTOPLT DIDN'T LEVEL. AFTER DSNDING 400 FT BELOW ASSIGNED, THE PF, FO, DISCONNECTED THE AUTOPLT AND FLEW BACK TO ASSIGNED ALT.

Narrative: WE WERE CLRED TO CROSS BRAID INTXN AT 9000 FT ON BUJ ARR TO DFW. I WAS FLYING AS FO, AND IT WAS MY LEG. PASSING APPROX 13000 FT, I ATTEMPTED TO ADJUST THE LEVELOFF POINT USING VERT SPD MODE. FLT MODE PANEL HAD BEEN IN LEVEL CHANGE. THE VERT SPD WINDOW REMAINED BLANK, AND LEVEL CHANGE LIGHT REMAINED ON. WE DECIDED TO IGNORE THE ANOMALY AND LET THE AUTOPLT LEVEL THE ACFT, AS IT WAS GOING TO INTERCEPT 9000 FT PRIOR TO BRAID. JUST PRIOR TO LEVELOFF, WE WERE GIVEN A HDG CHANGE, AND ALSO ENCOUNTERED WAKE TURB BEHIND AN S80 6 1/2 MI AHEAD. DISTRACTED, THE AUTOPLT FLEW THROUGH 9000 FT WITHOUT CAPTURING. AT APPROX 8700 FT, I DISCONNECTED THE AUTOPLT AND FLEW THE ACFT BACK TO 9000 FT. THE LOWEST ALT OBSERVED WAS 8600 FT. ATC DID NOT COMMENT ON THE DEV. NO FURTHER AUTOPLT ANOMALIES WERE OBSERVED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: CALLBACK IN CONJUNCTION WITH WAKE TURB STUDY. RPTR FO BELIEVES, THOUGH THE WAKE ENCOUNTER WAS A DISTR, IT WASN'T A MAJOR EVENT. HE SAYS IT WAS MORE OF A MODERATE BUMP. AT ALL TIMES HE HAD COMPLETE CTL OF THE ACFT, ONLY DISPLACEMENT BEING SLIGHT ROLL, LESS THAN 5 DEGS. BUT, NOT BEING ABSOLUTELY CERTAIN OF THE AUTOPLT MODE, IT WAS A DISTR JUST WHEN THE ACFT SHOULD HAVE BEEN LEVELING WITH THE AUTOPLT. SUPPLEMENTAL INFO FROM ACN 336205: WE HAD BEEN HAVING PROBS WITH THE VERT SPD MODE OF THE FLT MODE PANEL. WE WERE CLRED TO CROSS BRAID AT 9000 FT. WE SET AND CONFIRMED USING POINT AND SHOOT AS ALWAYS. FO WAS FLYING, I WAS ENTERING FLAP SETTING. WITH AUTOPLT ON, THE ACFT WENT THROUGH 9000 FT. FO TURNED OFF AUTOPLT AT 8600 FT AND CLBED BACK TO 9000 FT. APCH THEN CLRED US TO 5000 FT AND SAID NOTHING. WE COULD NOT FIGURE OUT WHY THIS HAPPENED, BUT WE DID GET INTO THE WAKE OF AN S80 RIGHT AT ALT CAPTURE, AND ENCOUNTERED A RAPID ROLL TO THE R. WHEN WE RECOVERED, THE FMC WAS IN LEVEL CHANGE MODE. AS USUAL, IT WAS A LONG DAY, LAST LEG OF TRIP, AND I GUESS WE COULD HAVE BEEN MORE VIGILANT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR PARTICIPATED IN THE WAKE TURB CALLBACK PROGRAM. RPTR FELT THIS WAS MINOR IN OCCURRENCE. IT WAS NOT SEVERE NOR LONG LASTING. RPTR DOESN'T SEE ANY WAY TO CHANGE THIS FROM HAPPENING AS ALL ACFT IN THIS AREA HAVE A STRUCTURED ROUTING, THEREFORE GND TRACKS ARE ALL THE SAME. ACFT HAD A HIGH SPD AT TIME OF ENCOUNTER (250 KIAS) WHICH GAVE GOOD FLT CTL RESPONSE TO RPTR'S ACFT. RPTR FELT THE MOST SIGNIFICANT CAUSE OF THE INCIDENT WAS ATC SPACING.

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.