Narrative:

Center had given us permission to deviate, as necessary, around a line of developing thunderstorms. A previous aircraft was reported to have successfully crossed this area at FL310, and reported tops as being at approximately that altitude. We were at FL330, and anticipated remaining in the clear, on top, with minor deviations to avoid crossing directly over the tops of the cells. Nearing the area, it became apparent that the higher tops were in excess of FL350. There was a clear area just east of the cell in front of us, and only minor deviating was required to circumnavigate the developing cumulus, remain in the clear, and cross into clear skies which were visible on the north side. Our doppler radar showed areas of turbulence in the cells, but nothing between them (though this would be expected with no moisture). Light turbulence was anticipated crossing the area, but moderate turbulence was encountered, at which point the autoplt pitch reverted from altitude hold to cws (control wheel steering), and commenced a descent reaching 1000 FPM. Pitch was altered to return to altitude, but the aircraft descended to approximately 32500 ft before a climb was established. Though in cws pitch, the autoplt was still in command mode and should have captured the altitude in the MCP window. This did not happen, and when altitude acquisition did not engage, reaching FL330, the pitch was altered to level the aircraft. The rate of climb had increased to slightly in excess of 1000 FPM, and altitude reached approximately 33500 ft. Before establishing a descent, and crossing, again, into smooth air. I reported the encounter with moderate turbulence to center, as we leveled off (the autoplt captured the altitude, this time), center said to report reaching FL330 and that we showed 500 ft high. By this time, we were level at altitude. No other aircraft were displayed on our TCASII, and I do not believe that there was any traffic conflict. Options to have avoided an altitude deviation: 1) a block altitude between FL310 and FL350. 2) a clearance to FL350 for a short time, may have kept us in smoother air. 3) deviating another 50 NM west of course through clrer airspace. 4) manually selecting altitude hold on the MCP when cws pitch mode was first observed may have minimized the altitude divergence.

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Original NASA ASRS Text

Title: AN ALT EXCURSION OCCURRED DURING AN AUTOPLT AUTOMATIC MODE SELECTION IN MODERATE TURB (IN THE VICINITY OF TSTMS).

Narrative: CTR HAD GIVEN US PERMISSION TO DEVIATE, AS NECESSARY, AROUND A LINE OF DEVELOPING TSTMS. A PREVIOUS ACFT WAS RPTED TO HAVE SUCCESSFULLY CROSSED THIS AREA AT FL310, AND RPTED TOPS AS BEING AT APPROX THAT ALT. WE WERE AT FL330, AND ANTICIPATED REMAINING IN THE CLR, ON TOP, WITH MINOR DEVS TO AVOID XING DIRECTLY OVER THE TOPS OF THE CELLS. NEARING THE AREA, IT BECAME APPARENT THAT THE HIGHER TOPS WERE IN EXCESS OF FL350. THERE WAS A CLR AREA JUST E OF THE CELL IN FRONT OF US, AND ONLY MINOR DEVIATING WAS REQUIRED TO CIRCUMNAVIGATE THE DEVELOPING CUMULUS, REMAIN IN THE CLR, AND CROSS INTO CLR SKIES WHICH WERE VISIBLE ON THE NORTH SIDE. OUR DOPPLER RADAR SHOWED AREAS OF TURB IN THE CELLS, BUT NOTHING BTWN THEM (THOUGH THIS WOULD BE EXPECTED WITH NO MOISTURE). LIGHT TURB WAS ANTICIPATED XING THE AREA, BUT MODERATE TURB WAS ENCOUNTERED, AT WHICH POINT THE AUTOPLT PITCH REVERTED FROM ALT HOLD TO CWS (CONTROL WHEEL STEERING), AND COMMENCED A DSCNT REACHING 1000 FPM. PITCH WAS ALTERED TO RETURN TO ALT, BUT THE ACFT DSNDED TO APPROX 32500 FT BEFORE A CLB WAS ESTABLISHED. THOUGH IN CWS PITCH, THE AUTOPLT WAS STILL IN COMMAND MODE AND SHOULD HAVE CAPTURED THE ALT IN THE MCP WINDOW. THIS DID NOT HAPPEN, AND WHEN ALT ACQUISITION DID NOT ENGAGE, REACHING FL330, THE PITCH WAS ALTERED TO LEVEL THE ACFT. THE RATE OF CLB HAD INCREASED TO SLIGHTLY IN EXCESS OF 1000 FPM, AND ALT REACHED APPROX 33500 FT. BEFORE ESTABLISHING A DSCNT, AND XING, AGAIN, INTO SMOOTH AIR. I RPTED THE ENCOUNTER WITH MODERATE TURB TO CTR, AS WE LEVELED OFF (THE AUTOPLT CAPTURED THE ALT, THIS TIME), CTR SAID TO RPT REACHING FL330 AND THAT WE SHOWED 500 FT HIGH. BY THIS TIME, WE WERE LEVEL AT ALT. NO OTHER ACFT WERE DISPLAYED ON OUR TCASII, AND I DO NOT BELIEVE THAT THERE WAS ANY TFC CONFLICT. OPTIONS TO HAVE AVOIDED AN ALTDEV: 1) A BLOCK ALT BTWN FL310 AND FL350. 2) A CLRNC TO FL350 FOR A SHORT TIME, MAY HAVE KEPT US IN SMOOTHER AIR. 3) DEVIATING ANOTHER 50 NM W OF COURSE THROUGH CLRER AIRSPACE. 4) MANUALLY SELECTING ALT HOLD ON THE MCP WHEN CWS PITCH MODE WAS FIRST OBSERVED MAY HAVE MINIMIZED THE ALT DIVERGENCE.

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.