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.