Narrative:

In cruise at FL310 en route from clt to hou and between vuz and jan (cleared direct from atl to aex), moderate to severe turbulence was encountered. Requested descent to FL280 (reported smoother). Set FL280 in the altitude preselect, selected flight level change mode on autoplt controller (honeywell spz 8000 afcs), and reduced thrust approximately 15% N1 to initiate a descent at the cruise speed of mach .77. Immediately after selection of this vertical mode (flight level change), the amber 'maximum speed' annunciation appeared on the pilot's ADI (although the aircraft was approximately mach .02 below vmo) and the stick puller began to raise the nose of the aircraft. Seeing the aircraft now climbing above FL310 rather than descending as per the clearance to FL280, I further reduced power as altitude chime sounded and aircraft reached FL313. I then disengaged the autoplt, allowed the aircraft to slow to mach .68 as we resumed descent to FL280. Autoplt was re-engaged and flight was uneventful from that point on. Center did not comment on the altitude excursion. In turbulence, it would have been better technique to first slow the aircraft, prior to initiating descent using the flight level change mode. The turbulence was greatest just as I selected flight level change for what I thought would be a normal descent.

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Original NASA ASRS Text

Title: CAPT OF A BAE125 SERIES 800 HS25 MOMENTARILY LOST CTL OF ACFT AFTER SELECTING DSCNT CLRNC ALT IN THE AUTOPLT FLT LEVEL CHANGE MODE RESULTING IN A CLB RATHER THAN DSCNT.

Narrative: IN CRUISE AT FL310 ENRTE FROM CLT TO HOU AND BTWN VUZ AND JAN (CLRED DIRECT FROM ATL TO AEX), MODERATE TO SEVERE TURB WAS ENCOUNTERED. REQUESTED DSCNT TO FL280 (RPTED SMOOTHER). SET FL280 IN THE ALT PRESELECT, SELECTED FLT LEVEL CHANGE MODE ON AUTOPLT CONTROLLER (HONEYWELL SPZ 8000 AFCS), AND REDUCED THRUST APPROX 15% N1 TO INITIATE A DSCNT AT THE CRUISE SPD OF MACH .77. IMMEDIATELY AFTER SELECTION OF THIS VERT MODE (FLT LEVEL CHANGE), THE AMBER 'MAX SPD' ANNUNCIATION APPEARED ON THE PLT'S ADI (ALTHOUGH THE ACFT WAS APPROX MACH .02 BELOW VMO) AND THE STICK PULLER BEGAN TO RAISE THE NOSE OF THE ACFT. SEEING THE ACFT NOW CLBING ABOVE FL310 RATHER THAN DSNDING AS PER THE CLRNC TO FL280, I FURTHER REDUCED PWR AS ALT CHIME SOUNDED AND ACFT REACHED FL313. I THEN DISENGAGED THE AUTOPLT, ALLOWED THE ACFT TO SLOW TO MACH .68 AS WE RESUMED DSCNT TO FL280. AUTOPLT WAS RE-ENGAGED AND FLT WAS UNEVENTFUL FROM THAT POINT ON. CTR DID NOT COMMENT ON THE ALT EXCURSION. IN TURB, IT WOULD HAVE BEEN BETTER TECHNIQUE TO FIRST SLOW THE ACFT, PRIOR TO INITIATING DSCNT USING THE FLT LEVEL CHANGE MODE. THE TURB WAS GREATEST JUST AS I SELECTED FLT LEVEL CHANGE FOR WHAT I THOUGHT WOULD BE A NORMAL DSCNT.

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.