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.