AFTER INITIATING AN AUTOPILOT DESCENT; A CESSNA CITATION 10 (CE-750) FLIGHT CREW SUDDENLY EXPERIENCED AN UNCOMMANDED RAPID CLIMB IN EXCESS OF 3000 FPM; REQUIRING CAPTAIN TO DISCONNECT THE AUTOPILOT TO REGAIN CONTROL.

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WHILE ON THE ABC ARR INTO ZZZ; WE HAD JUST BEEN GIVEN INSTRUCTIONS TO CROSS XXXXX INTXN AT AND MAINTAIN 13000 FT. THE CAPT SET 13000 FT IN THE ALT SELECTOR AND I CONFIRMED IT. I THEN SELECTED VERT SPD ON THE FLT GUIDANCE COMPUTER; COMMANDED A 1400 FPM DSCNT RATE AND VISUALLY CONFIRMED BOTH THE VERT SPD COMMAND BOX AND THE COMMAND BUG; SHOWING A DSCNT. ADDITIONALLY; I OBSERVED THE ACFT INITIATED A DSCNT. WE THEN LOOKED DOWN AT THE ARR TO CONFIRM THE CLRNC WE JUST RECEIVED MATCHED THE ARR. SUDDENLY WE FELT OURSELVES SINKING INTO OUR SEAT AND REALIZED WE WERE IN A RAPID CLB. WE LOOKED UP TO SEE A CLB IN EXCESS OF 3000 FPM. AT APPROX FL245; I DISCONNECTED THE AUTOPLT AND INITIATED A DSCNT. AT APPROX FL249; THE CLB WAS STOPPED AND WE THEN STARTED A DSCNT. WE RECEIVED A QUERY FROM CTR AS WE WERE PASSING THROUGH FL235 REQUESTING CONFIRMATION WE WERE DSNDING TO 13000 FT. THE CAPT REPLIED 'WE ARE OUT OF FL235 DSNDING TO 13000 FT.' WE WERE GIVEN A PHONE NUMBER TO CALL CTR. THE CAPT CALLED SOON AFTER WE LANDED AND WAS TOLD BY THE INDIVIDUAL AT CTR THAT HE REPLAYED THE EVENT AND SAW THE ACFT INITIALLY START A DSCNT; THEN CLB TO FL249. ACCORDING TO HIM; THIS SET OFF WARNINGS AND THAT THE EVENT CAUGHT THEM OFF-GUARD. HE SAID THAT WE NEVER LOST THE REQUIRED SEPARATION FROM ANOTHER ACFT NEAR US; BUT RATHER HE WAS VERY CURIOUS AS TO WHAT HAD HAPPENED. THE CAPT WROTE UP THE ACFT FOR THE UNCOMMANDED CLB. THE AVIONICS MECH FOUND 2 FAULTS IN THE AIR DATA COMPUTER RELATED TO INVALID VERT GUIDANCE DATA. WHAT I LEARNED FROM THIS EXPERIENCE IS THAT YOU CAN NEVER FULLY TRUST A FLT GUIDANCE COMPUTER/AUTOPLT TO DO WHAT HAS BEEN COMMANDED. SINCE A FAULT CAN OCCUR AT ANY TIME YOU MUST ALWAYS BE VIGILANT IN MONITORING ITS PROGRESS ESPECIALLY DURING TRANSITIONS.