C560 DEVIATED FROM ASSIGNED ALT DURING CRUISE AT FL310 DUE TO RUNAWAY TRIM MALFUNCTION.

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FLYING A CITATION ULTRA (CE560) FROM SANTA MONICA; CA; TO SAN JOSE; CA. FO WAS FLYING THIS LEG FROM THE L SEAT; NO PAX ABOARD. I TOLD FO I WAS GOING IN THE BACK TO USE THE BATHROOM. WE WERE LEVEL AT OUR CRUISE ALT OF FL310. AS I WAS RETURNING TO THE COCKPIT; I FELT THE ACFT PITCH UP EXCESSIVELY. I GOT TO THE FRONT AND ASKED FO WHAT THE PROB WAS. HE DIDN'T ANSWER ME RIGHT AWAY SO I IMMEDIATELY GOT INTO THE R SEAT. I NOTED THE ALT OF FL323 BEFORE THE ACFT STARTED DOWN AGAIN. ONCE LEVEL AT FL310 AGAIN; FO SAID THE PITCH TRIM RAN AWAY WHICH KICKED OFF THE AUTOPLT AND UP WE WENT. UNFORTUNATELY; I WASN'T THERE TO VERIFY THIS OR TO HELP. WE WERE ON ZLA'S FREQ. THEY NEVER SAID ANYTHING ABOUT THIS TO US AND WE DIDN'T SAY ANYTHING TO THEM EITHER. AROUND 5 MINS LATER; WE WERE GIVEN A FREQ CHANGE TO ZOA. UPON ARR AT SJC; I CALLED THE ACFT MANUFACTURER; TOLD THE PROGRAM MGR ON DUTY WHAT HAPPENED AND WHAT I SAW. I THEN HAD A LONG TALK WITH FO ABOUT TAKING MORE POSITIVE CTL OF THE ACFT IN AN EMER SIT. THE FO; I FEEL WAS NOT QUICK ENOUGH TO REACT. HE NEVER TOUCHED THE THROTTLES; NEVER BANKED THE ACFT AS HE PITCHED FORWARD; AND NEVER COMMUNICATED WITH ME WHEN I ASKED WHAT THE PROB WAS. THE COMPANY AIRLINED US BOTH HOME MONDAY MORNING. MGMNT MET WITH US BOTH SEPARATELY. THE FINAL OUTCOME WAS THEY LET THE FO GO. FO HAD SOME PREVIOUS PROBS AND THE COMPANY FELT HE WAS NOT PERFORMING UP TO OUR HIGH STANDARDS. CAPTS FOLLOW UP THIS PROCESS BY CAPT RPTS WHEN THE COPLT MAKES MISTAKES OR MAY NEED MORE TRAINING IN ANY AREA. SAFETY IS JOB #1.