BE400 CREW WAS HAVING DIRECTIONAL CTL PROBS WITH THE ACFT. THIS APPEARED TO CAUSE LESS THAN LEGAL SEPARATION WITH ANOTHER ACFT.

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ABOUT 2 MINS AFTER TKOF AND DURING HAND FLYING IN A VERY SMOOTH MORNING AND WHILE ACCELERATING TO 250 KIAS AND PASSING 3000 FT MSL; I STARTED TO HAVE A LITTLE DIFFICULTY MAINTAINING HDG. THIS DIFFICULTY WAS INCREASING RAPIDLY. INITIALLY; I WAS THINKING THAT THE CAUSE WAS A FLOATING SPOILER; SO I TOLD THE PNF TO CHK ON HER WING AND SEE IF SHE COULD SEE SOMETHING ABNORMAL; AND I WAS DOING THE SAME THING AT THE SAME TIME. WHILE DOING ALL OF THIS; ATC WAS GIVING US HDG AND ALT TO MAINTAIN; WHICH WE COULDN'T COMPLY EXACTLY WITH (ESPECIALLY WITH HDG) BECAUSE THE FACT OF TRYING TO KEEP THE AIRPLANE FLYING WAS KEEPING US REALLY BUSY AND IT WAS EXTREMELY HARD TO MAINTAIN HDG IN SUCH CONDITIONS. THERE WERE MOMENTS WHERE BOTH PLTS HAD TO BE AT THE CTLS AT THE SAME TIME. I TRIED TO APPLY DIFFERENTIAL PWR SETTINGS AND IT HELPED UP TO A CERTAIN POINT; BUT THEN WHEN WE NEEDED TO COMPLY WITH CLBING INSTRUCTIONS FROM ATC; IT WAS NECESSARY TO APPLY PWR ON BOTH SIDES AGAIN. IN THE MEANTIME; I REALIZED THAT THE CAUSE OR REASON OF THE FLT CTL PROB WAS A RUDDER TRIM RUNAWAY; BY LOOKING AT THE RUDDER TRIM INDICATOR. (IN THE BEECHJET 400A; THE RUDDER TRIM SYS IS THE ONLY ONE THAT DOES NOT HAVE A HORN WHEN IT IS RUNNING AWAY.) ATC ASKED US IF WE WERE DECLARING AN EMER; WHICH WE REPLIED 'NO;' THAT ALL WE WANTED WAS TO GO BACK TO THE ARPT AND ASKED FOR A DIFFERENT HDG. WE TOLD THEM THAT WE WERE EXPERIENCING PROBS; TO KEEP SPECIFIED HDG AND THAT WE WERE GOING BACK FOR A LNDG. WHILE TRYING TO MAINTAIN HDG; I ASKED FOR EMER CHKLIST (RUDDER TRIM RUNAWAY) AND WE EXECUTED THE APPROPRIATE PROC. WHILE SLOWING DOWN THE AIRPLANE; I STARTED TO GET MORE CTL OF THE AIRPLANE SO I ASKED FOR APCH AND BEFORE LNDG CHKLIST AND MADE A LNDG WITHOUT ANY PROB.