FLC OF AN LTT ON A MANUFACTURER TEST FLT FAILED TO MAINTAIN ASSIGNED TRACK DURING APCH PHASE DUE TO NOT SETTING THE PROPER MODE ON THE AUTOPLT APCH COUPLER.

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THE APCH ASSIGNED WAS THE LOC/BACK COURSE RWY 19L IN WICHITA; KS. THE ACFT BEING FLOWN WAS THE CITATION 5 ULTRA WITH THE (MANUFACTURER) AVIONICS PACKAGE. APCH CTL WAS VECTORING US PARALLEL TO THE FINAL APCH IN ORDER TO PASS A KING AIR DOING PRACTICE APCHS. WE HAD BEEN ASKED TO KEEP OUR SPD UP AND WE WERE INDICATING 250 KTS. THE KING AIR TFC HAD YET TO BE POINTED OUT TO US. (MANUFACTURE) HAS REDESIGNED THE FLT DIRECTOR PANEL ELIMINATING THE BACK COURSE BUTTON. NOW; THERE IS ONLY AN 'APCH' BUTTON THAT AUTOMATICALLY CHOOSES THE FRONT COURSE OR BACK COURSE BASED ON SELECTED INBOUND COURSE AND HDG AT TIME OF INTERCEPT. THUS; IF THE INTERCEPT IS TOO FAST OR THE ANGLE IS TOO GREAT; THE FLT DIRECTOR WILL COMMAND EXCESSIVE RATES OF TURN TO JOIN; S-TURN YOU DOWN FINAL OR EVEN TURN YOU AWAY FROM THE ARPT OUT THE FRONT COURSE. WE WERE ASSIGNED A 220 DEG HDG TO JOIN AND ASKED TO KEEP THE SPD UP. THE PF SELECTED 'NAV' INSTEAD OF 'APCH' SO THAT AS THE COURSE CAME ALIVE; THE FLT DIRECTOR COMMANDED A R TURN TO 270 DEGS AND STARTED TO TURN US NBOUND. THIS IS NOT UNCOMMON TO SELECT 'NAV' ON FRONT COURSES BECAUSE IT WILL TAKE MOST OF THE ERRATIC S-TURNING OUT OF THE COURSE AT HIGHER SPDS. BUT BOTH OF US FAILED TO REALIZE THAT WITHOUT SELECTING 'APCH;' BACK COURSE SENSING IS NOT ACTIVATED. BEFORE WE CAUGHT OUR MISTAKE AND TURNED INBOUND AGAIN; APCH CTL WAS ISSUING A 'TA' TO THE KING AIR WHICH WAS MAKING AN ILS TO RWY 19R. AT THE TIME OF THE ALERT; I WOULD ESTIMATE SEPARATION AT 500 FT AND 1 TO 1 1/2 MI. THIS COULD HAVE BEEN AVOIDED IF (MANUFACTURER) HAD JUST LEFT THE 'BC' SELECTION ON THE PANEL. HOWEVER; ALL AVIONICS MANUFACTURERS SEEM TO WANT THE PLT OUT OF LOOP AND AS A RESULT; IT'S HARDER FOR FLC'S TO FIGURE OUT WHAT THE AVIONICS ARE DOING.