AN MD80 CREW ALLOWED THE AUTOPLT TO BEGIN THE TURN EARLY; ON THE ARC APCH TO RWY 17 AT MTJ.

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S80 FALSE LOC CAPTURE. I WAS QUALIFYING CHK AIRMAN INTO MTJ AND WE DECIDED TO DO THE ARC TO ILS RWY 17 OFF OF PAGRE INTXN FOR TRAINING EVEN THOUGH IT WAS VFR. WE WERE ON THE ARC USING THE GPS TO NAV. THE ILS FREQ AND COURSE WERE IN THE L SIDE AND I WAS ON THE VOR FREQ TO XCHK THE ARC. THE DFGS WAS IN SIDE ONE. ABOUT 1/2 WAY AROUND THE ARC; MY PARTNER ARMED THE LOC. SHORTLY AFTER THAT WE GOT LOC CAPTURE AND THE ACFT TURNED OUTBOUND EVEN THOUGH WE WERE NOWHERE CLOSE TO THE INBOUND COURSE. WE CAUGHT IT AFTER ABOUT 5-10 DEGS OF HDG CHANGE AND CORRECTED BACK TO COURSE USING HDG SELECT. WE REINTERCEPTED THE ARC AND THEN THE LOC AND THE APCH WAS UNEVENTFUL AFTER THAT. THE CONCERN THAT I HAVE IS THAT THE ACFT TURNED TOWARD TERRAIN THAT IS OVER 1000 FT HIGHER ABOUT 10 MI AWAY. IF THIS HAPPENS IN THE WX AND THE RECOGNITION OF IT IS SLOW; YOU COULD HAVE A SERIOUS PROB SINCE THE TERRAIN RISES RAPIDLY. THIS IS ESPECIALLY TRUE IF YOU DON'T HAVE A NAV DISPLAY TO ENHANCE YOUR SITUATIONAL AWARENESS. ANOTHER CONCERN IS THAT IF THE NAV MODE IS USED TO CORRECT BACK TO COURSE YOU HAVE TO SWITCH TO NAV TO DO SO AND THEN BACK TO RADIAL TO INTERCEPT THE LOC. THIS IS TIME CONSUMING; PLUS; HOW DO YOU INTERCEPT A COURSE TO AN ARC? THERE ARE APCHS IN MEXICO THAT ALSO USE ACR'S TO GET ALIGNED WITH FINAL SO THIS COULD BE A PROB THERE ALSO. WHAT IS PARTICULARLY ONEROUS ABOUT MTJ THOUGH IS THAT THERE IS NO RADAR ON THE APCH SO YOU ARE REALLY ON YOUR OWN. HOPEFULLY IN MEX SOMEONE WOULD NOTIFY YOU. THERE IS NO INFO IN THE OPERATING MANUAL ABOUT WHEN TO ARM THE LOC. THE S80 IS PROBABLY DESIGNED TO PROPERLY INTERCEPT THE LOC WHEN IT IS ON AN INTERCEPT COURSE. WE BOTH FLEW THE F100 AND BOTH REMEMBER THERE IS NO PROB ARMING THE LOC WHEN USING THE NAV MODE. HOWEVER; IT APPEARS THAT THE S80 IS NOT. IF THIS IS THE CASE; I THINK THAT A MESSAGE SHOULD BE ADDED TO ALL S80 FLT PLANS SO THAT CREWS ARE MADE AWARE OF THE PROB. ALL THE APCHS THAT I CAN THINK OF THAT USE AN ARC ARE IN MOUNTAINOUS TERRAIN AND A MISTAKE COULD BE DISASTROUS.