CFIT; CFTT. CTLED FLT TOWARDS DESIGNATED MOUNTAINOUS TERRAIN WHEN COMMUTER FO MISREADS THE APCH CHART; MISLEADS OR MISINFORMS THE CAPT WHO GOES OVER TO COMPANY FREQ. FO THEN PERFORMS AN ALTDEV ALT EXCURSION FROM ASSIGNED; NOT CONFORMING TO CHART ALTS. NON ADHERENCE TO ATC PROC CLRNC STERILE COCKPIT PROC.

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WE WERE AT 15000 FT MSL; NBOUND TO THE JAC VOR. THE CTLR CLRED US TO CROSS JAC VOR AT 15000 FT. THEY SAID CLRED FOR THE ILS RWY 18 APCH. MY FO STATED THAT AFTER WE CROSS THE VOR WE ARE GOOD TO DSND TO 12000 FT. WITHOUT CHKING TO SEE IF HE WAS CORRECT; I CONTACTED OUR GND PEOPLE TO GIVE OUR FUEL OUTBOUND AND ETA. SHORTLY; I SAW THE FO CLBING QUICKLY BACK TO 15000 FT. I ASKED WHAT HE WAS DOING AND HE STATED THAT WE NEED TO BE AT 15000 FT AT THIS STAGE OF THE FLT. I LOOKED IT UP AT THIS TIME AND FOUND HE WAS CORRECT. THE OBSERVANCE THAT WE HAD DEVIATED BELOW PUBLISHED ALT WAS FIRST NOTED BY ZLC. SHE QUESTIONED THE FO ABOUT THE ALT DURING THAT SEGMENT OF THE APCH AND THEN HE NOTED THAT SHE WAS CORRECT AND STARTED AN IMMEDIATE CLB OUT OF 13700 FT BACK TO 15000 FT. LESSON LEARNED: ALWAYS DOUBLECHK; OUR ALTS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR WAS FLYING AN EMB120. HE HAD NOT RPTED THE INCIDENT TO THE COMPANY CHIEF PLT AND SAFETY DEPT BUT FELT; THAT AFTER OUR CONVERSATION; HE SHOULD AND WILL. HE WILL INCORPORATE THIS INCIDENT ALONG WITH OTHER DIFFICULT APCH STATIONS THAT ARE IN PROX OF MOUNTAINOUS TERRAIN. HE FEELS THAT THE COMPANY; WHO JUST WENT PART 121; IS INTIMIDATED BY THE FAA. THE FAA STATED THAT THIS COMPANY WOULD NOT HAVE THEIR PLTS PROTECTED FROM VIOLATION BY ANY 'NO FAULT' SAFETY RPTING SYS. RPTR FELT THAT THIS DID NOT PRODUCE A GOOD CLIMATE FOR SAFETY GROWTH. SUPPLEMENTAL INFO FROM ACN 355665: ON OUR DSCNT INTO JACKSON HOLE; WE INITIALLY BEGAN A VOR RWY 36 APCH; HOWEVER; OUR VOR NEEDLES (CAPT AND FO) WERE SWINGING +/-10 DEGS. WE BOTH AGREED THAT THE SIGNAL WAS UNRELIABLE AND CALLED CTR AND REQUESTED THE ILS RWY 18. THEY CLRED US FOR THE ILS RWY 18 AND TO CROSS THE VOR AT 15000 FT. I THEN BRIEFED THE APCH WITH THE CAPT. I SOMEHOW STATED THAT AFTER XING THE VOR WE COULD DSND TO 12000 FT OUTBOUND ON THE 004 DEG RADIAL AND THE CAPT AGREED. AS WE NEARED THE VOR THE CAPT SWITCHED RADIOS TO CALL COMPANY. I CROSSED THE VOR; INTERCEPTED THE 004 DEG RADIAL AND BEGAN A DSCNT TO 12000 FT WITH THE AUTOPLT ENGAGED. DURING THE DSCNT THE CTR CTLR CALLED. THE CAPT WHO WAS STILL ON A DIFFERENT RADIO; BECAME ALARMED WHEN I DISENGAGED THE AUTOPLT AND BEGAN CLBING. I BELIEVE THIS DEV WOULD NEVER HAVE OCCURRED IF: 1) I HAD READ AND BRIEFED THE APCH PROPERLY. 2) THE CAPT HAD READ THE APCH PROPERLY. 3) THE CAPT HAD MONITORED THE APCH MORE CLOSELY INSTEAD OF CALLING COMPANY. 4) THIS WAS AN UNFAMILIAR ARPT AND APCH TO ME; THE FIRST TIME I HAD EVER FLOWN THIS APCH. 5) FATIGUE MAY ALSO HAVE BEEN A FACTOR; BOTH OF US HAD VERY LITTLE SLEEP ON THE 2 NIGHTS OF THE 3 DAY TRIP. THIS WAS THE THIRD DAY. I'M GLAD THEY WERE MONITORING OUR APCH AND TACTFULLY POINTED OUT OUR MISTAKE. THIS COULD HAVE BEEN A MAJOR DISASTER HAD I NOT BEEN ON THE 004 DEG RADIAL SINCE THERE ARE HIGH MOUNTAINS IN THE AREA. I WILL USE THIS EXPERIENCE TO MONITOR ALL PHASES OF FLT MORE CLOSELY AND TO DOUBLECHK ANY APCH ALTS.