A LEAR 60; LNDG ON AN ICY RWY 18 SLIDES PAST THE END ONTO THE UNEVEN TERRAIN AT JAC; WY.

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UPON COMMENCING THE FULL APCH TO ILS RWY 18; WE HAD RECEIVED AWOS WHICH WAS 1200 FT OVCST; 10 MI VISIBILITY; WIND 020 DEGS AT 7 KTS; TEMP -9 DEGS C. DEWPOINT -13 DEGS C; ALTIMETER 30.19; WITH A BRAKING ACTION RPT OF FAIR. WE FIGURED THE LNDG DISTANCE DATA; AND DETERMINED THAT ILS RWY 18 WAS THE SAFEST APCH. WE WENT TO A VISUAL APCH AT APPROX 1000 FT AGL AND WE WERE CONFIGURED AND FLYING VREF; MADE A NORMAL TOUCHDOWN; WITHIN THE TOUCHDOWN ZONE. UPON LNDG AT APPROX 800-1000 FT DOWN THE RWY; WE NOTICED VERY LITTLE DECELERATION; APPLIED MAX BRAKING. ABOUT 1/2 WAY DOWN THE RWY; THE ACFT BEGAN TO SLIDE L AND R OF CTRLINE; RETURNED ACFT TO CTRLINE; WHILE CONTINUING MAX BRAKING. THE ACFT OVERRAN THE RWY BY APPROX 160 FT. THERE WERE SEVERAL CONTRIBUTING FACTORS THAT HAVE CONCLUDED THIS INCIDENT. WE HAD LATER DISCOVERED THAT THE LATEST BRAKING ACTION RPT WAS DONE AT AA16; OUR LNDG WAS MADE 3 HRS LATER. PRIOR TO US LNDG; 2 ACFT HAD DEPARTED AT APPROX AC30 AND AC45; BOTH ACFT WERE DELAYED DUE TO LOW VISIBILITY. WE WERE THE FIRST ACFT TO LAND AT JAC ON MAR/XA/00. BRAKING ACTION DURING OUR LNDG WAS NIL TO POOR. WE HAD ALSO DISCOVERED THAT THE RWY WAS 100% COVERED WITH THIN ICE. DURING THE DISCUSSION AND DECISION TO LAND AT THE JAC ARPT; WE HAD DETERMINED THAT A 'FAIR' BRAKING ACTION RPT MEANT THAT BRAKING ACTION WAS 'ADEQUATE' OR 'FAIRLY GOOD.' WE BASED OUR PRIMARY JUDGEMENT ON THAT INDICATION. HAVING KNOWN WHAT I KNOW NOW; IF BRAKING ACTION RPT HAD SAID '100% ICE ON RWY' I WOULD HAVE ASSESSED THE SIT IN A TOTALLY DIFFERENT ASPECT. I WOULD HAVE REQUESTED THAT ANOTHER BRAKING ACTION RPT BE DONE; WITH A BREAKDOWN OF MU NUMBERS; AND THAT THE RPT BE BROKEN DOWN INTO RWY PORTIONS. WHAT IS MEANT BY THAT IS: FAIR -- POOR -- FAIR. THIS INCIDENT HAS GIVEN MYSELF A NEW PERSPECTIVE WITH RWY CONDITIONS AND BRAKING ACTION RPTS. I FEEL THAT ALL PLTS SHOULD BE AWARE OF THIS SIT AND BE AWARE OF THE CHAIN OF EVENTS THAT LED MYSELF TO MAKE THE DECISIONS THAT I HAVE MADE.