SNOW COVERED RWY; BRAKING ACTION RPTED AS FAIR; BUT POOR AT THE END. ACFT HVY AND CARRIED EXTRA SPD ON FINAL DUE TO TURB. ACFT DECELERATED SLOWLY AND BY THE LAST THIRD OF RWY WAS NIL. CAPT ATTEMPTED TO STEER ACFT ONTO TXWY S1; THE LAST TXWY; BUT ACFT SLID OFF AT TXWY S1.

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I HAVE NEVER BEEN TRAINED ON HOW TO LAND ON A SLIPPERY RWY. I HAVE SINCE BEEN TRAINED (DURING A SIMULATOR SESSION) TO USE MAX BRAKING RIGHT AWAY; NOT TO WAIT UNTIL 100 KTS. AND DURING THE ROLLOUT; AFTER THE 80 KTS CALL; DO NOT COME OUT OF REVERSE. USE AS MUCH REVERSE THRUST AS IS NEEDED. DEP FROM ST LOUIS WITH COLD WX; LIGHT SNOW AND DEICING REQUIRED -- NORMAL. ENRTE -- NORMAL. FORECAST VISIBILITY WAS BELOW 1 MI AND ORD'S ATIS RPTED 1 MI VISIBILITY WITH LIGHT SNOW AND FOG. CAPT (PF) ELECTED TO FLY A MONITORED CAT 1 APCH TO RWY 22L. OUR LNDG WT WAS 149000 LBS AND THE WINDS WERE GUSTY. BUG SPD WAS 132 KTS. I FLEW THE AUTOPLT DOWN WITH BUG PLUS 10-15 KTS BECAUSE OF THE GUSTY WINDS. THE AUTOPLT FLEW THE APCH WELL. CAPT TOOK CTL AT 300 FT AGL. HE RELEASED THE AUTOPLT AT APPROX 100 FT; AND FLEW THE ACFT DOWN TO MAKE A NORMAL LNDG. NORMAL ROLLOUT WITH REVERSE THRUST AND NORMAL DECELERATION FOLLOWED. THE FIRST 1/3 OF THE RWY APPEARED TO BE SNOW COVERED; MIDDLE 1/3 WAS PATCHY SNOW AND LAST 1/3 WAS SNOW COVERED. I MADE THE 80 KT CALL AND SAW THE LAST HIGH SPD TURNOFF COME AND GO. THEN I SAW THE SNOW COVERED RWY WHICH DIDN'T APPEAR TO HAVE ANY TIRE MARKS IN THE SNOW. AFTER MY 80 KT CALL; CAPT EASED THE THROTTLES TOWARDS REVERSE IDLE. BRAKING ACTION SEEMED NORMAL UNTIL WE ROLLED ONTO THE SNOW COVERED LAST 1/3 OF THE RWY. AT THIS POINT CAPT WAS EASING THE THROTTLES BACK INTO REVERSE THRUST. I COULD FEEL THE BRAKES GRAB VERY SLIGHTLY AND THEN RELEASE NUMEROUS TIMES. I COULD SEE THE END OF THE RWY AHEAD. CAPT BEGUN A TURN TO THE R; TOWARDS THE LAST TURNOFF. THE ACFT ONLY TURNED APPROX 20 DEGS R; THEN SLID STRAIGHT AHEAD AND OFF THE TXWY AT APPROX 5-10 KTS. I AUTOMATICALLY STARTED APPLYING BRAKES ABOUT THE TIME CAPT STARTED THE TURN. WE CAME TO REST ON A FLAT GRASSY SURFACE. CAPT IMMEDIATELY PULLED ALL FUEL LEVERS TO FUEL CUTOFF AND ALL T-HANDLES. WE THEN OPENED THE COCKPIT DOOR TO CHK WITH THE FLT ATTENDANTS AND SEE HOW THE PAX WERE DOING. ALL WAS OK. AS WE DEPARTED THE TXWY; I RADIOED THE TWR TO SAY WE SLID OFF THE RWY AND INTO THE GRASS AND THAT WE NEEDED THE EMER EQUIP. WITHIN A FEW MINS THE FIRST EMER EQUIP ARRIVED AND WE SWITCHED TO GND CTL TO TALK TO THEM. THERE WERE NO ABNORMALITIES -- SMOKE; FIRE; ETC. WE CALLED FOR BUSES AND AIRSTAIRS.