AN ACR MLG SLID OFF THE SIDE OF THE RWY IN A SNOW STORM.

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CLRED FOR APCH TO CAT 1 RWY WITH RVR 2000 FT RWY 36. ATIS WINDS 34/18. UNPLOWED RWY. BRAKING ACTION RPTED AS 'POOR' BY COMMUTER AIRPLANE. THE RWY WAS SIGHTED STRAIGHT AHEAD WHEN WE REACHED THE APCH MINIMUMS. THE CAPT WAS FLYING AND TOUCHED DOWN IN THE TOUCHDOWN ZONE ON RWY CTRLINE WITHOUT ANY CRAB. AUTOBRAKES WERE USED AND BOTH THRUST REVERSERS WERE DEPLOYED IMMEDIATELY UPON TOUCHDOWN. THE ACFT BEGAN TO SKID AT AN ANGLE OF ABOUT 15 DEGS WITH THE RWY CTRLINE (NOSE TURNED L INTO THE WIND). THEN THE ACFT STARTED TO DRIFT TOWARD THE R SIDE OF THE RWY. THE CAPT REDUCED THE REVERSE THRUST TO STOP THE RIGHTWARD DRIFT. THE TOTAL BRAKING EFFECT WAS 'NIL' TO THIS POINT. NOSEWHEEL STEERING WAS INEFFECTIVE. THE ACFT STRAIGHTENED SLIGHTLY AND STARTED MOVING TOWARD THE L SIDE OF THE RWY. THE CAPT DECIDED TO USE FORWARD THRUST IN AN ATTEMPT TO STRAIGHTEN THE ACFT. THE ACFT DID STRAIGHTEN SLIGHTLY BUT CONTINUED MOVING TOWARD THE L AND FINALLY LEFT THE RWY ABOUT 3500 FT TO 4500 FT DOWN THE RWY. THE ACFT WENT THROUGH 1 GRASS AREA; CROSSED AN INTERSECTING RWY AND THEN QUICKLY STOPPED IN A GRASS AREA AS THE GEAR OF THE ACFT EMBEDDED IN THE MUD. THE CAPT STARTED THE APU AND SHUT DOWN BOTH ENGS. THE ACFT WAS COMPLETELY INTACT SO THE CAPT DECIDED NOT TO EVAC. I NOTIFIED ATC OF OUR SIT AND POS. I THEN NOTIFIED OUR COMPANY OPS AS THE CAPT INSTRUCTED THE FLT ATTENDANTS AND PAX. PAX WERE DEPLANED VIA PORTABLE STAIRS AND BUSSED TO THE TERMINAL. THE CAPT AND I ACCOMPLISHED THE AFTER LNDG AND PARKING-AND- SECURING CHKLISTS AS WE WAITED FOR PAX TO BE DEPLANED. EMER VEHICLES WERE PRESENT ON THE SCENE. BRAKING ACTION RPTS FROM PRIOR LNDG AIRPLANES ARE JUDGEMENT CALLS BASED ON EXPERIENCE. ACCEPTING OTHER PLT'S ASSESSMENTS ARE BASED ON TRUST. IN OUR CASE; WHAT WAS 'POOR' TO A COMMUTER ACFT WAS 'NIL' TO OUR 110;000 LB TURBOJET. WHEN ARPT AUTHORITIES LEAVE UNPLOWED RWYS OPEN FOR LNDG WHILE SNOW CONTINUES TO FALL AND THEN DEPEND UPON 'GUINEA PIG' LNDG ACFT TO PROVIDE BRAKING RPTS; EVENTUALLY ONE; LIKE US; WILL LET THEM KNOW IT IS TIME TO CLOSE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS WAS CLASSIFIED AS AN 'INCIDENT' BY THE FAA AND NTSB AS THERE WERE NO INJURIES AND MINIMAL DAMAGE. THERE WERE A FEW POPPED RIVETS IN THE NOSEWHEEL AREA AND SOME ENG FAN BLADE DAMAGE THAT MIGHT NOT HAVE BEEN CAUSED BY THIS INCIDENT. THERE WAS A RIPPLE IN THE FUSELAGE THAT MAY HAVE BEEN CAUSED BY PULLING THE ACFT OUT OF THE MUD OR GOING INTO THE MUD OR MAY HAVE BEEN ALREADY THERE. BOTH CREWMEN WERE GIVEN A '609' SIMULATOR CHKRIDE WITH THE FAA AS AN OBSERVER. MANY OF THE NORMAL PROCS WERE NOT FOLLOWED: NO DRUG TEST WAS SCHEDULED; THE TWR DID NOT NOTIFY FSDO OF THE INCIDENT; ATC WX WAS NOT COMPLETE. THERE WAS AN FAA INSPECTOR OF SOME SORT ON THE ACFT AS A PAX WITH HIS WIFE WHO NOTIFIED FSDO OF THE INCIDENT. THIS WAS THE SECOND APCH TO THE ARPT FOR THIS ACFT.300 FT AGL WAS THE FIRST MENTION OF SNOW ON THE RWY; NO MENTION OF GUSTS; AND THE BRAKING ACTION RPTS FOCUSED ON TXWYS RATHER THAN THE RWY. THE RPTING ACFT TRAVELED ABOUT 1 MI DOWN THE RWY AND EXITED THE SIDE OF THE RWY AT 80 TO 100 KTS. SUPPLEMENTAL INFO FROM ACN 236112: IN RETROSPECT; THE RWY SHOULD HAVE BEEN CLOSED DUE TO THE SNOW ACCUMULATION AND LACK OF BRAKING ACTION.