CORP ACFT LNDG ON RWY NOT PLOWED FULL WIDTH STRUCK SNOWBANK LOST CONTROL AND SLID OFF RWY.

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LAST WEATHER RECEIVED FROM TWR: 5 OB 1 MILE VIS. LS F 31/28; WIND -010/7 RVR 4000 1/2 INCH LOOSE SNOW ON RWY PLOWED 30 FT EITHER SIDE OF CTR. I WAS NOT INFORMED OF ANY IRREGULARITIES IN PLOWING OF RWY. LAST BRAKING ACTION FAIR TO POOR BY MLG MINUTES PRIOR (APPROX XB10L). VREF 134 KTS COMPUTED LNDG DISTANCE DRY 3100 FT. RWY REQUIRED PART 121 AND IBM PROCEDURES: DRY 5325 FT; FAIR BRAKING 6125 FT; AND POOR BRAKING 7100 FT. RWY AVAILABLE 11700 FT; 150 FT WIDE. A NORMAL ILS APCH WAS EXECUTED AND THE RWY WAS ACQUIRED AT APPROX 500 FT AGL. THE RWY CTRLINE LIGHTS WERE OBSCURED BY SNOW. A NORMAL TOUCH DOWN WAS MADE AT APPROX 300 FT FROM THRESHOLD AT VREF 134 KTS. RIGHT OUTSIDE TIRE 12 FT RIGHT OF CTR AND LEFT OUTSIDE TIRE 4 FT RIGHT OF CTR. IDLE REVERSE WAS SELECTED AND GND SPOILERS DEPLOYED. THE ACFT CONTINUED FURTHER RIGHT AND AT 1000 FT AFTER TOUCH DOWN THE RIGHT OUTSIDE TIRE WAS 25 FT FROM THE CTR LINE AND THE LEFT OUTSIDE TIRE WAS 17 FT FROM CTR. I PARALLELED THE RIGHT EDGE FROM THIS POINT. I WAS HAVING TROUBLE SEEING THE CTR LINE LIGHTS. THEY WERE OBSCURED BY SNOW. I WAS USING THE RIGHT PLOWED EDGE AS A REFERENCE AND HAD APPROX 10 FT OF CLRNC TO THE RIGHT TIRES. AT APPROX 1900 FT FROM TOUCH DOWN AND AT A SPEED 90 TO 100 KTS; THE RIGHT PLOWED EDGE SWUNG 11 FT TOWARD THE CTR OF THE RWY. THE RIGHT SIDE HAD BEEN PLOWED 35 FT WIDE AND NOW WENT TO 25 FT FROM THE CTR LINE. THE RIGHT TIRES IMPACTED THE PLOW BERM (8' TO 10' HIGH) AND PULLED THE ACFT INTO THE UNPLOWED AREA OF THE RIGHT SIDE OF THE RWY. BRAKES; RUDDER; AND DIFFERENTIAL REVERSE THRUST WERE INEFFECTIVE IN CTLING THE ACFT AND IT EXITED THE RIGHT SIDE OF THE RWY AND CONTINUED SEVERAL HUNDRED FEET THROUGH SNOW; SOFT TURF; AND MUD. FULL REVERSE THRUST WAS USED. THE ACFT CAME TO REST APPROX 75 FT FROM THE RIGHT SIDE OF THE RWY EDGE. THE ENGINES WERE SHUT DOWN USING THE FIRE 'T' HANDLES; THE TWR WAS NOTIFIED AND CRASH EQUIP WAS REQUESTED. INVERTORS AND BATTERIES WERE SHUT OFF AND THE CREW ESCORTED THE PASSENGERS FROM THE ACFT USING THE MAIN ENTRANCE DOOR. DAMAGE: TWO LNDG LIGHTS WERE CRACKED. INJURIES: NONE. I FEEL THE UNEVEN PLOWING OF THE RIGHT SIDE OF THE RWY AND THE DRASTIC INTRUSION OF THE RIGHT SIDE PLOW BERM INTO OUR LNDG PATH CAUSED LOSS OF CTL OF THE ACFT. HAD THE PLOW LINE BEEN UNIFORM OR HAD I BEEN INFORMED OF THE EXISTING CONDITION; I FEEL THIS UNFORTUNATE INCIDENT COULD HAVE BEEN AVOIDED. SUPPLEMENTAL INFORMATION FORM ACN 81607: SNOW REMOVAL HAD BEEN IN PROGRESS FOR SEVERAL HRS. THIS INCIDENT COULD HAVE BEEN AVOIDED HAD THE SNOW REMOVAL BEEN DONE CORRECTLY. THE MAIN CONTRIBUTING FACTOR WAS INCONSISTENT PLOWING; GRADUALLY NARROWING THE RWY WIDTH.