ACFT FOLLOWS PLOWED TXWY ONLY TO GET STUCK IN THE MUD. TXWY PLOWING DOES NOT FOLLOW ACTUAL TXWY.

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GSO WAS RECOVERING FROM A RECENT BLIZZARD. WE ARRIVED IN THE LATE AFTERNOON WITH TEMP OF 40 DEGS F. OUR CARGO WAS DELAYED BY BAD ROAD CONDITIONS SO THE ACFT WAS NOT LOADED UNTIL TWO OR SO HRS AFTER SUNSET. WE RECEIVED TAXI INSTRUCTIONS VIA A TXWY THAT HAD BEEN PLOWED; BUT UNUSED; GND REQUESTED A BRAKING ACTION RPT. WHILE THE TXWY HAD BEEN PLOWED; THERE WAS ABOUT 1 INCH OF HARD-PACKED SNOW COVERING IT. AS WE PROCEEDED DOWN THE TXWY I FELT THE ACFT PULL TO THE R; L TURN COMMANDS WITH NOSEWHEEL STEERING WERE COMPLETELY INEFFECTUAL AS WELL AS THE LIMITED DIFFERENTIAL PWR THE ENGS PRODUCE IN LOW (GND) RPM. I STOPPED THE ACFT BECAUSE IT WAS HEADED FOR THE EDGE OF THE TXWY. I THEN SHIFTED THE ENGS TO NORMAL (HIGH) RPM WHERE FULL PWR IS AVAILABLE. AFTER TRYING SEVERAL METHODS (POWERING-OUT FORWARD; MAX REVERSE DIFFERENTIAL PWR); IT WAS OBVIOUS THAT WE WERE STUCK. THIS WAS HARD TO ACCEPT SINCE WE WERE DEAD CTR ON THE PLOWED TXWY. AFTER CONTACTING GND AND REQUESTING ASSISTANCE; I PONDERED THE PREDICAMENT I WAS IN. IT BECAME APPARENT THAT THE TXWY WAS NOT PLOWED DOWN THE CTRLINE; BUT ABOUT 15 FT OR 20 FT R OF CTR. LATER INSPECTION ON THE GND PROVED THIS TO BE SO. THE TRACK OF THE R MAIN TIRES SHOWED THAT EVEN THOUGH THE ACFT WAS IN THE CTR OF THE PLOWED TXWY; THE R MAIN WAS NEVER ON THE TXWY. THE R SIDE TRAVELED OVER AN AREA OF GRAVEL 'SUB- STRAIGHT' WHICH SUPPORTED THE WEIGHT; BUT WHEN THE GRAVEL ENDED; THE UNDERLYING MUD COULD NOT SUPPORT THE WEIGHT AND THAT'S WHERE WE STOPPED. ANOTHER FACTOR WAS THE TXWY WAS UNSAFE TO WALK ON LET ALONE TAXI ON. THE HARD PACKED SNOW HAD STARTED TO MELT IN THE WARMTH OF THE DAY AND REFROZE WHEN THE SUN SET MAKING IT A SHEET OF ICE. THE ACFT COULD NOT BE EXTRACTED UNTIL THE FOLLOWING DAY WHEN THE ARPT AUTH CHEMICALLY DEICED THE TXWY. I AM BASED IN THE GREAT LAKES AREA AND I HAVE FLOWN IN WINTER CONDITIONS ALL OF MY FLYING CAREER. IT NEVER ENTERED MY MIND THAT THE TXWY WOULD BE PLOWED OFF CTR (NOW IT WILL). THIS IS A GOOD EXAMPLE OF THE MANY 'TRAPS' OUT WAITING TO GET YOU.