L1011 FLC HAS TXWY EXCURSION AND L MAIN GEAR BECAME STUCK OFF THE PAVEMENT AT DTW.

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UPON TAXI IN TO RAMP AREA; L MAIN TRUCK LEFT SNOW COVERED TXWY. AFTER LNDG AT DTW; CREW WAS INSTRUCTED TO TAXI FROM RWY 4L VIA TXWY V; TXWY A; TXWY Z12 TO MAINT RAMP AT HANGAR. ALL TXWYS WERE COVERED WITH PATCHY SNOW AND ICE AND CTRLINES WERE NOT VISIBLE BUT LIGHTED ON EDGE. UPON TURNING FROM TXWY Z TO TXWY Z12; I REFED THE MARSHALER WHO WAS STANDING ACROSS A PLOWED RAMP IN FRONT OF HANGAR. HE WAS STANDING LINED UP WITH THE THROAT OF THE PLOWED TXWY BUT WE WERE GOING TO TURN ONTO AN UNPLOWED (5 INCHES SNOW APPROX) SECTION. APPARENTLY; THE COVERED SECTION HAD A SLIGHT JOG TO IT AND DID NOT HAVE A WIDENED THROAT AREA ON ONE SIDE AS IT DID ON THE OTHER. MY TURN WAS TO BE 90 DEGS TO THE L. AFTER DRIVING THE ACFT OUT OVER WHAT I THOUGHT WAS THE GRASS; I BEGAN THE L TURN. APPARENTLY; WHAT I THOUGHT WAS GRASS WAS ACTUALLY SNOW COVERED PAVEMENT DUE TO THE JOG AND MY OBSCURED CTRLINE REF. I TURNED THE ACFT THROUGH 70-80 DEGS AND AT THAT POINT THE L TRUCK DEPARTED THE TXWY. NORMAL SHUTDOWN OCCURRED WITH APPROPRIATE CHKLIST. PAX AND BAGS WERE DEPLANED AND WITHOUT INCIDENT. NO ACTIVE TXWYS WERE BLOCKED. ACFT WAS EVENTUALLY EXTRACTED AFTER MANY HRS WITHOUT DAMAGE TO ACFT OR PROPERTY.