AN ACR FGT MISJUDGED THE SIDE OF THE RWY AND GOT ITS NOSE GEAR IN THE MUD TRYING TO TURN AROUND ON THE RWY.

Narrative

AFTER LNDG ON RWY 35; WE WERE INSTRUCTED TO 'MAKE A 180' ON THE RWY AND TAXI TO THE RAMP. DURING THE 180 DEG TURN ON THE RWY; THE NOSE GEAR LEFT THE RWY AND SANK INTO THE DIRT APPROX 1-2 FT. THE MINIMUM RWY WIDTH TO MAKE A 180 DEG TURN ON THIS ACFT IS 132 FT. THE RWY IS 150 FT WIDE. THIS GIVES 18 FT FOR MARGIN. WHEN I REVIEWED THE DEP NOTAMS; I NOTED THAT TXWY A WAS CLOSED. ON THE DSCNT INTO HOUSTON; I BRIEFED AN APCH TO RWY 17 SINCE THE WIND WAS RPTED CALM. AFTER ARRIVING IN THE LCL AREA; WE WERE TOLD TO EXPECT RWY 35. SINCE I WAS CONCENTRATING ON THE EFIS; I COMPLETELY FORGOT ABOUT TXWY A BEING CLOSED. WE MADE A NORMAL LNDG AND SLOWED TO TAXI SPD; EXPECTING TO TURN L AT TXWY A. AFTER PASSING TXWY B; THE TWR INSTRUCTED US TO MAKE A 180 DEG TURN AND TAXI TO THE RAMP. I MOVED THE ACFT AS FAR TO THE R EDGE OF THE RWY AS I FELT COMFORTABLE WITH AND BEGAN A HARD TURN TO THE L. DURING THE TURN; THE NOSE GEAR 'SCRUBBED' A LITTLE; HOWEVER; SINCE THE CONFINES OF THE RWY SEEMED SO CLOSE; I KEPT THE NOSE STEERING FULLY DEFLECTED. AS THE TURN PROGRESSED APPROX 160 DEGS TO 170 DEGS OF TURN; THE NOSE GEAR LEFT THE PAVEMENT BY APPROX 1-2 FT AND SANK APPROX 1-2 FT. WE CONTACTED THE TWR; OUR OPS; AND FLT CTL; THEN SHUT THE AIRPLANE DOWN. NO DAMAGE OCCURRED TO THE ACFT AND IT WAS TOWED BACK ONTO THE RWY AND BACK TO THE GATE. TIGHT 180 DEG TURNS ARE DONE VERY INFREQUENTLY AND IN THIS AIRPLANE MUST BE DONE PRECISELY TO STAY WITHIN THE 18 FT MARGIN. ATC COULD ALSO HELP PREVENT INCIDENTS LIKE THIS SINCE THEY KNEW THE TXWY WAS CLOSED; THE WIND WAS CALM; AND A HVY AIRPLANE WOULD MORE THAN LIKELY USE MOST OF THE AVAILABLE RWY.