A300 FLC ON FAIRGROUNDS VISUAL APCH TO SJC RWY 31 TURNED TOWARD HIGH TERRAIN AND RECEIVED A GPWS WARNING.

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WE WERE BEING VECTORED UP THE WESTERN SIDE OF THE SAN FRANCISCO BAY AT 6000 FT; HDG 110 DEGS; IN THE VICINITY OF THE SJC 210 DEG RADIAL. THE BAY APCH CTLR ASKED US IF WE HAD THE ARPT IN SIGHT. WE CONFIRMED THE ARPT IN SIGHT AND HE CLRED US FOR A FAIRGROUNDS VISUAL TO SJC RWY 30L. AT THE TIME WE WERE BTWN THE INITIAL FIX OF THE FAIRGROUNDS VISUAL AND RWY 30L. WE DSNDED TO 5000 FT; THE RECOMMENDED ALT ON DOWNWIND FOR THE FAIRGROUNDS VISUAL APCH PRIOR TO ENTERING THE TERMINAL AREA. WE HAD PREVIOUSLY INPUT THE SJC 170 DEG RADIAL FOR 10.5 MI AS A PBD IN THE FMS TO HELP ORIENT OURSELVES ON THE VISUAL. WHEN CLRED FOR THE FAIRGROUNDS VISUAL; THE CAPT DIRECTED ME TO TURN R TO GO DIRECT TO THE PBD; APPROX 130 DEGS. IN THE VICINITY OF THE PBD; WE WERE SWITCHED TO SJC TWR. WE CHKED IN AND TOLD THE TWR CTLR WE WERE TURNING L BASE TO RWY 30L. HE THEN CLRED US TO LAND. IN THE BASE TURN WE STARTED DSNDING FROM 5000 FT TO 2500 FT AS DIRECTED ON THE FAIRGROUNDS VISUAL. SHORTLY AFTER STARTING THE TURN; THE CAPT DIRECTED ME TO TIGHTEN UP THE BASE TURN SINCE WE WERE OVER HIGH TERRAIN. ABOUT 3500 FT; THE GPWS WARNING ACTIVATED AND I IMMEDIATELY EXECUTED THE ESCAPE MANEUVER. ABOUT THE SAME TIME; THE CTLR GAVE US AN ALT ALERT. WE CLBED UP TO 4500 FT AND THE GPWS WENT SILENT. WHEN THE ALERT SOUNDED; WE WERE APPROX 13 DME FROM THE SJC VORTAC. THE FAIRGROUNDS VISUAL TO RWY 30L IS A DANGEROUS PROC; ESPECIALLY AT NIGHT. IF ONE FOLLOWS RECOMMENDED ROUTING AND OVERFLIES THE SJC 170 DEG RADIAL 10.5 DME; HE WILL BE IN TROUBLE WITH THE TERRAIN AROUND MOUNT UMUNHUM. THERE IS NO WAY THIS PROC SHOULD BE FLOWN AT NIGHT. ON DARK NIGHTS; YOU CANNOT SEE ANY OF THE POINTS DEPICTED ON THE APCH PLATE: THE CEMENT PLANT; PWR XMISSION LINES; LEXINGTON RESERVOIR; OR MOUNT UMUNHUM -- ESPECIALLY WITH THE MINIMUM RECOMMENDED WX OF 2500 FT 5 MI.