AN AIRBUS FLC RECEIVED A GPWS 'TERRAIN' WARNING; AND A TWR ISSUED ALT ALERT; ON A VISUAL APCH TO SJC.

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THE APCH CTLR ASKED US IF WE HAD THE ARPT (SAN JOSE) IN SIGHT. I TOLD HIM 'YES' AND HE CLRED US FOR A FAIRGROUNDS VISUAL TO RWY 30L. WE THEN DSNDED TO 5000 FT; THE RECOMMENDED ALT ON DOWNWIND FOR THE FAIRGROUNDS VISUAL. PRIOR TO ENTERING THE TERMINAL AREA I TOLD THE FO TO LOAD THE SJC 170 DEG RADIAL 11 MI FIX INTO THE FMS. I TOLD THE FO TO TURN R TO OVERFLY THE (SJC/170/11). IT TOOK A HEADING OF ABOUT 130 DEGS. THIS WAS A BIG MISTAKE. AS WE STARTED THE L TURN TO THE BASE LEG WE ALSO STARTED A DSCNT TO 2500 FT; THE RECOMMENDED BASE LEG ALT ON THE FAIRGROUNDS VISUAL. SHORTLY AFTER STARTING THE TURN I TOLD THE FO TO TIGHTEN UP THE TURN BECAUSE IT LOOKED TO ME LIKE WE WERE IN AN AREA OF HIGH TERRAIN. SURE ENOUGH; SHORTLY AFTER I SAID THAT WE RECEIVED A GPWS ALERT 'TOO LOW -- TERRAIN.' THE FO IMMEDIATELY PERFORMED THE GPWS ESCAPE MANEUVER. SHORTLY AFTER THE GPWS ALERT THE TWR TOLD US THEY HAD RECEIVED AN ALT ALERT ON US. WE CLBED FROM 3500 FT TO 4500 FT AND THE REST OF THE APCH WAS UNEVENTFUL. WHEN THE GPWS ALERT SOUNDED WE WERE 13 DME FROM THE SJC VOR AT 3500 FT. THE FAIRGROUNDS VISUAL TO RWY 30L IS A DANGEROUS PROC; ESPECIALLY AT NIGHT. THERE IS NO WAY THIS PROC SHOULD BE AUTH AT NIGHT. ON THE NIGHT WE FLEW THIS PROC THERE WAS NO MOONLIGHT WHATSOEVER AND WE COULD NOT SEE ANY OF THE POINTS DEPICTED ON THE APCH PLATE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR ATTEMPTED TO MEASURE THE DEPICTED BASE TURNING POINT DISTANCE UTILIZING THE APCH PLATE SCALE. IT IS UNCERTAIN IF THE DEPICTED POINT WAS INTENDED TO REPRESENT AN ACTUAL DISTANCE OR NOT; HOWEVER. IT MAY BE MISINTERPRETED AS SUCH BY USERS OF THIS APCH.