AN ACR LGT CREW; WITH A 'GLASS' COCKPIT; RESPONDED TO A TCASII ALERT TO DSND WHILE ON A PARALLEL VISUAL APCH TO SFO.

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ON APCH INTO SFO; WE WERE ESTABLISHED ON SFO 095 DEGS R FOR A VISUAL TO RWY 28R. WE WERE THEN CLRED FOR A VISUAL TO 28L TO FOLLOW AN ACR WHICH WE HAD IN SIGHT. FOR SPACING ON HIM; WE WERE INSTRUCTED TO REDUCE TO FINAL SPD RIGHT BEFORE THE SAN MATEO BRIDGE; AND DID SO. A TA WAS GIVEN TO US OF AN ACFT IN OUR 5 O'CLOCK 1 MI ON A VISUAL TO 38R; A COMMUTER; THAT HAD US IN SIGHT. WE THEN RECEIVED A TCASII RA TO 'DSND; DSND.' WE DID COMPLY. IN FACT; THE RATE REQUIRED TO FLY TO THE 'GREEN ARC' WAS SUCH THAT WE RECEIVED A GPWS WARNING OF OUR HIGH DSCNT RATE. THEN TO OUR AMAZEMENT; AND IN DISBELIEF; THE COMMUTER APPEARED AT OUR 1-2 O'CLOCK HIGH; PASSING US AND DIVING STEEPLY ON HIS VISUAL. WE ESTIMATED HIS SPACING TO BE 500-1000 FT. TO TOP IT OFF; WITH ALL THESE RA'S AND GPWS WARNINGS; VISUAL SIGHTINGS; KEEPING OUR TFC IN SIGHT; MANUALLY FLYING A VISUAL TO A RWY CHANGE (SELECTING NEW ILS FREQ FOR GLIDE PATH GUIDANCE); WE WERE GIVEN A FREQ CHANGE TO TWR! THE ACTION OF THIS COMMUTER IS TYPICAL OF WHAT CAN HAPPEN AT SFO WITH THEIR MARGINALLY SAFE VISUAL APCH POLICIES. NO ACFT SHOULD EVER PASS ANOTHER AT SFO BECAUSE THE RWYS ARE TOO CLOSE TOGETHER!