WHILE DSNDING ON A DOWNWIND VECTOR TO THE FINAL APCH COURSE IN IMC; A B737 FLC EXECUTES A GPWS MANEUVER DUE TO A MOUNTAIN PEAK ON THE DOWNWIND VECTOR. THE PIC'S CALL TO THE TRACON DOES NOT PROVIDE A SUFFICIENT ANSWER TO HIS QUESTIONS.

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WE WERE BEING VECTORED BY SOCAL APCH FOR A LOC RWY 27 APCH. WE WERE DSNDING TO ASSIGNED ALT OF 3800 FT AND HOLDING ASSIGNED SPD OF 170 KTS WITH FLAPS 10 DEGS. WE WERE VECTORED TOWARDS THE LOC FROM THE N SIDE. WE WERE VECTORED OVER THE HIGHEST POINT ON THE APCH CHART. THE HILL IS 2791 FT HIGH AND WE CROSSED IT AT ASSIGNED ALT OF 3800 FT. WE RECEIVED A GND PROX WARNING; 'TERRAIN; TERRAIN; PULL UP; PULL UP;' WHICH WE DID. WE WERE IN IMC. WE TOLD THE CTLR AND HE SAID 'THAT HAPPENS A LOT.' WE MADE A NORMAL LNDG AND CALLED THE SUPVR. HE SAID; 'IT HAPPENS FREQUENTLY AND IT'S WORSE AT NIGHT BECAUSE THEY GET A LOT OF TELEPHONE CALLS COMPLAINING ABOUT NOISE;' BECAUSE THE JETS ARE GOING TO MAX PWR AND CLBING. I ASKED HIM; WHY DO THEY VECTOR US OVER THAT TERRAIN AT 3800 FT? HE SAID BECAUSE IT WOULD SLOW THINGS DOWN TO VECTOR US FARTHER OUT AND VECTORING US INSIDE THE HILL WOULD CAUSE SPACING PROBS. PERHAPS SOMEONE CAN EXPLAIN TO ME WHY WE CONTINUE WITH A PROC THAT CAUSES US TO HAVE TO USE EMER AUTH AND DEVIATE FROM ASSIGNED ALTS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: A TRACON SPECIALIST ADVISED THAT MOUNT SAN MIGUEL IS A KNOWN AREA FOR GPWS ALERTS. HE ADVISED USUALLY 3 GPWS ACFT ESCAPE MANEUVERS ARE OBSERVED DAILY. THE MOUNTAIN JUST HAPPENS TO BE THE PERFECT LOCATION FOR BASE LEG WHEN VECTORING FOR THE RWY 27 LOC APCH. DUE TO CLASS B REGS; ACFT VECTORED ON A N DOWNWIND ARE KEPT IN CLASS B AT 5000 FT UNTIL A SEQUENCE IS ESTABLISHED; THEN TURNED ONTO BASE LEG AND DSNDED TO THE MVA FOR THE AREA -- 3800 FT; MOUNT SAN MIGUEL. ALT ASSIGNMENT MUST ALSO TAKE INTO ACCOUNT THE CRITERION OF PUTTING THE ACFT INTO A POS TO INTERCEPT THE FINAL AT OR BELOW THE PROJECTED GS. ANOTHER CONSIDERATION IS THE VFR FLYWAY XING E OF MOUNT SAN MIGUEL. EITHER THE CTLR MUST SET THE PLT UP FOR A TIGHT TURN-ON SHORT OF THE MOUNTAIN; ABOVE THE PROJECTED GS; OR VECTOR THE ACFT WELL BEYOND THE OBSTRUCTION AT THE LOWEST MVA THAT MAY AVERT A GPWS ALERT. THE SPECIALIST ADVISED THAT PLT TECHNIQUE ALSO MUST BE TAKEN INTO ACCOUNT.