Narrative:

We were IMC being vectored for the localizer to runway 27 at san. Socal approach told us to fly a 250 degree heading to intercept the localizer and to maintain 4000 ft. He told us that due to terrain we would not be given lower than 4000 ft until established on the localizer. Our GPWS issued a 'terrain, terrain, pull up' warning. The first officer disconnected the autoplt, went to toga thrust, and climbed to 6000 ft. I told socal what had happened and requested vectors back to the east to another approach to runway 27. The airplane was landed without further incident. According to TCASII information, there was no conflict with other traffic. We made a logbook write-up and maintenance checked the airplane's GPWS for any malfunction. Supplemental information from acn 433086: the GPWS warning appears to be a software anomaly, peculiar to the localizer 27 approach to san, because of mountainous terrain (san miguel mountain) near the swatt intersection. Callback conversation with reporter revealed the following information: the reporter said that the flight crew discussed the high terrain in the vicinity of the final approach course, but felt that their original intercept heading was going to avoid most of the potential terrain conflict. However, the new intercept heading took them directly over a small mountain triggering the warning. The crew had little choice, but to respond due to the night, IMC situation. Maintenance personnel checked the GPWS and it performed as designed. The flight crew's airline is in the process of fitting the enhanced GPWS to their entire fleet and this more capable system will take into account the aircraft's planned trajectory, therefore, reducing these unnecessary warnings.

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Original NASA ASRS Text

Title: AN ACR A320 FLC RECEIVED A GPWS TERRAIN WARNING WHEN THEY WERE VECTORED OVER A MOUNTAIN WHILE ON APCH TO RWY 27 AT SAN.

Narrative: WE WERE IMC BEING VECTORED FOR THE LOC TO RWY 27 AT SAN. SOCAL APCH TOLD US TO FLY A 250 DEG HDG TO INTERCEPT THE LOC AND TO MAINTAIN 4000 FT. HE TOLD US THAT DUE TO TERRAIN WE WOULD NOT BE GIVEN LOWER THAN 4000 FT UNTIL ESTABLISHED ON THE LOC. OUR GPWS ISSUED A 'TERRAIN, TERRAIN, PULL UP' WARNING. THE FO DISCONNECTED THE AUTOPLT, WENT TO TOGA THRUST, AND CLBED TO 6000 FT. I TOLD SOCAL WHAT HAD HAPPENED AND REQUESTED VECTORS BACK TO THE E TO ANOTHER APCH TO RWY 27. THE AIRPLANE WAS LANDED WITHOUT FURTHER INCIDENT. ACCORDING TO TCASII INFO, THERE WAS NO CONFLICT WITH OTHER TFC. WE MADE A LOGBOOK WRITE-UP AND MAINT CHKED THE AIRPLANE'S GPWS FOR ANY MALFUNCTION. SUPPLEMENTAL INFO FROM ACN 433086: THE GPWS WARNING APPEARS TO BE A SOFTWARE ANOMALY, PECULIAR TO THE LOC 27 APCH TO SAN, BECAUSE OF MOUNTAINOUS TERRAIN (SAN MIGUEL MOUNTAIN) NEAR THE SWATT INTXN. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THAT THE FLC DISCUSSED THE HIGH TERRAIN IN THE VICINITY OF THE FINAL APCH COURSE, BUT FELT THAT THEIR ORIGINAL INTERCEPT HDG WAS GOING TO AVOID MOST OF THE POTENTIAL TERRAIN CONFLICT. HOWEVER, THE NEW INTERCEPT HDG TOOK THEM DIRECTLY OVER A SMALL MOUNTAIN TRIGGERING THE WARNING. THE CREW HAD LITTLE CHOICE, BUT TO RESPOND DUE TO THE NIGHT, IMC SIT. MAINT PERSONNEL CHKED THE GPWS AND IT PERFORMED AS DESIGNED. THE FLC'S AIRLINE IS IN THE PROCESS OF FITTING THE ENHANCED GPWS TO THEIR ENTIRE FLEET AND THIS MORE CAPABLE SYS WILL TAKE INTO ACCOUNT THE ACFT'S PLANNED TRAJECTORY, THEREFORE, REDUCING THESE UNNECESSARY WARNINGS.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.