Narrative:

Inbound to san on the barrt 4 arrival (STAR), we were outbound from karro intersection descending through FL180. We were cleared then 'direct to swatt intersection to intercept the localizer for runway 27, descend and maintain 12000 ft.' continuing our descent through 15000 ft about 35-40 NM northeast of swatt intersection, the captain depressed the localizer button on the MCP and continued to fly in accordance with our clearance. Approximately 35- 40 mi from swatt around 15000 ft descending, about 1 min after arming the localizer, the aircraft locked onto the localizer and turned right in an effort to capture the inbound course. It was as if the localizer at san was capturing out this far. I immediately instructed the captain to turn left and head direct to swatt as we were too far out to be on the localizer. I turned off the flight directors and autoplt and reprogrammed the FMS direct to swatt. Armed LNAV on the MCP, turned on the flight directors and autoplt. While doing this, ATC advised us that we looked like we were not heading to swatt. I advised them of the navigation equipment problem and that we were en route now. They gave us a heading (vector) to intercept, which we did. Approach and landing were uneventful. On nov/xa/98 I flew the same leg and confirmed a lock 35 NM out of san on frequency 110.9. In IMC this could be a dangerous situation and where ever the same ILS frequency is coming from, it should be changed. Callback conversation with reporter revealed the following information: reporter is convinced that they were receiving a localizer signal that was not san. He said he duplicated the problem a few days later on another flight and got the same results. He agreed that they may have armed the localizer farther out and higher than they should have.

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Original NASA ASRS Text

Title: B757 CREW DSNDING INBOUND TO SAN HAS PROB WHEN THE ARMED LOC STARTS THE ACFT IN A R TURN AWAY FROM INTENDED TRACK.

Narrative: INBOUND TO SAN ON THE BARRT 4 ARR (STAR), WE WERE OUTBOUND FROM KARRO INTXN DSNDING THROUGH FL180. WE WERE CLRED THEN 'DIRECT TO SWATT INTXN TO INTERCEPT THE LOC FOR RWY 27, DSND AND MAINTAIN 12000 FT.' CONTINUING OUR DSCNT THROUGH 15000 FT ABOUT 35-40 NM NE OF SWATT INTXN, THE CAPT DEPRESSED THE LOC BUTTON ON THE MCP AND CONTINUED TO FLY IN ACCORDANCE WITH OUR CLRNC. APPROX 35- 40 MI FROM SWATT AROUND 15000 FT DSNDING, ABOUT 1 MIN AFTER ARMING THE LOC, THE ACFT LOCKED ONTO THE LOC AND TURNED R IN AN EFFORT TO CAPTURE THE INBOUND COURSE. IT WAS AS IF THE LOC AT SAN WAS CAPTURING OUT THIS FAR. I IMMEDIATELY INSTRUCTED THE CAPT TO TURN L AND HEAD DIRECT TO SWATT AS WE WERE TOO FAR OUT TO BE ON THE LOC. I TURNED OFF THE FLT DIRECTORS AND AUTOPLT AND REPROGRAMMED THE FMS DIRECT TO SWATT. ARMED LNAV ON THE MCP, TURNED ON THE FLT DIRECTORS AND AUTOPLT. WHILE DOING THIS, ATC ADVISED US THAT WE LOOKED LIKE WE WERE NOT HDG TO SWATT. I ADVISED THEM OF THE NAV EQUIP PROB AND THAT WE WERE ENRTE NOW. THEY GAVE US A HDG (VECTOR) TO INTERCEPT, WHICH WE DID. APCH AND LNDG WERE UNEVENTFUL. ON NOV/XA/98 I FLEW THE SAME LEG AND CONFIRMED A LOCK 35 NM OUT OF SAN ON FREQ 110.9. IN IMC THIS COULD BE A DANGEROUS SIT AND WHERE EVER THE SAME ILS FREQ IS COMING FROM, IT SHOULD BE CHANGED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR IS CONVINCED THAT THEY WERE RECEIVING A LOC SIGNAL THAT WAS NOT SAN. HE SAID HE DUPLICATED THE PROB A FEW DAYS LATER ON ANOTHER FLT AND GOT THE SAME RESULTS. HE AGREED THAT THEY MAY HAVE ARMED THE LOC FARTHER OUT AND HIGHER THAN THEY SHOULD HAVE.

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.