A B737-700 AUTOPLT GOES INTO CTL WHEEL STEERING MODE DURING AN ILS-LOC COURSE INTERCEPTION. FLT CREW EXPERIENCES A HDG TRACK DEV AND A MISSED APCH FROM SCT CTLR 9 MI W OF BUR; CA.

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ON BEING VECTORED FOR ILS RWY 8 AT BUR; WE WERE GIVEN OUR FINAL LOC INTERCEPT HDG; AND CLRED TO INTERCEPT THE LOC. I WAS THE PF AND HAD THE AUTOPLT ENGAGED. THE MCP WAS SET UP IN THE HDG MODE. ONCE I TURNED TO THE INTERCEPT HDG USING THE BUG; I SELECTED VOR/LOC TO CAPTURE THE LOC. BOTH THE PNF AND I OBSERVED LOC CAPTURED AND THE ACFT TURNING TO INTERCEPT. AS THE ACFT APCHED THE INBOUND COURSE; I GLANCED AT THE ALTIMETER TO CALCULATE WHERE MY ALT CALLOUTS WOULD BE MADE. WHEN I LOOKED BACK OVER TO MY PRIMARY FLT INSTS; I NOTICED WE HAD FLOWN THROUGH OUR INBOUND COURSE; AND THE ACFT WAS IN A 30 DEG L-HAND BANK. I NOTICED IT SHOWED CTL WHEEL STEERING IN THE LATERAL MODE. AT APPROX THE SAME TIME I WAS CALCULATING MY CALLOUTS; THE PNF WAS TUNING IN THE TWR FREQ. WE BOTH LOOKED UP TO NOTICE THE DISCREPANCY AT THE SAME TIME. I IMMEDIATELY INITIATED A 30 DEG BANK R-HAND TURN BACK TOWARD THE INBOUND COURSE. WE WERE APPROX 30 DEGS OFF COURSE. BEFORE WE COULD TELL APCH WE WERE GOING TO MISS THE APCH; THE CTLR IMMEDIATELY ASSIGNED A CLB AND HDG. WE WERE VECTORED BACK AROUND AND EXECUTED AN UNEVENTFUL ILS RWY 8 AND LANDED WITHOUT INCIDENT. I FEEL THE MOST IMPORTANT FACTOR WAS THE FACT THAT I ALLOWED MYSELF TO BECOME DISTR DURING A CRITICAL PHASE OF FLT. WE ALL HAVE SEEN THE AUTOPLT CAPTURE A LOC NUMEROUS TIMES WITHOUT INCIDENT; AND I FEEL THIS LEAD ME TO TAKE IT FOR GRANTED. I SHOULD HAVE DONE ALL MY CALCULATIONS BEFORE BEGINNING THE APCH. I AM NOT SURE WHY THE MCP WENT FROM A VOR/LOC CAPTURE TO CTL WHEEL STEERING MODE. WE LATER TALKED TO THE FLT ATTENDANTS; AND THEY OBSERVED A PAX WHO MAY HAVE BEEN USING THEIR CELL PHONE. I SUPPOSE RADIO INTERFERENCE IS POSSIBLE. I BELIEVE THIS INCIDENT HAS TAUGHT ME TO TAKE MY ALREADY HEIGHTENED SENSE OF AWARENESS TO A HIGHER LEVEL. ESPECIALLY DURING A CRITICAL PHASE OF FLT. SUPPLEMENTAL INFO FROM ACN 619585: I SAW THAT THE ACFT WAS STILL IN A 30 DEG L BANK AND THE HDG HAD GONE THROUGH LOC COURSE AND WAS PASSING 030 DEGS. I NOTICED SOME LOC NEEDLE MODULATIONS ON THE MISSED APCH. IN TALKING WITH THE FLT ATTENDANTS; THEY STATED THAT THEY SUSPECTED A PAX OF HIDING BELOW SEAT LEVEL AND USING HIS CELL PHONE DURING THE APCH. I AM NOT CERTAIN THE CAUSE OF THE EVENT BUT; I SUSPECT THAT AS THE AUTOPLT HAD THE ACFT IN A 30 DEG L BANK TO INTERCEPT THE LOC; CELL PHONE INTERFERENCE CAUSED THE LOC NEEDLE TO DEFLECT FULL SCALE TO THE L. THE AUTOPLT CONTINUED THE TURN; TRYING TO INTERCEPT THE DEFLECTED NEEDLE AND WHEN THE NEEDLE SWUNG BACK TO A FULL R DEFLECTION; THE AUTOPLT REVERTED BACK TO CTL WHEEL STEERING; LEAVING US IN A 30 DEG BANKED L TURN.