Narrative:

On being vectored for ILS runway 8 at bur, we were given our final localizer intercept heading, and cleared to intercept the localizer. I was the PF and had the autoplt engaged. The MCP was set up in the heading mode. Once I turned to the intercept heading using the bug, I selected VOR/localizer to capture the localizer. Both the PNF and I observed localizer captured and the aircraft turning to intercept. As the aircraft approached the inbound course, I glanced at the altimeter to calculate where my altitude callouts would be made. When I looked back over to my primary flight instruments, I noticed we had flown through our inbound course, and the aircraft was in a 30 degree l-hand bank. I noticed it showed control wheel steering in the lateral mode. At approximately the same time I was calculating my callouts, the PNF was tuning in the tower frequency. We both looked up to notice the discrepancy at the same time. I immediately initiated a 30 degree bank r-hand turn back toward the inbound course. We were approximately 30 degrees off course. Before we could tell approach we were going to miss the approach, the controller immediately assigned a climb and heading. We were vectored back around and executed an uneventful ILS runway 8 and landed without incident. I feel the most important factor was the fact that I allowed myself to become distraction during a critical phase of flight. We all have seen the autoplt capture a localizer 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 approach. I am not sure why the MCP went from a VOR/localizer capture to control wheel steering mode. We later talked to the flight attendants, and they observed a passenger 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 flight. Supplemental information from acn 619585: I saw that the aircraft was still in a 30 degree left bank and the heading had gone through localizer course and was passing 030 degrees. I noticed some localizer needle modulations on the missed approach. In talking with the flight attendants, they stated that they suspected a passenger of hiding below seat level and using his cell phone during the approach. I am not certain the cause of the event but, I suspect that as the autoplt had the aircraft in a 30 degree left bank to intercept the localizer, cell phone interference caused the localizer needle to deflect full scale to the left. The autoplt continued the turn, trying to intercept the deflected needle and when the needle swung back to a full right deflection, the autoplt reverted back to control wheel steering, leaving us in a 30 degree banked left turn.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

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.