Narrative:

Navigation interference. Over cht, cleared '10 degrees right intercept localizer runway 31L plan circle runway 22L.' upon tuning localizer frequency and setting course, it appeared we were on the localizer, although visually we appeared south of course. ATC asked if we had intercepted and said we were south of course. The CDI then swung full scale to the other side indicating we were north of course. I turned to center the CDI and we switched to tower. My CDI swung right indicating we were south of course. I noticed the first officer's CDI was swinging the same direction as mine, but moving about half as far. When we saw the runway, we were north of course with CDI's indicating we were south of course. We were high and well north of course when tower asked if we could get down from there. We asked to be turned out to re-enter the pattern. Upon turning outbound we made a PA asking people to please make sure their cell phones and other equipment were turned off. The CDI's immediately became steady and we completed a normal ILS runway 31C circle runway 22L with normal indications and the flight attendants reported that a woman in the forward lounge was talking on her cell phone. As soon as she turned her phone off, our CDI indicated normally.

Google
 

Original NASA ASRS Text

Title: A B737-500 FLC HAS TO ASK FOR A SECOND APCH WHEN THE ILS LOC AND GS FOR RWY 31L IS SCALLOPING AT MDW, IL.

Narrative: NAV INTERFERENCE. OVER CHT, CLRED '10 DEGS R INTERCEPT LOC RWY 31L PLAN CIRCLE RWY 22L.' UPON TUNING LOC FREQ AND SETTING COURSE, IT APPEARED WE WERE ON THE LOC, ALTHOUGH VISUALLY WE APPEARED S OF COURSE. ATC ASKED IF WE HAD INTERCEPTED AND SAID WE WERE S OF COURSE. THE CDI THEN SWUNG FULL SCALE TO THE OTHER SIDE INDICATING WE WERE N OF COURSE. I TURNED TO CTR THE CDI AND WE SWITCHED TO TWR. MY CDI SWUNG R INDICATING WE WERE S OF COURSE. I NOTICED THE FO'S CDI WAS SWINGING THE SAME DIRECTION AS MINE, BUT MOVING ABOUT HALF AS FAR. WHEN WE SAW THE RWY, WE WERE N OF COURSE WITH CDI'S INDICATING WE WERE S OF COURSE. WE WERE HIGH AND WELL N OF COURSE WHEN TWR ASKED IF WE COULD GET DOWN FROM THERE. WE ASKED TO BE TURNED OUT TO RE-ENTER THE PATTERN. UPON TURNING OUTBOUND WE MADE A PA ASKING PEOPLE TO PLEASE MAKE SURE THEIR CELL PHONES AND OTHER EQUIP WERE TURNED OFF. THE CDI'S IMMEDIATELY BECAME STEADY AND WE COMPLETED A NORMAL ILS RWY 31C CIRCLE RWY 22L WITH NORMAL INDICATIONS AND THE FLT ATTENDANTS RPTED THAT A WOMAN IN THE FORWARD LOUNGE WAS TALKING ON HER CELL PHONE. AS SOON AS SHE TURNED HER PHONE OFF, OUR CDI INDICATED NORMALLY.

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.