Narrative:

We were given a 240 degree vector at 3800' to intercept the altitude 26R ILS course. The captain was flying the aircraft using the autoplt with the navigation/localizer function selected for an automatic intercept. His (the captain's) VOR receiver was properly tuned to 110.1 with 272 degrees in his course selector. I personally tuned and idented his NAVAID as well as redan, the marker. My NAVAID was left on atl (116.9) to receive DME information to properly identify balli, the ATC published intercept point. Though ATIS was calling 20 mi visibility, the haze gave limited ground references (airport) at 10 DME. ATC called informing us that we were overshooting 26R and were getting close to the approach path for 27L. We initiated a turn 25-30 degrees back to 26R and completed the approach. At the time of the ATC call the captain's INS still showed him well right of course. I immediately switched to 110.1 (ILS frequency) and it indicated well left of course. ATC advised that we needed to have the INS checked. At the gate we had maintenance check the INS. At that time I noticed the captain/first officer guarded INS switch was in the first officer position, which would explain the indications on the captain's INS. He was monitoring the atl VOR instead of the ILS. How the switch got in the open/first officer position is still a mystery. It is possible maintenance switched it, but that wouldn't explain the approach and the subsequent operational check of both vors. I suggest that the switch be safety wired to prevent future problems with it.

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Original NASA ASRS Text

Title: HEADING AND TRACK DEVIATION OCCURS ACCOUNT OF COCKPIT SWITCH SET UP DURING APCH PHASE.

Narrative: WE WERE GIVEN A 240 DEG VECTOR AT 3800' TO INTERCEPT THE ALT 26R ILS COURSE. THE CAPT WAS FLYING THE ACFT USING THE AUTOPLT WITH THE NAV/LOC FUNCTION SELECTED FOR AN AUTOMATIC INTERCEPT. HIS (THE CAPT'S) VOR RECEIVER WAS PROPERLY TUNED TO 110.1 WITH 272 DEGS IN HIS COURSE SELECTOR. I PERSONALLY TUNED AND IDENTED HIS NAVAID AS WELL AS REDAN, THE MARKER. MY NAVAID WAS LEFT ON ATL (116.9) TO RECEIVE DME INFO TO PROPERLY IDENT BALLI, THE ATC PUBLISHED INTERCEPT POINT. THOUGH ATIS WAS CALLING 20 MI VISIBILITY, THE HAZE GAVE LIMITED GND REFS (ARPT) AT 10 DME. ATC CALLED INFORMING US THAT WE WERE OVERSHOOTING 26R AND WERE GETTING CLOSE TO THE APCH PATH FOR 27L. WE INITIATED A TURN 25-30 DEGS BACK TO 26R AND COMPLETED THE APCH. AT THE TIME OF THE ATC CALL THE CAPT'S INS STILL SHOWED HIM WELL RIGHT OF COURSE. I IMMEDIATELY SWITCHED TO 110.1 (ILS FREQ) AND IT INDICATED WELL LEFT OF COURSE. ATC ADVISED THAT WE NEEDED TO HAVE THE INS CHKED. AT THE GATE WE HAD MAINT CHK THE INS. AT THAT TIME I NOTICED THE CAPT/FO GUARDED INS SWITCH WAS IN THE F/O POS, WHICH WOULD EXPLAIN THE INDICATIONS ON THE CAPT'S INS. HE WAS MONITORING THE ATL VOR INSTEAD OF THE ILS. HOW THE SWITCH GOT IN THE OPEN/FO POS IS STILL A MYSTERY. IT IS POSSIBLE MAINT SWITCHED IT, BUT THAT WOULDN'T EXPLAIN THE APCH AND THE SUBSEQUENT OPERATIONAL CHK OF BOTH VORS. I SUGGEST THAT THE SWITCH BE SAFETY WIRED TO PREVENT FUTURE PROBS WITH IT.

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.