Narrative:

Cleared for ILS approach, given a heading to intercept ILS 23L at rdu. NAVAID was idented (ILS and NDB) on both #1 and #2 receivers, along with NDB. #3 receiver was tuned to rdu VOR, idented and the DME was held from the #2 for distance reference from VOR. Just prior to OM we were handed off to tower. I questioned either approach or tower when I noted some fluctuations of the needle left and right which seemed strange with light winds (3 dots to full scale). As we chased the needle around it seemed like it may be returning to normal when at about 1000' MSL it went full left and right. At that time I commanded my first officer to go around and at that moment we got ground contact and parts of the airport environment in sight. We continued to a normal landing. The problem was we were about halfway between the parallel runways, maybe as much as 1/8 of a mi to the right of the centerline of runway 23L. It seems that the only thing we could have done was to execute a missed at the first sign of problems, although this happens fairly often as you are just intercepting the ILS and may cause false or unnecessary go around's.

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Original NASA ASRS Text

Title: PIC OF COMMUTER LTT REPORTERS LOCALIZER FLUCTUATIONS AT OUTER MARKER WHILE IAP ILS APCH BEING MADE.

Narrative: CLRED FOR ILS APCH, GIVEN A HDG TO INTERCEPT ILS 23L AT RDU. NAVAID WAS IDENTED (ILS AND NDB) ON BOTH #1 AND #2 RECEIVERS, ALONG WITH NDB. #3 RECEIVER WAS TUNED TO RDU VOR, IDENTED AND THE DME WAS HELD FROM THE #2 FOR DISTANCE REF FROM VOR. JUST PRIOR TO OM WE WERE HANDED OFF TO TWR. I QUESTIONED EITHER APCH OR TWR WHEN I NOTED SOME FLUCTUATIONS OF THE NEEDLE LEFT AND RIGHT WHICH SEEMED STRANGE WITH LIGHT WINDS (3 DOTS TO FULL SCALE). AS WE CHASED THE NEEDLE AROUND IT SEEMED LIKE IT MAY BE RETURNING TO NORMAL WHEN AT ABOUT 1000' MSL IT WENT FULL LEFT AND RIGHT. AT THAT TIME I COMMANDED MY F/O TO GO AROUND AND AT THAT MOMENT WE GOT GND CONTACT AND PARTS OF THE ARPT ENVIRONMENT IN SIGHT. WE CONTINUED TO A NORMAL LNDG. THE PROB WAS WE WERE ABOUT HALFWAY BTWN THE PARALLEL RWYS, MAYBE AS MUCH AS 1/8 OF A MI TO THE RIGHT OF THE CENTERLINE OF RWY 23L. IT SEEMS THAT THE ONLY THING WE COULD HAVE DONE WAS TO EXECUTE A MISSED AT THE FIRST SIGN OF PROBS, ALTHOUGH THIS HAPPENS FAIRLY OFTEN AS YOU ARE JUST INTERCEPTING THE ILS AND MAY CAUSE FALSE OR UNNECESSARY GAR'S.

Data retrieved from NASA's ASRS site as of August 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.