Narrative:

WX: 900 ceiling and 20 mi visibility. While on an IFR flight and receiving vector to the VOR-a final approach course during continuous light to mod turbulence, I was vectored very close to the final approach fix. The cross radial being used to determine the decot intersection was 051 R-024. As I approached decot the CDI showing the osi, R-024 began to fluctuate +/- 6 either side of center. I continued the approach estimating the osi, R-024 to be in the center of the fluctuations. Abut 1 minute after starting final descent from the decot the tower advised 'low altitude alert' and restated the altimeter setting. I confirmed the altimeter setting and my altitude of 1200 MSL and tower had no further comment. If I was truly past decot inbound then I was not below minimums. But if the fluctuation VOR-CDI average deviation did not accurately show decot, then my final descent to minimums was started before the final approach fix (decot). Approach was continued (breaking out of the overcast at 900' MSL) without further unusual event. Corrective action: 1) execute the missed approach when the CDI fluctuates. 2) next time try using sfd R-076 to determine decot. 3) have 'flight check' check out the use of osi R-024. At least 2 other times while shooting this approach in VFR conditions I have shown the same type of degree of CDI fluctuation while approaching decot. There may be some type of interference causing this CDI fluctuation to take place.

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Original NASA ASRS Text

Title: CTLR ADVISED 'LOW ALT ALERT'. RESTATED ALTIMETER SETTING. PLT STATED HE HAD ALTIMETER AND HIS PRESENT ALT. NO FURTHER COMMENT.

Narrative: WX: 900 CEILING AND 20 MI VISIBILITY. WHILE ON AN IFR FLT AND RECEIVING VECTOR TO THE VOR-A FINAL APCH COURSE DURING CONTINUOUS LIGHT TO MOD TURBULENCE, I WAS VECTORED VERY CLOSE TO THE FINAL APCH FIX. THE CROSS RADIAL BEING USED TO DETERMINE THE DECOT INTXN WAS 051 R-024. AS I APCHED DECOT THE CDI SHOWING THE OSI, R-024 BEGAN TO FLUCTUATE +/- 6 EITHER SIDE OF CENTER. I CONTINUED THE APCH ESTIMATING THE OSI, R-024 TO BE IN THE CENTER OF THE FLUCTUATIONS. ABUT 1 MINUTE AFTER STARTING FINAL DSCNT FROM THE DECOT THE TWR ADVISED 'LOW ALT ALERT' AND RESTATED THE ALTIMETER SETTING. I CONFIRMED THE ALTIMETER SETTING AND MY ALT OF 1200 MSL AND TWR HAD NO FURTHER COMMENT. IF I WAS TRULY PAST DECOT INBND THEN I WAS NOT BELOW MINIMUMS. BUT IF THE FLUCTUATION VOR-CDI AVERAGE DEVIATION DID NOT ACCURATELY SHOW DECOT, THEN MY FINAL DSCNT TO MINIMUMS WAS STARTED BEFORE THE FINAL APCH FIX (DECOT). APCH WAS CONTINUED (BREAKING OUT OF THE OVERCAST AT 900' MSL) WITHOUT FURTHER UNUSUAL EVENT. CORRECTIVE ACTION: 1) EXECUTE THE MISSED APCH WHEN THE CDI FLUCTUATES. 2) NEXT TIME TRY USING SFD R-076 TO DETERMINE DECOT. 3) HAVE 'FLT CHECK' CHECK OUT THE USE OF OSI R-024. AT LEAST 2 OTHER TIMES WHILE SHOOTING THIS APCH IN VFR CONDITIONS I HAVE SHOWN THE SAME TYPE OF DEG OF CDI FLUCTUATION WHILE APCHING DECOT. THERE MAY BE SOME TYPE OF INTERFERENCE CAUSING THIS CDI FLUCTUATION TO TAKE PLACE.

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.