Narrative:

I had requested and was told to expect ILS runway 9L at ZZZ. A few mi from the OM; ATC switched me to ILS 12 for noise abatement. I changed the approach in my avionics but did not sufficiently brief the approach because of the last min notice of the change and the immediate need to intercept the localizer. The controller was so busy that it was almost impossible to break in to the chatter. When I was seconds from blowing through the localizer; I broke in to request turn to intercept. He gave me the turn; told me to maintain 1700 ft until established and cleared for the ILS. I then canceled IFR. He acknowledged. I changed frequency. I appeared to be established on localizer but too high for GS. I descended quickly to catch the GS. I pulled power; speed brakes up; descending more than 1000 FPM and couldn't catch the GS. Thank god it was VMC so at 500 ft AGL; when I could not identify the runway environment; I reapplied power and climbed. When I looked at the GPS; I was still 5 mi from the airport. I had followed a bad GS. False GS's are not supposed to exist below the real GS; but standing water or interference may have played a part. In the end; nothing bad happened. My only potential violation was being too low over people and structures. Had I been IMC; the result may not have been as happy. Probable cause: the rushed changing of the approach did not allow me sufficient time to brief it properly. Had I briefed properly; I would have known to not begin my descent as early as I did. The false GS indication was a contributing factor.

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Original NASA ASRS Text

Title: IN AN ATTEMPT TO DESCEND TO CATCH A FALSE GS; A LANCAIR PLT DESCENDED TO 500 FT BEFORE REALIZING THAT HE WAS 5 MILES FROM THE ARPT.

Narrative: I HAD REQUESTED AND WAS TOLD TO EXPECT ILS RWY 9L AT ZZZ. A FEW MI FROM THE OM; ATC SWITCHED ME TO ILS 12 FOR NOISE ABATEMENT. I CHANGED THE APCH IN MY AVIONICS BUT DID NOT SUFFICIENTLY BRIEF THE APCH BECAUSE OF THE LAST MIN NOTICE OF THE CHANGE AND THE IMMEDIATE NEED TO INTERCEPT THE LOC. THE CTLR WAS SO BUSY THAT IT WAS ALMOST IMPOSSIBLE TO BREAK IN TO THE CHATTER. WHEN I WAS SECONDS FROM BLOWING THROUGH THE LOC; I BROKE IN TO REQUEST TURN TO INTERCEPT. HE GAVE ME THE TURN; TOLD ME TO MAINTAIN 1700 FT UNTIL ESTABLISHED AND CLRED FOR THE ILS. I THEN CANCELED IFR. HE ACKNOWLEDGED. I CHANGED FREQ. I APPEARED TO BE ESTABLISHED ON LOC BUT TOO HIGH FOR GS. I DSNDED QUICKLY TO CATCH THE GS. I PULLED PWR; SPD BRAKES UP; DSNDING MORE THAN 1000 FPM AND COULDN'T CATCH THE GS. THANK GOD IT WAS VMC SO AT 500 FT AGL; WHEN I COULD NOT IDENT THE RWY ENVIRONMENT; I REAPPLIED PWR AND CLBED. WHEN I LOOKED AT THE GPS; I WAS STILL 5 MI FROM THE ARPT. I HAD FOLLOWED A BAD GS. FALSE GS'S ARE NOT SUPPOSED TO EXIST BELOW THE REAL GS; BUT STANDING WATER OR INTERFERENCE MAY HAVE PLAYED A PART. IN THE END; NOTHING BAD HAPPENED. MY ONLY POTENTIAL VIOLATION WAS BEING TOO LOW OVER PEOPLE AND STRUCTURES. HAD I BEEN IMC; THE RESULT MAY NOT HAVE BEEN AS HAPPY. PROBABLE CAUSE: THE RUSHED CHANGING OF THE APCH DID NOT ALLOW ME SUFFICIENT TIME TO BRIEF IT PROPERLY. HAD I BRIEFED PROPERLY; I WOULD HAVE KNOWN TO NOT BEGIN MY DSCNT AS EARLY AS I DID. THE FALSE GS INDICATION WAS A CONTRIBUTING FACTOR.

Data retrieved from NASA's ASRS site as of May 2009 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.