Narrative:

Descending to ILS approach runway 22 reported WX 4600 scattered, higher (7000?) broken, light rain expecting high rate of descent (2500 FPM) to below clouds and proceed visually, while monitoring ILS on autoplt. Heavy rain experienced after localizer intercept high descent rate through GS caused autoplt to pull up quickly after going 3 dots below GS, autoplt pull-up resulted in too quick climb and I disconnected autoplt to fly manually. After 30-45 seconds of maintaining within 1 DOT localizer and a consistent 1 DOT high on GS, GS starting fluctuating wildly full scale both directions. I then called tower to ask if there had been any GS problems, and he said no, and that I was over the airport and to say intentions (I was about 4000-4500 ft AGL in IMC). I asked for vectors for another approach, and made an uneventful hand-flown ILS. Ceiling was about 1200 ft visibility 4600 scattered that had been reported. Since the event, I contacted a former instructor of mine and related this episode to him, since I was confused as to what had happened, as I thought I was maintaining a good approach path (though I did know I had passed the OM, I did not rechk my altitude -- only airspeed, vertical speed, and HSI/GS). He immediately assessed the situation as a 'false GS', something that apparently occurs during an ILS capture from a high altitude. I was not aware of this anomaly. To prevent further occurrences I will: 1) not try either a manual or autoplt high descent rate ILS capture. 2) expect worse WX than reported (I was not mentally prepared for the full procedure). 3) xchk altitude at OM, and during final approach. 4) respect the possibility of a 'false GS', and not attempt capturing GS from high altitude.

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Original NASA ASRS Text

Title: SMT IS FORCED TO EXECUTE MISSED APCH AFTER FINDING HIMSELF MORE THAN 4000 FT ABOVE THE ARPT WHILE FLYING AN ILS APCH.

Narrative: DSNDING TO ILS APCH RWY 22 RPTED WX 4600 SCATTERED, HIGHER (7000?) BROKEN, LIGHT RAIN EXPECTING HIGH RATE OF DSCNT (2500 FPM) TO BELOW CLOUDS AND PROCEED VISUALLY, WHILE MONITORING ILS ON AUTOPLT. HVY RAIN EXPERIENCED AFTER LOC INTERCEPT HIGH DSCNT RATE THROUGH GS CAUSED AUTOPLT TO PULL UP QUICKLY AFTER GOING 3 DOTS BELOW GS, AUTOPLT PULL-UP RESULTED IN TOO QUICK CLB AND I DISCONNECTED AUTOPLT TO FLY MANUALLY. AFTER 30-45 SECONDS OF MAINTAINING WITHIN 1 DOT LOC AND A CONSISTENT 1 DOT HIGH ON GS, GS STARTING FLUCTUATING WILDLY FULL SCALE BOTH DIRECTIONS. I THEN CALLED TWR TO ASK IF THERE HAD BEEN ANY GS PROBLEMS, AND HE SAID NO, AND THAT I WAS OVER THE ARPT AND TO SAY INTENTIONS (I WAS ABOUT 4000-4500 FT AGL IN IMC). I ASKED FOR VECTORS FOR ANOTHER APCH, AND MADE AN UNEVENTFUL HAND-FLOWN ILS. CEILING WAS ABOUT 1200 FT VISIBILITY 4600 SCATTERED THAT HAD BEEN RPTED. SINCE THE EVENT, I CONTACTED A FORMER INSTRUCTOR OF MINE AND RELATED THIS EPISODE TO HIM, SINCE I WAS CONFUSED AS TO WHAT HAD HAPPENED, AS I THOUGHT I WAS MAINTAINING A GOOD APCH PATH (THOUGH I DID KNOW I HAD PASSED THE OM, I DID NOT RECHK MY ALT -- ONLY AIRSPD, VERT SPD, AND HSI/GS). HE IMMEDIATELY ASSESSED THE SITUATION AS A 'FALSE GS', SOMETHING THAT APPARENTLY OCCURS DURING AN ILS CAPTURE FROM A HIGH ALT. I WAS NOT AWARE OF THIS ANOMALY. TO PREVENT FURTHER OCCURRENCES I WILL: 1) NOT TRY EITHER A MANUAL OR AUTOPLT HIGH DSCNT RATE ILS CAPTURE. 2) EXPECT WORSE WX THAN RPTED (I WAS NOT MENTALLY PREPARED FOR THE FULL PROC). 3) XCHK ALT AT OM, AND DURING FINAL APCH. 4) RESPECT THE POSSIBILITY OF A 'FALSE GS', AND NOT ATTEMPT CAPTURING GS FROM HIGH ALT.

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.