Narrative:

Descent below minimums while on ILS instrument approach in IMC. Had intercepted localizer. Approach had advised me to tell tower when I was LOM inbound. I had the VOR/DME, ADF and marker beacon indicators set up to tell me when I reached the LOM. The GS started going down rapidly. I advised tower 'OM inbound' and began to descend to catch the GS. After a while, I descended slightly below the GS. I added power. I believe that I may have descended almost 1000 ft below MDA. I continued to add power. I noticed that my descent had stopped. I started to climb. The ADF needle was pointing straight ahead. I continued to climb until I was back up to the MDA. I was concerned and confused until I saw that the DME indicated that I had not reached the LOM. I continued to the LOM, made the approach and landed. As best I can tell, I must have gotten a false signal on the GS and in my rush to chase it down, I forgot to notice that the marker beacon indicators never went off. (In some cases it is normal to begin your descent before reaching the LOM, but not on this particular approach.) this was my first solo instrument approach to minimums. I had been in the aircraft for over 5 hours. Lesson learned: don't trust the GS -- use your xchks.

Related reports:

Google
 

Original NASA ASRS Text

Title: PLT OF AN SMA SEL ACFT DSNDED BELOW APCH OM MINIMUMS AFTER FOLLOWING AN ERRONEOUS GS INDICATION.

Narrative: DSCNT BELOW MINIMUMS WHILE ON ILS INST APCH IN IMC. HAD INTERCEPTED LOC. APCH HAD ADVISED ME TO TELL TWR WHEN I WAS LOM INBOUND. I HAD THE VOR/DME, ADF AND MARKER BEACON INDICATORS SET UP TO TELL ME WHEN I REACHED THE LOM. THE GS STARTED GOING DOWN RAPIDLY. I ADVISED TWR 'OM INBOUND' AND BEGAN TO DSND TO CATCH THE GS. AFTER A WHILE, I DSNDED SLIGHTLY BELOW THE GS. I ADDED PWR. I BELIEVE THAT I MAY HAVE DSNDED ALMOST 1000 FT BELOW MDA. I CONTINUED TO ADD PWR. I NOTICED THAT MY DSCNT HAD STOPPED. I STARTED TO CLB. THE ADF NEEDLE WAS POINTING STRAIGHT AHEAD. I CONTINUED TO CLB UNTIL I WAS BACK UP TO THE MDA. I WAS CONCERNED AND CONFUSED UNTIL I SAW THAT THE DME INDICATED THAT I HAD NOT REACHED THE LOM. I CONTINUED TO THE LOM, MADE THE APCH AND LANDED. AS BEST I CAN TELL, I MUST HAVE GOTTEN A FALSE SIGNAL ON THE GS AND IN MY RUSH TO CHASE IT DOWN, I FORGOT TO NOTICE THAT THE MARKER BEACON INDICATORS NEVER WENT OFF. (IN SOME CASES IT IS NORMAL TO BEGIN YOUR DSCNT BEFORE REACHING THE LOM, BUT NOT ON THIS PARTICULAR APCH.) THIS WAS MY FIRST SOLO INST APCH TO MINIMUMS. I HAD BEEN IN THE ACFT FOR OVER 5 HRS. LESSON LEARNED: DON'T TRUST THE GS -- USE YOUR XCHKS.

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.