Narrative:

This was an IFR training flight. We had intercepted the localizer, and ATC had cleared us for the approach with instructions to maintain 3000 ft until established, which we did. I noted mentally that the GS flag had pulled and that we had not, at that time, intercepted the GS. Something diverted my attention and when I next looked at the instruments I observed that the instrument student pilot had the aircraft in a normal 500 FPM descent, and that the GS needle was level (which I interpreted as on GS). I knew, however, that I had not heard the marker sound off nor had I seen the marker light illuminate. My first thought was that the marker receiver had failed (as this had happened to me a week before in another airplane). After the aircraft had descended to 2200 ft MSL (800 ft below where we should have been at that point), I realized that the GS receiver flag was showing and that we had not yet crossed the OM (which was a LOM) as confirmed by the ADF needle still pointing in the same direction we were traveling. I immediately told the student to stop the descent, and notified ATC that our GS had failed. Even though we descended 800 ft below the appropriate altitude, ATC never did mention our altitude excursion. What concerns me more than anything, though, is the fact that a ridge is between the middle and OM which we surely would have hit had I not realized the GS flag was not pulled.

Google
 

Original NASA ASRS Text

Title: STUDENT AND INSTRUCTOR ON PRACTICE ILS DSND BELOW ASSIGNED ALT DUE TO GS FAILURE.

Narrative: THIS WAS AN IFR TRAINING FLT. WE HAD INTERCEPTED THE LOC, AND ATC HAD CLRED US FOR THE APCH WITH INSTRUCTIONS TO MAINTAIN 3000 FT UNTIL ESTABLISHED, WHICH WE DID. I NOTED MENTALLY THAT THE GS FLAG HAD PULLED AND THAT WE HAD NOT, AT THAT TIME, INTERCEPTED THE GS. SOMETHING DIVERTED MY ATTN AND WHEN I NEXT LOOKED AT THE INSTS I OBSERVED THAT THE INST STUDENT PLT HAD THE ACFT IN A NORMAL 500 FPM DSCNT, AND THAT THE GS NEEDLE WAS LEVEL (WHICH I INTERPRETED AS ON GS). I KNEW, HOWEVER, THAT I HAD NOT HEARD THE MARKER SOUND OFF NOR HAD I SEEN THE MARKER LIGHT ILLUMINATE. MY FIRST THOUGHT WAS THAT THE MARKER RECEIVER HAD FAILED (AS THIS HAD HAPPENED TO ME A WK BEFORE IN ANOTHER AIRPLANE). AFTER THE ACFT HAD DSNDED TO 2200 FT MSL (800 FT BELOW WHERE WE SHOULD HAVE BEEN AT THAT POINT), I REALIZED THAT THE GS RECEIVER FLAG WAS SHOWING AND THAT WE HAD NOT YET CROSSED THE OM (WHICH WAS A LOM) AS CONFIRMED BY THE ADF NEEDLE STILL POINTING IN THE SAME DIRECTION WE WERE TRAVELING. I IMMEDIATELY TOLD THE STUDENT TO STOP THE DSCNT, AND NOTIFIED ATC THAT OUR GS HAD FAILED. EVEN THOUGH WE DSNDED 800 FT BELOW THE APPROPRIATE ALT, ATC NEVER DID MENTION OUR ALT EXCURSION. WHAT CONCERNS ME MORE THAN ANYTHING, THOUGH, IS THE FACT THAT A RIDGE IS BTWN THE MIDDLE AND OM WHICH WE SURELY WOULD HAVE HIT HAD I NOT REALIZED THE GS FLAG WAS NOT PULLED.

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.