Narrative:

I was flying an small aircraft on an IFR flight plan from alw to geg, landing geg, which was reporting ceiling 300-500', 1 mi in fog, ILS 21 approach in use. Flight proceeded west/O problem and I was handed off from ZSE to geg approach, which began giving me vectors and altitudes to intercept the localizer. I set up my radios for the ILS and I idented the ILS by its code. Neither the navigation nor the G/south warning flags were visible. Everything going well. Eventually I was cleared to maintain 4000', heading 270 degrees. While turning and descending I was given a heading of 220 degrees, maintain 4000' intercept localizer, cleared for approach. The published G/south intercept altitude was 4300'. Inbound course was 205 degrees.das I was turning to 220 degrees it was obvious that I would need to continue turning to 205 degrees to be on the localizer course. I also noticed then that I had a G/south warning flag. I did manage to get on the localizer course, but that G/south warning flag held my attention. While focused on the warning flag, I allowed the airplane to descend below 4000'--even below the published localizer only minimum altitude. As soon as I noticed that--perhaps only seconds, perhaps longer--I climbed back to the localizer only published altitude for the intermediate segment of the approach. (I was still outside the FAF). I planned to fly the approach as a localizer west/O G/south. On passing the FAF I began descent and was preparing to call tower to report losing G/south when the G/south came alive and the warning flag disappeared. I then continued the approach as a full ILS and made an uneventful landing. All in all, a lousy approach. I was preoccupied with the G/south warning flag when it appeared. I should have concentrated on my basic flying of the airplane rather than staring at the flag wondering what had happened. I question the practice of vectoring an airplane to an ILS at below the published G/south intercept altitude, especially in IMC. This, plus the turn apparently blanked out the G/south receiver antenna, at least temporarily. When the warning flag appeared I should have immediately converted to localizer only procedure or declared a missed approach and figure things out later. I would be very interested to hear from someone re: the practice of vectoring aircraft to intercept the ILS at below the published G/south intercept altitude. Callback conversation with reporter revealed the following: the reporter has discussed the questioned procedure with other pilots and satisfied himself that all was normal and acceptable.

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Original NASA ASRS Text

Title: GA SMA ALT DEVIATION EXCURSION FROM CLRNC ALT WHILE BEING VECTORED FOR LOCALIZER INTERCEPT DUE TO PLT DISTR OF GLIDE SLOPE WARNING FLAG.

Narrative: I WAS FLYING AN SMA ON AN IFR FLT PLAN FROM ALW TO GEG, LNDG GEG, WHICH WAS RPTING CEILING 300-500', 1 MI IN FOG, ILS 21 APCH IN USE. FLT PROCEEDED W/O PROB AND I WAS HANDED OFF FROM ZSE TO GEG APCH, WHICH BEGAN GIVING ME VECTORS AND ALTS TO INTERCEPT THE LOC. I SET UP MY RADIOS FOR THE ILS AND I IDENTED THE ILS BY ITS CODE. NEITHER THE NAV NOR THE G/S WARNING FLAGS WERE VISIBLE. EVERYTHING GOING WELL. EVENTUALLY I WAS CLRED TO MAINTAIN 4000', HDG 270 DEGS. WHILE TURNING AND DSNDING I WAS GIVEN A HDG OF 220 DEGS, MAINTAIN 4000' INTERCEPT LOC, CLRED FOR APCH. THE PUBLISHED G/S INTERCEPT ALT WAS 4300'. INBND COURSE WAS 205 DEGS.DAS I WAS TURNING TO 220 DEGS IT WAS OBVIOUS THAT I WOULD NEED TO CONTINUE TURNING TO 205 DEGS TO BE ON THE LOC COURSE. I ALSO NOTICED THEN THAT I HAD A G/S WARNING FLAG. I DID MANAGE TO GET ON THE LOC COURSE, BUT THAT G/S WARNING FLAG HELD MY ATTN. WHILE FOCUSED ON THE WARNING FLAG, I ALLOWED THE AIRPLANE TO DSND BELOW 4000'--EVEN BELOW THE PUBLISHED LOC ONLY MINIMUM ALT. AS SOON AS I NOTICED THAT--PERHAPS ONLY SECONDS, PERHAPS LONGER--I CLBED BACK TO THE LOC ONLY PUBLISHED ALT FOR THE INTERMEDIATE SEGMENT OF THE APCH. (I WAS STILL OUTSIDE THE FAF). I PLANNED TO FLY THE APCH AS A LOC W/O G/S. ON PASSING THE FAF I BEGAN DSCNT AND WAS PREPARING TO CALL TWR TO RPT LOSING G/S WHEN THE G/S CAME ALIVE AND THE WARNING FLAG DISAPPEARED. I THEN CONTINUED THE APCH AS A FULL ILS AND MADE AN UNEVENTFUL LNDG. ALL IN ALL, A LOUSY APCH. I WAS PREOCCUPIED WITH THE G/S WARNING FLAG WHEN IT APPEARED. I SHOULD HAVE CONCENTRATED ON MY BASIC FLYING OF THE AIRPLANE RATHER THAN STARING AT THE FLAG WONDERING WHAT HAD HAPPENED. I QUESTION THE PRACTICE OF VECTORING AN AIRPLANE TO AN ILS AT BELOW THE PUBLISHED G/S INTERCEPT ALT, ESPECIALLY IN IMC. THIS, PLUS THE TURN APPARENTLY BLANKED OUT THE G/S RECEIVER ANTENNA, AT LEAST TEMPORARILY. WHEN THE WARNING FLAG APPEARED I SHOULD HAVE IMMEDIATELY CONVERTED TO LOC ONLY PROC OR DECLARED A MISSED APCH AND FIGURE THINGS OUT LATER. I WOULD BE VERY INTERESTED TO HEAR FROM SOMEONE RE: THE PRACTICE OF VECTORING ACFT TO INTERCEPT THE ILS AT BELOW THE PUBLISHED G/S INTERCEPT ALT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: THE RPTR HAS DISCUSSED THE QUESTIONED PROC WITH OTHER PLTS AND SATISFIED HIMSELF THAT ALL WAS NORMAL AND ACCEPTABLE.

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.