Narrative:

We were on an assigned heading to intercept the approach course just outside of hasdo at 3100 ft. We were cleared for the approach ILS runway 12R. I selected VOR/localizer initially then selected app as the aircraft (on autoplt) intercepted the localizer. I confirmed localizer and GS captured on the flight director annunciator panel. At approximately 2200 ft MSL; I heard the captain/pm state my name and say; 'we are low; start a climb back up to 2800 ft.' as I was climbing back up to 2800 ft I heard 'caution obstacle' and say same annunciation. Threats and errors 1) did not know GS was reported unusable on the ACARS WX report. I hear tower clear another aircraft for the ILS GS unusable approach; but didn't verbalize it and then forgot about it when I was cleared for the approach. 2) WX was marginal VMC so I briefed ILS runway 12R and the visual approach planning on a visual approach. 3) I verbalized VOR/localizer selected but not app selected. 4) when I confirmed localizer and GS were captured; I apparently believed I was on the proper GS and had tunnel vision on that even though after the fact I realized I had intercepted a false GS. I still cannot believe I made such a stupid mistake. 5) ATC did not verbalize 'GS unusable' to us when cleared for the approach. 6) the pm told me after we were on the ground that the tower controller told us twice to climb back up to 2800 ft. I never heard any of those calls. Why I didn't hear them; I do not know. Follow our procedures all the time.

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Original NASA ASRS Text

Title: B737-700 FO ON APCH TO SAT FOLLOWS UNRELIABLE GS AND GETS LOW ALTITUDE WARNING FROM CAPT AND TOWER.

Narrative: WE WERE ON AN ASSIGNED HDG TO INTERCEPT THE APCH COURSE JUST OUTSIDE OF HASDO AT 3100 FT. WE WERE CLRED FOR THE APCH ILS RWY 12R. I SELECTED VOR/LOC INITIALLY THEN SELECTED APP AS THE ACFT (ON AUTOPLT) INTERCEPTED THE LOC. I CONFIRMED LOC AND GS CAPTURED ON THE FLT DIRECTOR ANNUNCIATOR PANEL. AT APPROX 2200 FT MSL; I HEARD THE CAPT/PM STATE MY NAME AND SAY; 'WE ARE LOW; START A CLB BACK UP TO 2800 FT.' AS I WAS CLBING BACK UP TO 2800 FT I HEARD 'CAUTION OBSTACLE' AND SAY SAME ANNUNCIATION. THREATS AND ERRORS 1) DID NOT KNOW GS WAS RPTED UNUSABLE ON THE ACARS WX RPT. I HEAR TWR CLR ANOTHER ACFT FOR THE ILS GS UNUSABLE APCH; BUT DIDN'T VERBALIZE IT AND THEN FORGOT ABOUT IT WHEN I WAS CLRED FOR THE APCH. 2) WX WAS MARGINAL VMC SO I BRIEFED ILS RWY 12R AND THE VISUAL APCH PLANNING ON A VISUAL APCH. 3) I VERBALIZED VOR/LOC SELECTED BUT NOT APP SELECTED. 4) WHEN I CONFIRMED LOC AND GS WERE CAPTURED; I APPARENTLY BELIEVED I WAS ON THE PROPER GS AND HAD TUNNEL VISION ON THAT EVEN THOUGH AFTER THE FACT I REALIZED I HAD INTERCEPTED A FALSE GS. I STILL CANNOT BELIEVE I MADE SUCH A STUPID MISTAKE. 5) ATC DID NOT VERBALIZE 'GS UNUSABLE' TO US WHEN CLRED FOR THE APCH. 6) THE PM TOLD ME AFTER WE WERE ON THE GND THAT THE TWR CTLR TOLD US TWICE TO CLB BACK UP TO 2800 FT. I NEVER HEARD ANY OF THOSE CALLS. WHY I DIDN'T HEAR THEM; I DO NOT KNOW. FOLLOW OUR PROCS ALL THE TIME.

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.