Narrative:

On a vector to intercept the final approach for the ILS to runway 8 at sju in between patty OM and wesen intersection, the autoplt captured what appeared to be a false GS signal. The MCP was on approach, the autoplt engaged, and the aircraft was at about 2000 ft descending to an assigned altitude of 1800 ft about 7 NM from the runway and intercepting the localizer on the extended centerline. The GS indicated on glide path before MCP approach selection. The aircraft was only about 2.5 NM from the OM descending through 2000 ft with a GS crossing altitude at the marker of 1552 ft with an on glide path indication and joining the localizer when approach was selected on the MCP. Immediately after selection of approach, the GS deviation scale went from no deviation to more than 1 DOT below glide path and at the selected MCP airspeed of 180 KTS the autoplt started a climb for the new GS indication from leveling at the assigned altitude of 1800 ft. Autoplt and autothrottles were disconnected and the aircraft was returned to the proper altitude, but not before first climbing to 2500 ft. I was the PF and was totally taken by surprise by the automation actions. I immediately disconnected autoplt and autothrottles but it took me a few seconds to interpretation the instruments and this delay caused the altitude deviation. I have become automation dependent and was not monitoring the autoplt as I should have been. I'll make efforts to improve in this area immediately.

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Original NASA ASRS Text

Title: B737 FLC SLOW TO REACT TO ERRONEOUS GS INDICATION DEVIATES FROM ASSIGNED ALT.

Narrative: ON A VECTOR TO INTERCEPT THE FINAL APCH FOR THE ILS TO RWY 8 AT SJU IN BTWN PATTY OM AND WESEN INTXN, THE AUTOPLT CAPTURED WHAT APPEARED TO BE A FALSE GS SIGNAL. THE MCP WAS ON APCH, THE AUTOPLT ENGAGED, AND THE ACFT WAS AT ABOUT 2000 FT DSNDING TO AN ASSIGNED ALT OF 1800 FT ABOUT 7 NM FROM THE RWY AND INTERCEPTING THE LOC ON THE EXTENDED CTRLINE. THE GS INDICATED ON GLIDE PATH BEFORE MCP APCH SELECTION. THE ACFT WAS ONLY ABOUT 2.5 NM FROM THE OM DSNDING THROUGH 2000 FT WITH A GS XING ALT AT THE MARKER OF 1552 FT WITH AN ON GLIDE PATH INDICATION AND JOINING THE LOC WHEN APCH WAS SELECTED ON THE MCP. IMMEDIATELY AFTER SELECTION OF APCH, THE GS DEV SCALE WENT FROM NO DEV TO MORE THAN 1 DOT BELOW GLIDE PATH AND AT THE SELECTED MCP AIRSPD OF 180 KTS THE AUTOPLT STARTED A CLB FOR THE NEW GS INDICATION FROM LEVELING AT THE ASSIGNED ALT OF 1800 FT. AUTOPLT AND AUTOTHROTTLES WERE DISCONNECTED AND THE ACFT WAS RETURNED TO THE PROPER ALT, BUT NOT BEFORE FIRST CLBING TO 2500 FT. I WAS THE PF AND WAS TOTALLY TAKEN BY SURPRISE BY THE AUTOMATION ACTIONS. I IMMEDIATELY DISCONNECTED AUTOPLT AND AUTOTHROTTLES BUT IT TOOK ME A FEW SECONDS TO INTERP THE INSTS AND THIS DELAY CAUSED THE ALTDEV. I HAVE BECOME AUTOMATION DEPENDENT AND WAS NOT MONITORING THE AUTOPLT AS I SHOULD HAVE BEEN. I'LL MAKE EFFORTS TO IMPROVE IN THIS AREA IMMEDIATELY.

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.