Narrative:

On an ILS final approach to bna, #1 navigation (GPS), which is linked to an HSI, did not activate and capture the localizer and GS. I had set up #2 navigation (GPS), which is linked to a conventional CDI, to function as a normal localizer receiver, which did capture the localizer and GS. Realizing I had conflicting information between the 2 navs, I turned onto the final approach course using the #2 navigation and waited for the #1 navigation to activate and capture the localizer and GS. It never did. While attempting to resolve the information conflict between the 2 navs, I inadvertently descended below the GS. Just as I realized I was low and began to climb back up to the GS I was informed by bna tower that I had set off the low altitude alert. I regained the GS using the #2 navigation and landed uneventfully. After landing I analyzed what had happened and determined that although the #1 navigation was properly set for the approach it did not automatically switch to the proper localizer frequency and to localizer mode as it has done numerous times before, and as I had expected it to do. As a result, on all ILS and localizer approachs I now manually switch it to the correct frequency, and mode, when appropriate to do so during the approach.

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Original NASA ASRS Text

Title: PLT OF CIRRUS SR22 DSNDED BELOW GS ON APCH TO BNA WHILE TROUBLESHOOTING MALFUNCTIONING GPS-HSI. ATCT LCL CTLR RECEIVED MSAW AND ALERTED PLT WHO SALVAGED THE APCH AND LANDED SAFELY.

Narrative: ON AN ILS FINAL APCH TO BNA, #1 NAV (GPS), WHICH IS LINKED TO AN HSI, DID NOT ACTIVATE AND CAPTURE THE LOC AND GS. I HAD SET UP #2 NAV (GPS), WHICH IS LINKED TO A CONVENTIONAL CDI, TO FUNCTION AS A NORMAL LOC RECEIVER, WHICH DID CAPTURE THE LOC AND GS. REALIZING I HAD CONFLICTING INFO BTWN THE 2 NAVS, I TURNED ONTO THE FINAL APCH COURSE USING THE #2 NAV AND WAITED FOR THE #1 NAV TO ACTIVATE AND CAPTURE THE LOC AND GS. IT NEVER DID. WHILE ATTEMPTING TO RESOLVE THE INFO CONFLICT BTWN THE 2 NAVS, I INADVERTENTLY DSNDED BELOW THE GS. JUST AS I REALIZED I WAS LOW AND BEGAN TO CLB BACK UP TO THE GS I WAS INFORMED BY BNA TWR THAT I HAD SET OFF THE LOW ALT ALERT. I REGAINED THE GS USING THE #2 NAV AND LANDED UNEVENTFULLY. AFTER LNDG I ANALYZED WHAT HAD HAPPENED AND DETERMINED THAT ALTHOUGH THE #1 NAV WAS PROPERLY SET FOR THE APCH IT DID NOT AUTOMATICALLY SWITCH TO THE PROPER LOC FREQ AND TO LOC MODE AS IT HAS DONE NUMEROUS TIMES BEFORE, AND AS I HAD EXPECTED IT TO DO. AS A RESULT, ON ALL ILS AND LOC APCHS I NOW MANUALLY SWITCH IT TO THE CORRECT FREQ, AND MODE, WHEN APPROPRIATE TO DO SO DURING THE APCH.

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.